模拟氮沉降对杉木人工林碳库及其化学机理的影响
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摘要
碳是组成生物体最重要的元素之一,碳的循环是全球各种生命活动最重要的形式之一。森林是陆地生态系统的主体,蕴藏着陆地碳库总量的46.3%,森林植被所维持的碳库占到陆地植被碳库的比例高达77.0%,因此森林生态系统碳循环是全球最主要的碳循环,对于维护全球生态安全,进而维护人类社会的可持续发展具有十分重要的作用。
人类影响全球碳循环的方式主要有三种:一是大肆砍伐森林使林地变成了农业用地及农业耕作方式的改变;二是人类社会大量使用化石燃料;三是人类生活、生产排放的一些化学物质。前两种方式直接导致了全球碳库的损失,成为碳源;第三种方式目前主要指的是酸沉降,酸沉降包括硫沉降和氮沉降;两者在全球碳循环中的作用,与酸沉降的程度、持续时间及其所在的地理生态环境有关联。随着人类社会的发展,不断增长的氮沉降量对全球碳循环的负面作用越来越大,受到了国际社会的关注,尤其是在欧洲和北美温带地区。我国有些地区也存在高氮沉降问题,有些学者甚至指出,我国已成为世界三大氮沉降集中区之一,有关氮沉降研究的项目已经开始启动。
杉木是我国南方重要的用材树种之一,自然分布和人工栽培都很辽阔。杉木生长快,意味着对碳的吸收也快,因此对降低大气中的CO2浓度有十分明显的效果,从这个角度看,杉木栽培对减缓全球变化有十分重要的作用。研究氮沉降对杉木人工林生态系统的影响,既可以弥补我国该研究领域的不足,为杉木人工林的可持续经营提供理论指导,同时为人们进一步开展全球变化研究打下基础。本文对福建省三明市一片立地条件相似的12年生杉木人工人进行为期5年的模拟氮沉降,氮沉降水平分为N0、N1、N2和N3等4种水平,氮沉降量依次为0、60、120和240 kg·hm~(-2)·a-1,每处理重复三次,以期研究氮沉降对森林生态系统的碳库及其有关化学生态机理的影响,研究结果如下:
1.氮沉降对森林生态系统碳库的影响
1.1氮沉降对森林乔木层活立木生物量及碳储量的影响
经过5年的模拟氮沉降试验,各处理样地林分乔木层活立木生物量增长量大小表现为N1>N0>N2>N3。N0、N1、N2和N3处理样地林分杉木活立木生物量增量分别为53.690、60.663、43.402和41.051 t·hm~(-2),N1、N2和N3处理的样地林分杉木活立木生物量增量分别比对照处理增大了12.99%、-19.16%和~(-2)3.54%;N1处理促进了林分乔木层杉木生物量的增加;N2、N3处理有不同程度地抑制样地林分杉木乔木层生物量的增加;氮沉降处理5年后,各处理样地林分乔木层碳储量增量大小顺序表现为N1>N0>N2>N3;N0、N1、N2和N3处理样地林分乔木层杉木碳储量分别增长了24.048、26.810、20.215和18.746 t·hm~(-2),与对照处理相比,N1、N2和N3处理样地林分乔木层碳储量增长量提高了11.49%、-15.94%和~(-2)2.05%。结果表明,与对照处理相比,N1处理促进了林分乔木层杉木碳储量的增长,N2、N3处理对林分杉木碳储量的增加有抑制作用。
1.2氮沉降对杉木人工林均凋落物碳通量的影响
经过N0、N1、N2和N3处理三年(2006.12)后的各林分凋落物碳储量比处理一年(2004.12)后的各林分凋落物碳储量分别增加了52.9、114.4、157.2和92.6 kg·hm~(-2),分别增加了4.89%、11.53%、15.38%和8.46%;与对照相比,N1、N2和N3处理林分凋落物碳素量分别提高了6.64%、10.49%和3.57%,由此计算得,由于氮沉降而使得各林分的凋落物碳储量分别增加了65.9、107.2和39.0 kg·hm~(-2)。如果把这一数据折算成CO2,则N1、N2、N3处理使得林分每年以凋落物的形式分别额外储存了CO2120.8、196.6和71.6 kg·hm~(-2)。
1.3氮沉降对林下植被碳储量的影响
通过四年的模拟氮沉降试验后, N1、N2和N3处理林分林下草本植物碳储量分别比对照处理(N0处理)降低了0.072、0.136和0.167 t·hm~(-2);而N1、N2和N3处理林分林下灌木碳储量分别比对照处理降低了0.092、0.237和0.314 t·hm~(-2);综合来看,经过四年氮沉降试验后,各处理林分林下植被碳储量大小表现为N0>N1>N2>N3,与对照相比,N1、N2、N3处理林分林下植被碳储量分别降低了0.164、0.373和0.482 t·hm~(-2),平均每年减少0.041、0.093和0.120 t·hm~(-2),换算成CO2的量,相当N1、N2和N3处理的林分林下植被每年吸存的二氧化碳量减少了0.151、0.342和0.441 t·hm~(-2)。结果表明,氮沉降水平越高,所处理样地林分林下植被碳储量就越少.
1.4氮沉降对森林土壤碳库的影响
土壤表层的碳含量受到氮沉降的影响最大,而且随着氮沉降的持续,表层土壤碳含量不断降低,但降低的速度渐趋缓慢;氮沉降水平越高,表层土壤碳含量降低越多;中层和底层土壤碳含量在氮沉降的前两年逐渐降低,但在随后的两年里,却有不断升高。综合起来,整个土壤层的碳含量在氮沉降试验的前两年,有降低的趋势,随后逐渐升高。经过四年的试验后发现,N1、N2和N3处理林分土壤碳含量略有降低,N2处理的降低幅度最大,其次是N3处理,N1处理降低最小。从氮沉降的高低水平来看,似乎没有规律性,说明氮沉降量对土壤碳库影响的非线性关系。虽然氮沉降降低了土壤的碳含量,但由于氮沉降同时增加了土壤的密度,所以综合起来,氮沉降反而提高了土壤碳库的碳储量。经过四年的氮沉降处理,N0、N1、N2、N3处理的林分样地土壤的碳储量分别增加了5.97、7.33、6.42和14.89 t·hm~(-2),与对照处理比较,四年的N1、N2和N3处理分别使林分样地土壤碳储量增加了1.36、0.45和4.84 t·hm~(-2)。结果表明,不同的氮沉降处理会显著影响林分土壤碳储量,各氮沉降水平处理之间没有明显的规律性。如果换算成CO2的量,经过N1、N2和N3处理,与对照处理相比,在本研究四年期间,各林分土壤每年分别吸收了1.25、0.41和4.44 t·hm~(-2)的CO2。
2.氮沉降对森林化学生态过程的影响
2.1氮沉降对森林土壤pH值的影响
氮沉降对森林各层土壤pH的影响呈现以下特点:0～20cm土层较20～40cm、40～60cm土层敏感,上层土壤比下层土壤越容易引起酸化;pH值的下降程度与氮沉降量呈正相关,氮沉降时间越长,土壤pH值下降程度越大,土壤酸化也就越明显。在氮沉降试验过程中,各处理样地pH值都出现了降低的现象,但模拟氮沉降试验初期土壤pH值的下降幅度要明显低于氮沉降两年后的土壤pH的下降幅度,尤其是N1处理下的20～60cm土壤层,在氮沉降初期,变化极不明显。
2.2氮沉降对森林土壤有效养分含量的影响
在整个氮沉降过程中,与对照相比,各处理样地有效氮含量(氨态氮和硝态氮)都呈增长趋势,且氮沉降水平越高,其增长程度越大。各处理样地土壤有效氮含量随着时间的推进,与对照的差距越来越大,由此可以说明氮沉降对土壤有效氮的影响存在累积性效应;氮沉降对杉木人工林土壤中的速效磷含量的影响如下:氮沉降造成土壤中的速效磷的衰减;在氮沉降初期,氮沉降水平越高,土壤速效磷的衰减程度更大;但土壤中的速效磷并不随氮沉降量的增大而加速衰减;氮沉降导致0—20cm土层速效钾更大程度的淋失,而20—40cm、40—60cm土层出现了速效钾含量低于对照的变化,土层的速效钾含量随氮沉降量的增加而上升。总体来说,越大程度的氮沉降引起表层土壤更大程度的速效钾淋失;氮沉降加剧土壤速效钾向更深一层土壤运移;土壤速效钾的淋失随氮沉降的持续而加强;同层土壤中交换性Ca~(2+)、Mg~(2+)含量因不同氮沉降水平而异,不同氮沉降水平土壤交换性Ca~(2+)、Mg~(2+)含量从高到低的顺序为:N0>N1>N3>N2,且N0、N1、N3与N2处理水平之间差异显著(p<0.05),说明各土层对N2水平氮沉降反应更为敏感。同氮沉降水平同层土壤交换性Ca~(2+)、Mg~(2+)含量随着时间的变化有下降的趋势:氮沉降初期是交换性Ca~(2+)、Mg~(2+)快速淋失阶段,但随着氮沉降的继续,土壤酸化的同时,也会导致土壤中的一些矿物释放出盐基离子,从而在氮沉降近两年后土壤交换性Ca~(2+)、Mg~(2+)的淋失程度降低。
2.3氮沉降对林木营养状态的影响
经过四年的模拟氮沉降试验,相对于N0处理,N1、N2、N3处理在一定程度上提高了杉木针叶氮素含量,各处理林分杉木针叶氮素含量比对照处理高出18.25%、11.68%和13.14%,但其作用随时间推移有减弱的趋势,在本试验的第三年,增加氮的输入,林分杉木针叶的氮增加率不能继续提高;在本研究试验中,虽然经N1、N2、N3处理的林分杉木针叶中磷的含量呈现出上升的趋势,但N1、N3处理增长幅度明显低于对照处理,表明N1、N3对针叶磷元素的提高有一定的抑制作用,N2有明显的促进作用;在四年的大部分时间里,氮沉降表现出一定抑制针叶中K、Mg含量增加的作用;氮沉降对林分杉木针叶中N含量与C、K、P、Mg含量的比值的影响,表现出相似的规律,即先升高后降低的特性,但在本研究的四年时间里,各林分的比值均处在杉木生长所需的最适值范围之内,说明经过四年的处理,杉木还没有表现出营养失衡的问题;氮沉降对林分杉木针叶中的微量元素的影响特点:中长期的氮沉降抑制了杉木针叶中锰含量的提高,且氮沉降水平越高,这一作用就越明显;N1、N2处理林分杉木针叶锌含量先降低后升高;N3处理林分杉木针叶锌含量表现出逐年增加的趋势;氮沉降抑制了处理林分杉木针叶中的铁含量的提高,但随时间推进,其抑制作用越来越不明显。
2.4氮沉降对森林土壤呼吸及土壤酶活性的影响
氮沉降抑制了林地表层土壤的呼吸作用,但明显促进了中层和底层土壤的呼吸作用。土壤呼吸速率在N1、N2和N3处理下,表层分别降低了28.34%、2.04%和15.31%,而中层土壤分别增加了53.44%、62.22%和20.20%,底层分别增加了117.46%、42.72%和72.86%。氮沉降在初始的2年内使森林土壤纤维素酶活性提高,而在第3年,N1和N2处理对土壤纤维素酶活性的促进作用减弱,而高氮沉降(N3)则显著降低了土壤纤维素酶活性。总体上看,经中、低氮处理(N1、N2)后,土壤呼吸速率与土壤纤维素酶活性存在正相关性,但高氮沉降(N3)下两者的关系不显著;在本试验监测的四年时间里,N0处理林分土壤过氧化氢酶活性波动很小,N1、N2处理的林分土壤过氧化氢酶活性逐年升高,N3处理林分土壤过氧化氢活性在处理一年后,升高了32.58%,但随后三年时间里,逐年降低。经过四年的模拟氮沉降处理,2007年12月测得N0、N1、N2和N3处理林分土壤过氧化氢酶活性分别比2003年提高了4.34%、41.21%、55.56%和-20.13%;与对照处理相比,N1、N2、N3处理分别提高了33.38%、53.13%和-20.31%,方差分析表明,各处理之间差异显著(p<0.05),显然N1、N2处理提高了土壤过氧化氢酶活性,而N3处理降低了土壤过氧化氢酶活性。在本试验监测的三年时间里,N0处理林分土壤脲酶活性波动很小,N1、N2处理的林分土壤脲酶活性逐年升高,N3处理林分土壤脲酶活性在处理一年后,升高了32.58%,但随后两年时间里,逐年降低。经过三年的模拟氮沉降处理,2006年12月测得N0、N1、N2和N3处理林分土壤脲酶活性分别比2003年提高了6.01%、99.82%、86.08%和-14.29%;与对照处理相比,N1、N2处理大大提高了土壤脲酶酶活性,而N3处理降低了土壤脲酶活性。
2.5氮沉降对凋落物有关化学成分及其分解过程的影响
氮沉降使林分杉木落叶中的氮含量显著增加。与对照相比,N3、N2和N1处理分别使落叶中的氮含量增加32.5%、19.3%和10.2%。凋落物其它组分中的N含量对氮处理的响应不尽相同,但都没有达到统计上的显著差异。氮沉降量水平与生态系统氮素归还量具有明显的相关性,即氮沉降量越高,凋落物中氮素的总归还量就越高。与对照(N0)相比,N2和N3处理分别使氮归还量增加10.9%和32.6%,而N1处理对氮素归还量的影响不显著;凋落物各组分中,微量元素含量大小顺序表现为Fe>Mn>Zn>Cu。各处理林分凋落物中铜元素含量高低顺序为N3>N0>N1>N2;各处理林分凋落物中Zn元素含量高低顺序为N0>N3>N2>N1;各处理林分凋落物中Mn元素含量高低顺序为N1>N2>N3≈N0;各处理林分凋落物中Fe元素含量高低顺序为N1>N2>N3>N0。
N1、N2处理对凋落物分解有不同程度的促进作用。N2处理对凋落物的分解的促进作用最大,N3处理表现出轻微的抑制作用。N0、N1、N2和N3的周转期分别为3.99、3.95、3.06和4.11a。四种氮沉降水平处理凋落物,在各分解阶段的碳素释放率有较大差异。在初始阶段,N1处理的碳释放率最小,碳素释放率仅为6.78%,是对照组的61.94%;在分解试验中期,N2、N3处理的凋落物碳释放率明显小于对照组;在试验的后期,N1、N2、N3处理的碳释放率分别为16.11%、19.73%、16.30%,都明显大于对照的12.60%的释放率。从总体看,不同氮沉降水平处理凋落物,其碳释放率的大小顺序为:N2>N0>N1>N3。
Carbon is one of the most important elements in all life forms, and the carbon cycle is one of the most important processes in all life forms. Forest are the main terrestrial ecosystems which loading 46.3% of the total carbon pool in the earth land, and forest vegetation carbon pools maintained as high as 77.0% of the terrestrial vegetation carbon pools, so the cycle of the forest carbon are the world’s most important carbon cycle, and it plays an important role in maintaining global ecological security and the sustainable development of human society. The global carbon cycle has been impacted by human society through three main ways: first, large-scale deforestation to forest land into agricultural land and agricultural farming methods change; second, large-scale using of ore fuel by human society, the third are chemical substance emissions produced by human life. The first two directly led to the loss of the global carbon pool and become carbon source, and the third is usually mainly referring to the acid deposition including deposition of sulfur and nitrogen deposition. The role of the both acid deposition at the global carbon cycle are relevant to the geographical and the ecological environment. With the development of human society, the negative effects of growing nitrogen deposition on global carbon cycle became more and more serious, so it is concerned by the international community, especially in the temperate regions of Europe and North America. High nitrogen deposition also occurred in some areas of china, even some scholars pointed out that China has become one of the world's three major focus areas on nitrogen deposition. Some researching projects related to nitrogen deposition have been started.
Chinese fir, which natural distribution and artificial cultivation is very vast, is one of important timber species in southern China. Chinese Fir grows fast, and its absorption of carbon is also fast, so that it has an obvious effect on reducing atmospheric CO2 concentration. From this perspective, fir cultivation has a very important role on mitigating the global change. Researching the effects of nitrogen deposition on the Chinese fir plantation ecosystems, not only can make up China's deficiencies in the areas of the study, as well as provides theoretical guidance for the sustainable management of Chinese fir plantation, and furthermore makes a base for study the global change. In this paper, to investigate the response of forest ecosystem to increased nitrogen deposition, a field experiment was conducted in a 12-year-old Chinese fir plantation forest in Sanming, northwestern Fujian. Nitrogen loadings were designed at four levels as N0, N1, N2, and N3, at the doses of 0, 60, 120 and 240 kg N hm~(-2) yr-1, respectively, with three replicates in each treatment. Based on three or five years of manipulation, the research results responding to nitrogen loading are as follows:
1. Effects of nitrogen deposition on carbon stock of Chinese fir plantation
1.1 Effects of nitrogen deposition on biomass and carbon reservation in forest tree layer
After five years of nitrogen deposition simulation, the growth of tree layer biomass in plot stands was as follow order: N1> N0> N2> N3. The growth of tree layer biomass were up to 53.690, 60.663, 43.402 and 40.051 t·hm~(-2) respectively in plot stands treated by N0, N1, N2 and N3, and compared to the control treatment, respectively, the growth of tree layer biomass increase 12.99%, -19.16% and ~(-2)3.54% more than that of the plot stands treated by N0; Variance analysis showed that, there was significantly different between N1 and the control treatment, and N1 promote the Chinese fir to increase tree layer biomass; Also N2, N3 and the control treatment reached the level of the difference (p <0.10), but the N2, N3 treatment cause fir tree layer biomass increasing less than that in the control treatment.; After five years of nitrogen deposition in the Chinese fir plantation, the carbon storage of tree layer in treated plot increased in the following order:N1>N0>N2>N3, and the carbon storage of tree layer in the plot stands treated with N0, N1, N2 and N3 increased respectively by 24.048, 26.810, 20.215 and 18.746 t·hm~(-2), and compared to the control treatment, respectively, the growth of tree layer carbon reserves increase more 11.49, -15.94% and ~(-2)2.05% than that of the plot stands treated by N0. Variance analysis showed that there is significant difference between N1, N3 and N0, but no obvious difference between N2 and N3. The results showed that after five years of nitrogen deposition, compared with the control treatment, N1 promoted the carbon storage growth of tree layer in the plot stands of Chinese fir, and N2,N3 inhibited the carbon storage growth of tree layer in Chinese fir stands.
1.2 Impaction of nitrogen deposition on year carbon flux in forest litter After three years ( in 2006.12), the carbon flux of the forest litter in stands treated by N0,N1,N2 and N3 increased by 52.9, 114.4, 157.2 and 92.6 kg ? hm~(-2), respectively, compared to its in 2004.12, when the stands were treated for just one year, and respectively, had an increase of 4.89%, 11.53%, 15.38% and 8.46%. Compared with the control, the litter carbon flux in stands treated by N1,N2 and N3 were increased by 6.64%, 10.49 and 3.57%, respectively. So we could conclude that the litter carbon flux in stands treated by N1, N2 and N3 were increased by 65.9, 107.2 and 39.0 kg?hm~(-2) respectively because of nitrogen deposition. If the above data were converted to CO2, the forest in stands treated by N1, N2, N3 stored another 120.8, 196.6 kg.hm~(-2) and 71.6 kg.hm~(-2) of CO2 every year by the forms of litter compared to the control (N0) treatment.
1.3 Effects of nitrogen deposition on vegetation carbon storage of understory in Chinese fir plantation
After four years of nitrogen deposition simulation, the carbon storage of understory herbs in stands treated by N1, N2, N3 had reduced respectively by 0.072, 0.136, 0.167 t/hm2 compared to the control (N0) treatment, and the carbon storage of undergrowth in stands treated by N1, N2, N3 had reduced respectively by 0.092, 0.237, 0.314 t·hm~(-2) compared to the control (N0) treatment. General view, after four years of nitrogen deposition simulation, the carbon storage size of understory vegetation in the stands treated with nitrogen deposition are as follows: N0> N1> N2> N3, and compared to the control treatment, the carbon storage of understory vegetation in the stands treated by N1, N2 and N3 reduced respectively by 0.164, 0.373 and 0.480 t·hm~(-2), and respectively again, about annual reduced by 0.041, 0.093 and 0.120 t·hm~(-2) in average. When we converted the carbon storage into the amount of CO2, the reduced carbon storage of the understory vegetation in stands which treated by N1, N2 and N3 is equal to release CO2 0.151, 0.342 and 0.441 t·hm~(-2) respectively every year. The results showed that the higher level of the nitrogen deposition is, the more vegetation carbon stock reduced in the understory of Chinese fir.
1.4 Effects of nitrogen deposition on forest soil carbon pools
The carbon content of surface soil was impacted the most greatly by nitrogen deposition, along with continued nitrogen deposition, the carbon content of surface soil decreased, but the decreasing speed reduced gradually; the higher level of nitrogen deposition, the more the carbon content of surface soil reduced; In the first two years of nitrogen deposition, the carbon content of middle and bottom soil reduced gradually, but rising in the subsequent two years. Taking together, the carbon content of soil layer in 0~60cm depth had a trend to lower in first two years of simulation testing, and then gradually increased. After four years of tests, it was found that N1, N2, and N3 reduced slightly the carbon content of stand soils, and N2 reduced mostly, followed by the N3, N1 reduced the minimum, so it seems that there is no law in the impaction of all nitrogen deposition levels on the soil carbon pools, and there is non-linear relationship between the volume of nitrogen deposition and its impaction on soil carbon pools. Although nitrogen deposition reduced the soil carbon content, at the same time it had increased the density of the soil for more, and instead of reducing the soil carbon content, nitrogen deposition increased the carbon reserves in soil pools. After a four- year simulation of nitrogen deposition, the forest soil carbon stock increased by 5.79, 7.33, 6.42 and 14.89 t·hm~(-2) respectively in soil pools treated with N0, N1, N2 and N3. Compared to the control treatment, respectively, the soil carbon stock increased by 1.36, 0.45 and 4.84 t·hm~(-2) in the soil treated by N1, N2, and N3 after four years. This results show that the difference nitrogen deposition will significantly affect the forest soil carbon reserves, but there is no apparent regularity on soil carbon reserves affected by the increasing level of nitrogen deposition. If converting the data into the amount of CO2, respectively, the stands soil pools treated by N1, N2, and N3 absorbed 1.25, 0.410 and 4.44 t·hm~(-2) of CO2 every year, compared to the control treatment.
2. Effects of nitrogen deposition on process of forest chemical ecology
2.1 Effects of nitrogen deposition on pH value of forest soils
The following characters are presented in the effects of nitrogen deposition on different layers of forest soil: the soil in depth of 0~20 cm is more sensitive than that in depth of 20~40cm and 40~60cm responding to nitrogen deposition, and the upper layer of soil is easier to be acidified than the lower layer of soil; Descending degree of pH value is positively correlated with the amount of nitrogen deposition, the longer nitrogen deposition, the greater extent of soil pH values descending and the more obvious the soil acidification. During the experimental process of nitrogen deposition, the soil pH values of all plots decrease, however, the descending degree of soil pH value at the beginning of the nitrogen deposition simulation is significantly lower than that two years later. In particular, for soil in depth of 20~60cm treated with N1, the decline of pH value is not very obvious at the beginning.
2.2 Effects of nitrogen deposition on effective nutrient content in forest soil During the whole process of nitrogen deposition, effective nitrogen content (ammonium and nitrate) increase in the plot soil treated with nitrogen deposition compared to the control treatment, and the higher level of nitrogen deposition, the greater extent of its growth. With time passing, there is a growing gap between the effective nitrogen content in plot soil treated with nitrogen deposition and that in control treated soil, so we can conclude that nitrogen deposition have an accumulative effect on the effective nitrogen content in soil. The nitrogen deposition has the following effects on soil available phosphorus content in Chinese fir plantation. The nitrogen deposition causes attenuation of available phosphorus content in soil; At the beginning of nitrogen deposition, the higher the level of nitrogen deposition, the greater extent of attenuation of available phosphorus in soil; however, the attenuation of available phosphorus in the soil does not speed up with the increasing amount of nitrogen deposition. Nitrogen deposition result in a greater extent of available potassium leaching from the soil at the depth of 0~(-2)0cm , but in soil at the depth of 20—40cm and 40—60cm , the change of available potassium content is lower than that in contrast, on the contrary, it increased with the nitrogen deposition. Generally speaking, the greater extent of nitrogen deposition causes a greater degree of leaching of available potassium; The nitrogen deposition aggravates the migration of available potassium to the deeper layer of soil, and the leaching of available potassium strengthens with nitrogen deposition continuous; The exchangeable Ca~(2+) and Mg~(2+) contents in the same layer of soil are different due to the different levels of nitrogen deposition. The high-to-low order of exchangeable Ca~(2+) and Mg~(2+) contents in soils treated with different levels of nitrogen deposition is N0>N1>N3>N2, moreover, the difference of treatment levels between N0,N1,N3 and N2 is very obvious(p<0.05), so it can be that every layer of soil is more sensitive to N2 . The exchangeable Ca~(2+) and Mg~(2+) contents in the same layer of soil with the same level of nitrogen deposition decline with time passing; the exchangeable Ca~(2+) and Mg~(2+) in soil leach fast in the initial of nitrogen deposition, but with continued nitrogen deposition and accompanying soil acidification, the soil will also release some of the base mineral ions, and thus the leaching degree of soil exchangeable Ca~(2+) and Mg~(2+)reduced after about two years of nitrogen deposition.
2.3 Effects of nitrogen deposition on nutritional status of trees
Compared to N0, N1, N2 and N3 improve the nitrogen content of fir needles to some extent after four years of nitrogen deposition simulation, and the nitrogen content of fir needles in stands treated with nitrogen deposition increased 18.25 % , 11.68 % and 13.14 % respectively, compare to that in the contrast treatment., however, the effect declines with time passing. At the third year of this experiment, even if nitrogen input is increased, increment rate of nitrogen in fir needles can not be improved continuously. In this study experiment, although phosphorus content in Chinese fir needles treated by N1, N2, and N3 showed an upward trend, the increasing amplitude in fir needles treated by N1 or N2 was significantly lower than that of the contrast treatment, which displayed that N1 or N3 nitrogen deposition has an inhibitory effect on phosphorus absorption in the Chinese fir. At most of four years, the nitrogen deposition showed a certain extent of inhibitory effect on the increase of K and Mg content in needles. The influence of nitrogen deposition on ratios of N and C, K, P, Mg content in Chinese fir needles presents similar law, which ascending in first and descending at last, but among 3 years of the study, the ratios are all inside the pale of optimal values needed by the growth of firs, and it illuminates that Chinese fir has not yet shown a nutritional imbalance problem after three years of treatment; The influence of nitrogen deposition on trace element in Chinese fir needles has the following characters: mid-and-long term nitrogen deposition inhibits the increase of Mn content in Chinese fir needles, moreover, the higher the level of nitrogen deposition, the more obvious the effect; The Zn content of needles in Chinese fir through N1 and N2 treatment ascended in first and descended at last; The Zn content of needle in Chinese fir through N3 treatment showed an increasing trend year by year; Nitrogen deposition inhibits the increase of Fe content of needle in Chinese fir, but with time passing, the inhibitory effect is more and more inconspicuous.
2.4 Effects of nitrogen deposition on soil respiration and activity of soil enzymes in forest
Based on three years of manipulation, nitrogen loading was found to inhibit soil respiration rate in the 0-20 cm depth, but promot respiration rate in the 20-60 cm horizon. N1, N2 and N3 treatments decreased surface soil respiration rates by 28.34%, 2.04% and 15.31%, respectively, but increased soil respiration rate by 53.44%, 62.22% and 20.20% within the 20-40 cm depth, respectively, and by 117.46%, 42.72% and 72.86% at the soil depth of 40-60 cm, respectively.
In the first two years of treatment, nitrogen deposition was observed to elevate soil cellulose enzyme activity, but in the third year the magnitude of enhancement declined in N1 and N2, and significant inhibition was detected for N3. Significant positive linear relationship was developed between soil respiration rate and for the treatments of N1 and N2, but failed for N3. The effect of nitrogen deposition on catalase activity was monitored continually for four years in forest soil treated by N0, N1, N2, and N3. The results show that little change was found for catalase activity in soil after treated with N0, but the catalase activity upgraded year by year in the soil treated with N1 and N2, as for N3, the catalase activity step up 32.58% in first year and gradually step down in the following three years. After four-year simulation of nitrogen deposition, December in 2007, catalase activity in the forest soils treated with N0, N1, N2 and N3,,were raised about 4.34%, 41.21%, 55.56% and ~(-2)0.13%, respectively, compared with the activity in 2003 respectively. Compared to control treatment, the treatment of N1, N2, N3 enhanced the forest soil catalase activity about 33.38%, 53.13% and ~(-2)0.31% respectively. The variance analysis show that, there is significant difference between various treatments (p<0.05), and it was obvious that the catalase activity be raised in forest soil after treated with N1 and N2, while N3 made the activity be lower. Among three years of monitor, the activity of urea enzyme in forest soil treated by N0 changed little , but the activity of urea enzyme in forest soil treated with N1 and N2 raised year by year; and the activity of urea enzyme in forest soil treated with N3 was increased about 32.58% in the first year of simulation, but gradually step down in the following two years. After 3-year simulation of nitrogen deposition, December in 2006, we detected that the activity of urea enzyme in soil from forest was raised about 6.01%, 99.82%, 86.08% and -14.29%, after treated with N0, N1, N2 and N3 respectively, compared with the activity in 2003. Compared to the control treatment, the activity of urea enzyme in soil were raised remarkably after treated with N1 and N2, while N3 made the activity be lower.
2.5 Effects of nitrogen deposition on litter fall composition and its decomposition process Nitrogen deposition increased the nitrogen content of deciduous leaf markedly. Comparing with the control treatment, the treatment of N1, N2, N3 made the nitrogen content in deciduous leaf increase 32.5%, 19.3% and 10.2% respectively. Although nitrogen content in other components of litter fall showed different response, but there is no significant difference on statistical. There is obvious positive relationship between nitrogen deposition levels and the return of volume of nitrogen in ecological system, that is, the higher nitrogen deposition level accompanying the higher total amount of nitrogen restitution in litter fall. Compared with control treatment (N0), the total amount of nitrogen restitution in litter fall increased about 10.9% and 32.6% respectively in forest treated by N2 and N3, while the effect of N1 on it was non-significant. The content of microelements in compositions of litter fall showed a sequence of Fe>Mn>Zn>Cu. In this research period, the nitrogen deposition didn’t significantly change the main characteristic of trace nutrient recycle in fir wood. The magnitude of copper content of litter fall in stands after treatment is N3>N0>N1>N2, for zinc content is N0>N3>N2>N1, and for manganese content is N1>N2>N3≈N0, and for ferrum content is N1>N2>N3>N0.
The treatment of N1 and N2 promoted the decomposition of litter fall with different extent. N2 promoted the decomposition most significantly, The effect of N1 on decomposition was positive too, but N3 appeared some little depressant effect on it. The turnover time for litter fall decomposition treated by N0, N1, N2, N3 was 3.99, 3.95, 3.06, 4.11a, respectively. The carbon release rates of litter falls in plots treated by four levels of nitrogen deposition has comparatively large difference in the different decomposition phase. In initial phase of decomposition, the carbon release rate of litter fall treated with N1 was only 6.78%, the least among all treatments, and just being 61.94% as much as that in control treatment. In intermediate stage of the decomposing experiment, the carbon release rate of litter fall treated with N2 and N3 were obviously smaller than that in control treatment. In the later stage of decomposition, the carbon release rate of litter fall treated with N1,N2, N3 were 16.11%, 19.73%,16.30% respectively, all of which were larger than 12.60%, the data of control treatment. Overall, with treatment of different nitrogen deposition, the carbon release rate of litter fall has a sequence of N2>N0>N1>N3.
人类影响全球碳循环的方式主要有三种:一是大肆砍伐森林使林地变成了农业用地及农业耕作方式的改变;二是人类社会大量使用化石燃料;三是人类生活、生产排放的一些化学物质。前两种方式直接导致了全球碳库的损失,成为碳源;第三种方式目前主要指的是酸沉降,酸沉降包括硫沉降和氮沉降;两者在全球碳循环中的作用,与酸沉降的程度、持续时间及其所在的地理生态环境有关联。随着人类社会的发展,不断增长的氮沉降量对全球碳循环的负面作用越来越大,受到了国际社会的关注,尤其是在欧洲和北美温带地区。我国有些地区也存在高氮沉降问题,有些学者甚至指出,我国已成为世界三大氮沉降集中区之一,有关氮沉降研究的项目已经开始启动。
杉木是我国南方重要的用材树种之一,自然分布和人工栽培都很辽阔。杉木生长快,意味着对碳的吸收也快,因此对降低大气中的CO2浓度有十分明显的效果,从这个角度看,杉木栽培对减缓全球变化有十分重要的作用。研究氮沉降对杉木人工林生态系统的影响,既可以弥补我国该研究领域的不足,为杉木人工林的可持续经营提供理论指导,同时为人们进一步开展全球变化研究打下基础。本文对福建省三明市一片立地条件相似的12年生杉木人工人进行为期5年的模拟氮沉降,氮沉降水平分为N0、N1、N2和N3等4种水平,氮沉降量依次为0、60、120和240 kg·hm~(-2)·a-1,每处理重复三次,以期研究氮沉降对森林生态系统的碳库及其有关化学生态机理的影响,研究结果如下:
1.氮沉降对森林生态系统碳库的影响
1.1氮沉降对森林乔木层活立木生物量及碳储量的影响
经过5年的模拟氮沉降试验,各处理样地林分乔木层活立木生物量增长量大小表现为N1>N0>N2>N3。N0、N1、N2和N3处理样地林分杉木活立木生物量增量分别为53.690、60.663、43.402和41.051 t·hm~(-2),N1、N2和N3处理的样地林分杉木活立木生物量增量分别比对照处理增大了12.99%、-19.16%和~(-2)3.54%;N1处理促进了林分乔木层杉木生物量的增加;N2、N3处理有不同程度地抑制样地林分杉木乔木层生物量的增加;氮沉降处理5年后,各处理样地林分乔木层碳储量增量大小顺序表现为N1>N0>N2>N3;N0、N1、N2和N3处理样地林分乔木层杉木碳储量分别增长了24.048、26.810、20.215和18.746 t·hm~(-2),与对照处理相比,N1、N2和N3处理样地林分乔木层碳储量增长量提高了11.49%、-15.94%和~(-2)2.05%。结果表明,与对照处理相比,N1处理促进了林分乔木层杉木碳储量的增长,N2、N3处理对林分杉木碳储量的增加有抑制作用。
1.2氮沉降对杉木人工林均凋落物碳通量的影响
经过N0、N1、N2和N3处理三年(2006.12)后的各林分凋落物碳储量比处理一年(2004.12)后的各林分凋落物碳储量分别增加了52.9、114.4、157.2和92.6 kg·hm~(-2),分别增加了4.89%、11.53%、15.38%和8.46%;与对照相比,N1、N2和N3处理林分凋落物碳素量分别提高了6.64%、10.49%和3.57%,由此计算得,由于氮沉降而使得各林分的凋落物碳储量分别增加了65.9、107.2和39.0 kg·hm~(-2)。如果把这一数据折算成CO2,则N1、N2、N3处理使得林分每年以凋落物的形式分别额外储存了CO2120.8、196.6和71.6 kg·hm~(-2)。
1.3氮沉降对林下植被碳储量的影响
通过四年的模拟氮沉降试验后, N1、N2和N3处理林分林下草本植物碳储量分别比对照处理(N0处理)降低了0.072、0.136和0.167 t·hm~(-2);而N1、N2和N3处理林分林下灌木碳储量分别比对照处理降低了0.092、0.237和0.314 t·hm~(-2);综合来看,经过四年氮沉降试验后,各处理林分林下植被碳储量大小表现为N0>N1>N2>N3,与对照相比,N1、N2、N3处理林分林下植被碳储量分别降低了0.164、0.373和0.482 t·hm~(-2),平均每年减少0.041、0.093和0.120 t·hm~(-2),换算成CO2的量,相当N1、N2和N3处理的林分林下植被每年吸存的二氧化碳量减少了0.151、0.342和0.441 t·hm~(-2)。结果表明,氮沉降水平越高,所处理样地林分林下植被碳储量就越少.
1.4氮沉降对森林土壤碳库的影响
土壤表层的碳含量受到氮沉降的影响最大,而且随着氮沉降的持续,表层土壤碳含量不断降低,但降低的速度渐趋缓慢;氮沉降水平越高,表层土壤碳含量降低越多;中层和底层土壤碳含量在氮沉降的前两年逐渐降低,但在随后的两年里,却有不断升高。综合起来,整个土壤层的碳含量在氮沉降试验的前两年,有降低的趋势,随后逐渐升高。经过四年的试验后发现,N1、N2和N3处理林分土壤碳含量略有降低,N2处理的降低幅度最大,其次是N3处理,N1处理降低最小。从氮沉降的高低水平来看,似乎没有规律性,说明氮沉降量对土壤碳库影响的非线性关系。虽然氮沉降降低了土壤的碳含量,但由于氮沉降同时增加了土壤的密度,所以综合起来,氮沉降反而提高了土壤碳库的碳储量。经过四年的氮沉降处理,N0、N1、N2、N3处理的林分样地土壤的碳储量分别增加了5.97、7.33、6.42和14.89 t·hm~(-2),与对照处理比较,四年的N1、N2和N3处理分别使林分样地土壤碳储量增加了1.36、0.45和4.84 t·hm~(-2)。结果表明,不同的氮沉降处理会显著影响林分土壤碳储量,各氮沉降水平处理之间没有明显的规律性。如果换算成CO2的量,经过N1、N2和N3处理,与对照处理相比,在本研究四年期间,各林分土壤每年分别吸收了1.25、0.41和4.44 t·hm~(-2)的CO2。
2.氮沉降对森林化学生态过程的影响
2.1氮沉降对森林土壤pH值的影响
氮沉降对森林各层土壤pH的影响呈现以下特点:0～20cm土层较20～40cm、40～60cm土层敏感,上层土壤比下层土壤越容易引起酸化;pH值的下降程度与氮沉降量呈正相关,氮沉降时间越长,土壤pH值下降程度越大,土壤酸化也就越明显。在氮沉降试验过程中,各处理样地pH值都出现了降低的现象,但模拟氮沉降试验初期土壤pH值的下降幅度要明显低于氮沉降两年后的土壤pH的下降幅度,尤其是N1处理下的20～60cm土壤层,在氮沉降初期,变化极不明显。
2.2氮沉降对森林土壤有效养分含量的影响
在整个氮沉降过程中,与对照相比,各处理样地有效氮含量(氨态氮和硝态氮)都呈增长趋势,且氮沉降水平越高,其增长程度越大。各处理样地土壤有效氮含量随着时间的推进,与对照的差距越来越大,由此可以说明氮沉降对土壤有效氮的影响存在累积性效应;氮沉降对杉木人工林土壤中的速效磷含量的影响如下:氮沉降造成土壤中的速效磷的衰减;在氮沉降初期,氮沉降水平越高,土壤速效磷的衰减程度更大;但土壤中的速效磷并不随氮沉降量的增大而加速衰减;氮沉降导致0—20cm土层速效钾更大程度的淋失,而20—40cm、40—60cm土层出现了速效钾含量低于对照的变化,土层的速效钾含量随氮沉降量的增加而上升。总体来说,越大程度的氮沉降引起表层土壤更大程度的速效钾淋失;氮沉降加剧土壤速效钾向更深一层土壤运移;土壤速效钾的淋失随氮沉降的持续而加强;同层土壤中交换性Ca~(2+)、Mg~(2+)含量因不同氮沉降水平而异,不同氮沉降水平土壤交换性Ca~(2+)、Mg~(2+)含量从高到低的顺序为:N0>N1>N3>N2,且N0、N1、N3与N2处理水平之间差异显著(p<0.05),说明各土层对N2水平氮沉降反应更为敏感。同氮沉降水平同层土壤交换性Ca~(2+)、Mg~(2+)含量随着时间的变化有下降的趋势:氮沉降初期是交换性Ca~(2+)、Mg~(2+)快速淋失阶段,但随着氮沉降的继续,土壤酸化的同时,也会导致土壤中的一些矿物释放出盐基离子,从而在氮沉降近两年后土壤交换性Ca~(2+)、Mg~(2+)的淋失程度降低。
2.3氮沉降对林木营养状态的影响
经过四年的模拟氮沉降试验,相对于N0处理,N1、N2、N3处理在一定程度上提高了杉木针叶氮素含量,各处理林分杉木针叶氮素含量比对照处理高出18.25%、11.68%和13.14%,但其作用随时间推移有减弱的趋势,在本试验的第三年,增加氮的输入,林分杉木针叶的氮增加率不能继续提高;在本研究试验中,虽然经N1、N2、N3处理的林分杉木针叶中磷的含量呈现出上升的趋势,但N1、N3处理增长幅度明显低于对照处理,表明N1、N3对针叶磷元素的提高有一定的抑制作用,N2有明显的促进作用;在四年的大部分时间里,氮沉降表现出一定抑制针叶中K、Mg含量增加的作用;氮沉降对林分杉木针叶中N含量与C、K、P、Mg含量的比值的影响,表现出相似的规律,即先升高后降低的特性,但在本研究的四年时间里,各林分的比值均处在杉木生长所需的最适值范围之内,说明经过四年的处理,杉木还没有表现出营养失衡的问题;氮沉降对林分杉木针叶中的微量元素的影响特点:中长期的氮沉降抑制了杉木针叶中锰含量的提高,且氮沉降水平越高,这一作用就越明显;N1、N2处理林分杉木针叶锌含量先降低后升高;N3处理林分杉木针叶锌含量表现出逐年增加的趋势;氮沉降抑制了处理林分杉木针叶中的铁含量的提高,但随时间推进,其抑制作用越来越不明显。
2.4氮沉降对森林土壤呼吸及土壤酶活性的影响
氮沉降抑制了林地表层土壤的呼吸作用,但明显促进了中层和底层土壤的呼吸作用。土壤呼吸速率在N1、N2和N3处理下,表层分别降低了28.34%、2.04%和15.31%,而中层土壤分别增加了53.44%、62.22%和20.20%,底层分别增加了117.46%、42.72%和72.86%。氮沉降在初始的2年内使森林土壤纤维素酶活性提高,而在第3年,N1和N2处理对土壤纤维素酶活性的促进作用减弱,而高氮沉降(N3)则显著降低了土壤纤维素酶活性。总体上看,经中、低氮处理(N1、N2)后,土壤呼吸速率与土壤纤维素酶活性存在正相关性,但高氮沉降(N3)下两者的关系不显著;在本试验监测的四年时间里,N0处理林分土壤过氧化氢酶活性波动很小,N1、N2处理的林分土壤过氧化氢酶活性逐年升高,N3处理林分土壤过氧化氢活性在处理一年后,升高了32.58%,但随后三年时间里,逐年降低。经过四年的模拟氮沉降处理,2007年12月测得N0、N1、N2和N3处理林分土壤过氧化氢酶活性分别比2003年提高了4.34%、41.21%、55.56%和-20.13%;与对照处理相比,N1、N2、N3处理分别提高了33.38%、53.13%和-20.31%,方差分析表明,各处理之间差异显著(p<0.05),显然N1、N2处理提高了土壤过氧化氢酶活性,而N3处理降低了土壤过氧化氢酶活性。在本试验监测的三年时间里,N0处理林分土壤脲酶活性波动很小,N1、N2处理的林分土壤脲酶活性逐年升高,N3处理林分土壤脲酶活性在处理一年后,升高了32.58%,但随后两年时间里,逐年降低。经过三年的模拟氮沉降处理,2006年12月测得N0、N1、N2和N3处理林分土壤脲酶活性分别比2003年提高了6.01%、99.82%、86.08%和-14.29%;与对照处理相比,N1、N2处理大大提高了土壤脲酶酶活性,而N3处理降低了土壤脲酶活性。
2.5氮沉降对凋落物有关化学成分及其分解过程的影响
氮沉降使林分杉木落叶中的氮含量显著增加。与对照相比,N3、N2和N1处理分别使落叶中的氮含量增加32.5%、19.3%和10.2%。凋落物其它组分中的N含量对氮处理的响应不尽相同,但都没有达到统计上的显著差异。氮沉降量水平与生态系统氮素归还量具有明显的相关性,即氮沉降量越高,凋落物中氮素的总归还量就越高。与对照(N0)相比,N2和N3处理分别使氮归还量增加10.9%和32.6%,而N1处理对氮素归还量的影响不显著;凋落物各组分中,微量元素含量大小顺序表现为Fe>Mn>Zn>Cu。各处理林分凋落物中铜元素含量高低顺序为N3>N0>N1>N2;各处理林分凋落物中Zn元素含量高低顺序为N0>N3>N2>N1;各处理林分凋落物中Mn元素含量高低顺序为N1>N2>N3≈N0;各处理林分凋落物中Fe元素含量高低顺序为N1>N2>N3>N0。
N1、N2处理对凋落物分解有不同程度的促进作用。N2处理对凋落物的分解的促进作用最大,N3处理表现出轻微的抑制作用。N0、N1、N2和N3的周转期分别为3.99、3.95、3.06和4.11a。四种氮沉降水平处理凋落物,在各分解阶段的碳素释放率有较大差异。在初始阶段,N1处理的碳释放率最小,碳素释放率仅为6.78%,是对照组的61.94%;在分解试验中期,N2、N3处理的凋落物碳释放率明显小于对照组;在试验的后期,N1、N2、N3处理的碳释放率分别为16.11%、19.73%、16.30%,都明显大于对照的12.60%的释放率。从总体看,不同氮沉降水平处理凋落物,其碳释放率的大小顺序为:N2>N0>N1>N3。
Carbon is one of the most important elements in all life forms, and the carbon cycle is one of the most important processes in all life forms. Forest are the main terrestrial ecosystems which loading 46.3% of the total carbon pool in the earth land, and forest vegetation carbon pools maintained as high as 77.0% of the terrestrial vegetation carbon pools, so the cycle of the forest carbon are the world’s most important carbon cycle, and it plays an important role in maintaining global ecological security and the sustainable development of human society. The global carbon cycle has been impacted by human society through three main ways: first, large-scale deforestation to forest land into agricultural land and agricultural farming methods change; second, large-scale using of ore fuel by human society, the third are chemical substance emissions produced by human life. The first two directly led to the loss of the global carbon pool and become carbon source, and the third is usually mainly referring to the acid deposition including deposition of sulfur and nitrogen deposition. The role of the both acid deposition at the global carbon cycle are relevant to the geographical and the ecological environment. With the development of human society, the negative effects of growing nitrogen deposition on global carbon cycle became more and more serious, so it is concerned by the international community, especially in the temperate regions of Europe and North America. High nitrogen deposition also occurred in some areas of china, even some scholars pointed out that China has become one of the world's three major focus areas on nitrogen deposition. Some researching projects related to nitrogen deposition have been started.
Chinese fir, which natural distribution and artificial cultivation is very vast, is one of important timber species in southern China. Chinese Fir grows fast, and its absorption of carbon is also fast, so that it has an obvious effect on reducing atmospheric CO2 concentration. From this perspective, fir cultivation has a very important role on mitigating the global change. Researching the effects of nitrogen deposition on the Chinese fir plantation ecosystems, not only can make up China's deficiencies in the areas of the study, as well as provides theoretical guidance for the sustainable management of Chinese fir plantation, and furthermore makes a base for study the global change. In this paper, to investigate the response of forest ecosystem to increased nitrogen deposition, a field experiment was conducted in a 12-year-old Chinese fir plantation forest in Sanming, northwestern Fujian. Nitrogen loadings were designed at four levels as N0, N1, N2, and N3, at the doses of 0, 60, 120 and 240 kg N hm~(-2) yr-1, respectively, with three replicates in each treatment. Based on three or five years of manipulation, the research results responding to nitrogen loading are as follows:
1. Effects of nitrogen deposition on carbon stock of Chinese fir plantation
1.1 Effects of nitrogen deposition on biomass and carbon reservation in forest tree layer
After five years of nitrogen deposition simulation, the growth of tree layer biomass in plot stands was as follow order: N1> N0> N2> N3. The growth of tree layer biomass were up to 53.690, 60.663, 43.402 and 40.051 t·hm~(-2) respectively in plot stands treated by N0, N1, N2 and N3, and compared to the control treatment, respectively, the growth of tree layer biomass increase 12.99%, -19.16% and ~(-2)3.54% more than that of the plot stands treated by N0; Variance analysis showed that, there was significantly different between N1 and the control treatment, and N1 promote the Chinese fir to increase tree layer biomass; Also N2, N3 and the control treatment reached the level of the difference (p <0.10), but the N2, N3 treatment cause fir tree layer biomass increasing less than that in the control treatment.; After five years of nitrogen deposition in the Chinese fir plantation, the carbon storage of tree layer in treated plot increased in the following order:N1>N0>N2>N3, and the carbon storage of tree layer in the plot stands treated with N0, N1, N2 and N3 increased respectively by 24.048, 26.810, 20.215 and 18.746 t·hm~(-2), and compared to the control treatment, respectively, the growth of tree layer carbon reserves increase more 11.49, -15.94% and ~(-2)2.05% than that of the plot stands treated by N0. Variance analysis showed that there is significant difference between N1, N3 and N0, but no obvious difference between N2 and N3. The results showed that after five years of nitrogen deposition, compared with the control treatment, N1 promoted the carbon storage growth of tree layer in the plot stands of Chinese fir, and N2,N3 inhibited the carbon storage growth of tree layer in Chinese fir stands.
1.2 Impaction of nitrogen deposition on year carbon flux in forest litter After three years ( in 2006.12), the carbon flux of the forest litter in stands treated by N0,N1,N2 and N3 increased by 52.9, 114.4, 157.2 and 92.6 kg ? hm~(-2), respectively, compared to its in 2004.12, when the stands were treated for just one year, and respectively, had an increase of 4.89%, 11.53%, 15.38% and 8.46%. Compared with the control, the litter carbon flux in stands treated by N1,N2 and N3 were increased by 6.64%, 10.49 and 3.57%, respectively. So we could conclude that the litter carbon flux in stands treated by N1, N2 and N3 were increased by 65.9, 107.2 and 39.0 kg?hm~(-2) respectively because of nitrogen deposition. If the above data were converted to CO2, the forest in stands treated by N1, N2, N3 stored another 120.8, 196.6 kg.hm~(-2) and 71.6 kg.hm~(-2) of CO2 every year by the forms of litter compared to the control (N0) treatment.
1.3 Effects of nitrogen deposition on vegetation carbon storage of understory in Chinese fir plantation
After four years of nitrogen deposition simulation, the carbon storage of understory herbs in stands treated by N1, N2, N3 had reduced respectively by 0.072, 0.136, 0.167 t/hm2 compared to the control (N0) treatment, and the carbon storage of undergrowth in stands treated by N1, N2, N3 had reduced respectively by 0.092, 0.237, 0.314 t·hm~(-2) compared to the control (N0) treatment. General view, after four years of nitrogen deposition simulation, the carbon storage size of understory vegetation in the stands treated with nitrogen deposition are as follows: N0> N1> N2> N3, and compared to the control treatment, the carbon storage of understory vegetation in the stands treated by N1, N2 and N3 reduced respectively by 0.164, 0.373 and 0.480 t·hm~(-2), and respectively again, about annual reduced by 0.041, 0.093 and 0.120 t·hm~(-2) in average. When we converted the carbon storage into the amount of CO2, the reduced carbon storage of the understory vegetation in stands which treated by N1, N2 and N3 is equal to release CO2 0.151, 0.342 and 0.441 t·hm~(-2) respectively every year. The results showed that the higher level of the nitrogen deposition is, the more vegetation carbon stock reduced in the understory of Chinese fir.
1.4 Effects of nitrogen deposition on forest soil carbon pools
The carbon content of surface soil was impacted the most greatly by nitrogen deposition, along with continued nitrogen deposition, the carbon content of surface soil decreased, but the decreasing speed reduced gradually; the higher level of nitrogen deposition, the more the carbon content of surface soil reduced; In the first two years of nitrogen deposition, the carbon content of middle and bottom soil reduced gradually, but rising in the subsequent two years. Taking together, the carbon content of soil layer in 0~60cm depth had a trend to lower in first two years of simulation testing, and then gradually increased. After four years of tests, it was found that N1, N2, and N3 reduced slightly the carbon content of stand soils, and N2 reduced mostly, followed by the N3, N1 reduced the minimum, so it seems that there is no law in the impaction of all nitrogen deposition levels on the soil carbon pools, and there is non-linear relationship between the volume of nitrogen deposition and its impaction on soil carbon pools. Although nitrogen deposition reduced the soil carbon content, at the same time it had increased the density of the soil for more, and instead of reducing the soil carbon content, nitrogen deposition increased the carbon reserves in soil pools. After a four- year simulation of nitrogen deposition, the forest soil carbon stock increased by 5.79, 7.33, 6.42 and 14.89 t·hm~(-2) respectively in soil pools treated with N0, N1, N2 and N3. Compared to the control treatment, respectively, the soil carbon stock increased by 1.36, 0.45 and 4.84 t·hm~(-2) in the soil treated by N1, N2, and N3 after four years. This results show that the difference nitrogen deposition will significantly affect the forest soil carbon reserves, but there is no apparent regularity on soil carbon reserves affected by the increasing level of nitrogen deposition. If converting the data into the amount of CO2, respectively, the stands soil pools treated by N1, N2, and N3 absorbed 1.25, 0.410 and 4.44 t·hm~(-2) of CO2 every year, compared to the control treatment.
2. Effects of nitrogen deposition on process of forest chemical ecology
2.1 Effects of nitrogen deposition on pH value of forest soils
The following characters are presented in the effects of nitrogen deposition on different layers of forest soil: the soil in depth of 0~20 cm is more sensitive than that in depth of 20~40cm and 40~60cm responding to nitrogen deposition, and the upper layer of soil is easier to be acidified than the lower layer of soil; Descending degree of pH value is positively correlated with the amount of nitrogen deposition, the longer nitrogen deposition, the greater extent of soil pH values descending and the more obvious the soil acidification. During the experimental process of nitrogen deposition, the soil pH values of all plots decrease, however, the descending degree of soil pH value at the beginning of the nitrogen deposition simulation is significantly lower than that two years later. In particular, for soil in depth of 20~60cm treated with N1, the decline of pH value is not very obvious at the beginning.
2.2 Effects of nitrogen deposition on effective nutrient content in forest soil During the whole process of nitrogen deposition, effective nitrogen content (ammonium and nitrate) increase in the plot soil treated with nitrogen deposition compared to the control treatment, and the higher level of nitrogen deposition, the greater extent of its growth. With time passing, there is a growing gap between the effective nitrogen content in plot soil treated with nitrogen deposition and that in control treated soil, so we can conclude that nitrogen deposition have an accumulative effect on the effective nitrogen content in soil. The nitrogen deposition has the following effects on soil available phosphorus content in Chinese fir plantation. The nitrogen deposition causes attenuation of available phosphorus content in soil; At the beginning of nitrogen deposition, the higher the level of nitrogen deposition, the greater extent of attenuation of available phosphorus in soil; however, the attenuation of available phosphorus in the soil does not speed up with the increasing amount of nitrogen deposition. Nitrogen deposition result in a greater extent of available potassium leaching from the soil at the depth of 0~(-2)0cm , but in soil at the depth of 20—40cm and 40—60cm , the change of available potassium content is lower than that in contrast, on the contrary, it increased with the nitrogen deposition. Generally speaking, the greater extent of nitrogen deposition causes a greater degree of leaching of available potassium; The nitrogen deposition aggravates the migration of available potassium to the deeper layer of soil, and the leaching of available potassium strengthens with nitrogen deposition continuous; The exchangeable Ca~(2+) and Mg~(2+) contents in the same layer of soil are different due to the different levels of nitrogen deposition. The high-to-low order of exchangeable Ca~(2+) and Mg~(2+) contents in soils treated with different levels of nitrogen deposition is N0>N1>N3>N2, moreover, the difference of treatment levels between N0,N1,N3 and N2 is very obvious(p<0.05), so it can be that every layer of soil is more sensitive to N2 . The exchangeable Ca~(2+) and Mg~(2+) contents in the same layer of soil with the same level of nitrogen deposition decline with time passing; the exchangeable Ca~(2+) and Mg~(2+) in soil leach fast in the initial of nitrogen deposition, but with continued nitrogen deposition and accompanying soil acidification, the soil will also release some of the base mineral ions, and thus the leaching degree of soil exchangeable Ca~(2+) and Mg~(2+)reduced after about two years of nitrogen deposition.
2.3 Effects of nitrogen deposition on nutritional status of trees
Compared to N0, N1, N2 and N3 improve the nitrogen content of fir needles to some extent after four years of nitrogen deposition simulation, and the nitrogen content of fir needles in stands treated with nitrogen deposition increased 18.25 % , 11.68 % and 13.14 % respectively, compare to that in the contrast treatment., however, the effect declines with time passing. At the third year of this experiment, even if nitrogen input is increased, increment rate of nitrogen in fir needles can not be improved continuously. In this study experiment, although phosphorus content in Chinese fir needles treated by N1, N2, and N3 showed an upward trend, the increasing amplitude in fir needles treated by N1 or N2 was significantly lower than that of the contrast treatment, which displayed that N1 or N3 nitrogen deposition has an inhibitory effect on phosphorus absorption in the Chinese fir. At most of four years, the nitrogen deposition showed a certain extent of inhibitory effect on the increase of K and Mg content in needles. The influence of nitrogen deposition on ratios of N and C, K, P, Mg content in Chinese fir needles presents similar law, which ascending in first and descending at last, but among 3 years of the study, the ratios are all inside the pale of optimal values needed by the growth of firs, and it illuminates that Chinese fir has not yet shown a nutritional imbalance problem after three years of treatment; The influence of nitrogen deposition on trace element in Chinese fir needles has the following characters: mid-and-long term nitrogen deposition inhibits the increase of Mn content in Chinese fir needles, moreover, the higher the level of nitrogen deposition, the more obvious the effect; The Zn content of needles in Chinese fir through N1 and N2 treatment ascended in first and descended at last; The Zn content of needle in Chinese fir through N3 treatment showed an increasing trend year by year; Nitrogen deposition inhibits the increase of Fe content of needle in Chinese fir, but with time passing, the inhibitory effect is more and more inconspicuous.
2.4 Effects of nitrogen deposition on soil respiration and activity of soil enzymes in forest
Based on three years of manipulation, nitrogen loading was found to inhibit soil respiration rate in the 0-20 cm depth, but promot respiration rate in the 20-60 cm horizon. N1, N2 and N3 treatments decreased surface soil respiration rates by 28.34%, 2.04% and 15.31%, respectively, but increased soil respiration rate by 53.44%, 62.22% and 20.20% within the 20-40 cm depth, respectively, and by 117.46%, 42.72% and 72.86% at the soil depth of 40-60 cm, respectively.
In the first two years of treatment, nitrogen deposition was observed to elevate soil cellulose enzyme activity, but in the third year the magnitude of enhancement declined in N1 and N2, and significant inhibition was detected for N3. Significant positive linear relationship was developed between soil respiration rate and for the treatments of N1 and N2, but failed for N3. The effect of nitrogen deposition on catalase activity was monitored continually for four years in forest soil treated by N0, N1, N2, and N3. The results show that little change was found for catalase activity in soil after treated with N0, but the catalase activity upgraded year by year in the soil treated with N1 and N2, as for N3, the catalase activity step up 32.58% in first year and gradually step down in the following three years. After four-year simulation of nitrogen deposition, December in 2007, catalase activity in the forest soils treated with N0, N1, N2 and N3,,were raised about 4.34%, 41.21%, 55.56% and ~(-2)0.13%, respectively, compared with the activity in 2003 respectively. Compared to control treatment, the treatment of N1, N2, N3 enhanced the forest soil catalase activity about 33.38%, 53.13% and ~(-2)0.31% respectively. The variance analysis show that, there is significant difference between various treatments (p<0.05), and it was obvious that the catalase activity be raised in forest soil after treated with N1 and N2, while N3 made the activity be lower. Among three years of monitor, the activity of urea enzyme in forest soil treated by N0 changed little , but the activity of urea enzyme in forest soil treated with N1 and N2 raised year by year; and the activity of urea enzyme in forest soil treated with N3 was increased about 32.58% in the first year of simulation, but gradually step down in the following two years. After 3-year simulation of nitrogen deposition, December in 2006, we detected that the activity of urea enzyme in soil from forest was raised about 6.01%, 99.82%, 86.08% and -14.29%, after treated with N0, N1, N2 and N3 respectively, compared with the activity in 2003. Compared to the control treatment, the activity of urea enzyme in soil were raised remarkably after treated with N1 and N2, while N3 made the activity be lower.
2.5 Effects of nitrogen deposition on litter fall composition and its decomposition process Nitrogen deposition increased the nitrogen content of deciduous leaf markedly. Comparing with the control treatment, the treatment of N1, N2, N3 made the nitrogen content in deciduous leaf increase 32.5%, 19.3% and 10.2% respectively. Although nitrogen content in other components of litter fall showed different response, but there is no significant difference on statistical. There is obvious positive relationship between nitrogen deposition levels and the return of volume of nitrogen in ecological system, that is, the higher nitrogen deposition level accompanying the higher total amount of nitrogen restitution in litter fall. Compared with control treatment (N0), the total amount of nitrogen restitution in litter fall increased about 10.9% and 32.6% respectively in forest treated by N2 and N3, while the effect of N1 on it was non-significant. The content of microelements in compositions of litter fall showed a sequence of Fe>Mn>Zn>Cu. In this research period, the nitrogen deposition didn’t significantly change the main characteristic of trace nutrient recycle in fir wood. The magnitude of copper content of litter fall in stands after treatment is N3>N0>N1>N2, for zinc content is N0>N3>N2>N1, and for manganese content is N1>N2>N3≈N0, and for ferrum content is N1>N2>N3>N0.
The treatment of N1 and N2 promoted the decomposition of litter fall with different extent. N2 promoted the decomposition most significantly, The effect of N1 on decomposition was positive too, but N3 appeared some little depressant effect on it. The turnover time for litter fall decomposition treated by N0, N1, N2, N3 was 3.99, 3.95, 3.06, 4.11a, respectively. The carbon release rates of litter falls in plots treated by four levels of nitrogen deposition has comparatively large difference in the different decomposition phase. In initial phase of decomposition, the carbon release rate of litter fall treated with N1 was only 6.78%, the least among all treatments, and just being 61.94% as much as that in control treatment. In intermediate stage of the decomposing experiment, the carbon release rate of litter fall treated with N2 and N3 were obviously smaller than that in control treatment. In the later stage of decomposition, the carbon release rate of litter fall treated with N1,N2, N3 were 16.11%, 19.73%,16.30% respectively, all of which were larger than 12.60%, the data of control treatment. Overall, with treatment of different nitrogen deposition, the carbon release rate of litter fall has a sequence of N2>N0>N1>N3.
引文
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