摘要
全球陆地生态系统土壤有机碳(Soil organic carbon,SOC)储量约为1500 PgC (1 Pg = 1015 g),是陆地生物碳库(560 PgC)的2.8倍,大气圈碳库(760 PgC)的2倍。全球土壤通过呼吸的碳排放量(75 PgC a-1)约占大气二氧化碳(CO2-C)总量的10%,是化石燃料燃烧释放CO2-C的10倍。SOC储量和土壤呼吸的微小变化都能引起全球CO2浓度显著变化。因此,提高SOC储量、减少土壤CO2排放对减缓大气CO2浓度升高具有重要意义。
黄土高原处于干旱半干旱区,为我国典型的雨养农业区和生态脆弱区之一。由于多种不利因素,如降雨量低且年季多变、植被退化、土壤侵蚀和流失严重等,严重制约了该地区SOC积累。SOM含量(1%)均处于国内最低水平。尽管黄土区有关SOC已有大量研究,但主要集中于点位尺度单一治理措施与SOC含量变化的关系,而对SOC储量演变机理以及土壤CO2排放还知之甚少。小流域是黄土区进行水土流失综合治理的基本单元。研究小流域SOC储量和CO2排放影响因素有助于揭示区域SOC储量变化机理具有重要意义。为此,本研究以黄土高原沟壑区的王东沟流域为平台开展了以下两方面研究工作:
一、以小流域内塬面、坡地和沟道等地貌单元和土地利用方式为对象,通过野外调查、试验分析,并结合实地调查与遥感、GIS技术,重点研究地貌单元与土地利用的交互作用对小流域SOC分布的影响,以期揭示大比例尺小尺度条件下地貌单元与SOC含量变化及其空间分布特征的关系,为准确估算黄土区大比例尺度SOC储量提供参考,为探索水土流失治理条件下流域SOC储量演变提供新思路和理论依据。
二、以小流域内塬面上农田土壤系统和坡地上林地土壤系统为对象,监测土壤呼吸、土壤温度和土壤水分动态变化特征,研究土壤温度、土壤水分对土壤呼吸的相关关系,研究土壤CO2排放的不同过程和机理,探讨小流域内不同地貌和土地利用条件下影响土壤CO2排放的环境和人为因素。
主要结果如下:
地貌单元和土地利用对王东沟小流域表层SOC含量空间分布影响显著(P<0.05)。0-20 cm土层SOC含量,沟道>塬面>塬坡。塬面表层SOC含量的变化平缓(6.49-8.56 g kg-1);塬坡和沟道SOC含量变化显著。不同地貌单元条件下土地利用方式对表层SOC含量空间分布的影响也存在明显差异。塬面区,人工草地SOC含量亦明显高于农田和果园;在塬坡上,不同土地利用类型间,SOC含量也存在显著差异(P<0.05)。沟道内,林草两种土地利用类型间的SOC含量无显著差异。
在王东沟小流域,地貌显著影响SOC垂直分布特征。除表层SOC含量沟道大于塬面和塬坡外,塬面均显著高于塬坡和沟道;塬坡和沟道SOC含量随深度增加而降低,而塬面上呈现SOC随深度增加降低-升高-降低的变化趋势。土地利用方式对SOC垂直分布的影响存在显著差异:塬面上,SOC含量基本呈现人工草地>农田>果园的趋势,影响深度为表层40 cm;塬坡上,呈现自然草地>人工林地>人工草地>果园的趋势,影响深度达到100 cm;而沟道地区林草地利用方式对整个垂直剖面分布的差异无显著影响。黄土高原沟壑区深层SOC储量巨大,20 -100 cm土层SOC储量占0-100 cm储量的67.6%;100-200 cm土层SOC储量占0-200 cm储量的37.3%,相当于0-100 cm的63.8%。
王东沟小流域坡地人工刺槐林,去除灌丛群落可使土壤呼吸作用显著降低,去除灌丛群落的对照处理、去除、添加凋落物处理较保留灌丛群落的相应处理分别降低19%、15%和17%。对凋落物处理而言,添加凋落物使土壤呼吸速率增加,去除凋落物使土壤呼吸速率降低,且去除凋落物使土壤呼吸降低程度小于添加凋落处理土壤呼吸增加程度。灌丛群落条件下,与对照处理土壤呼吸速率相比,添加凋落物处理增加31.15%,去除凋落物处理则降低27.23%;去除灌丛群落条件下,与对照处理土壤呼吸速率相比,添加凋落物处理增加34.51%,去除凋落物处理则降低23.77%。土壤温度与土壤水分呈抛物线相关,与土壤温度呈指数相关。改变凋落物的输入量并没有显著改变土壤呼吸与土壤温度与水分的相关关系程度;而去除灌丛群落却明显提高了土壤呼吸与土壤温度和水分的相关程度。
王东沟小流域农田休闲期(7月7日至9月9日),不同施肥处理土壤呼吸速率大小为:化肥有机肥配施处理(NMP)>有机肥处理(M)>化肥氮磷处理(NP)>化肥氮处理(N)和不施肥处理(CK),平均值分别为3.07、2.56、1.87、1.25和1.23μmol m-2 s-1。依据观测的实验数据,CK、N、NP、M和NPM处理休闲期土壤CO2排放量,依次为312、324、477、651和780 g m-2。黄土旱塬区农田休闲期土壤呼吸与土壤水分呈现极显著抛物线关系(P<0.01),可解释55%以上的土壤呼吸变异性;土壤呼吸与土壤温度呈极显著线性相关(P<0.01),但仅能解释呼吸作用变异性的19%-39%。休闲期土壤呼吸受土壤水分和土壤温度共同作用,但土壤水分是控制呼吸作用的主导因子,土壤温度仅为次要限制因素。土壤呼吸对耕作的响应强度,与微生物量碳极显著线性正相关(P <0.01),与SOC显著线性正相关(P<0.05),与全氮、可溶性碳无明显关系(P >0.05)。降雨对土壤呼吸的影响与土壤微生物量碳、SOC、可溶性碳、全氮显著线性以及土壤水分状况密切相关。
Global warming is one of the urgent environmental problems. Carbon cycle takes a major role in climate changes, which becomes a hot issue interesting more and more scientists. SOC (Soil organic carbon) pools in the terrestrial ecosystem is about 1500 Pg, which is more than 2.8 times than terrestrial biological carbon pool and 2 times than global atmospheric carbon pool. Soil carbon emission at the global scale annual is 75 Pg, accounting on tem percents of total amount of atmospheric CO2. All these show that the fractional change of SOC pool and soil respiration may cause voilent changes in the climate of the Earth. Increasing SOC sequestration and reducing soil carbon dioxide emissions play an important role for reliefing global warming.
The Loess Plateau, one of the areas with the most serious soil erosion in the world, is a typical rainfed agriculture area and an ecological fragile zone lying in arid and semiarid regions. Because of soil erosion and sweeping, vegetation degeneration, and low rainfall,the region has the lowest SOC content and density. Presently, the researches main centralize on the SOC content under the rehabilitation of small river basins, but the SOC pools and dynamic in the future, even soil respiration are neglected, for which we take two jobs based on the Wanggonggou watershed on the Loess Plateau. (1) Soil sampling units from 448 sites (0-20cm) and 33 sites (0-200cm) defining on the basis of land formand land use, were used to analyze the effect of land use, landform on SOC, and on the vertical distribution characteristic of SOC sampled to a depth of 200cm. (2) Based on the long-term fertilization experiment in the arid upland fields on the Loess Plateau that started in 1984 and manmade wood(Robinia pseudoacia L.), soil respiration, soil temperature and soil moisture were monitored with the dynamic closed chamber method (LI-8100 USA), and dynamics of soil respiration and its relationships with environmental factors were investigated.
Results were as follows:
There was significant variation in SOC concentration across the landforms (P<0.01). For the top soil of 0-20 cm, SOC concentration was greatest at the gully position (9.1 g kg-1), next was at the tableland position (7.8 g kg-1), and least (6.8 g kg-1) at the slope position. On the tableland, SOC in 0-20 cm varied between 6.49 and 8.56 g kg-1, and SOC content under manmade grass was significantly higher than cropland and orchard; on the slopleland, the range of SOC concentrations increased (5.79-9.95 g kg-1), and there was significant difference among landuse, while varied most in the gully (5.79-10.64 g kg-1), but there was no significant derrerence between grassland and woodland.
Landform and lanuse had significant effect on the vertical distribution of SOC. For the subsoil, SOC in tableland was higher than that in gully and slopeland. For slopeland and gully, SOC decreased with depth increasing, while for tableland, SOC decreased initially, then increased, lastly decreased. Meanwhile, for tableland, the order of SOC appeared approximately manmade grassland > cropland > orchard with the effecting depth of land uses for 40cm, and for slopeland the order was native grassland > manmade woodland > manmade grassland > orchardwith the depth for 100cm, while for gully, there was no significantly difference (P > 0.05) among different land uses. SOC storage in the profile of 20-200 cm accounted for 88.3% sampled to a depth of 100cm, while for 100-200cm, SOC storage accounted 37.3% in 0-200cm equaling to 63.8% of the SOC storage in 0-100cm. The results revealed that landforms and land uses highly significantly (P< 0.05) affected the vertical distribution of SOC at a small watershed scale and considerable amounts of C were stored at deeper depths.
At the gully region of Loess Plateau, soil respiration in soils under the removal of shrub under wood. For control treatment, exclusion and addition litter treatment, soil respiration decreased by 19%, 15%, and 17%, respectively. The amount change of litter also could significantly effected soil respiration. Soil respiration increased under litter addition treatment, while decreased under litter exclusion treatment. For shrud, the litter adding could increase soil respiration by 31.15%, while decreaed by 27.23% under exclusing litter. For removal of shrub, the values of the two litter treatment were 34.51% and 23.77%. Correlstion analysis showed that the litter change did not influence the relation between soil respiration and soil water content and soil temperature, but removal of shrub cound significantly altered the correlation level of soil respiration and soil water content and soil temperature. Significant exponential relation were found between soil respiration and soil temperature, meanwhile soil respiration has cubic parabola relation with soil moisture.
Soil respiration varied drastically within the range from 0.06 to 5.05μmol m~(-2) s~(-1) with a variation coefficient of 116.5% and a mean of 2.00μmol m~(-2) s~(-1). For the whole study period, in terms of soil respiration, the treatments followed a decreasing order of NPM > M > NP > N and CK, with cumulative CO_2-C emission being 2.0, 1.6, 1.2, 0.8 and 0.8 Mg hm-2, respectively. The relationship between soil respiration and soil moisture appeared in an extremely significant parabolic curve (P < 0.01), which explains 55% of the variation of the soil respiration. Although soil respiration was in extremely significant linear relationship with soil temperature (P < 0.01),soil temperature could explain only 19% to 39% of the variation of soil respiration. Strength of the response of soil respiration to ploughing significantly (P <0.05) was positively related to soil microbial biomass carbon and soil organic carbon showing a linear relationship, but not to total nitrogen or soil dissolved organic carbon (P > 0.05).
引文
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