摘要
土壤碳是陆地生态系统的重要组成部分。土壤微生物是土壤中有机质和养分转化与循环的主要动力,微生物在推动地球生物化学元素循环过程中起着重要作用。本研究选取美国东部几种典型的生态系统,研究了人为管理因素和自然因素对土壤碳氮动态变化,微生物和蚯蚓的生物量、活性和种群结构的影响。实验分别测定了草坪生态系统中管理时间和管理强度对土壤碳,微生物量和微生物活性的影响,同时通过测定碳同位素(13)~C研究土壤不同密度组分中碳的稳定性以及C3和C4植物对各组分碳的贡献率;研究了管理时间、管理强度和农药使用对草坪系统蚯蚓生物量、活性和群落结构的影响;分析了农田生态系统中不同田间管理方式下土壤碳的动态变化过程和微生物量、微生物活性、微生物种群结构和稳定性;测定了不同土地利用系统下的土壤碳、微生物量、微生物活性、微生物种群结构和稳定性变化;田间模拟研究了全球变化情况下温室效应气体CO_2和O_3对土壤微生物量和微生物活性、菌根侵染率的影响。
研究的目的在于探讨人为管理和自然环境因素对土壤碳动态变化和稳定性的影响程度,揭示土壤微生物在碳氮循环中的作用,为土地资源的合理有效利用、温室效应气体的减少和实现生态系统的可持续发展提供理论依据和科学技术支撑。主要研究结果如下:
1.管理时间和管理强度对草坪土壤碳和微生物的影响
管理时间长短对土壤碳氮累积有着明显的作用,1 m深土层中碳氮累积速率分别为71.89 g m~2y~(-1)和10.63 g m~2y~(-1)。管理时间长短、土地利用方式改变和气候变化对土壤微生物量、微生物活性有着一定的影响。在气候正常情况下,微生物碳氮在松林地系统中最高,但在干旱季节出现的情况下在管理时间最长的草坪系统中最高。管理强度对土壤碳、土壤氮、微生物碳、微生物氮和微生物活性都有影响,所有的这些指标在中等管理强度系统中最高。研究结果表明长期中等管理强度的草坪系统可以累积更多的碳,有利于减少大气中二氧化碳浓度。
2.管理时间、管理强度和农药使用对草坪蚯蚓的影响
80年草坪比10年草坪中的蚯蚓种群密度和生物量高,而松树林地中蚯蚓种群密度和生物量均比10年和80年草坪系统要高。总体来看,蚯蚓数量和总生物量在高尔夫球场发球区高于其它区域。但是在极端干旱炎热的2007年,边缘深草区的蚯蚓数量比球道区的多,而在湿润年份2008年,结果则相反,这也表明土壤湿度和土壤温度对蚯蚓密度和活动起着主导作用。农药对蚯蚓种群动态变化和活动行为也有影响。按照农药生产商所提供的最大使用剂量,单独使用杀虫剂Sevin和Merit这两种农药,在不引起蚯蚓死亡的情况下可以在最少三周的时间内明显抑制蚯蚓的挖穴行为。随着农药使用次数增多,农药对蚯蚓的毒害性也在增加。选取三种农药Sevin,Merit和T-methyl,每周使用一次,在连续使用4周后发现蚯蚓的死亡率分别为35 %、45 %和80 %。田间试验中每周使用一次农药T-methyl和Sevin,把农药使用停止6周后农药对蚯蚓的抑制作用作为对照,连续使用5次后的蚯蚓数量和生物量比对照明显减少。在草坪生态系统中,蚯蚓的活动行为可以通过适当的灌溉和农药使用来进行管理控制,研究结果同时也表明,入侵物种在环境条件较好的地区更容易入侵,应该加强这类地区物种的监管和控制。
3.长期田间管理方式对土壤碳和微生物的影响
尽管有机肥的使用对土壤中碳及氮产生很大的影响,但耕作对土壤中碳及氮影响最为明显。免耕及使用有机物极大的增强了微生物总量及其活动,提升了土壤中碳及氮沉积总量。通过采用免耕及有机管理结合的方式能够将土壤中的有机碳含量提高140 %,此时土壤中碳含量就与未受干扰的草地相似。免耕及有机耕种也能改变不同密度组分中的土壤有机碳分布。经过免耕及有机投入处理,有机碳含量在轻组降低,重组增加,这表明有机碳从不稳定态转化为稳定态。在管理方式中,有机投入对土壤微生物多样性及群落结构稳定性的影响占据主导地位,尽管免耕做法同样也产生巨大的影响。免耕及有机投入极大的促进土壤微生物多样性及群落稳定性。与传统的耕种控制相比,采用免耕及有机管理能够将土壤微生物多样性提升6.46 %,与未受干扰的草原相似。在不同的生产方式中,免耕及有机耕种能够改变微生物群落结构。在经过免耕及有机投入处理之后,土壤中微生物群落结构与自然草原体系更为接近。通过减少耕作以及增加有机碳投入这种有效管理方式能够增强土壤碳储积能力,改善土壤肥性,同时也缓和空气二氧化碳的上升;通过减少耕种,增加有机碳投入这样的有效管理方式能够提升土壤微生物多样性及群落稳定性,提高土壤的健康与质量。
4.农业系统对土壤碳和微生物的影响
极端干旱气候对影响土壤微生物生存条件的土壤水分和浸提态碳有明显的负面影响;土壤碳、土壤氮在中等扰动强度情况下高于扰动最强和扰动最弱的农业生态系统;浸提态碳在弱扰动系统中含量最高;碳的稳定性也受到扰动强度的影响,轻组有机碳在弱扰动农业生态系统中最高,在强扰动系统中最低,而超重组有机碳在中等扰动强度的农业生态系统中最高;微生物碳、微生物氮和微生物活性对不同扰动强度的反应不同,在中等扰动强度下均高于最强和最弱两种农生态系统;磷酸脂肪酸分析结果的主成分分析表明微生物多样性和种群结构在不同扰动强度的生态系统中发生了变化,中等扰动强度的农业生态系统中微生物多样性最高,低扰动农业生态系统的土壤环境最有利于真菌群落的发展。研究结果表明在农业生产过程中,即能维持土壤生物多样性,又能增加土壤碳储量的农业生态系统是可以实现的。
5.温室效应气体对土壤微生物的影响
高浓度CO_2可以明显的提高土壤水分、浸提态碳、微生物碳氮、微生物活性和菌根感染率,而高浓度的O_3则对这些测定指标有负作用,高浓度CO_2对O_3的负影响有一定的缓解作用。
Soil carbon is an important part of the terrestrial ecosystem. Soil microbe is the main driving force to carbon dynamic and nutrient turnover in the biogeochemical cycling. The effect of management practice and environmental factors on soil carbon and nitrogen dynamic, biomass and diversity and community structure of microbe and earthworm were tested in typical ecosystems in the eastern US. We tested the effect of management duration and intensity on soil carbon, microbial biomass and activity in turfgrass system, we also determine carbon stability in different density fraction and carbon proportion derived from C3 and C4 plant by test C13; the effect of management duration, management intensity and chemical usage on biomass and community structure of earthworm was tested in golf course and phytotron environment; the effect of field management practice on soil carbon dynamic, microbial biomass, microbial activity, microbial community structure and stability was tested; soil carbon, microbial properties in different agriculture system was tested; the effect of greenhouse gases on microbial was simulate in open top chamber.
The objective of this research were to determine if and how management practice and environmental factors influenced carbon dynamic and stability, and the function of microbe in carbon and nitrogen cycling. The results of this research were showed as following:
1. The effect of management duration and intensity on soil carbon and microbial properties in turfgrass system
Our result showed that the management duration positively affect the carbon sequestration and nitrogen retention. The carbon and nitrogen accumulate rate was 71.89 g m2 y-1 and 10.63 g m~2y~(-1) respectively in 1 m depth. The management duration, land-use change and climate change all influenced the soil microbial biomass and activity. Microbial carbon and nitrogen was highest in the native pine tree system in normal climate condition but highest in the longest management system when the dry season occurred. Microbial activity was highest in the longest management system. Management intensity was also affect the soil carbon, soil nitrogen, microbial carbon, microbial nitrogen and microbial activity. All these indicator were highest in the intermediate intensity systems. Our research indicate that long and moderate management turfgrass system can sequestration more carbon than native pine system, which can facilitate to decrease the concentration of carbon dioxide in the atmosphere.
2. The effect of management duration, management intensity and chemical usage on earthworm in turfgrass system
Population density and biomass of earthworms tended to be higher in the 80-year than in the 10-year areas, which were all lower than the control under pine trees. Greater abundance and biomass were found in the tee than other areas. However, earthworms were more abundant in the rough than in the fairway in the dry and hot year (2007) while the opposite was true for the wet year (2008), indicating that soil moisture and temperature had a dominant effect on earthworm density and activities. Pesticides affected earthworm population dynamics and activities. Single application of insecticides Sevin and Merit at the manufacturers’maximum dosages significantly inhibited burrowing behaviors of earthworms for at least three weeks without leading to any earthworm death. The toxicity to earthworms, however, increased as application frequency increased. Weekly applications of chemicals Sevin, Merit and T-methyl for four weeks led to the mortality at 35, 45 and 80 %, respectively. In field, consecutive weekly applications of T-methyl and Sevin for 5 times reduced the population and biomass of earthworms as compared to the control with suppressive effect lasting for at least 6 weeks after chemical application stopped. Taken together, these findings suggest that earthworm activities in turfgrass systems may be managed through appropriate irrigation and pesticide applications; our result also indicate that the invasive species were favorite with the area with more resource.
3. The effect of field management practice on soil carbon and microbial properties
After 15 years of continuous treatments with six different management regimes, soil C and N pools and microbial parameters diverged. Tillage dominated the effect of management practices on soil C and N, although organic practice also exerted significant impacts. Both no tillage and organic inputs significantly promoted soil microbial biomass and activity, and enhanced total soil C and N accumulation. The combination of no tillage and organic management increased soil organic C by 140 % over the conventional tillage control, leading to soil C content comparable to an un-disturbed grassland. No tillage and organic farming also altered the distribution of soil organic C among different density fractions. Under the no tillage plus organic input treatment, the percentage of organic C was reduced in the light fraction but increased in the heavy fraction, suggesting an increased transformation of organic C from the labile into the stabile pool. Organic input dominated the effect of management practices on soil microbial diversity and community structure, although no-tillage practice also exerted significant impacts. Both no-tillage and organic inputs significantly promoted soil microbial diversity and community stability. The combination of no-tillage and organic management slightly increased 6.46 % soil microbial diversity over the conventional tillage control and so similar to undisturbed grassland. No-tillage and organic farming also altered the microbial community structure among different production systems. Under the no-tillage plus organic input treatment, the microbial community structure was more similar to the natural grassland systems. These results indicate that effective management through reducing tillage and increasing organic C inputs can enhance soil C accumulation, improving soil fertility while mitigating the atmospheric CO_2 rise. Effective management through reducing tillage and increasing organic C inputs can enhance soil microbial diversity and the community stability, which improving soil health and quality.
4. The influence of agroecosystems on soil carbon and microbial properties
Result showed that extreme dry climate event can significantly decreased soil moisture condition and available resource, extractable carbon, which many affect microbial living. Soil carbon and nitrogen were highest in moderately disturbance agroecosystems than other intensive and lowly disturbed agroecosystems. Extractable carbon was highest in lowly disturbed system. Carbon dynamic was significantly affected by the management disturb, Light fraction carbon was highest in lowly disturbed system and lowest in intensive agroecosystems, while Very heavy fraction carbon was highest in intermediately disturbed ecosystem, no-tillage system and lowest in lowly disturbance system. Microbial biomass carbon, biomass nitrogen, microbial activity was also affected by the different disturbance gradient. Generally, these microbial properties were highest in the moderately than other two extreme disturbance agroecosystems. Principle component analyses of PLFA data showed microbial diversity and community structure diverged among the soils of the six agroecosystems. High microbial diversity was found in the moderately than intensive and lowly disturbed agroecosystems, and the lower disturbance systems more facilitate the development of microbial communities that favor fungi than other groups. Our research indicate that the agroecosystems, which not only facility microbial diversity, but also enhanced soil carbon pool could be achieved.
5. The effect of greenhouse gas on soil microbial properties
The result showed that high level CO_2 was favorite soil moisture, soil microbial biomass and activity than ambient CO_2. O_3 was suppressed these tested indicators but high level CO_2 can alleviate the negative effects.
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