蚯蚓对土壤温室气体排放的影响及机制研究进展

土壤是温室气体的重要源和汇。蚯蚓是土壤物质循环的重要参与者,能够直接或间接影响土壤CO2、N2O和CH4等温室气体的产生和释放。蚯蚓呼吸产生的CO2,是土壤呼吸的重要组成部分;蚯蚓自身肠道、分泌液、消化物和排泄物等微环境促进反硝化过程释放N2O。蚯蚓还通过取食、掘穴、排泄等活动,改变土壤理化性质、微生物组成和活性及其他土壤动物的组成,影响地上植物生长,调节土壤分解、矿化、硝化、反硝化和甲烷生成及氧化等生态过程,间接影响土壤温室气体的排放。蚯蚓对土壤温室气体排放的影响逐渐受到重视,但目前研究仍以室内培养和单因子环境条件的模拟为主,缺少野外原位实验和多环境因子的交互实验研究。长期监测和同位素示踪技术,是深入探讨蚯蚓影响温室气体排放机制的重要手段。温室气体类型上,CO2和N2O是研究热点,CH4研究比较罕见。未来研究,应重视不同生态类群蚯蚓与土壤理化特征、微生物组成、其他类群土壤动物和地上植物间的交互作用,加强机制研究,并关注土壤污染环境下蚯蚓功能性状的变化;综合评价蚯蚓对土壤温室气体排放和土壤碳氮固定的影响,科学评估蚯蚓活动对土壤碳氮释放的促进或减缓作用。

Advancement in study on effect of earthworm on greenhouse gas emission in soil and its mechanism

Soil is one of the major sources or sinks of the greenhouse gases (GHGs). Earthworms are a major component of the soil fauna communities in most ecosystems and comprise a large proportion of macrofauna biomass. Earthworms are termed soil ecosystem engineers, as they play an crucial role in the soil physical-chemical properties, soil structure and nutrient cycling through their feeding, burrowing and casting activities. So their activity would potentially affect the produce and emission of the GHGs (CO2, N2O and CH4) from soil. On one hand, the respiration of earthworm is an important part of soil respiration. In the earthworm guts, mucus or casts, conditions are ideal for denitrifying bacteria as they are anaerobic microsites where enough C and N supply and favorable moisture all stimulate denitrifier activity. Thus, earthworm can directly emission 2.5~25ng N2O h-1 g-1 fresh earthworm, as shown in recent studies. And N2O emissions from their casts were significant higher than from bulk soils. On the other hand, earthworms also affect the produce and release of soil GHGs indirectly by their activities by modifying the soil physicochemical and microbial properties and ecological processes (e.g. decomposition, nitrification and denitrification). Through the mixing of soil particles and plant residues they reshape the soil pores and aggregates, thereby affecting soil moisture dynamics, gas diffusivity, nutrient content and availability. Aerobic and anaerobic microsites within earthworm-made aggregates may affect rates of decomposition and denitrification, and a higher proportion of macroaggregates newly formed by earthworms contain more C and protect C from exposure and erosion in the long term. Earthworms stimulate the microbial populations through the increase of the contact of microorganisms with organic matter during gut transit, while they also modify the microbial community structures through digestion and dispersion in casts and burrows. Interactions between earthworms and denitrifying and methanotrophic microbes lead to increased N2O and decreased CH4 emission from the "drilosphere". These effects usually depend on soil moisture, organic matter content and earthworm species. Based on their feeding and burrowing behaviors, earthworms are typically subdivided into three ecological groups: epigeic, endogeic and anecic species. Food accessibility and the delivery distance of gases from soil to the atmosphere makes their functions quite different, and the interactions between two or three groups lead to more complex effects on soil GHGs. Earthworms may impact the biomass, activity and community structure of other soil fauna, such as mites, collembola, nematodes, isopods, and enchytraeids in their habitats. Earthworms might actually increase plant growth through altering soil nutrient availability as well. By these indirect ways, earthworms transform the whole soil ecosystem compositions and functions, but little is known about the effects of earthworm-plant interactions on the soil GHGs balance. Further researches are needed to fully understand the potential roles of different earthworm species, and their positive or negative interactions with soil physical, chemical and microbial properties in affecting GHGs emissions form soils. Overall, earthworms affect CO2, CH4 and N2O emissions mainly by regulating the carbon and nitrogen processes, such as decomposition, mineralization, nitrification, denitrification, methanogenesis and methanotrophy. Although much research has been done on the impacts of earthworms on soil CO2 and N2O emissions, there is still substantial gaps in our knowledge on CH4 efflux. In consideration of the soil pollution problems, it is urgent to study the effects and the potential role changes of earthworms in the contaminated soils. As for research techniques, current studies are mainly based on microcosm and mesocosm experiments in laboratories, while little were based on in-situ field studies and long-term investigations. In the field, the earthworm effects on the GHGs are usually long-lasting and seasonally variable; however, most of the laboratory studies are highly manipulated and simplified and could not really represent the natural field conditions. In addition, molecular and isotopic techniques are increasingly used to reveal the biological and ecological mechanisms on earthworms effects on the emissions of GHGs. And we should assess whether earthworms predominantly affect soils to act as net source or sink of GHGs by thoroughly considering their roles in carbon and nitrogen sequestration and gas emissions.