How Do Earthworms, Soil Texture and Plant Composition Affect Infiltration along an Experimental Plant Diversity Gradient in Grassland?

Background
Infiltration is a key process in determining the water balance, but so far effects of earthworms, soil texture, plant species diversity and their interaction on infiltration capacity have not been studied.
Methodology/Principal Findings
We measured infiltration capacity in subplots with ambient and reduced earthworm density nested in plots of different plant species (1, 4, and 16 species) and plant functional group richness and composition (1 to 4 groups; legumes, grasses, small herbs, tall herbs). In summer, earthworm presence significantly increased infiltration, whereas in fall effects of grasses and legumes on infiltration were due to plant-mediated changes in earthworm biomass. Effects of grasses and legumes on infiltration even reversed effects of texture. We propose two pathways: (i) direct, probably by modifying the pore spectrum and (ii) indirect, by enhancing or suppressing earthworm biomass, which in turn influenced infiltration capacity due to change in burrowing activity of earthworms.
Conclusions/Significance
Overall, the results suggest that spatial and temporal variations in soil hydraulic properties can be explained by biotic processes, especially the presence of certain plant functional groups affecting earthworm biomass, while soil texture had no significant effect. Therefore biotic parameters should be taken into account in hydrological applications.

背景
入渗（infiltration）是决定水量平衡的核心过程，但截至目前，学界尚未探究蚯蚓、土壤质地（soil texture）、植物物种多样性（plant species diversity）及其交互作用对入渗能力的影响。
方法/主要结果
我们在嵌套于不同植物物种（1、4及16种）、植物功能群（plant functional group）丰富度与组成（1至4个功能群：豆科植物、禾本科植物、小型草本植物、高大草本植物）样地内、设置有自然密度与降低密度蚯蚓处理的小样方（subplot）中，测定了入渗能力。夏季时，蚯蚓的存在可显著提升入渗能力；而秋季，禾本科与豆科植物对入渗的影响则源于植物介导的蚯蚓生物量变化。禾本科与豆科植物对入渗的影响甚至逆转了土壤质地的作用效应。我们提出两条作用路径：其一为直接路径，可能通过调控土壤孔隙谱（pore spectrum）实现；其二为间接路径，通过提升或抑制蚯蚓生物量，而蚯蚓生物量的变化会通过改变其掘穴活动（burrowing activity）进而影响入渗能力。
结论/意义
总体而言，本研究结果表明，土壤水力特性（soil hydraulic properties）的时空变异可由生物过程解释，尤其是某些植物功能群通过影响蚯蚓生物量所介导的过程；而土壤质地未产生显著影响。因此，在水文应用研究中应纳入生物参数。