Soil community structure: Effect of different organic agroecosystems and edaphic properties.

摘要

Because of their agronomic importance and sensitivity to management changes soil organisms are potentially robust indicators of soil quality. This study examined the sensitivity of decomposer, nematode, and collembolan communities to different organic management practices and to chemical and physical edaphic properties. Two study sites were used: the Long-term Organic Farming Systems Research and Demonstration Site, Puyallup, WA and Full Circle Farm, Carnation, WA.;The experiment at Puyallup is designed to compare cropping system, amendment type, and tillage mechanism. Amendment with C:N of 11:1 increased bacterial-feeding nematodes without decreasing omnivorous/predacious nematodes compared to amendment with C:N of 15:1. Including a one-year pasture in rotation reduced bacterial-feeding nematodes, increased nematode maturity index and fungal dominance and increased Collembola by a factor 3.4-4.5. However, after cultivation, none of these changes to the soil community persisted.;At Full Circle Farm we evaluated farm-scale and field-scale variation of soil communities and their relation to edaphic properties. Regression trees with chemical, physical, and management parameters explained more than 60% farm-scale variation in microbial biomass, N-mineralization potential, and nematode abundance, but were less effective at modeling collembolan abundance, and nematode structure and enrichment indices. Field-scale spatial analyses indicated nematodes, bacterial to fungal biomass ratio, and collembolans were autocorrelated in the sandier, but not in the clay-rich field. Microbial biomass was autocorrelated in the clay-rich field. We conclude soil physical and chemical properties can have significant, but inconsistent effects on biological variation, and mapping inherent soil quality parameters should precede monitoring soil communities.;We also evaluated selective inhibition of substrate-induced respiration (SIR), for determining the ratio of fungal to bacterial biomass in a replicated field study. We optimized inhibitor concentrations using soil from one plot of the Puyallup experiment by evaluating the percent inhibition and inhibition additivity ratio (IAR). When optimized concentrations of cycloheximide (fungal inhibitor) and streptomycin (bacterial inhibitor) were applied to soils from 17 other plots, significant inhibitor overlap was recorded. We conclude that using selective inhibition of SIR to identify treatment effects in a replicated field experiment requires optimization for antibiotic level be completed for each plot, making it too cumbersome for routine use.