Multi-Scale Variation of Soil Quality Indices and Association with Edaphic Properties.

Soil organisms are indicators of dynamic soil quality because their community structure and population density are both sensitive to management changes.  However, edaphic properties and high spatial variability can also affect soil biology and confound their utility for soil evaluation.  In the present study we evaluate decomposer, nematode, and collembolan communities, in conjunction with chemical and physical edaphic properties, at 81 sites across a 25-ha area of a working organic farm in Western WA.  In addition we identified two areas with similar management and contrasting soil texture for field-scale (0.09 ha) analysis (42 sites per field).  We built regression trees with chemical, physical, and management parameters to explain the farm-scale variation in microbial biomass (r²=0.74), N-mineralization potential (r²=0.67), nematode density (r²=0.61), collembolan density (r²=0.46), nematode structure index (SI, r²=0.39), and the nematode enrichment index (EI, r²=0.42).  Spatial analysis of field scale biological populations indicated spatial structure (99% of variance explained by autocorrelation) for nematodes in the sandier (mean=41%), less clay-rich (mean=13%) field, but a lack of spatial structure in the less sandy (mean=14%) more clay-rich (mean=25%) field.  Both bacterial to fungal biomass and collembolan density also showed more spatial structure in the sandier field, but microbial biomass was more structured in the clay-rich field.  We conclude soil physical and chemical data are not a priori evidential of soil biological populations, but within certain ranges the variation in these properties can affect biological variation.  Careful mapping of inherent soil quality parameters should be the first step in monitoring soil biological populations.