Farm-scale variation of soil quality indices and association with edaphic properties

Farm-scale variation of soil quality indices and association with edaphic properties. Soil organisms can be used as indicators of dynamic soil quality because their community structure and population density are sensitive to management changes. However, edaphic properties can also affect soil organisms and spatial variability can confound their utility for soil evaluation. We evaluate the relationship between two important agronomic functions, N-mineralization potential and aggregate stability, and biological, chemical, and physical edaphic properties. Decomposers, nematodes, collembolans, total C, total N, pH, bulk density (D b), and texture were evaluated at 81 sites across 25-ha of an organic farm in western Washington. We built regression trees with biological, chemical, physical, and management parameters to explain the farm-scale variation in microbial biomass (r 2 = 0.74), nematode density (r 2 = 0.61), collembolan density (r 2 = 0.36), nematode structure index (SI, r 2 = 0.41), nematode enrichment index (EI, r 2 = 0.54), proportion of soil as aggregates > 0.25 mm (r 2 = 0.60) and N-mineralization potential (r 2 = 0.58). Soils with microbial biomass > 597 μg C mic g -1 formed a homogeneous group with the greatest N-mineralization potential, and soils with >13.5% clay formed a homogeneous group with the greatest proportion of soil aggregates > 0.25 mm. Increased soil aggregation was associated with larger nematode SI, though much of the variability in SI remained unexplained by the data. Tillage had a strong effect on both decomposer and nematode populations; soils not tilled for 5 yr had the largest microbial biomass and soils not tilled 2 wk before sampling had the largest nematode populations. Comparisons of soil quality indicators across farms should be sensitive to the association of indicators with soil texture and recent management practices.