Soil Enzyme Activities and Carbon and Nitrogen Availability In Organic Agroecosystems Across An Intensively-Managed Agricultural Landscape.

Variability in organic management practices and soil characteristics across farms may influence soil microbial metabolic function and hence the suite of soil-derived ecosystem functions (e.g. nitrogen [N] availability and [C] transformations) on which organic production relies. In order to investigate how this variability affects soil microbial activity, we measured nine soil enzyme activities across 13 organically-managed Roma-type tomato (Solanum lycopersicum L.) fields in California's Sacramento Valley. Soil samples (0-15 cm depth) were taken around the stage of tomato anthesis, when nutrient demand is highest. These fields spanned a three-fold gradient in soil C (6.7 to 20.0 g C kg^-1 soil) with a range of C:N ratio from 8.05 to 9.71. Soils had similar parent material (mixed alluvium), similar soil texture (loams and silt loams), and no significant variation in pH (mean = 6.66). Nutrient inputs varied across farms with two general groups based on primary organic matter amendment applied (manure or composted green material). Soil inorganic N (NH₄⁺ and NO₃^-) and Olsen P were highly variable across the fields. Redundancy analysis (RDA) revealed distinct profiles of enzyme activities in these fields with the majority of variation (65%) explained by microbial biomass C and N, soil organic matter (SOM), and inorganic N. For example, fields with the highest soil C had low inorganic N and the highest activity of N cycling enzymes (aspartase and L-asparaginase), while fields with intermediate levels of soil C had higher inorganic N and the highest activity of C cycling enzymes (β-glucosidase, α-galactosidase, and β-glucosaminidase). Fields with the lowest soil C (less than 10 g C kg^-1 soil) had the lowest overall soil enzyme activity. Our study suggests that opposing patterns in the activities of C vs. N cycling enzymes across a gradient of SOM have potential implications for N availability, N retention, C stabilization, and ultimately, productivity and environmental quality, in organic agroecosystems.