Factors Influencing Soil Inorganic Nitrogen Levels in an Organic Vegetable Cropping Rotation Study.

Effective agronomic management of soil mineral nitrogen is critical for crop performance and mitigating environmental impact. In organic cropping systems, which rely on mineralization processes for in-season availability of amendment and soil inorganic N, understanding the environmental and management factors which influence the rate and periodicity of these processes is especially important. A field experiment has been conducted since 2011 on the University of California Santa Cruz farm in Santa Cruz, California to elucidate drivers of soil inorganic N in 16 crop rotation and fertility management regimes representative of the typical organic vegetable/strawberry production systems in California’s central coast region. Soil inorganic N (NO₃ + NH₄ mg/kg dry soil) was measured from these rotations on a frequent basis and matched with the environmental and management contexts from each treatment and sampling date. Using a random forest regression method, data from the 2011-2013 cropping seasons were analyzed to distinguish the contribution of environmental and management variables to soil N mineralization. Observed values varied broadly (0.03 – 175.8 mg/kg dry soil) through the measurement period. Pre-plant fertilizer N (<174 vs >174 kg/ha N)input was found to be the most important driver of inorganic N levels, followed by days since cultivation (<30, 30-150, >150 days), crop type (lettuce, broccoli, cauliflower, strawberry), and soil moisture levels (<15.06, >15.06 gravimetric water content). Despite strongly contrasted fertility management approaches included in the treatment cohort, baseline soil total-N levels also ranked highly in explanatory importance, suggesting a legacy effect that persists longer than the 3 years that treatments have been imposed. This work provides a preliminary assessment of important factors which influence mineral N dynamics in organic vegetable cropping systems and suggests that management of soil moisture levels and cultivation frequency could be important tools for controlling in-season mineralization.