Explaining the Rotation Effect: Do Soil N Dynamics Contribute to Higher Yields in Diverse Crop Systems?.

High-yielding maize-based crop systems require maize to take up large quantities of nitrogen over short periods of time. Nitrogen management in conventional crop systems assumes that soil N mineralization alone cannot meet rapid rates of crop N uptake, and thus large pools of inorganic N, typically supplied as fertilizer, are required to meet crop N demand. Net soil N mineralization data support this assumption; net N mineralization rates are typically lower than maize N uptake rates. However, net N mineralization does not fully capture the flux of N from organic to inorganic forms. Here we utilize a long-term cropping systems experiment in Iowa, USA to compare the peak rate of N accumulation in maize biomass to the rate of inorganic N production through gross ammonification of soil organic N. Peak maize N uptake rates averaged 4.4 kg N ha-1 d -1 , while gross ammonification rates over the 0-80 cm depth averaged 23 kg N ha-1 d -1 . Gross ammonification was highly stratified, with 63% occurring in the 0-20 cm depth and 37% in the 20-80 cm depth. Across the three cropping systems with varied rotation lengths and fertilizer inputs, neither peak maize N uptake rate nor gross ammonification rate differed significantly. Gross ammonification rate was 3.4 to 4.5 times greater than the peak maize N uptake across the cropping systems, indicating that inorganic N mineralized from soil organic matter may be able to satisfy a large portion of crop N demand and explicit consideration of gross N mineralization may contribute towards strategies that reduce the reliance crops on large soil inorganic N pools that are easily lost to the environment.