Aims

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. Gross ammonification may better represent the absolute flux of inorganic N produced by soil N mineralization. Methods

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. Results

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. Neither peak maize N uptake rate nor gross ammonification rate differed significantly among three cropping systems with varied rotation lengths and fertilizer inputs. Conclusions

Gross ammonification rate was 3.4 to 4.5 times greater than 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 that explicit consideration of gross N mineralization may contribute to development of strategies that reduce crop reliance on large soil inorganic N pools that are easily lost to the environment.