Influence of Cropping Intensity and Nitrogen Fertilizer Rates on In Situ Nitrogen Mineralization

Cycling of N through an agroecosystem can be managed more effectively if effects of N management and cropping sequence on soil N microbial processes are understood. Effects of cropping intensity and N fertilizer rate on net soil N mineralization were studied as well as their correlation with precipitation, air temperature and soil water content. Net soil N mineralization was measured by incubating undisturbed soil cores (15‐cm depth) containing anion and cation exchange resins at their bottoms. Cores were incubated during each of five time periods (3–4 wk each) during the fallow phase (mid‐April to mid‐September) of two no‐till cropping systems, wheat ( Triticum aestivum L.)‐fallow (WF) and wheat‐corn ( Zea mays L.)‐fallow (WCF). Past N fertilizer applications were over four rates with total amounts applied during the previous 6 yr of 0, 95, 190, and 286 kg N ha ‐1 in WF and 0, 134, 269, and 403 kg N ha ‐1 in WCF. Soils were an Aridic Paleustoll at Sterling and an Aridic Argiustoll at Stratton in eastern Colorado. Total net N mineralization in WCF was half that in WF (22 vs. 43 kg N ha ‐1 ; 2‐site average), probably due to greater immobilization as evidenced by nearly three times greater accumulation of crop residue on the soil surface after 6 yr of no‐till management. Greater conservation of applied N and soil N can be expected in the more intensive WCF system. Total mineralized N increased with N rate by = 0.2 kg ha ‐1 for each kg ha ‐1 of previously applied N. Precipitation in combination with air temperature and their interaction term gave the best prediction of average daily N mineralization at both sites.