Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/1999
Publication Date: N/A
Interpretive Summary: Mineralization of N is affected by cropping system management, i.e. the number of crops grown over a period of years, and fertilizer applications of previous years. Nitrogen mineralization was measured during the fallow season in two rotations, wheat-fallow (WF) and wheat-corn-fallow (WCF) at two locations in eastern Colorado. Soils were a Weld loam near Sterling and a Keith clay loam near Stratton. Measurements were also taken over four N fertilizer rates applied during the previous six years under no-till management. Total amounts of N mineralized in the more intensive WCF rotation were half those in WF. Greater amounts of crop residue (nearly 3 times) in WCF than WF are likely the cause of lesser N being mineralized. Consequently, excess N present in WCF has a greater chance of being conserved for use by the following crop as it is gradually released through subsequent mineralization in comparison to WF. Also, for every unit increase in N previously applied, the total amount of N mineralized increased 0.2 units. This implies that careful attention should be paid to supplying only the current N needs of the crop as any excess increases the possibility of greater mineralization during fallow and a possible loss to the environment. A simple linear regression model of total monthly precipitation and average monthly air temperature plus their product gave the best predictor of average daily N mineralization.
Technical Abstract: 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 its correlation with precipitation, air temperature and soil water content. Net soil N mineralization was measured 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) over four N fertilizer rates (total amounts applied over previous 6 yr were 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. Undisturbed soil cores (15-cm depth) containing anion and cation exchange resins at the bottom were incubated during each of 5 time periods (3-4 wk each). 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 approximately 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