Nitrogen Fertilizer Inputs and Soil Organic Matter Dynamics in Maize-Based Cropping Systems.

Nitrogen fertilizer inputs to maize-based cropping systems increase organic matter inputs and organic matter mineralization. The balance between these effects determines whether nitrogen fertilizer increases or decreases soil organic matter stocks. We investigated the 10-year effect of nitrogen fertilizer inputs (0-269 kg N ha-1) on soil organic matter stocks in continuous corn and corn-soybean cropping systems at four locations in Iowa.  Nitrogen was applied to the corn phase only. Organic matter inputs, rather than cropping system or nitrogen fertilizer rate, appeared to have the greatest effect on soil organic matter storage over time. At three of four research sites at the economic optimum N fertilizer rate, soil organic carbon balances were positive in continuous corn systems, but negative in corn-soybean rotation systems. Using data from seven fertilizer rates, two cropping systems and four geographic locations we build a conceptual model for soil organic matter storage that emphasizes interactions between soil carbon saturation, cropping system and nitrogen fertilizer rate.