Previous research has indicated that long-term no-till systems where heavy crop residue accumulations occur may require additional N fertilizer to compensate for the N immobilized by soil microorganisms in decomposing the residue. Little research has been reported in the literature addressing this issue. To gain better understanding of the N cycling and crop N availability under high residue conditions, three soils and six crop residues (corn, flax, soybean, spring wheat, winter wheat, winter pea and radish) were incubated for 20 weeks to evaluate the contribution of N mineralization from these residues to crops. The incubations were conducted at a constant temperature of 25°C with 3 replications. The treatments included soil alone or soil with residue. Incubation tubes were leached at two week intervals followed by addition of a nutrient solution without N to maintain a favorable environment for soil microbial activity. With the exception of winter pea and forage radish (components of a cover crop mixture), the residues had carbon to nitrogen (C:N) ratios of 53:1 to 101:1. The forage radish and winter pea had C:N ratios of 8:1 and 18:1, respectively. A 20 week (140 day) incubation period was chosen to represent a period slightly longer than the normal North Dakota growing season (115-130 days).
Across the incubation period, all of the soil plus crop residue treatments with the exception of winter pea and forage radish, showed much lower mineralization rates (N immobilization) than the soil alone across the incubation period. Only the winter pea and forage radish had a total mineralization rate equivalent to or greater than the soil alone. The radish also appears to “dump” nitrate-N after it dies (freezes) and dries out. Unfortunately, winter pea and forage radish are utilized in cover crop mixes and generate a relatively small amount of biomass relative to the accumulated crop residues. Immobilization of soil N during the decomposition of crop residues competes with growing crops for available mineral N and may require applications of additional fertilizer N to compensate for heavy residues. Although this research is not conclusive, it appears that immediate N mineralization as crop residues decompose is not a major source of plant available N in following seasons for high N using crops (corn, small grains). We are currently conducting additional incubations with residues on the soil surface to simulate a no-till system (second year of funding). Future research will also look at different environmental conditions (crop residue leachate, added ammonium-N) to try to define conditions required to minimize the potential need for added N in long-term no-till production systems.