Nitrogen Dynamics of Soybeans and Soybean Residues in Long-term No-till Production Systems : Final Report

Larry Cihacek and Rashad Alghamdi
Soil Science Department
North Dakota State University
Fargo, ND 58108

Executive Summary

In 2017, approximately 7,050,000 acres of soybeans were harvested in North Dakota, making the state the 5th largest producer of soybeans in the nation. In the region, most soybeans are grown in a rotation with spring wheat or corn which is expected to provide a part of the N needs of the following crop. No-till conditions often result in heavy crop residue accumulation. The cooler climate and shorter growing season of the region may slow down decomposition and inhibit nitrogen availability and mineralization by plants when certain crop residues are present. Instead, an immobilization of N may be taking place in these systems. Several states in the northern U.S. have recommended that N should be added in long-term no till systems when heavy crop residues are present (Bundy, 1998; Ketterings et al., 2003; Jokela et al., 2004; Pariera Dinkins and Jones, 2019). To better understand crop fertilizer N needs, nitrogen mineralization and immobilization trends were examined taking into consideration climatic conditions such as freeze and thaw, as well as crop rotational effects. In nature, the C:N ratios of post-harvest crop residues differ by species. When crops are rotated in a field, the C:N ratios of the system may be altered resulting in soil N mineralization or immobilization values which vary from those observed by each crop on its own (Chatterjee et al., 2016). Incorporating a late season cover crop in a rotation where non-leguminous crop residue exists may alter the soil N immobilization effects of non-leguminous crops to accelerate soil or residue N mineralization. In this research, a series of soil incubations were conducted with additions of corn (C), soybean (S), spring wheat (SW), and radish (R) crop residues, to simulate common cropping systems with a 2-year, 3-year and 3-year with cover crop rotation. Cropping systems and rotations included: C-C-C, R-R-R, C-S-C, S-C-S, SW-S-SW, S-SW-S, SW-S-C, S-C-SW, C-SW-S, SW/R-S/R-C, S/R-C-SW/R, C-SW/R-S/R and S/R-C-SW/R. Soils were incubated for 12 weeks (the length of a normal North Dakota growing season), frozen for 3 weeks to simulate winter freezing and were then thawed. Samples were leached and nitrate was recorded. After thaw, the subsequent crop for the sequence was applied and the process was repeated. Nitrate mineralization and immobilization trends were recorded with the bare soil as a control and baseline standard to determine the trend. Findings of this research determined that crops were only mineralizing where the radish cover crop was incorporated for the sequence. For all other sequences, immobilization trends were evident. High C:N ratio crop residues (C:N>40) did not demonstrate N mineralization patterns over a period of time with similar climatic conditions in a field setting. Evaluation of N mineralization soybean plant residues at senescence or at harvest Showed a general trend of N immobilization for most plant parts. Results of this study indicate a need for field validation research to verify these findings and should lead North Dakota soybean farmers to reconsider N fertilizer credit recommendations for their fields with long-term no-till conditions.