A soybean crop is given a 40 lb./A nitrogen (N) credit toward the nitrogen requirement for subsequent crops (Franzen, 2018). North Dakota farmers harvested 7,050,000 acres of soybeans in 2017 ranking 5th in soybean production, nationally. Most soybeans in North Dakota are grown in rotation with spring wheat or corn and are expected to provide a portion of the N needs of the following crop. However, our recent and ongoing research on N mineralization from heavy crop residue accumulations in long-term no-till cropping systems has indicated that perhaps more immobilization of N rather than mineralization of N may be taking place in these systems. A part of the reason of the high residue accumulations in North Dakota is the cooler climate and shorter growing season than many other parts of the corn-soybean-wheat growing regions of the US. Our observations have noted that at times soybean residue after harvest has a higher C:N ratio than corn residue which is directly related to how rapidly N will mineralize from the residue. This work is proposed to fill in gaps in our knowledge about how the soybean crop and its residues dynamically interact with N availability in North Dakota no-till cropping systems and how this relates to soil health and will look at N transformations occurring in the plant and soil environment around the plant root zone during the late maturity, plant senescence, and at or immediately post-harvest.

a. Identify the N dynamics in soil root zone as the crop reaches maturity, senesces and is harvested;
b. Identify plant part contribution to soil N and N mineralization as it senesces prior to and at harvest, and,
c. Determine whether other factors such as root exudates (amino containing compounds) in the root zone may influence soil N dynamics and availability to succeeding crops.

Update:

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Update:

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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 and are expected to provide a part of the N needs of following crops. No-till conditions often result in heavy residue accumulation while a cooler climate and short growing season slows down decomposition, inhibiting N availability and mineralization from crop residues and N immobilization may be occurring in these systems. 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 for each individual crop.

To better understand crop fertilizer N needs, N mineralization or immobilization were studied for crop rotation and season effects. A series of laboratory soil incubations were conducted by adding 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 (an average North Dakota growing season), frozen for 3 weeks to simulate winter freezing and thawed. Samples were leached and nitrate determined every two weeks. After thaw, the next crop for the sequence was applied and the process was repeated. Nitrate mineralization or immobilization was determined using bare soil as a control and baseline to determine the trend.

We found that crops were only mineralizing N where the radish cover crop was included in the sequence with all other crop sequences showing immobilization. High C:N ratio crop residues (C:N>40) did not show N mineralization patterns over the study period. Nitrogen mineralization of soybean plant residues at senescence or at harvest generally showed N immobilization for most plant parts. This study shows a need for field validation research to verify these findings and help North Dakota farmers in utilizing N fertilizer credit recommendations for fields under long-term no-till management.

The results of this research will provide information to farmers, consultants and Extension personnel so they can modify or adapt soil test recommendation and fertilizer practices in longterm no-till (>5 years) to economically account for heavy crop residues in corn-soybean or wheat-soybean production systems. This knowledge will also provide information to better understand soil health in long-term no-till production systems.