Recent research by Chatterjee et al. (2015) and Aher et al. (2016) have shown that significant levels of crop residues can accumulate in crop rotations in which high residue crops such as wheat or corn are frequent component crops. This residue accumulation is partly due to the relatively cool climate in the northern Great Plains of the U.S. High residue producing crops often have high N requirements. N fertilizer is a major input in the cost of crop production but little information is available in the literature about the rate of N mineralization and the N contribution of residue decomposition to the N requirements of subsequent grain crops in long-term no-till culture. We have just completed laboratory research that shows that most crop residues after harvest have wide C:N ratios which encourage N immobilization rather than N mineralization. We have also noted that adding a high N-mineralizing cover crop such as forage radish may negate the immobilization through supplementing microbial N needs during the rapid growth phase of crop development. Since this previous work was done under optimum laboratory temperature and moisture conditions, we need to validate our findings under field conditions. Knowledge of the mechanisms of N cycling from post-harvest crop residues in combination with effects of cover crops in long-term no-till culture will information growers on improved decisions about the most efficient use of their fertilizer N applications.

The objective of this research is to:
(1) evaluate and validate our laboratory research on N-mineralization/immobilization processes in a field environment;
(2) determine how varying temperature and moisture conditions in a field environment affect crop residue decomposition and N mineralization under a no-till culture, and
(3) evaluate whether a cover crop (e.g. radish) can help mitigate N immobilization under field conditions.

This study is currently providing information on soil moisture and temperature changes during the growing season. Soil sampling during the season will provide information on soil plant-available N related to the residue(s) accumulating on the soil surface. This information will be presented at farmer/grower workshops and incorporated in research and extension publications providing farmers with information to make sound fertilizer management decisions.

Updated January 5, 2024:
FIELD VALIDATION OF MINERAL N CYCLING FROM MIXED CROP RESIDUES IN LONG-TERM NO-TILL SYSTEMS
2023 Fall Progress Report
Larry Cihacek and Rashad Alghamdi
NDSU SNRS – Soil Science
Fargo, ND 58108

Introduction
Soil test N fertilizer recommendations developed in North Dakota within the last decade suggest that up to 50 lbs. N/A credit can be taken for fields in long-term no-till culture if no-till has been practiced in that field for more than 6 years (Franzen, 2018). Recent laboratory work on nitrogen (N) mineralization/immobilization in North Dakota has shown that post-harvest crop residues are low in N and promote N immobilization as they decompose (Alghamdi and Cihacek, 2021a; Alghamdi et al., 2021b; Alghamdi et al., 2022). This study was established to validate our observations under field conditions with the specific objective of evaluating seasonal soil moisture and temperature conditions that influence both soil microbial activity as well as the rate of residue breakdown across at least 3 growing seasons.
Materials and Methods
A series of 96 microplots were established on a Fargo silty clay soil (fine, smectitic, frigid Typic Epiaquerts) on the NDSU campus in May of 2021. Each microplot was 15 inches in diameter and was surrounded by a ring constructed with landscape edging. In 64 of these micro plots, a time-domain reflectometry (TDR) probe was installed at a depth of 5 cm near the center of the microplot. The microplots were arranged in “nests” of 5 plots in order to accommodate a data logger to collect continuous temperature and moisture data from the 5 plots. Residues for all crops were collected from research plots or fields in the Fargo area, dried and passed through a garden leaf shredded to reduce the residue particle size that is easier to handle for the conditions in this study.
Crop residues at a calculated rate of 6.25 T/A were applied to all plots except the bare soil controls. The residue was either left on the surface (no-till) or mixed with the soil (conventional tillage) in three replications. A set of microplots also received the same treatments but with the equivalent of 30 lbs. N/A as a urea solution to evaluate the potential priming effect of N fertilizer. The residue treatments represented the same treatments used in previous laboratory incubation studies and included: a) No residue-bare soil control, b) soybean, c) corn, d) spring wheat and e) radish residues, Crop residues (soybean, corn and spring wheat) with 30% radish by weight were included to evaluate the ability of quickly N mineralizing materials (radish) to help offset the N immobilization normally observed in our lab studies. The residues are applied annually utilizing a corn-soybean-spring wheat (C-S-SW) rotation with each crop being represented in each year. The residues are applied at the beginning of the growing season and follow the C-S-SW sequence in subsequent years depending on the entry crop in 2021. The entire study is designed to last for three years in order to evaluate the effects of the whole rotation sequence. Soil N samples were collected at the beginning of the study, July 2021, and October 2021 (end of season). This was repeated in May, July and September 2022. In 2023, the samples were collected in late May, July and October. All samples except the 2023 soil samples have been analyzed in the laboratory and these are being analyzed at the writing of this report. 2023 is the last season for this study. End of season temperature and moisture sensor data was collected in October of 2023 and this data is currently being summarized and analyzed.
Results and Discussion
The summer of 2021 at Fargo was very dry with low rainfall and above average temperatures through early August. Consequently, very little residue decomposition appeared to have occurred during the growing season. The 2022 growing season started out wet and cool, thus providing a contrast to the 2021 season but turned dry later in the season. The 2023 season has started out with snow extending into late April followed by a short period of saturated soils. However, the summer turned out relatively dry for the rest of the growing season.
At this time, the data is highly variable due to the highly variable weather conditions (drought in 2021, wet spring in 2022, late spring in 2023). We expect that N mineralization rates will be lower under actual field conditions than under controlled conditions in the laboratory Acknowledgements
We thank Kaylie Carver and Jordan Whitterall for helping in establishing the plots and initial sample collection. We also thank Maria Batool, Muhammad Asfaq, Maksat Batyrbek and Anand Gupta for sample collection and sample analysis. We also thank Joel Bell for maintaining wheat and soybean crops on the study site.
References
Alghamdi, R., and L. Cihacek. 2021a. Do post-harvest crop residues in no-till systems provide for nitrogen needs of following crops? Agron. J. 2021:1-18. doi: 10.1002/agj2.20885.

Alghamdi, R., L. Cihacek, A. Daigh and S. Rahman. 2021b. Post-harvest crop residue contribution to soil N availability or unavailability in North Dakota. Agrosys. Geosci. Environ. 2021(4):e20221. doi: 10.1002/agg2.20221

Alghamdi, R., L. Cihacek and Q. Wen. 2022. Simulated cropping season effects on N mineralization from accumulated no-till crop residues. Nitrogen 3(2):149-160. https://doi.org/10.3390/nitrogen3020011.

Franzen, D. 2018. North Dakota Fertilizer Recommendations Tables and Equations. NDSU Ext. Bull. SF882 (Revised). NDSU Extension Service, Fargo, ND. February 2018.

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Most soybean growers grow the soybean in rotation combinations with other crops (spring wheat, corn) as a means of providing crop diversity to control weeds, insects and diseases as well as providing an N credit for their subsequent crop’s fertility needs. Our research shows that heavy crop residue accumulation in long-term no-till culture may be responsible for N immobilization in subsequent crops. Soybean can lend itself to seeding a fall cover crop after harvest that may help offset N immobilization and mitigate the immobilization by the residue. However, soil moisture and temperature are variables that cannot be controlled under field conditions. This work will provide a clearer picture of how the crop cam be managed to improve N availability to subsequent crops.