**4. Conclusions**

Accumulation of crop residue on the soil surface did not increase soil NO3-N mineralization. Narrow C/N ratio crop residue treatments influenced the rate of soil NO3-N mineralization, as was evident with the forage radish and pea crop residue treatments being the only ones to mineralize. This can help to inform freeze and thaw effects on NO3-N mineralization in a frigid environment such as North Dakota, where producers are limited to short growing seasons. The number of incubation cycles may have had a direct effect on the mineralization process for all the crop residue treatments, where cycle 4 and cycle 5 were noted to release the most NO3-N, particularly in the mid to late growing season as cycles increase. Wide C/N ratio crop residue such as corn, soybean, spring wheat, and winter wheat showed a net immobilization effect for every incubation cycle. The bare, unamended soil showed a cumulative increase in NO3-N mineralization cycle-over-cycle. It would have been expected that the addition of a carbon source to the soil surface, in the form of crop residue, would have been at or above the rate of the control as carbon would act as a new nutrient source for microorganisms to decompose and release nutrients back to

the soil (e.g., nitrate). This was not found to be true and indicates a state where degradation of the native SOM occurs, which may eventually contribute to soil degradation. In other words, the absence of a carbon source addition may lead microorganisms to seek a carbon source from the soil itself. When this occurs, it is due to the lack of nutrient substrate from organic matter. Thus, the microorganisms themselves release nutrients, including nitrogen, from the organic matter in the bare, unamended soil. Future studies may consider examining the addition of nitrogen fertilizer to determine whether it may aid in facilitating soil NO3-N mineralization in fields under a long-term no-till system with wide C/N ratio crop residues, offsetting the mineralization of N by decomposing crop residues or releasing N from the death of microorganisms.

There are several directions in future research that can be informed by our results. Further studies should examine the effect of the freeze and thaw cycle as well as the microbial count and community to better understand soil NO3-N mineralization source and sink. Microbes require nitrogen as a nutrient source; thus, limited nutrients in a cropping system may be observed where the volume of crop residue and the recommended fertilizer additions or credits are not equitable. In addition, further studies incorporating increased fertilizer may provide insights on the impacts of wide C/N ratio crop residues on NO3-N mineralization source and sink. A limitation of this study was the homogenous crop residue on the soil surface, whereas in a field setting, a mixed residue application (i.e., a sequence of different crops in a rotation) would more closely align with field conditions in a production environment.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nitrogen3020011/s1, Table S1: Mean and cumulative NO3-N mineralization for nine incubation cycles and overall mean for corn, flax, pea, radish, soybean, spring wheat, and winter wheat crop residues for Heimdal-Emrick, Fargo, and Forman soil series in North Dakota.

**Author Contributions:** Conceptualization, L.C. and R.S.A.; methodology, L.C. and R.S.A.; validation, L.C.; formal analysis, Q.W.; investigation, R.S.A.; resources, L.C.; data curation, R.S.A.; writing—original draft preparation, R.S.A.; writing—review and editing, R.S.A., L.C., and Q.W.; supervision, L.C.; funding acquisition, L.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the North Dakota State Board of Research and Extension Corn Committee (grant # 18-5-0090 and 19-11-0090), Soybean Committee (grant #19-6-0139), and Wheat Committee (grant # 19-38-0162) with matching funding from the North Dakota Corn Council (grant # 18-5-0090 and 19-11-0090), North Dakota Soybean Council (grant #19-06-0139), and North Dakota Wheat Commission (grant #19-38-0162). This work was conducted in conjunction with NDSU Hatch Project No. FARG008572 contributing to Multistate Research Project NC-1178 (Land Use and Management Practices Impacts on Soil Carbon and Relate Agroecosystems Services) of the National Institute of Food and Agriculture, U.S. Department of Agriculture.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data supporting reported results can be accessed by contacting rashadalgamdi@hotmail.com.

**Acknowledgments:** This research is part of Alghamdi's doctoral thesis under the supervision of Cihacek, and it was published at North Dakota State University database.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
