**4. Conclusions**

The soil incubation study reported herein supports the view that N availability is an important factor in the decomposition of corn stover, as cumulative CO2 production after 60 d was directly affected by residue N content and a substantial increase occurred from the application of mineral N, although there was no significant impact on gross N mineralization. A stimulatory effect of exogenous N was observed for active biomass and cellulase activity but not for MBC, MBN, or protease. With respect to C mineralization, this effect was much more pronounced in the first than in the second month of incubation, as would be expected due to a greater decline in substrate availability.

These findings have practical implications for modern corn production that relies on high planting rates and intensive N fertilization to increase grain yield and is often assumed to promote SOC storage by enhancing residue inputs. On the contrary, five decades of synthetic N fertilization led to a net decline in profile storage of SOC, relative to treatmentspecific baseline data collected for the historic Morrow Plots [26], which is consistent with similar evidence from numerous other long-term cropping experiments throughout the world. Such findings would be expected if fertilizer and residue N promote heterotrophic C utilization during microbial decomposition, as demonstrated by the short-term incubation approach adopted for the present project.

**Author Contributions:** Conceptualization, T.J. and R.L.M.; methodology, T.J. and R.L.M.; software, T.J.; validation, T.J. and R.L.M.; formal analysis, T.J. and R.L.M.; investigation, T.J., D.T.M. and R.L.M.; resources, R.L.M.; data curation, T.J.; writing—original draft preparation, T.J. and R.L.M.; writing—review and editing, T.J. and R.L.M.; visualization, T.J. and R.L.M.; supervision, R.L.M.; project administration, R.L.M.; funding acquisition, R.L.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

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

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** Appreciation is expressed to Michelle Wander, Department of NRES, and Lynn Connor, USDA Agricultural Research Service, for providing access to some of the laboratory equipment used in our work. We also thank Andrew Margenot, Department of Crop Sciences, for providing expertise regarding enzyme assay techniques; Vander Nunes, Department of NRES, for assistance with electronic technical illustration; Tim Smith, Cropsmith, for assisting with soil sample collection; Wenyan Li, South China Agricultural University, for constructive suggestions regarding experimental design; and Kelsey Griesheim, Lucas Goldschmidt, Jill Allen, and Jenna Mattes for technical support with various phases of the project.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are not available from the authors.

#### **References**

