Lack of nitrogen fertilizer rate effects on soil carbon and nitrogen supply capacity: evidence from a 10-year trial

Background and aims The long-term effects of annual fertilizer nitrogen (N) applications on soil organic carbon (SOC) has important implications for cropping system productivity and climate change mitigation. To understand the impacts of N fertilizer on SOC and other soil properties in a corn-based cropping system, a 10-year (2009–2018) study was initiated in Elora, Ontario, Canada. Methods Corn was grown every year and crop management was held constant aside from the N rate applied. Plots received one of six long-term N rates (range: 30 to 260 kg-N ha -1 ). SOC (0–20 cm) was measured in every plot at the start and end of the study. Results N fertilizer rate increased both grain yield (range: 5.6 to 11.0 Mg ha -1 ) and residue biomass (range: 5.0 to 7.6 Mg ha -1 ). Harvest index also increased significantly with N application (range: 0.49 to 0.55). Assuming a fixed harvest index would have overestimated residue returns. SOC increased by a similar amount after 10 years in all N rate treatments (mean: 4.5 Mg-C ha -1 ). Soil N supply capacity assessed at the end of the experiment was also unaffected by long-term N rate as measured by total soil N (0-20 cm), corn yield planted after trial conclusion (2019) and results from a 10-week incubation study. Conclusions N rate had no detectable effect on SOC, N supply capacity or aggregate stability after 10 years. While yield and residue C inputs did increase with N rate, assuming a constant harvest index of 0.5 would have overestimated actual residue C returns.