A long‐term nitrogen fertilizer gradient has little effect on soil organic matter in a high‐intensity maize production system

• DOI: 10.1111/gcb.12519
• Publication Date: 2014-01-07T08:10:25Z
• Authors: Kimberly H. Brown; Elizabeth M. Bach; Rhae A. Drijber; Kirsten S. Hofmockel; Elizabeth S. Jeske; John E. Sawyer; Michael J. Castel...
• Keywords: Crops, Agricultural; 2. Zero hunger; 330; Nitrogen; Ecology and Evolutionary Biology; Agriculture; 04 agricultural and veterinary sciences; 15. Life on land; Iowa; Zea mays; 630; Carbon; Soil; 13. Climate action; 0401 agriculture, forestry, and fisheries; Biomass; Fertilizers; Soil Microbiology

AbstractGlobal maize production alters an enormous soil organic C (SOC) stock, ultimately affecting greenhouse gas concentrations and the capacity of agroecosystems to buffer climate variability. Inorganic N fertilizer is perhaps the most important factor affectingSOCwithin maize‐based systems due to its effects on crop residue production andSOCmineralization. Using a continuous maize cropping system with a 13 year N fertilizer gradient (0–269 kg N ha−1 yr−1) that created a large range in crop residue inputs (3.60–9.94 Mg dry matter ha−1 yr−1), we provide the first agronomic assessment of long‐term N fertilizer effects onSOCwith direct reference to N rates that are empirically determined to be insufficient, optimum, and excessive. Across the N fertilizer gradient,SOCin physico‐chemically protected pools was not affected by N fertilizer rate or residue inputs. However, unprotected particulate organic matter (POM) fractions increased with residue inputs. Although N fertilizer was negatively linearly correlated withPOMC/N ratios, the slope of this relationship decreased from the least decomposedPOMpools (coarsePOM) to the most decomposedPOMpools (fine intra‐aggregatePOM). Moreover, C/N ratios of protected pools did not vary across N rates, suggesting little effect of N fertilizer on soil organic matter (SOM) after decomposition ofPOM. Comparing a N rate within 4% of agronomic optimum (208 kg N ha−1 yr−1) and an excessive N rate (269 kg N ha−1 yr−1), there were no differences betweenSOCamount,SOMC/N ratios, or microbial biomass and composition. These data suggest that excessive N fertilizer had little effect onSOMand they complement agronomic assessments of environmental N losses, that demonstrate N2O andNO3emissions exponentially increase when agronomic optimum N is surpassed.