title: A long-term nitrogen fertilizer gradient has little effect on soil organic matter in a high-intensity maize production system.
abstract: Global 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 affecting SOC within maize-based systems due to its effects on crop residue production and SOC mineralization. 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 on SOC with direct reference to N rates that are empirically determined to be insufficient, optimum, and excessive. Across the N fertilizer gradient, SOC in 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 with POM C/N ratios, the slope of this relationship decreased from the least decomposed POM pools (coarse POM) to the most decomposed POM pools (fine intra-aggregate POM). 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 of POM. 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 between SOC amount, SOM C/N ratios, or microbial biomass and composition. These data suggest that excessive N fertilizer had little effect on SOM and they complement agronomic assessments of environmental N losses, that demonstrate N2 O and NO3 emissions exponentially increase when agronomic optimum N is surpassed.
authors.affiliation: Department of Agronomy, Iowa State University, Ames, IA, 50011, USA.
authors.forename: Kimberly H
authors.initials: KH
authors.lastname: Brown
authors.forename: Elizabeth M
authors.initials: EM
authors.lastname: Bach
authors.forename: Rhae A
authors.initials: RA
authors.lastname: Drijber
authors.forename: Kirsten S
authors.initials: KS
authors.lastname: Hofmockel
authors.forename: Elizabeth S
authors.initials: ES
authors.lastname: Jeske
authors.forename: John E
authors.initials: JE
authors.lastname: Sawyer
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Castellano
mesh_terms: D000383:Agriculture / Q000379:methods*; D018533:Biomass; D002244:Carbon / Q000032:analysis; D018556:Crops, Agricultural; D005308:Fertilizers; D007484:Iowa; D009584:Nitrogen / Q000032:analysis; D012987:Soil / Q000737:chemistry*; D012988:Soil Microbiology; D003313:Zea mays
chemical_list: D005308:Fertilizers; D012987:Soil; D002244:Carbon; D009584:Nitrogen