Longitudinal Study of the Microbial Basis of N2O Emissions within a Long-Term Agricultural Experiment

Abstract

Much of the global nitrous oxide emissions are derived from agricultural management driving microbial N transformations. Crop rotation, no-till, and cover cropping are feasible agronomic strategies to prevent N losses to the environment, though their effect on soil microbial N cycling at the field scale remains relatively unknown. Our goal was to determine the effect of crop rotation (continuous corn, CCC; and continuous soybean, SSS), tillage (no-till, NT; and chisel tillage, T), and cover crops (cover crop mixture, CC; and no cover crop, NCC) on the quantification of functional genes related to the N cycle from different times throughout the growing season. The study was conducted during the growing season of the cash crops following a first season of cover crops introduced after 23 years of management. Using quantitative polymerase chain reaction (qPCR) techniques, we quantified nifH (N2 fixation), amoA (nitrification) and nirK, nirS, and nosZ (denitrification). Our results show that CCC increased nitrous oxide emissions by 44% compared to SSS and reduced soil pH by nearly 1 unit. The reduction in soil pH, coupled with an increase in fertilizer-derived ammonium, caused ammonia-oxidizing bacteria (AOB) and nirK copy numbers to increase. The SSS rotation showed opposite results. Chisel tillage was found to increase all N cycle gene counts compared to no-till. The cover crop mixture of cereal rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth) significantly reduced soil nitrate levels, though they did increase nitrous oxide emissions, possibly due to the inclusion of a legume in the cover crop mixture. In corn-dominated systems, more fertilizer N is required to maintain yields, leading to increased nitrous oxide emissions through bacterial nitrification and nirK denitrification caused by high ammonium concentrations and acidic soil conditions.