Impact of cover cropping and landscape positions on nitrous oxide emissions in northeastern US agroecosystems

The environmental benefits of organic farming compared to conventional agriculture are well documented, but relatively few studies have assessed their differences in emissions of nitrous oxide (N2O), a potent greenhouse gas (GHG). The objective of the study was to assess the interactive impact of management and landscape positions on soil characteristics and N2O emissions. A field-scale experiment was conducted in two adjacent grain farms in upstate New York that have both undergone the same management for 20 years. In the conventional field (CNV), inorganic fertilizer was the only nitrogen (N) source, but in the organic fields (ORG), a legume cover crop, red clover (Trifolium pratense), was frost-seeded into a winter grain (spelt, Triticum spelta), and then incorporated in spring as a N source for the subsequent maize plants (Zea mays). Measurements of soil properties and N2O emissions were conducted at shoulder and toeslope positions on both CNV and ORG fields in 2012. Based on Principal Component Analysis, landscape position, management regime, and rotation phases explained 67% of the variation in the soil properties; these three major sources of variation in soil properties (principal components) were correlated significantly with seasonal average N2O emissions. Comparable N2O emissions were found from the clover-maize (ORG Cl-M) phase in the ORG field and the bare fallow-maize phase in the CNV field. The spelt-clover phase in the ORG field had the lowest N2O emissions due to low N availability. In the CNV field, seasonal average N2O emissions were driven mainly by the elevated gas fluxes after fertilizer application. High soil moisture and inorganic N pools towards the end of the growing season probably resulted in increased denitrification rates. The impact of landscape position on N2O emissions was mainly found in the CNV field, probably because greater moisture and pH drove greater rates of complete denitrification at toeslope positions. In the ORG Cl-M phase, the seasonal average N2O emissions were dominated by the emission peaks that immediately followed incorporation of clover. Greater clover biomass at shoulder slope positions resulted in greater N2O peaks there, but the position effect was not statistically significant. Our study suggests that ecosystem state factors, such as landscape characteristics, interacted with management practices to impact soil properties, crop growth, and microbial communities and, therefore, had interactive effects on N dynamics, including N2O emissions.