Influence of Tillage Systems, and Forms and Rates of Nitrogen Fertilizers on CO2 and N2O Fluxes from Winter Wheat Cultivation in Oklahoma  
[PERSON]  
1 Noble Research Institute, LLC, 2510 Sam Noble Parkway,  
Regardless of crop usage, a 3-4 month period of summer fallow exists in continuous monocultures of winter wheat, to conserve limited and sporadic amounts of precipitation as soil moisture to support winter wheat [2]. However, legumes could be grown during the summer to serve as sources of green N for winter wheat [3]. The use of annual legumes as cover crops during summer represents one of a series of systems of intensified production [3; 4; 5] that may improve efficiencies in use of available soil water in the SGP [6], increase soil carbon (C), suppress weed populations, and reduce nutrient runoff or leaching [7]. In the SGP, winter wheat is primarily cultivated with conventional tillage, though different systems of reduced tillage are increasing in use to conserve soil moisture, increase soil aggregation, and decrease erosion [8; 9]. As tillage influences soil physical, chemical, biological and environmental conditions, emissions of CO\({}_{2}\) and N\({}_{2}\)O from soils can be influenced by tillage systems [10; 11]. In general, systems of reduced tillage contribute to decreased soil emissions of CO\({}_{2}\) and increased soil organic carbon (SOC) over the long term [12]. Tillage operations induce large peaks of soil respiration due to increased soil aeration and increased contact of crop residues with soil [13]. In contrast, no-till systems avoid such tillage-induced emissions and delay decomposition of residues, but can have higher rates of soil respiration for longer periods due to increased availability of carbon substrates and soil moisture [14]. Increased soil efflux of N\({}_{2}\)O and CO\({}_{2}\) are expected after inclusion of legume-based cover crops in cropping systems applied to winter wheat, due to increased soil pools of C and N. However, the dynamics and magnitude of these fluxes largely depend on quality and quantity of biomass provided by the cover crop at termination [15]. Additionally, environmental conditions in the soil, such as temperature and moisture, control C and N mineralization and thereby CO\({}_{2}\) and N\({}_{2}\)O fluxes from decomposing green manures [16]. Legume-based cover crops grown during the summer period of monocultures of winter wheat in the SGP are generally terminated 1-2 months prior to planting of winter wheat, to save soil moisture which is the critical factor for winter wheat production in the region [17]. Additionally, the rates of N fixation by legumes decrease after flowering, so delayed termination may not add significant amounts of N inputs to the system. Alternatively, early-terminated legumes with less structural components can mineralize rapidly after termination, and the mineralized nutrients may be more efficiently transferred to winter wheat. However, if proper synchronization between N mineralization from green manures and uptake by the recipient crop is not achieved, yields of winter wheat may decrease and emissions of N\({}_{2}\)O increase outside the growth period of winter wheat [18]. Although interest for conservation agricultural practices such as no-till and legume green manures is increasing in the US SGP, there is limited information on influence of these managements on fluxes of CO\({}_{2}\) and N\({}_{2}\)O from winter wheat cropping system in this region. Therefore, in this study, we tested the influences of tillage system, rates of applied N fertilizers and cowpea as a legume green manure on canopy and soil fluxes of CO\({}_{2}\) and soil fluxes of N\({}_{2}\)O during a growing period of winter wheat cultivated in the US SGP. Fluxes of CO\({}_{2}\) and N\({}_{2}\)O were monitored in a side-by-side small-plot experiment using closed chamber methods.