## 4 Discussion  
We observed minimal and inconsistent influences of tillage system on canopy fluxes of CO2 from winter wheat cultivated in central Oklahoma during this study. Albeit influence of tillage system on GPP was statistically significant, the difference was only ~10%, and opposite responses were recorded on the two sites. As with GPP, ER differed by ~10% among tillage systems, and opposite responses were noted on the two sites. Consistent differences in GPP and ER rates on the two sites indicated the difference in ER rates between tillage systems was mostly due to differences in plant growth. Figure 7: Soil concentrations of (**a**) ammonium (NH4+) and (**b**) nitrate (NO3–) in the 0–0.15 m soil depth in upland (left panels) and bottomland (right panels) sites during two sampling dates. Data are shown as average and standard errors (\(n=3\)). The different letters above the bars represent significance difference among N treatments within a sampling date. Influence of tillage system was not significant at any sampling dates for NH4+ and NO3– concentrations. A possible reason for similar canopy CO\({}_{2}\) fluxes from the contrasting tillage systems might be related to the lower intensity tillage applied under CT, which was restricted to the upper 20 cm of soil, and residues were left on the soil surface. Previous studies in the SGP have documented similar yield responses of winter wheat to NT and CT systems [3; 22; 23], and were corroborated by similar levels of GPP for tillage systems in this study. The short distance between sites, and difference in soil properties and fertility, allowed a test of effects of nitrogen and tillage treatments across a broad set of landscapes. Generally, soil in the bottomland areas are considered highly fertile in the region, and hence highly productive. In the upland site, which is less fertile and has a shallower profile than the bottomland site, a CT system would have contributed for better cycling of nutrients through decomposition of biomass residues. This might have contributed for better growth of winter wheat under CT system which contributed for greater canopy CO\({}_{2}\) fluxes. Contrary to system of tillage, large and consistent responses of canopy CO\({}_{2}\) fluxes to N rates were seen on both sites. Canopy fluxes of CO\({}_{2}\) increased with increasing rates of N fertilizer, which indicated strong growth responses of winter wheat to amounts of applied N fertilizer. Increased GPP fluxes with increasing level of N fertilizer corroborate results from previous studies in the SGP, where most studies reported strong positive responses of grain or forage yield of winter wheat to amount of applied N fertilizer [4; 24; 25]. The results of canopy CO\({}_{2}\) fluxes also corroborate results on crop growth and yield of winter wheat, which was reported in a parallel study [26]. Higher rates of ER fluxes in fertilized treatments was mostly contributed by increased plant respiration due to better crop growth, as influence of rates of N fertilizer on soil respiration was not significant. The similar magnitudes and dynamics of canopy CO\({}_{2}\) fluxes from cowpea and 0-N treatments indicated that cowpea was not an effective source of green N for winter wheat. Additionally, the flux rates from cowpea treatments were significantly lower than from the 45-N and 90-N treatments. These results corroborate the previous findings in the region, as poor growth and yield responses of winter wheat to legume-based green N are reported frequently [3; 4; 25; 27; 28]. Lack of interaction effects between the tillage systems and N treatments indicated that cowpea-based green manure could not replace inorganic N fertilizers in either tillage system. In 2016, cowpea produced 1.1 (\(\pm\)0.2) and 1.9 (\(\pm\)0.5) Mg ha\({}^{-1}\) aboveground dry biomass (mean \(\pm\) standard deviation) on CT and NT plots at bottomland site, 1.7 (\(\pm\)0.7) and 2.0 (\(\pm\)0.6) Mg ha\({}^{-1}\) aboveground dry biomass on CT and NT plots at upland site.