Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2020
Publication Date: 2/26/2020
Citation: Kandel, T.P., Gowda, P.H., Northup, B.K. 2020. Influence of tillage systems, and forms and rates of nitrogen fertilizers on CO2 and N2O fluxes from winter wheat cultivation in Oklahoma. Agronomy. 10(3):320. https://doi.org/10.3390/agronomy10030320.
Interpretive Summary: Cropping systems in agriculture can be a significant source greenhouse gasses (GHG) in the U.S., which makes cropping systems potentially large contributors to climate variability. A large portion of the nearly 80% of emissions from agriculture are contributed by management of cropping systems, including tillage system, type (organic, inorganic), and amount of fertilizer applied. Growing green sources of nitrogen (N) on croplands during fallow (non-cropped) periods may provide many benefits to producers and the environment, such as improved water use, preventing soil erosion, and providing N for subsequent crops. However, they may also generate GHG emissions in the form of carbon dioxide (CO2) and nitrous oxide (N2O) after soil incorporation. In this paper, we compared the effect of a summer green manure (cowpea) grown to supply N for following wheat crops, managed by either conventional or no-till systems, to wheat responses to an unfertilized control and two different amounts of inorganic N (40, and 80 lb N/acre). We followed the amounts of N2O and CO2 that were produced during the growing season of winter wheat (October through June). We found the exchange (flux) of CO2 between the canopy of wheat and the atmosphere was similar for both conventional and no-till management, and that the amount of CO2 flux into winter wheat increased with rate of N fertilization. We also found that the flux of CO2 into wheat in response to the green N and unfertilized control treatments were similar, and the lowest recorded during the study due to poor growth of winter wheat compared to the N fertilized treatments. The movement of N2O from soil into the atmosphere approximated zero from all treatments throughout the study, and we could not find any response to N fertilizer or tillage system. In conclusion, this study showed that the fluxes of CO2 from stands of winter wheat are controlled more by the effects of type (organic, inorganic), and amount, of nitrogen fertilizer than by the form of tillage.
Technical Abstract: Cultivation of winter wheat under reduced tillage systems is increasing in the U.S. southern Great Plains. Likewise, there is revived interest for including summer legumes in monocultures of winter wheat as green sources of nitrogen (N). This study investigated the influence of tillage system (no- and conventional tillage), and source and rates of N fertilizer (0, 45 and 90 kg N ha–1 yr–1 in inorganic N fertilizer, and cowpea as green manure) on emissions of carbon dioxide (CO2) and nitrous oxide (N2O). The study was conducted within a long-term field experiment (initiated in 2011) at upland and bottomland sites near El Reno, Oklahoma during the 2016–2017 growing season of winter wheat. The experiment was conducted on a site-wise basis as split-plots in a completely randomized design, with N treatment as main plots and tillage system as subplots Thus, there were a total of 8 treatment combinations with 3 replicated plots (4 m × 10 m) in each combination in both sites. Net ecosystem exchange (NEE) of CO2 was measured by a closed chamber connected to an infra-red gas analyzer, and fluxes were partitioned to gross primary production (GPP) and ecosystem respiration (ER). Heterotrophic soil respiration (SR) was measured on areas of bare soil. Fluxes of N2O were measured with an opaque closed chamber system with a portable gas analyzer. Dynamics of canopy CO2 fluxes (NEE, GPP and ER) were similar between tillage systems, while canopy CO2 fluxes increased with rate of N fertilization. Canopy CO2 fluxes from cowpea and an unfertilized control were similar, and the lowest, due to poor growth of winter wheat compared to the N fertilized treatments. Fluxes of N2O approximated zero from all treatments throughout the study and no response of N fertilizer or tillage system was seen. In conclusion, the results from this study indicated canopy fluxes of CO2 from winter wheat are controlled by form and rate of nitrogen fertilizer rather than form of tillage.