Abstract – Incorporation and harvest management of how hairy vetch-based green manure influences nitrous oxide emissions
In this study, we measured nitrous oxide (N2O) emissions from plots of fall-planted hairy vetch (HV, Vicia villosa) grown as a green nitrogen (N) source for following summer forage crabgrass (Digitaria sanguinalis). Two treatments were compared: (i) HV grown solely as green manure where all biomass was incorporated by tillage, and (ii) harvesting of aboveground HV biomass prior to planting of crabgrass. Fluxes of N2O were measured with closed chamber systems on 27 dates during a 2-month growth period of crabgrass after the termination of HV in early May. At termination, the average aboveground biomass yield of HV was 4.6 Mg ha−1 with 146 kg N ha−1 content. The N2O emissions were as high as 66 g N2ON ha−1 day−1 on day 1 after HV incorporation, but reached close to zero within a week. Emissions of N2O increased with subsequent rainfall and irrigation events from both treatments but emission peaks were not observed during the rapid growth of crabgrass. Twomonth cumulative emission of N2O (mean ± S.E., n = 4) from HV incorporated plots (921 ± 120 g N2O-N ha−1) was three times (P < 0.05) of HV harvested plots (326 ± 30 g N2O-Nha−1). However, crabgrass biomass yields, N concentrations and total biomass N uptake were decreased significantly by harvesting HV. In conclusion, the results suggested that whereas removal of HV biomass for use as forage may significantly reduce N2O emissions, quantity and quality of the following recipient crops may be constrained.
The cultivation of legume-based cover crops as green sources of nitrogen (N) is practiced in many parts of the world, including the US Southern Great Plains (SGP) (Tonitto et al., 2006; Bergtold et al., 2019; Kandel et al., 2018). Summer crops such as corn (Zea mays), sorghum (Sorghum bicolor) and annual summer grasses are important components of agriculture in the region. These crops are generally cultivated from June to September, and have long fallow periods (October through May). Winter hardy annual legumes such as hairy vetch (HV, Vicia villosa) can be cultivated as green manures in these production systems (Kaufman et al., 2013; Kandel et al., 2018). High yielding winter legumes cultivated in the SGP may fix enough atmospheric N in biomass to meet the requirements of following summer crops (Tonitto et al., 2006; Kandel et al., 2018). The biomass N of winter legumes can be effectively transferred to the following summer crop after soil incorporation due to generally low C/N ratio of biomass. Such a low ratio is a conducive biomass trait for rapid decomposition and N mineralization after termination (Rosecrance et al., 2000; Melkonian et al., 2017). However, rapid decomposition and mineralization of biomass N can also contribute to elevated emissions of N2O which is a highly potent greenhouse gas (Huang et al., 2004; Basche et al., 2014). An Intergovernmental Panel on Climate Change (IPCC) estimate suggests that on average 1% of N in biomass residue is converted into N2O (IPCC 2007). However, the emission factor for legume-based green manures can be higher due to rapid decomposition and N mineralization rates, compared to non-legume residues (Sanz-Cobena et al., 2014).