Influence of contrasting soil moisture conditions on carbon dioxide and nitrous oxide emissions from terminated green manures

Authors: SINGH, HARDEEP - Oklahoma State University; KANDEL, TANKA - Oklahoma State University; Gowda, Prasanna; SOMENAHALLY, ANIL - Texas A&M University; Northup, Brian; KAKANI, VIJAYA - Oklahoma State University

Submitted to: Agrosystems, Geosciences & Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/27/2019
Publication Date: 10/10/2019
Citation: Singh, H., Kandel, T.P., Gowda, P.H., Somenahally, A., Northup, B.K., Kakani, V.G. 2019. Influence of contrasting soil moisture conditions on carbon dioxide and nitrous oxide emissions from terminated green manures. Agrosystems, Geosciences & Environment. 2(1):190012. https://doi.org/10.2134/age2019.03.0012
DOI: https://doi.org/10.2134/age2019.03.0012

Interpretive Summary: Cover crops may contribute to increase soil carbon, and decrease soil and nutrient leaching. Legume cover crops in particular can also substitute inorganic N fertilizers. In the Southern Great Plains, summer crops are usually grown between May and October. During the fallow periods, fall or spring planted legumes can be grown as green N manure. The fall- and spring-planted legumes have different level of maturity at termination which influence decomposition and N mineralization rates of biomass after soil incorporation. Additionally, effective N transfer largely depends on soil moisture during incorporation. Thus, timing of termination and soil incorporation of legumes is crucial management for effective N transfer to the following recipient summer crops. Biomass incorporated at moist soil conditions usually decompose rapidly and release N to the soil. However, such rapid increase of soil N prior to establishment of recipient crops may also contribute for increased N2O emissions during rainfall events through denitrification, which is a potent greenhouse gas with 265-298 times higher warming potential than CO2. This experiment was designed to understand the effects of prevailing soil moisture at termination of two legumes [fall grown hairy vetch (HV) and spring grown grasspea (GP)] on biomass decomposition and N2O emissions. We hypothesized that large N2O emissions can be avoided by incorporating legumes during dry periods based on short term rain forecast. Results indicated that avoiding heavy rainfall events at soil incorporation of legumes may not be useful to avoid large N2O emissions.

Technical Abstract: Carbon dioxide (CO2) and nitrous oxide (N2O) emissions from decomposing legume green manures largely depend on soil moisture at incorporation. A potential management to mitigate N2O emissions could be to terminate and incorporate legumes during dry periods based on short term-rainfall forecast. The present mesocosm study was designed to examine the impact of soil moisture at soil incorporation of hairy vetch (Vicia villosa) and grass pea (Lathyrus sphaericus) on CO2 and N2O emissions. Fall-planted hairy vetch had higher concentrations of fiber fractions and lower concentrations of nitrogen than spring planted grass pea at termination in early summer. Aboveground biomass of both species (8 Mg dry matter ha-1) was incorporated within 0-10 cm soil depth of 25 cm soil cores which had 15% of water filled pore space. Two timings of rainfall were simulated as early and late rainfall that received 80 mm deionized water at or after a week of soil incorporation of the legumes. Additional 20 mm water was added after two weeks of the first simulated rainfalls. CO2 and N2O gas fluxes were measured on 2-3 times per week using a closed chamber method for 28 days incubation assay. Soil concentrations of NH4+ and NO3-, concentrations of N in undecomposed legume biomass, abundances of denitrifier bacterial genes (nirS, nirK and nosZ) and arbuscular mycorrhiza fungi (AMF) were also determined at weekly intervals. CO2 emissions increased immediately after the first simulated rainfall events and peaked around day 2-3 from both legume treatments. After the first rainfall simulations, soil NH4+ and NO3- concentrations increased while biomass N concentrations decreased rapidly. Abundance of nirK, nosZ and AMF were positively correlated to N2O emissions. In both rainfall timing treatments, N2O emissions increased immediately after the first simulated rainfall and reached to peak level around day 8-10. The dynamics and magnitude of emissions after the first rainfall events remained similar irrespective of the timing of simulated rainfall. Thus, the results indicated that avoiding heavy rainfall events at soil incorporation of legumes may not be useful to avoid large N2O emissions.