Prescribed defoliation strategies influence soil carbon dynamics and nitrous oxide emission potential in West Virginia pastures

Prescribed defoliation strategies influence soil carbon dynamics and nitrous oxide emission potential in West Virginia pastures Jordan M.A. KoosGrowing interest in sustainable agriculture has driven inquiry into the impacts of grazing on cool-season perennial grasslands, which are commonly utilized as forage for ruminant livestock in West Virginia and throughout the Central Appalachian region.However, current understanding of the below-ground impacts of forage defoliation (i.e., one selective pressure applied by grazing livestock) management strategies is nascent.The aim of this thesis is to investigate how defoliation management strategies affect above-ground (e.g., forage production) and below-ground processes (e.g., soil organic matter (SOM) accumulation, microbial carbon, nitrogen, and phosphorus (P) cycling capacity, and nitrous oxide (N 2 O) emission potential).I hypothesized that the rotational (i.e., moderate frequency) defoliation at low severity will increase SOM storage and the microbial capacity to cycle C, N, and P.However, I also hypothesize that the greatest N 2 O emission capacity will also come from plots defoliated at a moderate frequency and low severity, due to greater below-ground C availability that will stimulate microbial activity.To address these hypotheses, data were collected from two West Virginia University Research Farms, one organically managed and one conventionally managed, each with experimental plots where forage defoliation treatments of increasing intensity and frequency have been implemented since May 2016.At the completion of two growing seasons, above-and below-ground parameters of interest were recorded, including leaf area index, forage utilization, stable and recalcitrant C pools, extracellular enzyme activities associated with microbial C, N, and P acquisition, and N 2 O emission potential via quantification of microbial genes by qPCR.At the Organic farm, greater defoliation severity decreased labile carbon content in the soil (-21%).Further, at the Organic Farm, defoliation treatments at rotational frequency and low severity resulted in a 39% reduction in β-glucosidase activity, however β-glucosidase activity was not impacted by defoliation frequency or severity at the Reedsville Farm.In addition, N 2 O emission potential was differentially affected by defoliation treatments at the two sites.At the Organic Farm, defoliation severity impacted N 2 O emission potential, whereby low defoliation severity resulted in 82% decrease in potential N 2 O emissions compared to high defoliation severity.At the Reedsville Farm, defoliation frequency impacted N 2 O emission potential, whereby continuously defoliated plots experienced a 71% decrease in potential N 2 O emissions compared to less frequent defoliation treatments.Taken together, these results suggest that defoliation management is capable of increasing soil C storage at the cost of increased nitrous oxide emission potential.However, management prescriptions must consider site specific nutrient dynamics to most accurately predict defoliation management impacts on soil C.