The Effects of Compost Application on Grazed Grasslands’ Greenhouse Gas Budget, Water Budget and Net Primary Productivity

Abstract:

Compost application has the potential to facilitate soil carbon sequestration through the direct input of organic carbon and enhanced photosynthesis while modulating ecosystem water cycling by increasing water holding capacity and decreasing water losses via evapotranspiration. Yet, it lacks direct observations of carbon and water cycles at ecosystems with compost applications to support our quantitative predictability. This pioneering study is part of the Working Lands Innovation Center’s effort to scale, sustain, and quantify carbon dioxide capture and greenhouse gas emissions reductions by deploying soil amendments on Californian agroecosystems. This study quantifies the effects of a compost application on a grazed annual grassland’s greenhouse gas budget, water budget, and microbial communities. The studied grassland is divided into control and treatment areas. The treatment area will recieve a compost applicaton in October of 2020. We utilize multiscale measurements ranging from an eddy covariance flux tower, chamber measurements, and lab and field experiments to quantify scale-emergent properties like net ecosystem exchange of CO₂ and water vapor, microbial and plant community dynamics, and soil carbon sequestration. We present the results from the first 1.25 years of measurements as a baseline of the grassland’s carbon and water budget prior to the compost application. We show how changes in temperature, rainfall, cattle grazing, and other environmental factors influence the seasonal dynamics and annual budget of CO₂ at the grazed annual grassland. The annual CO₂ budget is -0.0197gC m-² yr-¹, indicating the grassland is near carbon neutral. Further, we utilize an analytical footprint model to trace the source areas of measured fluxes at each time and partition the data into treatment and control groups based on the wind directions. Our preliminary results show the control and treatment areas have compatible CO₂ and water vapor fluxes after normalizing the differences in net radiation, air, and soil temperatures (ANCOA, p>0.05). These baseline analyses reveal that our treatment and control areas are similar, giving us confidence that any future differences between the two areas post-compost application could be attributable to the compost application.