Simulating soil carbon and greenhouse gas dynamics in grasslands amended with compost

Compost amendment to grasslands has been proposed as a way to mitigate climate change through carbon (C) sequestration, yet little research has been done exploring the source-sink potential of this management strategy. We used the ecosystem biogeochemical model, DAYCENT, to investigate the climate change mitigation potential of compost amendments to three grassland sites in California, including a valley grassland in the Sierra foothills and two coastal grasslands that differed in soil texture (e.g. sandy loam and loam texture) in northern California. The model was parameterized using site-specific characteristics, including long-term weather records and edaphic characteristics. Model validation was conducted by comparing simulated above- and belowground net primary production (NPP) and soil C with that from a three-year field experiment at each site and iteratively adjusting crop parameters. We then used the model to test ecosystem responses and source-sink potential of a variety of compost qualities and application rates. We found that ecosystem C and N responded rapidly to amendments, but the effects tended to be down-regulated by higher compost C:N ratios. Carbon sequestration rates were greater with low C:N compost, but soils amended with low C:N ratio compost experienced greater N2O fluxes relative to composts with higher C:N ratios. These results suggest a trade-off between maximizing plant production and minimizing N2O losses. We also found that the source-sink potential varied greatly when considered over short (10 year), medium (30 year), or long (100 year) time periods. We conclude that compost amendments to rangeland soils can result in significant C sinks, but that the full suite of soil greenhouse gas emissions and timeframes for C sequestration must be explicitly considered.