Soil amendments can increase net primary productivity (NPP) and soil carbon (C) sequestration in grasslands, but the net greenhouse gas fluxes of amendments such as manure, compost, and inorganic fertilizers remain unclear. To evaluate opportunities for climate change mitigation through soil amendment applications, we designed a field-scale model that quantifies greenhouse gas emissions (CO2, CH4, and N2O) from the production, application, and ecosystem response of soil amendments. Using this model, we developed a set of case studies for grazed annual grasslands in California. Sensitivity tests were performed to explore the impacts of model variables and management options. We conducted Monte Carlo simulations to provide estimates of the potential error associated with variables where literature data were sparse or spanned wide ranges. In the base case scenario, application of manure slurries led to net emissions of 14 Mg CO2e ha−1 over a 3-year period. Inorganic N fertilizer resulted in lower greenhouse gas emissions than the manure (3 Mg CO2e ha−1), assuming equal rates of N addition and NPP response. In contrast, composted manure and plant waste led to large offsets that exceeded emissions, saving 23 Mg CO2e ha−1 over 3 years. The diversion of both feedstock materials from traditional high-emission waste management practices was the largest source of the offsets; secondary benefits were also achieved, including increased plant productivity, soil C sequestration, and reduced need for commercial feeds. The greenhouse gas saving rates suggest that compost amendments could result in significant offsets to greenhouse gas emissions, amounting to over 28 MMg CO2e when scaled to 5% of California rangelands. We found that the model was highly sensitive to manure and landfill management factors and less dependent on C sequestration, NPP, and soil greenhouse gas effluxes. The Monte Carlo analyses indicated that compost application to grasslands is likely to lead to net greenhouse gas offsets across a broad range of potential environmental and management conditions. We conclude that applications of composted organic matter to grasslands can contribute to climate change mitigation while sustaining productive lands and reducing waste loads.


  • Grasslands cover 25% of the Earth’s land surface and are the dominant land-use globally (Asner and others 2004)

  • We developed a model to calculate greenhouse gas emissions and offsets resulting from amendment production and the effects of soil amendment application on net primary productivity (NPP), soil C storage, and factors associated with ruminant grazers

  • We evaluated the case where amendments were added at different rates: compost was added at 1,250 kg N ha-1 (1.27 cm, Ryals and Silver 2013), but manure and inorganic fertilizer were added at 250 and 125 kg N ha-1, respectively

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Grasslands cover 25% of the Earth’s land surface and are the dominant land-use globally (Asner and others 2004) These ecosystems occur in a biome characterized by periodic drought and high belowground allocation of plant tissues, leading to significant soil carbon (C) sequestration potential (Conant and others 2001, 2011). The application of soil amendments has been proposed as a means to increase net primary productivity (NPP) and soil C storage in grasslands (Paustian and others 1997; Conant and others 2001; Lal 2004a, b; Smith and others 2008; Cabrera and others 2009; Conant 2011; Ryals and Silver 2013). Organic fertilizers have co-benefits that include increased soil fertility, soil water holding capacity, and drought resistance (Hudson 1994)


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