Spatial Variation in Potential Greenhouse Gas Mitigation Across California.

Successful implementation of soil greenhouse gas (GHG) mitigation options will be highly dependent on spatial variation in soil C sequestration and emissions of trace gases including N₂O when converting conventional to alternative management practices. Policy and decision makers need risk information to achieve a sensible target of changes in GHG emissions by adopting alternative management practices. The objectives of this study were to predict spatial changes in soil C and net soil greenhouse gas (GHG) fluxes by converting conventional to alternative management practices within California’s Central Valley using the DAYCENT model. We evaluated each or combinations of conservation tillage, winter cover cropping, and manure application or a 25% reduction in N fertilizer input for all unique combinations of crop rotation, climate, and soil types for the period 1997–2006. The effects of adopting conservation tillage, winter cover cropping, manure application or reduced fertilizer input by itself on net soil GHG flux reductions were modest (on average -1 Mg CO₂-eq ha^-1 yr^-1). In comparison, combining two or more of these practices led to a more substantial decrease in net soil GHG fluxes of up to -3 Mg CO₂-eq ha^-1 yr^-1. However, variance of the changes greatly increased when combined with winter cover cropping, compared with when combined with manure application or conservation tillage. This would lead to highly uncertain effects of winter cover cropping across the region, which may not be cost-effective. This suggests that long-term GHG mitigation potentials should be evaluated at finer spatial scales to decide regional practicality of these alternative management practices as mitigation options.