Nutrient Management Research Database

Climate change predictions for California indicate that agriculture will need to
substantially adapt to reduced water availability, changing crops, and changes
in temperatures, in order to sustain the level and diversity of crop production in California. California legislators recently passed the California Global Warming Solutions Act of 2006 (AB 32) that requires all industries to reduce the three major greenhouse gases (GHGs) (CO2, N2O, and CH4) to 1990 levels by 2020. The great diversity of cropping systems and management practices in California agriculture leads, however, to greater uncertainties in estimates of GHG budgets compared to Midwest agriculture. In light of AB 32, we, here, synthesize all the available information on the potentials for California agriculture to sequester C and reduce GHG emissions through various alternative management practices: minimum or no tillage, organic, cover cropping, manuring, and
reduced chemical fertilizer management. Our review indicates that C sequestration
and GHG emission reductions are possible, but there is no single land management practice or change in inputs that could mitigate the C released from agricultural practices (e.g., fossil fuel usage, land-use changes, soil erosion, biomass burning, and N fertilizer associated emissions) and meet climate change commitments set out in AB 32. Therefore, it is only the integration of different management strategies that shows considerable potential for C mitigation as well as provides important co-benefits to ensure the future sustainability of California agriculture.

This review paper highlights current research focusing on GHG reduction in CA agriculture, discusses the basic principles regulating GHG fluxes, reviews the capacity of CA soils to sequester C and reduce GHG emissions and identifies research gaps.  The authors discuss the potential of various technologies to increase soil C, including conservation and zero tillage, organic farming, winter cover cropping, manure or sewage applications, n fertilizer application, and improved irrigation.

This review paper presents results of other research that assesses the impact of selected agricultural management practices on carbon storage in agricultural soils.  The influence of tillage, cover cropping, organic amendments and synthetic fertilizer usage are discussed.

The review summarizes a number of studies that suggest an increase in CO2 emissions after tillage in CA annual cropping systems, as well increased potential for nitrous oxide emissions. Although research on the potential of conservation tillage to increase soil C  has been studies less and more long-term research is needed, the review reports results of CA research that suggests that a transition from conventional to no tillage could increase soil microbial biomass C and reduce CO2 emissions associated with tractor passes.

Cover cropping in annual cropping systems and incorporation of crop residues into the soil, as well as adjusting nitrogen applications to account for mineralization of N from organic matter are also highlighted as potential management strategies to improve soil C sequestration.

The review then goes on to discuss predictive modeling approaches for GHG emissions, including a study based in California that estimated that on balance CA soils were sequestering C, but dynamics varied by crop and region. Modeling scenarios predicted that incorporation of crop residues and manure application led to an increase in soil C storage, but did not show conclusively what the effect of these practices on nitrous oxide emissions would be.  Another approach using the DAYCENT suggested that organic agriculture and winter cover cropping had a high potential to mitigate CO2 emissions from agriculture.

The potential cobenefits of carbon-mitigating agricultural practices, economics of conservation farming, as well as the challenges and recommendations for future research on the potential for California agricultural soils to sequester carbon are also addressed.