Management options for reducing CO2 emissions from agricultural soils

Crop-based agriculture occupies 1.7 billion hectares, globally, with a soil C stock of about 170 Pg. Of the past anthropogenic CO2 additions to the at mosphere, about 50 Pg C came from the loss of soil organic matter (SOM) in cultivated soils. Improved management practices, however, can rebuild C sto cks in agricultural soils and help mitigate CO2 emissions. Increasing soil C stocks requires increasing C inputs and/or reducing soil heterotrophic respiration. Management options that contribute to reduced so il respiration include reduced tillage practices (especially no-till) and i ncreased cropping intensity. Physical disturbance associated with intensive soil tillage increases the turnover of soil aggregates and accelerates the decomposition of aggregate-associated SOM. No-till increases aggregate sta bility and promotes the formation of recalcitrant SOM fractions within stab ilized micro- and macroaggregate structures. Experiments using C-13 natural abundance show up to a two-fold increase in mean residence time of SOM und er no-till vs intensive tillage. Greater cropping intensity, i.e., by reduc ing the frequency of bare fallow in crop rotations and increasing the use o f perennial vegetation, can increase water and nutrient use efficiency by p lants, thereby increasing C inputs to soil and reducing organic matter deco mposition rates. Management and policies to sequester C in soils need to consider that: soil s have a finite capacity to store C, gains in soil C can be reversed if pro per management is not maintained, and fossil fuel inputs for different mana gement practices need to be factored into a total agricultural CO2 balance.