Rapid Carbon Accumulation Across a Chronosequence of Land Conversion to Grazing Dairies.

In the southeastern U.S., management intensive grazing dairies (MiGD), involving rotating cows to new paddocks every 12 hours, are proliferating on irrigated row crop agricultural land that is typically depleted in soil carbon stocks. We measured C stocks and δ¹³C composition over 2 years (April 2011 – December 2013) on a MiGD conversion chronosequence comprising three farms in Burke, Co GA converted from row crop agriculture to MiGD in 2006, 2008 and 2009, respectively. This land-use change is stimulating an extremely rapid soil C accumulation rate of ~ 7 Mg C ha^-1yr^-1 that persists for at least the first six years of conversion. Although many aggregating tropical forests accumulate C in aboveground biomass at almost ten times this rate, the Row-crop to MiGD land use change in the southeastern US accumulates C in the soil on par with the fastest rates yet reported [FISHER et al. 1994, Nature 371, 236—238]. This high accumulation rate likely stems from a low initial C content due to till agriculture, a warm climate favorable to year round forage growth, management methods (irrigation and fertilization) to maximize forage production, and continuous return of manure to the grasslands. Ruminant methane emissions may offset much of these C sequestration gains from a net greenhouse-gas CO₂ balance perspective, but the C-cycle implications of this landuse change are more nuanced and include positive feedbacks on water-holding and nutrient retention.