Carbon accumulation across a chronosequence of land conversion to grazing dairies

Background/Question/Methods

Increasingly, the dairy industry in the eastern U.S. is transitioning from total confinement dairy systems (TCD) toward pasture-based, management intensive grazing dairy (MiGD) systems. This shift in land-use will likely drive considerable changes in ecosystem carbon cycling. MiGD systems involve fundamentally different land-use practices compared to conventional confinement dairies and conventional no-till pastures. The MiGD forage system involves rotational grazing at optimal digestibility, when the plants are immature and protein-rich (~20 days). In addition, MiGD cows spend >90% of their time in the field and deposit >90% of their waste directly to the soil surface. Thus, little above ground plant residues are directly returned to the soil, but rather substantial carbon inputs derive from bovine manure. In this study, we sampled a MiGD- chronosequence of row-crop to MiGD conversion established in 2007 in southeastern Georgia. All soils across the MiGD-chronosequence occur in relative close proximity (40 km) and are deep, well-drained, fine sandy loam Ultisols formed on Coastal Plain sediments. Prior to MiGD establishment, the soils were farmed for >50 years using conventional tillage techniques. We sampled soils at 0, 2, 3, and 5 years since conversion to 1 m depth for total and clay-associated carbon, and took monthly measurements of carbon dioxide flux.  Previous and ongoing research consists of identifying and characterizing soil carbon including stable isotopes, ¹³C NMR, pyrolysis GC-MS and chemolytic techniques.

Results/Conclusions

Total and clay-associated soil carbon increased following conversion and was limited to the upper 40cm of soil. The greatest increase occurred between 3 and 5 years after conversion with a 40% increase in bulk soil carbon and 30% increase in clay-associated carbon. Carbon in the surface horizon was isotopically enriched (δ¹³C; -22‰) in both bulk and clay fractions, and was consistent with increased carbon inputs from roots and manure. These trends in the clay fraction suggest that new carbon additions become rapidly associated with soil minerals. Overall, the oldest site (5 years since conversion) exhibited the highest carbon dioxide flux (p<0.005) and was approximately 50% higher than the other two sites (2 and 3 years since conversion). This data suggest that land-use change from row-crop agriculture to MiGD will likely improve soil health and increase carbon storage. Incorporation of a total ecosystem carbon flux (including ruminal methane emissions) as well as the composition and characterization of the carbon sequestered will fill a key knowledge gap for future southeastern U.S. carbon cycling estimates.