Since the early 2000’s, the southeastern United States has witnessed an accelerated conversion of row-crop land to management intensive grazing dairy (MiGD) systems utilizing multiple pastures and a 12-h rotation schedule. This land-use change represents a radical shift in the carbon cycle, one that differs fundamentally from what would be exhibited by conventional dairies or typical no-till pastures. MiGD cows spend over 90% of their time in the pasture in 20-day grazing rotations. To assess the impact of this land-use change on the carbon (C) cycle, we sampled a chronosequence of row-crop to MiGD conversion in southeastern Georgia, capturing pastures at 2, 3, and 5 years since conversion. We chose to focus on a representative soil pit form the oldest site to further investigate utilizing a H2O2 wet oxidation technique on both bulk and clay fractions. Subsamples were taken at regular intervals during the procedure and are undergoing 13C analysis to better understand the oxidation resistant fraction of the soil organic carbon. In addition, we are employing iron-oxide extractions and XRD analysis to constrain the composition of clay minerals and their potential for carbon sequestration.
Results/Conclusions
Total and clay fraction soil carbon increased since time of conversion in the top 40 cm, with the greatest increase occurring from 3-5 years since land-use change. This resulted in a 63% increase in surface (0-5cm) bulk soil carbon, a 24% increase in clay fraction soil carbon, and an increase in root mass from 1.9 g kg-1 to 45.9 g kg-1 soil. This increase from 2-5 years since conversion is an indication that long-term rotational grazing strategies increase belowground primary productivity. The accumulated carbon trended towards isotopic enrichment (δ13C; -21.5‰) in bulk and clay fractions, an indication of the increasing influence from belowground roots and evidence of MiGD mixed C3 and C4 forage production. These observations suggest a rapid increase of new carbon in the clay fraction. XRD analysis indicates quartz as the dominant mineral phase in the bulk material, with kaolinite, glauconite, and a mixed layered kaolin chlorite phase present in the <2um fraction with increasing depth. AAO extraction data shows SRO Fe phases to range from 4.74 ±.02% in the deepest clay fraction to 80.65 ±.02% in the shallowest bulk fraction of total CBD extractable iron. Current work involves analyzing the δ13C of the H2O2 resistant organic carbon and determining any correlations with the total free or SRO Fe, Al, and Si phases.