Rapid association of clay fraction carbon in soil from grazing dairy pastures

Background/Question/Methods

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.