Application of stable isotope analysis to quantify the retention of eroded carbon in grass filters at the North Appalachian experimental watersheds

The entrapment of eroded soil organic carbon (SOC) in grass filters could affect watershed C export, but the magnitude of the process is rarely quantified. In order to assess the retention of eroded C in these settings, SOC stock was measured in grass buffers receiving runoff from cropped watersheds under long-term (> 20 y): chisel-till (CT) corn ( Zea mays, L.)-soybean [ Glycine max, (L.) Merr.] rotation, moldboard plowing (MP) continuous corn (CC), and no-till (NT) CC. Adjacent reference grasslands not affected by erosion were also sampled. In the CC watersheds, the δ 13C of bulk soil and soil separates was determined to identify the source of SOC in the grass filters. After accounting for differences in the number of corn crops, SOC stock in the MP watershed showed a corn-C deficit of 9.3 Mg C ha − 1 compared to NT. Corn-C accounted for 2 to 16% (mean: 5.2 Mg C ha − 1 ) of the total SOC pool in the grass filter and, assuming water erosion as the main determinant of C distribution, this corn-C gain translates into the retention of 55% of eroded C in the grass strip. Relative to the reference grasslands, SOC stock in the grass filters was up to 30 Mg C ha − 1 higher, an amount deemed too large to be attributed solely to retention of eroded C (export rate: 0.05-0.08 Mg C ha − 1 y − 1 ). Periodic delivery of nutrients may have enhanced biomass production and indirectly contributed to the observed SOC accrual in grass filters. Higher extractable P and higher C:N ratios at these locations support that hypothesis. These results demonstrate the applicability of 13C isotope to trace SOC sources in buffers receiving runoff from areas supporting C 4 vegetation. They also underscore the need to incorporate in-situ biomass production and burial processes in assessing the temporal evolution of SOC stocks in terrestrial deposits and the contribution of these landscape segments to watershed C budget.