Sources of Carbon and Nitrogen behind Rock Detention Structures in Post-Wildfire Semiarid Montane Watersheds.

We are working to identify sources of and potential for sequestration of C and N in sediments trapped behind rock detention structures, which are frequently used in watershed restoration to reduce erosion. One of the largest potential losses of C and N occurs after wildfires via the processes of overland flow, streamflow, and wind transport. Isotopes of C and N may provide useful insights into processes controlling carbon cycling and sequestration behind rock detention structures. We sampled soils at two depths behind rock detention structures in two mountain watersheds, Turkey Pen and Coal Pit, Chiricahua Mountains, southeast Arizona. Each watershed burned in a wildfire in 2011. Prior to the wildfire, Turkey Pen was shrubbier and grassier with older detention structures than Coal Pit. Eighteen percent of Turkey Pen had either high or moderate burn severity. Coal Pit was more extensively forested prior to the wildfire, and twenty-eight percent of the watershed had high or moderate burn severity. The soils were analyzed for total N, organic C, and stable isotope ratios (d¹³C and d¹⁵N). In this ecosystem, values of d¹³C < -25‰ imply a predominance of woody debris. Values greater than this indicate mixing with grassy debris. More negative values of d¹⁵N probably result from preferential oxidation of ¹⁵N in high burn severity areas. In scatterplots of each pair of analytes, the two watersheds form two distinct clusters. Sediments behind rock detention structures in Coal Pit have greater percentages of organic C and N and more negative values of d¹³C and d¹⁵N than do those in Turkey Pen, which we attribute to the higher burn severity of the Coal Pit watershed. In the less severely burned Turkey Pen watershed, with older detention structures and hence less runoff, easily transported grass debris may be more abundant in sediments behind detention structures.