Fire changes the spatial distribution and sources of soil organic carbon in a grassland-shrubland transition zone

Abstract

In many mixed grass-shrub ecosystems, increased shrub biomass tends to promote overall carbon storage, but the distribution of carbon pools may be complicated by disturbances such as wildfires. We investigated the spatial distribution of surface soil organic carbon (SOC) and its relative contribution from grasses and shrubs after fires in a grass-shrub transition zone in the northern Chihuahuan Desert, USA. We used a prescribed fire to create a burned treatment, then collected soil and plant samples. The biogeochemical approaches, geostatistical analyses, and carbon partitioning analyses were used to quantify the SOC and soil δ13C spatial patterns. Before the prescribed fire, up to 98% of the spatial dependence of SOC was autocorrelated at a distance of 1.91 m, corresponding to the approximate average shrub canopy diameter, but the spatial dependence dropped to 81% at a larger autocorrelation distance (3.74 m) two windy seasons after the fire. C4 grasses and C3 shrubs contributed approximately equal amounts of carbon to the surface SOC pool before the prescribed fire. However, C4 grasses became the dominant source of SOC two windy seasons following the fire. For individual microsites, a substantially increased proportion of SOC was derived from C4 grasses at the shrub microsites following the fire. The higher proportion of C4 grasses-derived SOC at the shrub microsites post-fire suggests that SOC may have preferred pathways to move among different microsites following fire disturbance. The distinct spatial distribution patterns of δ13C, and the increased contribution of SOC from grasses may be explained by the rapid recovery of grasses following the fire. Overall, our results provide insights into how fire might be used as a management tool to alter soil carbon pools in the context of shrub encroachment.