Post-fire Redistribution of Soil Carbon and Nitrogen at a Grassland–Shrubland Ecotone

Abstract

The rapid conversion of grasslands into shrublands has been observed in many arid and semiarid regions worldwide. Studies have shown that fire can negatively affect shrub communities and promote resource homogenization, thereby providing some reversibility to the resource heterogeneity induced by shrub encroachment, especially in the early stages of encroachment. Here, we used prescribed fire in a grassland–shrubland transition zone in the northern Chihuahuan Desert to test the hypothesis that fire facilitates the remobilization of nutrient-enriched soil from shrub microsites to grass and bare microsites and thereby reduces the spatial heterogeneity of soil resources. Results show that the shrub microsites had the lowest water content compared to grass and bare microsites after fire, even when rain events occurred. Significant differences of total soil carbon (TC) and total soil nitrogen (TN) among the three microsites were not detected 1 year after the fire. The spatial autocorrelation distance increased from 1 to 2 m, approximately the mean diameter of an individual shrub canopy, to over 5 m 1 year after the fire for TC and TN. Patches of high soil C and N decomposed 1 year after the prescribed fire. Overall, fire stimulates the redistribution of soil C and N from shrub microsites to nutrient-depleted grass and bare microsites, leading to a decrease in spatial heterogeneity of these elements. The redistribution of soil C and N from shrub to grass and bare microsites, coupled with the reduced soil water content under the shrub canopies but not in grass and bare microsites, suggests that fire might influence the competition between shrubs and grasses, leading to a higher grass, compared to shrub, coverage in this ecotone.