Fire changes the spatial pattern and dynamics of soil nitrogen (N) and δ¹⁵N at a grassland-shrubland ecotone

Abstract

Fire disturbance represents a major driver of soil nitrogen (N) distribution in many arid and semiarid grasslands. The spatial patterns of soil N and δ 15 N at microsite scale following fires, however, are rarely studied. Here we investigated the spatial distribution of soil N and soil δ 15 N before and within three years after a prescribed fire in a grassland-shrubland ecotone in the northern Chihuahuan Desert. The spatial heterogeneity of soil N decreased significantly after the fire, with the autocorrelation distance increasing from 1.55 m to over 3 m, and the spatial dependence index decreasing from 99% to 73%. Soil δ 15 N was autocorrelated at 0.44 m after the fire, a much smaller scale that is similar to recovered grass patches, suggesting grasses tend to control the soil N cycling rather than shrubs. Different from the overall content of soil N, which merely changed before and after fire, soil δ 15 N increased substantially after the fire. We argued that high temperature and increased soil moisture at grass microsites post-fire may lead to an enhanced mineralization rate and 15 N-enriched soil ammonium at grass microsites, which further stimulates the rapid recovery of grasses after the fire.