On fertility island dynamics after creosote death

Creosote bush (Larrea tridentata), throughout its range in North American drylands, is well-established to accumulate organic matter under individual shrubs, which enhances biological activity and creates islands of fertility compared to interspace soils. Less documented is whether island effects depend on the shrub’s biological processes, or instead result from the physical presence of the shrub alone. We utilized a grassland restoration effort that chemically killed creosote, over three years, to evaluate nutrient and soil gas flux under dead shrubs, compared to high- and low-density live shrub sites and differences under shrub canopy compared to interspace soils. Soil organic carbon was consistently higher under all shrubs (live or dead). However, soil moisture was higher and temperature was consistently lower under live shrubs. Gas flux was not directly attributed to shrub cover or plant viability, but nutrient fluxes were highly variable. Across sampling dates, soil nitrate (NO3 -), ammonium (NH4 +) and phosphate (PO4 3-) were often measured in higher concentrations in the interspace soils compared to under creosote, for both live and dead shrub conditions. Nitrification rates did not substantially differ under any of the experimental conditions, but did positively correlate with soil PO4 3- and negatively with soil pH. These results recapitulate that creosote is an anchor of islands of fertility in the Chihuahuan Desert; individual shrubs facilitate organic matter accumulation, but moisture retention and lower soil temperature were only consistently observed under live shrub canopies. Our data suggest that the physical presence of dead creosote maintains aspects of fertility islands, live shrubs are more important for soil properties promoting microbial activity, and soil nutrient content is highly dynamic in these systems.