Shrubs have displaced grasses throughout much of the world’s arid and semiarid lands (drylands) over the past century. In the Chihuahuan Desert in southern New Mexico, the conversion of black gramma grasslands to mesquite shrublands over the past 100 years has been well documented. The extent to which this displacement of grasses by shrubs has altered leaf litter quality and the spatial distribution of litter inputs and thus nutrient turn over via decomposition is unknown. Furthermore, reductions in grass cover are accompanied by accelerated rates of nutrient redistribution and loss via wind and water erosion. The current generation of predictive ecosystem models, largely based on studies in mesic systems, cannot adequately quantify these changes in biogeochemistry that accompany grassland-to-shrubland conversions. Several recent studies suggest that unique drivers of decomposition not accounted for in models may influence dryland decomposition. One of these is soil-litter mixing, which may promote decomposition by (a) litter burial altering the microclimate surrounding leaf litter, enhancing suitable conditions for decomposer organisms;  (b) acting as a vector for colonization by microbes, and (c) physical abrasion. We quantified the effects of soil-litter mixing on leaf litter decomposition by placing litterbags in areas where grass cover was reduced 0, 50, 75, and 100%, and in un-manipulated response plots immediately downwind of our treatment plots. Previous work has documented an increase in soil flux when cover by grasses is reduced 50, 75 and 100%.
Results/Conclusions

Our results do not show a significant relationship between grass removal treatment and mass loss after six months (r²=0.0009; P=0.8189).  However, we found a strong negative linear relationship between the ash content of litter, an index of soil deposition into litterbags, and mass loss in litterbags (r²= 0.2570; P<0.0001). Results suggest that soil-litter mixing is an important driver of leaf litter decomposition in the drylands of southern New Mexico. Soil-litter mixing may be particularly influential on biogeochemical cycling in drylands where increased soil erosion has occurred as a consequence of grazing and shrub encroachment.