Shrub influence on soil carbon and nitrogen in a semi-arid grassland is mediated by precipitation and largely insensitive to livestock grazing

Dryland (arid and semi-arid) ecosystems globally provide more than half of
livestock production and store roughly one-third of soil organic carbon
(SOC). Biogeochemical pools are changing due toshrub encroachment,
livestock grazing, and climate change. We assessed how vegetation
microsite, grazing, and precipitation interacted to affect SOC and total
nitrogen (TN) at a site with long-term grazing manipulations and
well-described patterns of shrub encroachment across elevation and mean
annual precipitation (MAP) gradients. We analyzed SOC and TN in the
context of vegetation cover at ungrazed locations within livestock
exclosures, high-inten- sity grazing locations near water sources, and
moderate-intensity grazing locations away from water. SOC was enhanced by
MAP (p<0.0001), but grazing intensity had little effect regardless
of MAP (p = 0.12). Shrubs enhanced SOC (300–1279 g C m2) and TN (27–122 g
N m2), except at high MAP where the contribution or stabilization of shrub
inputs relative to grassland inputs was likely diminished. Cover of
perennial herbaceous plants and litter were significant predictors of SOC
(r2 = 0.63 and 0.34, respectively) and TN (r2 = 0.64 and 0.30,
respectively). Our results suggest that continued shrub encroachment in
drylands can increase SOC storage when grass production remains high,
although this response may saturate with higher MAP. In contrast, grazing
– at least at the intensities of our sites – has a lesser effect. These
effects underscore the need to understand how future climate and grazing
may interact to influence dryland biogeochemical cycling.