Long-term dynamics of soil organic matter and aboveground net primary production in a Chihuahuan Desert Grassland at the Sevilleta National Wildlife Refuge, New Mexico (1989-2014)

Drylands contain a third of the organic carbon stored in global soils;
however, the long-term dynamics of soil organic carbon and soil
organic matter (SOM) in drylands remain poorly understood relative to
dynamics of the vegetation carbon pool. We examined long-term patterns
in SOM against both climate and prescribed fire in a Chihuahuan Desert
grassland in central New Mexico, USA. SOM was measured each spring and
fall for 25 years (1989–2014) in unburned desert grassland and from
2003 to 2014 following a prescribed fire. SOM concentration from 0-20
cm depth did not show a clear long-term trend but fluctuated
seasonally at both burned and unburned sites, ranging from a minimum
of 0.9% to a maximum of 3.3%. SOM concentration declined nonlinearly
in wet seasons and peaked in dry seasons. These results not only
contrast with the positive relationships between aboveground net
primary production and precipitation for this region, but also with
previous reports of greater SOM in wetter sites across drylands
globally, suggesting that space is not a good substitute for time in
predicting the dynamics of dryland SOM. We suggest that declines in
SOM in wet periods are caused by increased soil respiration, runoff,
leaching, and soil erosion. In addition to tracking natural
variability in climate, SOM concentration also decreased by 14%
following prescribed fire, a response that magnified over time and has
persisted for nearly a decade due to the slow recovery of primary
production. Our results document the surprisingly dynamic nature of
soil organic matter and its high sensitivity to climate and fire in
this dryland ecosystem.