Soil carbon storage responses to expanding pinyon-juniper populations in southern Utah

Over the past several decades, the expansion and thickening of woodlands in the western United States has caused a range of ecological changes. Woody expansion often leads to increases in soil organic matter (SOM) pools with implications for both biogeochemical cycling and ecological responses to management strategies aimed at restoration of rangeland ecosystems. Here we directly measure C and N stocks and use simple non-steady-state models to quantify the dynamics of soil C accumulation under and around trees of varied ages in southern Utah woodlands. In the two pinyon-juniper forests of Grand Staircase Escalante National Monument studied here, we found approximately 3 kg C/m2 and approximately 0.12 kg N/m2 larger C and N stocks in soils under pinyon canopies compared to interspace sites. These apparent increases in soil C and N stocks under woody plant species were dominated by elevated SOM in the surface 10 cm of soil, particularly within non-mineral-associated organic fractions. The most significant accumulation of C was in the >850 microm fraction, which had an estimated C residence time of <20 yr. Rates of carbon accumulation following pinyon-juniper expansion appear to be dominated by changes in this fast-cycling surface soil fraction. In contrast, we found that after separating >850 microm organic matter from the remaining light fraction (LF), C had residence times of approximately 400 yr and mineral-associated (MA) soil C had residence times of approximately 600 yr. As a result, we calculate that input rates to the LF and MA pools to be 10 +/- 1 and 0.68 +/- 0.15 g m(-2) yr(-1) (mean +/- SE), respectively. These findings suggest that one consequence of management activities aimed at the reduction of pinyon-juniper biomass may be a relatively rapid loss of soil C and N pools associated with the >850 microm fraction. The temporal dynamics of the <850 microm pools suggest that carbon and nitrogen continue to accumulate in these fractions, albeit at very slow rates, and suggest that multidecadal storage of C following tree recruitment is limited to relatively small, subsurface fractions of the total soil C pool.