PARENT SESSION
Poster Session 32: Biogeochemistry
Thursday, August 11, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Impacts of woody plant encroachment and land-use change on soil carbon stocks of the Great Plains.

McCulley, Rebecca^*,1, Jackson, Robert², ¹ Dept. of Biology, Durham, NC, USA² Nicholas School of the Environment and Earth Sciences, Durham, NC, USA

ABSTRACT- The Great Plains of North America have experienced high levels of land use during the past 150 years. In the southern portion of the Plains, cattle grazing and historic fire suppression have contributed to grasslands being extensively converted to woodlands. Woody plant encroachment into grasslands has been shown to increase, decrease, and/or cause no net change in SOC storage, depending on the climate, soil texture, and species characteristics present at the site. In an effort to understand how this land-cover change has altered patterns of SOC storage across the region, we sampled adjacent grassland and woodlands at 4 sites spanning a sub-humid to semi-arid climatic gradient. We separated particulate organic matter (POM) fractions from soil collected to 1.5 m depth, and determined the organic C content and isotope ratio on each fraction. Despite the response of the total SOC pool to woody plant encroachment varying from small gains at the semi-arid sites to large losses at the sub-humid site, some general trends in the fractions of organic matter impacted were found. For example, the isotopic signatures of the POM fractions from woodland soils indicated significant inputs of woody plant C had occurred at all depths and sites. In general, across sites the proportion of woody plant C was highest in the larger sized POM fractions and at the 0-10 cm depth increment. However, the effect of woody plant encroachment on the C content of the different POM fractions varied by depth and across sites. Engeling, the site with the largest decrease in SOC with woody plant encroachment (-43%), was the most consistent with regard to changes in SOC pools. This site gained C in the largest POM fraction but lost C in the smallest, most recalcitrant SOC pool at all depths. Although the POM fractions and depth increments impacted by woody plant encroachment vary across sites, these results indicate that significant woody inputs and decomposition of grassland derived organic matter have occurred deep within the soil profile at all sites.

Key words: woody plant encroachment, land-use change, land-cover change, Great Plains