Spatial-temporal changes in soil properties following woody plant encroachment. Archer, Steve^*,1, Boutton, Thomas², Wu, X. Ben², Liu, Feng², Bai, Edith², ¹ School of Natural Resources, Tucson, AZ, USA² Rangeland Ecology & Management, College Station, TX

ABSTRACT- Encroachment of woody plants into drylands during the past century may have significantly influenced the terrestrial carbon cycle. However, the magnitude and sign of change in soil organic carbon (SOC) pools accompanying this vegetation change is highly uncertain, ranging from positive to neutral to negative. Some of the controversy over woody plant impacts on SOC pools may be an artifact how soil properties determined from point samples are area-weighted and extrapolated. If there is substantial spatial structure in properties of soils associated with woody and herbaceous communities, extrapolations from limited point samples that do not account for this may over- or underestimate actual SOC pools. To test this possibility, we quantified near surface (0-15 cm) soil properties (bulk density, SOC, total N [TN], root biomass) at seven locations along transects extending from the tree bole to canopy edge and into adjoining herbaceous zones in replicated woody communities known to have developed on grasslands over the past 100 y. A strong gradient was found to occur: root biomass, SOC, and TN decreased exponentially with distance from tree boles, while bulk density increased. These spatial changes are consistent with temporal changes expected to occur as shrub establish and their canopies grow through time. Given the strong spatial structure of the data, it appears that area-weighted extrapolations of SOC based on near-bole samples would overestimate woody plant influences, whereas sampling soils away from boles would tend to underestimate impacts. Implications for sampling strategies to efficiently and effectively represent this non-linear spatial variation will be discussed.

KEY WORDS: soil nitrogen, soil organic carbon, delta 13C, root biomass