Spatial patterns of soil C-13 and N-15 in a savanna parkland landscape. Bai, Edith^*,1, Boutton, Thomas², Liu, Feng¹, Wu, Ben¹, Archer, Steven², Jessup, Kirk¹, ¹ Department of Rangeland Ecology and Management, College Station, TX, USA² School of Renewable Natural Resources, Tucson, AZ, USA

ABSTRACT- d¹³C values of soil organic matter reflect relative productivity of C₃ vs. C₄ plants, while soil d¹⁵N values generally reflect the presence or absence of N₂-fixing plants, as well as rates of N-cycling processes. Therefore, spatial patterns of soil d¹⁵N and d¹³C should reflect plant community composition and dynamics, and attributes of the N-cycle. We investigated spatial variation of soil d¹³C and d¹⁵N and their relationships with vegetation in a savanna parkland landscape in southern Texas. A 100 m X 160 m plot was divided into 10 m X 10 m grid cells, and cell coordinates were measured using GPS. Soils (0-15 cm) were collected at two randomly selected points within each cell. A remote sensing based vegetation density index (VDI) was used to indicate woody plant abundance. Woody patches had lower d¹³C values than grasslands because they are comprised of C₃ plants. Woody patches also had lower d¹⁵N values than grasslands because they are dominated by the N₂-fixing plant Prosopis glandulosa. Plants capable of N₂-fixation generally have lower d¹⁵N values than non-N₂-fixing plants. Mantel tests and semivariograms revealed significant spatial structures in soil d¹³C, d¹⁵N, and VDI. Soil d¹³C and d¹⁵N had stronger pattern in the direction of southeast and weaker pattern in southwest, similar to that of VDI. The underlying cause of these spatial structures may be related to topographic variation across the landscape, which could influence soil texture, soil moisture, and other key soil properties that influence vegetation patterns.

KEY WORDS: geostatistics, spatial variation, carbon isotopes, nitrogen isotopes