Soil carbon and nitrogen storage in juniper-oak savanna: role of vegetation and geology

Abstract:

Grasslands and savannas cover 40% of the terrestrial surface, and store 32% of global soil organic carbon and 40% of global soil nitrogen. Woody plant encroachment into these ecosystems has been globally widespread during the past 150 yrs, likely driven by complex interactions between livestock grazing, fire suppression, rising atmospheric CO₂, and climate change. Throughout the US southern Great Plains, Juniperus (juniper) and Quercus (oak) species have increased in abundance. To evaluate the biogeochemical consequences of this vegetation change, we quantified soil organic C (SOC), total N (TN), and their isotopic composition (δ¹³C and δ¹⁵N) in samples taken from 6 soil trenches passing through grassland, juniper, and oak patches on soils developed on either Edwards or Buda limestone on the Edwards Plateau of central Texas. Trench depths varied from 0.3-2.0 m, depending on depth to bedrock. The δ¹³C values of SOC under grasslands were -19‰ while those under woody patches were -24 to -21‰, indicating woody areas were relatively recent components of the landscape. Soil δ¹⁵N values ranged from 3-7‰, and were affected by a complex depth x vegetation x geology interaction. Both SOC and TN were significantly higher under oak and juniper (SOC = 45-105 g kg^-1; TN = 3-8 g kg^-1) than under grasslands (SOC = 40-45 g kg^-1; TN = 3-4 g kg^-1) on both Buda and Edwards parent material. In addition, SOC and TN were significantly higher on Edwards than on Buda soils. LOWESS smoothing plots showed soils beneath oak and juniper had higher SOC and TN than grasslands throughout the profile on soils derived from both Edwards and Buda limestone; concentrations were highest in the interiors of woody patches, decreased towards the edges of those patches, and reached lowest values in the surrounding grasslands. The spread of juniper and oak into areas that were once grassland has altered spatial variation in soil C and N across this landscape and to considerable depth, suggesting significant changes in the relative rates of nutrient inputs vs. losses in this system following vegetation change. Woody encroachment has been geographically widespread throughout the world’s drylands during the past century, suggesting that changes in SOC and TN storage documented here may have implications for regional and global C and N cycles and potentially the climate system.