SEV Core Site Biogeochemical and Biophysical Monitoring

Novelty:

Assessments of terrestrial biogeochemistry are critical components of LTER monitoring across the network. Drylands contain a third of the organic carbon stored in global soils; however, the long-term dynamics of soil organic carbon and soil organic matter (SOM) in drylands remain poorly understood relative to dynamics of the vegetation carbon pool. Our data have uncovered novel patterns. Our soil N data revealed the dependence of nutrient limitation on climate. Soil N levels were greatest following a drought period in 1989, declined during wetter periods that followed, and remained relatively stable until another extended drought period. After a drought in 1995-6, both forms of soil N increased, indicating the potential for a pulse of increased plant productivity following drought that subsides during periods of normal precipitation. Our SOM data linked soil C accumulation with periods of severe drought, and revealed that soil C declined nonlinearly with precipitation. Declines in SOM during wet periods are likely caused by increased soil respiration, runoff, leaching, and soil erosion. In addition, SOM concentration decreased 14% following prescribed fire, a response that magnified over time and persisted for nearly a decade due to the slow recovery of vegetation. Short-term spatial sampling campaigns revealed key differences among ecosystem types: in grasslands, all elements were uniformly distributed, but in shrublands, plant-essential elements, nitrogen, phosphorus, and potassium, were concentrated under shrub canopies, whereas non-limiting elements were either concentrated in the intershrub spaces or were equally across the surface. These results revealed how state transitions from grassland to shrubland contribute to long-term, widespread changes in the structure and function of dryland ecosystems.