Biogeochemistry and elemental cycling in a vernal pool and grassland reserve

Background/Question/Methods

Most California vernal pools have been destroyed by agricultural and urban development. Remaining pools are threatened by aggressive non-native grasses and excessive N loading from atmospheric deposition. Cattle grazing is recognized as an important management tool for maintaining the pool hydroperiod by controlling the surrounding grasses; this also reduces competition with native, rare vernal pool plants.  Cattle graze the entire 2590 ha Reserve. However, grazing, combined with over fertilization of pools from nearby agricultural fields, could cause changes to this ecosystem in ways not yet understood. Soil types and their elemental compositions are other factors that may influence plant growth in these ecosystems. Therefore, we investigated ecosystem biogeochemistry by measuring residual dry matter (RDM); major, minor and trace elements; and stable isotope composition in plants and 12 soil types in the Merced Vernal Pool and Grassland Reserve. RDM, measured after the growing season, provides a measure of grazing intensity and possibly nutrient availability.

Results/Conclusions

Nitrogen concentrations in surface soils were high in every soil type throughout the reserve (mean 74.2 ppm; n=50). Heavy metals were in low concentrations. Some were in higher concentrations in pool soils than in grasslands within RbA and CkB soils, whereas the opposite was found In KaB and ReB soils. Metal concentrations in cattle stock ponds were the lowest of all soils. RDM of ungrazed areas outside the Reserve was 4050 kg/ha, 4x greater than within the Reserve. Lightly grazed areas, the majority of the Reserve, had a mean RDM of 1012 kg/ha. In vernal pools, mean RDM was 560 kg/ha, because grass density was low there. The RDM of over-utilized areas was 553, nearly half the RDM measured on most of the Reserve. RDM grasses were all C₃, annual grasses (d¹³C=-27.2±0.86‰; d¹⁵N=0.8±1.3‰), whereas living plants collected in summer included several C₄ species, including Swamp Timothy along with native C₃ plants (d¹³C=-25.9‰; d¹⁵N=5.3±3.7‰). Isotopic compositions of winter-growing grasses were different from those of summer plants indicating that plants used different nitrogen sources seasonally. Soils were uniform in d¹³C=-28.1±0.4‰, but variable in d¹⁵N=3.5±2.5‰. Soil organic matter is derived principally from C₃ annual grasses, rather than native species dominating in summer.