Response of the shortgrass steppe to changes in rainfall seasonality

Studies in temperate grassland ecosystems have shown that differences in co mposition of C-3 and C-4 plant functional types can have important influenc es on ecosystem pools and processes. We used a plant community dynamics mod el (STEPPE) linked to a biogeochemical cycling model (CENTURY) to determine how ecosystem properties in shortgrass steppe are influenced by plant func tional type composition. Because of phenological differences between C-3 an d C-4 plants, we additionally simulated the effects of precipitation season ality on plant communities and examined how C-3 and C-4 composition interac ts with precipitation to affect ecosystems. The model output suggests that differences in C-3 and C-4 composition can lead to differences in soil orga nic carbon (C) and nitrogen (N) within 1000 simulation years. Soil organic C and N (g C and N m(-2) to 0.2-m depth) were least in a 100% C-3 community compared with a 100% C-3 community and a mixed C-3-C-4 community. A change in the time of maxi-mum precipitation from summer to spring in a simulated shortgrass steppe slightly favored C-3 plants over C-4 plants. The proport ion of total net primary production accounted for by C-3 plants increased f rom 21% to 25% after 200 years, when 90 mm of precipitation was switched fr om summer to spring. Soil organic matter (SOM) was relatively stable in the C-4-dominated communities with respect to changes in precipitation seasona lity, whereas SOM in the C-3 community was sensitive to precipitation seaso nality changes. These results suggest an important interaction between plan t community composition and precipitation seasonality on SOM, with phenolog y playing a key role.