Climate change will impact grassland ecosystem processes such as primary productivity, N retention and soil C sequestration. Tallgrass prairie ecosystems can be co-limited by water and N, so altered precipitation patterns can have complex effects on microbial activity, nutrient cycling, and plant productivity (ANPP). Soil moisture regulates microbial N mineralization and immobilization, which may mediate responses of ANPP and C storage to precipitation changes. Plant and microbial processes may also adjust to long-term changes in precipitation amount, but chronic precipitation changes may alter sensitivity to drought, projected to increase in frequency and severity in the Central Plains.
Using a multi-decadal climate manipulation experiment, we explore: How do short- and long-term changes in precipitation affect ecosystem functioning? Do legacy effects of past climate mediate responses to altered rainfall? In this grassland, irrigation treatments initiated in 1991 increase growing-season rainfall by ~33%, resulting in sustained increases in ANPP. Reversal of irrigated and ambient treatments in a subset of plots, coupled with new drought treatments, offers new insights into how past and present climate shape grassland productivity and biogeochemistry. We quantified ANPP, soil CO2 efflux, soil available N, and net N mineralization and nitrification rates of prairie under contrasting long-and short-term precipitation regimes.
Results/Conclusions
Long-term irrigation decreased net N mineralization and nitrification rates in upland prairie, suggesting tighter N cycling with release of water limitation. Nitrogen mineralization returned to ambient levels two years after cessation of irrigation, but nitrification rates showed a lagged response to climate alterations. This distinction may result from high mobility and uptake rate of nitrate. Microbial biomass increased with long-term irrigation in lowland prairie, but decreased sharply by 20% after irrigation cessation. Drought treatments reduced ANPP and increased availability of soil inorganic N regardless of irrigation history, but ANPP was less sensitive to drought in plots with a legacy of irrigation. Likewise, in situ soil respiration increased with current irrigation and decreased with drought, but historically irrigated plots exhibited higher soil CO2 flux across all current treatments.
Together, these results indicate differential sensitivity of C and N cycling processes and suggest that the long-term implications of climate change on C and N dynamics are difficult to predict from short-term manipulations. Microbial N transformations and soil CO2 efflux respond on different timescales and show distinct sensitivities to experimental climate alterations, so ecosystem-level N loss and C sequestration in grasslands may show lagged, asynchronous or nonlinear responses to precipitation changes.