COS 186-2 - Historic precipitation regimes alter current N cycling in tallgrass prairie

Background/Question/Methods

Climate change is expected to shift precipitation patterns in the North American Central Plains, which will likely alter functioning in these water-limited ecosystems. Tallgrass prairies at the eastern edge of this grassland biome are co-limited by both water and N, so changes in N cycling with altered precipitation regimes may be especially important. Rates of N mineralization and nitrification—two key N supply processes—are sensitive to precipitation patterns, but how these processes respond to short- and long-term changes in precipitation and the implications for N availability to plants and microbes remains unclear. We used a long-term precipitation experiment at Konza Prairie (Kansas, USA) to assess how past and current precipitation regimes influence N cycling in tallgrass prairie. Since 1991, supplemental irrigation was added to reduce water deficits and increase growing-season rainfall inputs by ~32%; in 2017, we reversed treatments in half the plots and added a reduced rainfall treatment across both historic rainfall regimes. This generated plots with identical current rainfall inputs but distinct previous precipitation regimes.

Results/Conclusions

In lowland prairie, previous irrigation doubled N mineralization and nitrification rates the year following cessation of irrigation (2018). Reduced microbial C/N ratio and lower relative investment in N-acquiring enzymes suggested that a previously wetter climate led to a legacy of increased N availability for microbes. Likewise, while current (since 2017) irrigation and drought decreased and increased plant N concentration, respectively, plant N recovered to ambient levels following cessation of long-term irrigation. Together, these results suggest that long-term elevated precipitation in tallgrass prairie resulted in a legacy of accelerated N cycling, which may relieve N limitation and support higher plant and microbial functioning.