Growing evidence suggests that past climate conditions shape ecosystem structure and function in ways that alter responses to current climate variability, and understanding climate legacies may inform projections of ecosystem function. Although primary production (ANPP) often depends on past as well as current precipitation, the effects on nitrogen (N) cycling are unclear. In mesic grasslands such as tallgrass prairie, where water and N can co-limit ecosystem function, climate history may indirectly affect ANPP through changes in microbial N transformation rates.
In this study, we ask (1) Does climate history (long-term release from water stress) alter net supply rates of available N in tallgrass prairie? (2) How does climate history interact with current climate and with drought to affect N cycling? (3) How do these responses to altered water availability vary across years and landscape position? We used a multi-decadal irrigation experiment spanning a topographic gradient to assess effects of long-term release from water stress on N cycling. In 2017, we reversed irrigation and control treatments in a subset of plots to assess effects of climate history and potential legacy effects. We measured growing-season net N mineralization rates, nitrification rates, and microbial biomass in the years following treatment reversal.
Results/Conclusions
Climate history interacted with current climate and topography to affect N cycling. A legacy of release from water stress decreased net N mineralization in lowland locations, but this result was not seen until 2019, two years post-reversal. Drought conditions in 2017 and 2018 likely masked legacy effects initially, while subsequent wetter conditions in 2019 allowed for rapid plant N uptake and microbial immobilization. In contrast, decades of past irrigation affected nitrification rates only in 2018, with effects that varied with landscape position. After long-term irrigation and regardless of current treatment, nitrification increased in the wetter lowlands and decreased in the drier uplands. Long-term release from water stress may support a legacy of high nitrifier populations in lowlands, while inorganic N uptake or immobilization rates may have increased in the uplands at the expense of net nitrification. Moreover, in both 2018 and 2019 a legacy of irrigation increased the sensitivity of microbial populations to water stress in lowland locations. These results show that legacies of climate history can affect N transformations and the microbial organisms that drive them for multiple years, and these effects on N uptake and transformations may drive patterns in C cycling in these grasslands.