Warming had greater impact than drought or soil nitrogen for reducing Lupine survival in a Mediterranean grassland terracosm experiment

Climate change is bringing extensive changes to plant community composition throughout the globe with variable impacts on N-fixing species. Higher temperatures and drought can reduce nodulation and nitrogenase activity leading to decline in legume species. Soil nitrogen availability could be either increased or decreased with warming thereby impacting the competitive advantage of N-fixing species. Yet all these factors often change together so it can be difficult to assess their relative impacts.

Here, we performed a mesocosm warming experiment on constructed Mediterranean grassland ecosystems where treatments provided +3.5°C warming via both symmetric (SYM) and asymmetric daily temperature profiles (ASYM, +5°C at dawn changing minute by minute to +2°C by midday) and hence differential soil moisture. The differential warming and soil moisture in turn provided conditions that could lead to differential soil N availability. As such, this experiment allowed us to assess the relative impacts of warming, drought, and nitrogen availability on Lupine albicaulis, the primary N-fixer in this system.

Our results showed reduced lupine survivorship for both warming treatments despite the maintenance of soil moisture for ASYM at the same level as the ambient treatment (AMB). Reduced survival was persistent despite the low soil NO3-N (<2.9ppm) and NH4-N (<1.4ppm) below the low threshold for agricultural crops (10 and 2 ppm, respectively) with no differences between SYM, ASYM and AMB treatments for bulk soil or plant root simulator probes. Increased N-availability in the warmer chambers could therefore not have prevented lupine establishment.

Overall, our results point to the warmer temperatures as the primary factor accounting for reduced lupine establishment in the warmer chambers. Results are discussed considering the potential impacts of reduced N-fixation in a warmer world and the factors needed to help maintain and preserve N-fixing species in grassland ecosystems.