Nitrogen Loading Enhances Stress Impact of Drought on a Semi-natural Temperate Grassland

Abstract

Two important threats to the sustainable functioning of seminatural grasslands in temperate zones are (1) nutrient loading due to agricultural fertilization and pollution, and (2) the increase of extreme drought events due to climate change. These threats may cause substantial shifts in species diversity and abundance and considerably affect the carbon and water balance of ecosystems. The synergistic effects between those two threats, however, can be complex and are poorly understood. Here, we experimentally investigated the effects of nitrogen addition and extreme drought (separately and in combination) on a seminatural temperate grassland, located in Freiburg (South Germany). To study the grassland response, we combined eddy-covariance techniques with open gas exchange systems. Open gas exchange chambers were connected to an infrared gas analyzer and water isotope spectrometer, which allowed the partitioning of net ecosystem exchange and evapotranspiration. Vegetation parameters were described by species richness, species abundance, and leaf area index. Our results suggest that grassland communities, strongly weakened in their stress response by nitrogen loading, can substantially lose their carbon sink function during drought. While nitrogen addition caused a significant loss in forb species (−25%), precipitation reduction promoted a strong dominance of grass species at season start. Consequently, the grass-dominated and species-poor community suffered from a strong above-ground dieback during the dry summer months, likely caused by lower water use efficiency and weaker drought adaptations of the species community. Over the growing season (April-September), the carbon sequestration of the studied grassland was reduced by more than 60% as a consequence of nitrogen addition. Nitrogen addition in combination with precipitation reduction decreased carbon sequestration by 73%. Eutrophication can severely threaten the resilient functioning of grasslands, in particular when drought periods will increase as predicted by future climate scenarios. Our findings emphasize the importance of preserving high diversity of grasslands to strengthen their resistance against extreme events such as droughts.

Efficient Physiological and Nutrient Use Efficiency Responses of Maize Leaves to Drought Stress under Different Field Nitrogen Conditions

“…Drought and soil water deficits are increasingly limiting factors for plant growth in many regions of the world [22]. In our study, the total precipitation of the maize-growing season in 2017 was only 39.5% of that in 2018 ( Figure 1); thus, 2017 was considered the dry year, and 2018 was considered the wet year.…”

Combined experimental drought and nitrogen loading: the role of species‐dependent leaf level control of carbon and water exchange in a temperate grassland

“…Therefore, we investigated the plant physiological response of the abundant forb and grass species ( Centaurea jacea L., Arrhenatherum elatius (L.) P.Beauv. ex J.Presl & C.Presl, Dactylis glomerata L.) in the same grassland studied by Kübert et al (2019). We studied their response under natural, drought, N addition and combined drought and N addition conditions over the entire growing season.…”

“…Hofer et al (2017), on the other hand, discovered that N fertilization improved drought resistance of non‐legume species. In contrast, Kübert et al (2019) observed, at the community level, that N loading strongly enhanced the effect of drought stress on a grassland community. Drought‐related reduction of the grassland’s gross primary production and net ecosystem exchange was much stronger on plots with N loading, resulting in the ecosystem becoming a net carbon source.…”

“…With ongoing global change, N loading and extended drought will likely occur together more often and pose a multifactorial stress on plant communities. Nevertheless, comparatively few studies have examined the potential interacting effects of N loading and drought on grassland communities, either on the community (Hartmann & Niklaus 2012; Grant et al 2014; Carlsson et al 2017; Kübert et al 2019) or species (Gonzalez‐Dugo et al 2005; Bloor & Bardgett 2012; Hofer et al 2017) level. The focus of previous studies was predominantly on the impact on productivity, i.e .…”

“…Kübert et al (2019) observed a significant reduction of forb species richness and abundance under combined drought and N loading accompanied by a huge drop in ecosystem gross primary production during a naturally occurring summer drought on the grass‐dominated plots. Based on their findings, we expect that a more efficient N use efficiency of A. elatius and D. glomerata compared to C. jacea can somewhat ameliorate their drought sensitivity, but that this is strongly dependent on drought intensity, timing and duration.…”