Structures of intransitive competition network affect functional attributes of plant community under nitrogen enrichment

Atmospheric nitrogen (N) deposition is a potential danger factor for grassland ecology, and will cause unpredictable consequences to plant communities. However, how plant species interactions response to N enrichment and then affect ecological functions are not fully known. We investigated how intransitive competition network was related to the functional attributes of plant community under a 13-years N-deposition experiment. Results showed that intransitive competition network was not a single structure, but a complexly interwoven structure of various simple structures. Nested work was more common, accounting for 76.96%, and gained new species at a higher colonization rate than short network did. The network had a long-term mechanism to maintain the small-scale Alpha diversity, and a significant lag effect on the large-scale Gamma diversity. Under the conditions of N ≥ 2 g N·m-2·year-1, without mowing and under high fertilization frequency, the increase of network complexity significantly decreased plot biomass gradually. The relationship between biomass and network complexity is quadratic curves, also between abundancy and the complexity, but with the opposite bending directions, which indicated that biomass and abundance were complementary to each other, which may be a mechanism of maintaining the relative balance of species competition. In addition, the decrease of species asynchronism changing with the increase of N-enrichment gradually destroyed ecosystem stability. However, at medium N enrichment, intransitive network counteracted the negative effects of N enrichment and maintained or even improved the biomass ecosystem stability. Our results suggested that intransitive competition network is an internal mechanism of self-restoration of a grassland ecosystem. Under nitrogen enrichment conditions, competitive networks complexity is reduced, leading to a reduction in species diversity. These analyses emphasize the important role of intransitive network structure to stabilize grassland ecosystem. In order to achieve sustainable development of grassland, it is indispensable to control nitrogen addition rate.

Competition and Plant Trait Plasticity of Invasive (Wedelia trilobata) and Native Species (Wedelia chinensis, WC) under Nitrogen Enrichment and Flooding Condition

Nitrogen (N) is the important nutrition that regulatory plant functioning and environmental stability of invasive plant species under flooding (F) conditions. Little information clarifies the role of nitrogen enrichment and flooding on the invasive plant functional traits and competition with native competitors. Plant functional traits play an essential role in the successful growth of plants under different environmental conditions. Therefore, greenhouse pots experiment was conducted with invasive plant species (Wedelia trilobata, WT), and its native competitor (Wedelia chinensis, WC) in monoculture and cocultivation culture, along with flooding and nitrogen enrichment conditions. Considering the impact of flooding (F) and nitrogen (N) on an individual basis, the plant physiological traits of WC were nonsignificant compared to that of WT. However, in the combination of flooding × additional nitrogen (F.N, F.2N), plant physiological traits of WT were comparatively higher than those of WC, especially in cocultivation. In flooding × additional nitrogen (F.N and F.2N), better phenotypic plasticity at different plant traits makes WT more dominant in resource competition over WC. In conclusion, improved functional traits of WT under nitrogen enrichment and flooding conditions enhanced its competitiveness over native competitors.

Past climate conditions predict the influence of nitrogen enrichment on the temperature sensitivity of soil respiration

The response of soil carbon release to global warming is largely determined by the temperature sensitivity of soil respiration, yet how this relationship will be affected by increasing atmospheric nitrogen deposition is unclear. Here, we present a global synthesis of 686 observations from 168 field studies to investigate the relationship between nitrogen enrichment and the temperature sensitivity of soil respiration. We find that the temperature sensitivity of total and heterotrophic soil respiration increased with latitude. In addition, for total and autotrophic respiration, the temperature sensitivity responded more strongly to nitrogen enrichment with increasing latitude. Temperature and precipitation during the Last Glacial Maximum were better predictors of how the temperature sensitivity of soil respiration responds to nitrogen enrichment than contemporary climate variables. The tentative legacy effects of paleoclimate variables regulate the response through shaping soil organic carbon and nitrogen content. We suggest that careful consideration of past climate conditions is necessary when projecting soil carbon dynamics under future global change.