The potential bias of nitrogen deposition effects on primary productivity and biodiversity.

Atmospheric nitrogen (N) deposition is composed of both inorganic nitrogen (IN) and organic nitrogen (ON), and these sources of N may exhibit different impacts on ecosystems. However, our understanding of the impacts of N deposition is largely based on experimental gradients of INs or more rarely ONs. Thus, the effects of N deposition on ecosystem productivity and biodiversity may be biased. We explored the differential impacts of N addition with different IN:ON ratios (0:10, 3:7, 5:5, 7:3, and 10:0) on aboveground net primary productivity (ANPP) of plant community and plant diversity in a typical temperate grassland with a long-term N addition experiment. Soil pH, litter biomass, soil IN concentration, and light penetration were measured to examine the potential mechanisms underlying species loss with N addition. Our results showed that N addition significantly increased plant community ANPP by 68.33%-105.50% and reduced species richness by 16.20%-37.99%. The IN:ON ratios showed no significant effects on plant community ANPP. However, IN-induced species richness loss was about 2.34 times of ON-induced richness loss. Soil pH was positively related to species richness, and they exhibited very similar response patterns to IN:ON ratios. It implies that soil acidification accounts for the different magnitudes of species loss with IN and ON additions. Overall, our study suggests that it might be reasonable to evaluate the effects of N deposition on plant community ANPP with either IN or ON addition. However, the evaluation of N deposition on biodiversity might be overestimated if only IN is added or underestimated if only ON is added.

Effects of multi-resource addition on grassland plant productivity and biodiversity along a resource gradient.

The change of plant biodiversity caused by resource-enhancing global changes has greatly affected grassland productivity. However, it remains unclear how multi-resource enrichment induces the effects of multifaceted biodiversity on grassland productivity under different site resource constraints. We conducted a multiple resource addition (MRA) experiment of water and nutrients at three sites located along a resource gradient in northern China. This allowed us to assess the response of aboveground net primary productivity (ANPP), species (species richness and plant density), functional (functional richness and community-weighted mean of traits) and phylogenetic (phylogenetic richness) diversity to increasing number of MRA. We used structural equation model (SEM) to examine the direct and indirect effects of MRA and multifaceted biodiversity on ANPP. The combined addition of the four resources increased ANPP at all three sites. But with increasing number of MRA, biodiversity varied at the three sites. At the high resource constraint site, species richness, plant density and leaf nitrogen concentration (LNC) increased. At the medium resource constraint site, plant height and LNC increased, leaf dry matter content (LDMC) decreased. At the low resource constraint site, species, functional and phylogenetic richness decreased, and height increased. The SEM showed that MRA increased ANPP directly at all three sites, and indirectly by increasing plant density at the high constraint site and height at the medium constraint site. Independent of MRA, ANPP was affected by height at the high resource constraint site and LNC at the low resource constraint site. Our results illustrate that multi-resource addition positively affects productivity, while affects biodiversity depending on site resource constraint. The study highlights that site resource constraint conditions need to be taken into consideration to better predict grassland structure and function, particularly under the future multifaceted global change scenarios.

Dominant species control effects of nitrogen addition on ecosystem stability.

Increased nitrogen (N) deposition is known to reduce the ecosystem stability, while the underlying mechanisms are still controversial. We conducted an 8-year multi-level N addition experiment in a temperate semi-arid grassland to identify the mechanisms (biodiversity, species asynchrony, population stability and dominant species stability) driving the N-induced loss of temporal stability of aboveground net primary productivity (ANPP). We found that N addition decreased ecosystem, population, and dominant species stability; decreased species richness and phylogenetic diversity; increased species dominance; but had nonsignificant effects on community-wide species asynchrony. Structural equation model revealed that N-induced loss of ecosystem stability was mainly driven by the loss of dominant species stability and the reduction in population stability. Moreover, species relative instability was negatively related with species relative production and the slopes increase with N addition, indicating that N addition weakened the stabilizing effect of dominant species on ecosystem function. Overall, our results highlight that the dominant species control the temporal stability of ANPP in grassland ecosystem under N addition, and support 'dominance management' as an effective strategy for conserving ecosystem functioning in grassland under N deposition.