Functional structure mediates the responses of productivity to addition of three nitrogen compounds in a meadow steppe

Atmospheric nitrogen (N) deposition is altering grassland productivity and community structure worldwide. Deposited N comes in different forms, which can have different consequences for productivity due to differences in their fertilization and acidification effects. We hypothesize that these effects may be mediated by changes in plant functional traits. We investigated the responses of aboveground primary productivity and community functional composition to addition of three nitrogen compounds (NH4NO3, [NH4]2SO4, and CO[NH2]2) at the rates of 0, 5, 10, 20 g N m-2 yr-1. We used structural equation modeling (SEM) to evaluate how functional structure influences the responses of productivity to the three N compounds. Nitrogen addition increased community-level leaf chlorophyll content but decreased leaf dry matter content and phosphorus concentration. These changes were mainly due to intra-specific variation. Functional dispersion of traits was reduced by N addition through changes in spec..., We assessed the total aboveground biomass (g/m2) and the relative aboveground biomass of each species in a 1 m × 1 m randomly established quadrat in each plot in August 2017. All aboveground parts of vascular plants were harvested and sorted into species and then dried at 65℃ for 48h before weighing for dry biomass. After recording all species occurred in the studied quadrat of each plot, we collected fresh leaves for all those species within each plot, to measure their specific leaf area (SLA, the ratio of leaf area to leaf dry mass, cm2/g), leaf dry matter content (LDMC, the ratio of leaf dry mass to water saturated fresh mass, mg/g), leaf chlorophyll content (Chls, mg/g), and leaf phosphorus concentration based on mass (LPC, mg/kg) of all the emerging species.
In each plot, topsoil samples (0–10 cm) were taken from five random soil cores after removing litter and mixed to form one composite sample. Soil pH was determined in a 1:2.5 soil-to-water suspension with a pH electrode (S210 S..., Community weighted mean (CWM) for each functional trait was weighted by the relative biomass of each species (Garnier et al., 2004). Community functional dispersion (FDis) was calculated based on the four functional traits and every single trait. FDis was calculated as the average distance of each individual species to the centroid of all species in the community trait space, and FDis of multi-trait is multi-dimensional index based on multi-trait space.