DOI: 10.5061/dryad.02v6wwq1t
title: Functional structure mediates the responses of productivity to addition of three nitrogen compounds in a meadow steppe

description: 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 species composition. SEM revealed that fertilization effects were more
 important than soil acidification for the responses of productivity to
 CO(NH2)2 addition, which enhanced productivity by decreasing functional
 trait dispersion. In contrast, the effects of (NH4)2SO4 and NH4NO3 were
 primarily due to soil acidification, influencing productivity via
 community-weighted means of functional traits. Our results suggest that N
 forms with different fertilizing and acidifying effects influence
 productivity via different functional traits pathways. Our study also
 emphasizes the need for in situ experiments with the relevant N compounds
 to accurately understand and predict the ecological effects of atmospheric
 N deposition on ecosystems.
description: 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 SevenCompact™, Mettler, Gießen, Germany).
description: 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.