DOI: 10.5061/dryad.f996b
title: Data from: Chronic nitrogen addition induces a cascade of plant community responses with both seasonal and progressive dynamics

description: Short-lived herbaceous plants provide a useful model to rapidly reveal how
 multiple generations of plants in natural plant communities of sensitive
 desert ecosystems will be affected by N deposition. We monitored dynamic
 responses of community structure, richness, evenness, density and biomass
 of herbaceous plants to experimental N addition (2:1 NH4+:NO3− added at 0,
 0.5, 1, 3, 6 and 24 g N m− 2 a− 1) in three seasons in each of three years
 in the Gurbantunggut desert, a typical temperate desert of central Asia.
 We found clear rate-dependent and season-dependent effects of N deposition
 on each of these variables, in most cases becoming more obvious through
 time. N addition reduced plant richness, leading to a loss of about half
 of the species after three generations in the highest N application level.
 Evenness and density were relatively insensitive to all but the greatest
 levels of N addition for two generations, but negative effects emerged in
 the third generation. Biomass, both above and below ground, was
 non-linearly affected by N deposition. Low and intermediate levels of N
 deposition often increased biomass, whereas the highest level suppressed
 biomass. Stimulatory effects of intermediate N addition disappeared in the
 third generation. All of these responses are strongly interrelated in a
 cascade of changes. Notably, changes in biomass due to N deposition were
 mediated by declines in richness and evenness, and other changes in
 community structure, rather than solely being the direct outcome of
 release from limitation. The interrelationships between N deposition and
 the different plant community attributes change not only seasonally, but
 also progressively change through time. These temporal changes appear to
 be largely independent of interannual or seasonal climatic conditions.
description: Properties of annual
 plant communities subjected to N additionIn October 2008, sixty
 8×8m plots were randomly placed across an experimental area of 5,400 m2 in
 the center of the Gurbantunggut desert (44.876 N, 87.823 E), with an
 average separation distance of about 10 m. The plots had similar plant
 community composition and structure before the N treatments. From year
 2008 to 2011, five N concentrations plus one control (without N) were
 randomly applied on the plots, totaling 10 replicates of each of the six
 concentrations. The rates of six N treatments were 0, 0.5, 1, 3, 6 and 24
 g N m-2 a-1 (hereafter denoted as N0, N0.5, N1, N3, N6 and N24,
 respectively). The N treatments were applied in two equal pulses per year
 in March after snow thaw and October before snowfall every year,
 coinciding reasonably with a fall-spring pulses associated with rainy
 seasons, and with fertilization (and thus deposition) pulses in
 agricultural areas of the region. Each applied N treatment consisted of
 2:1 molar ratio of NH4+: NO3- (as NH4NO3 and NH4Cl), in 3 L of water per
 plot (about 0.037 mm of rainfall equivalent) applied using a spray.
 Controls received an equivalent amount of water only. The first treatment
 began in October 2008 and were repeated every year after that to simulate
 long-term effects of N deposition. In each plot, we established one 1× 1 m
 permanent quadrat for the investigation of community composition and
 structure. The richness (number of species per plot) and density (number
 of individuals per plot) were measured in mid-spring (April), late spring
 (May – June), and summer (July - August) in each year after N treatments.
 These seasons were chosen because various life forms of plants reach their
 peak biomass during these periods. We calculated the evenness index J’
 based on the number of individuals per species (Tuomisto, 2012). Peak
 aboveground biomass provides a good estimate of annual aboveground
 production in communities dominated by annual plants (Sala et al., 1988).
 We measured production three times each year, concurrent with community
 measures. We selected one 0.5×0.5 m quadrat (far from the permanent
 quadrat for community investigation) in each 8×8 m plot for the
 measurement of aboveground and belowground biomass. All plants from the
 quadrat were collected using spades. We separated shoot and root portions
 for each species in the lab (after washing adherent sand from roots using
 tap water), and measured the biomass after drying at 70 oC for 24 hours in
 the oven to obtain consistent weight. The aboveground biomass was the
 total shoot biomass of all annual plants (including ephemeroid plants).
 The belowground biomass was the total root biomass of all annual plants
 excluding ephemeroid plants. The ephemeroid plants were excluded because
 their roots were cloned together (two or more ramets fused together) and
 exist for several years. The root biomass of ephemeroids was significantly
 higher than other annual plants and significantly differed among
 quadrants, even before the start of the experiment. In order to use
 community structure as a variable in some analyses, data reduction was
 necessary. As a data reduction tool, we used nonmetric multidimensional
 scaling of total above and belowground biomass by species, based on the
 Bray-Curtis distance measure (McCune and Grace, 2002). Prior to
 ordination, we omitted any species that were present in fewer than 3
 samples to reduce noise and omitted any samples that lacked any plant
 biomass because empty samples are incompatible with this distance measure.
 After these modifications, we performed a general relativization,
 rescaling the abundance of all species within a sample such that they
 summed to 1. We obtained a two-axes ordination and rotated it so that Axis
 1 correlated with season, the apparent strongest driver of composition. We
 saved axis scores for each sample for use in our structural equation
 model.Zhang_data_dryad_version.xls