DOI: 10.5061/dryad.wwpzgmsjb
title: Nitrogen addition amplifies the nonlinear drought response of grassland productivity to extended growing-season droughts

description: Understanding the response of grassland production and carbon exchange to
 intra-annual variation in precipitation and nitrogen addition is critical
 for sustainable grassland management and ecosystem restoration. We
 introduced growing-season drought treatments of different lengths (15, 30,
 45 and 60 day drought) by delaying growing-season precipitation in a
 long-term nitrogen addition experiment in a low diversity meadow steppe in
 northeast China. Response variables included aboveground biomass (AGB),
 ecosystem net carbon exchange (NEE), and leaf net carbon assimilation rate
 (A). In unfertilized plots drought decreased AGB by 13.7% after a 45 day
 drought and 31.7% after a 60 day drought (47.6% in fertilized plots).
 Progressive increases in the drought response of NEE were also observed.
 The effects of N addition on the drought response of productivity
 increased as drought duration increased, and these responses were a
 function of changes in AGB and biomass allocation, particularly root to
 shoot ratio. However, no significant effects of drought occurred in
 fertilized or unfertilized plots in the growing season a year after the
 experiment, N addition did limit the recovery of AGB from severe drought
 during the remainder of the current growing season. Our results imply that
 chronic N enrichment could exacerbate the effects of growing-season
 drought on grassland productivity caused by altered precipitation
 seasonality under climate change, but that these effects do not carry over
 to the next growing season.
description: <b>Vegetation sampling </b>
 Vegetation surveys were conducted four times: (1) at the
 beginning of the drought treatments, on or around May 22, 2017, (2)
 immediately after each 15, 30, 45, or 60 day drought treatment ended, (3)
 at the end of the 2017 growing season (Sep. 4), and (4) approximately one
 year later in mid-August of 2018. In each plot, we surveyed the number of
 plant species, and counted the number of individuals for each species in 3
 randomly placed 0.5 m × 0.5 m quadrats. For each survey, twenty live
 shoots of <i>L. chinensis </i>were<i> </i>randomly
 collected from each plot to calculate the proportion of dead
 leaves. After the vegetation survey, one of the
 quadrats was randomly selected for the measurement of aboveground biomass
 (AGB) by harvesting all aboveground live plant shoots. Harvest quadrats
 were not allowed to overlap. For belowground biomass (BGB), we washed the
 roots out of three soil cores with a diameter of 10 cm and a depth of 30
 cm. The soil cores were randomly collected in the quadrat used for the
 harvesting of AGB. The harvested plant and root materials were oven-dried
 at 70°C to a constant weight (48 hours). Leaf C/N ratio was determined by
 an elemental analyzer (vario EL cube, Ele mentar, Langenselbold, Germany).
 The root:shoot (R/S) ratio was calculated as BGB/AGB.
 The densities of buds and tillering nodes were measured at the
 end of the 2017 growing season (Sep. 14). A soil sample with an area of
 0.25 m × 0.25 m and a depth of 0.2 m was collected from each plot. The
 collected soil was gently crushed by hand, and the numbers of buds and
 tillering nodes of <i>L. chinensis</i> were counted directly
 in the field to avoid damage of the buds by dehydration.
 <b>Leaf carbon exchange measurements</b>
 The measurements of leaf net carbon assimilation rate
 (<i>A</i>) and transpiration rate (<i>E</i>) were
 conducted from 08:00 to 10:00 am on a sunny day at the end of each drought
 treatment using a portable infrared gas analyzer (LI-6400, LiCor Inc.,
 Lincoln, NE, USA). Five representative intact <i>L.
 chinensis</i> leaves (uppermost, fully expanded) were randomly
 selected in each plot for leaf gas exchange measurements. Photon flux
 density and CO<sub>2</sub> concentration in the leaf chamber
 were set at 1500 μmol m<sup>-2</sup>
 s<sup>-1</sup> and 400 μmol mol<sup>-1</sup>,
 respectively. Leaf chamber temperature was set to match the ambient
 value. <b>Ecosystem carbon exchange
 measurements</b> Concurrently with leaf-level
 measurements, we measured short-term net ecosystem
 CO<sub>2</sub> exchange (NEE) and evapotranspiration (ET)
 using an infrared gas analyzer (LI-6400, LiCor Inc., Lincoln, NE, USA)
 attached to a transparent chamber (0.5 m × 0.5 m × 1 m polymethyl
 methacrylate). To stabilize the transparent chamber, an iron frame (0.5 m
 × 0.5 m) was fixed in the center of each plot in early 2017. For each
 measurement, 11 consecutive recordings of CO<sub>2</sub> and
 H<sub>2</sub>O concentration were logged at 10 s intervals
 during a 2 min period after the chamber was mounted on top of the iron
 frame, then the slope of the linear regression of
 CO<sub>2</sub> and H<sub>2</sub>O concentration
 was used to determine NEE<sub> </sub>and ET . Within the
 chamber, four small electric fans ran continuously to mix air during the
 measurement.
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