DOI: 10.5061/dryad.f7m0cfxsb
title: Herbivore assemblage as an important factor modulating grazing effects on ecosystem carbon fluxes in a meadow steppe in northeast China

description: A better understanding of how grazing by large herbivores, as the major
 land use worldwide, affects grassland carbon fluxes is critical for
 predicting future uptake of CO2 in terrestrial ecosystem.
 Previous studies have focused on individual herbivore species; it remains
 poorly understood as to if and how herbivore assemblage (single-
 vs. mixed-species) would alter the impact of grazers on grassland
 carbon fluxes. Here we examined the effects of moderate grazing by
 different herbivore assemblages on net ecosystem CO2 exchange
 (NEE), gross ecosystem productivity (GEP) and ecosystem respiration (ER)
 over the growing season in two types of grassland communities in a meadow
 steppe. We found that herbivore assemblages significantly affected NEE,
 and the effects varied in the two types of grassland communities. Compared
 to un-grazed grassland, mixed grazing of sheep and cattle increased
 NEE (i.e., increased CO2 uptake) over the
 growing season regardless of community type, while sheep- or cattle-only
 grazing increased NEE in the low diversity community and decreased it
 in the high diversity community. Herbivore assemblages
 altered the NEE primarily via changing GEP. The different effects
 of herbivore assemblages on GEP may be attributed mainly to
 grazing-induced changes in soil water availability and canopy light
 availability. Our study indicates that mixed grazing of sheep and cattle
 might be an important grazing management practice to
 improve plant aboveground productivity and help mitigate
 CO2 emissions during the growing season. It should be
 particularly used in diverse plant communities, where it
 might increase grassland carbon sequestration.
description: 1. Sampling design and measurements In 2014, after five years of grazing, ecosystem carbon fluxes over the growing season were measured. All the grazing herbivores were excluded from the experimental plots in order to accurately estimate aboveground biomass as it can be reduced by herbivore consumption. Four stationary quadrats (0.5 × 0.5 m) were randomly chosen for the measurement of ecosystem carbon fluxes in each plot per month. Each month over the growing season from May to September, a new quadrat near each quadrat where fluxes were measured was chosen for measurement of plant aboveground biomass and community leaf N assessment. Ecosystem carbon fluxes, plant and soil parameters were measured once a month. Thus, a total of twenty-four quadrats (0.5 × 0.5 m) were selected for measurement in each plot. 2. Net ecosystem carbon exchange The net ecosystem CO<sub>2</sub> exchange (NEE), gross ecosystem productivity (GEP), and ecosystem respiration (ER) were measured using a static-chamber method. Measurements of ecosystem carbon fluxes were conducted monthly around between 8:00 and 12:00 (local time) during cloud-free days, using a LI-COR 6400 portable photosynthesis system (LI-Cor, Lincoln, NE, USA). CO<sub>2</sub> concentrations were recorded at 10-s intervals for 2 minutes. NEE was calculated based on the change in CO<sub>2</sub> concentrations. Following each NEE measurement, the chamber was vented, then replaced on the surface of frame, and covered with an opaque cloth. The second set of measurements, with light eliminated by the opaque cloth, represented ER. GEP was then calculated as the difference between NEE and ER. Positive NEE values represent net CO<sub>2</sub> release from ecosystem and negative values represent net CO<sub>2</sub> uptake by it. 3. Plant and soil variables Every month from May to September, at the same time carbon fluxes were measured, all vegetation in each biomass survey quadrat was clipped to ground level to measure aboveground biomass. The live plant samples were separated into leaf and shoot according to the species, and then were dried at 65℃ for 48 hours to determine leaf biomass and aboveground biomass of each species, the total aboveground biomass of each quadrat, and community leaf N content. In addition, the abundance of each plant species was investigated in four quadrates for measuring carbon fluxes. Leaf N content for each species was analyzed using the Kjeldahl method. Leaf area index of the quadrats for NEE measurements was determined using a plant canopy analyzer. Three replicate 0-10 cm soil samples were randomly collected once a month near each frame using a 3.5-cm-diameter soil auger and then were pooled to homogenize the samples. Soil samples were passed through a 2-mm mesh sieve to remove roots and other visible debris. Soil water content was determined gravimetrically using 10 g of fresh soil samples dried at 105 ℃ for 24 hours to a constant weight.  
description: Chang et al. 2020 This data used to estimate
 the effects of herbivore assemblage (single- vs. mixed-species) on
 ecosystem carbon fluxes, and  related plant and soil variables in a meadow
 steppe in northeast China. All variables measured in the experiment are
 included in the file. data-fluxes.xlsx