DOI: 10.5061/dryad.t76hdr81t
title: Responses of soil temperature, moisture, and respiration to five-year warming and nitrogen addition in a semi-arid grassland

description: How climate warming interacts with atmospheric nitrogen (N) deposition to
 affect carbon (C) release from soils remains largely elusive, posing a
 major challenge in projecting climate change‒terrestrial C feedback. As
 part of a five-year (2006–2010) field manipulative experiment, this study
 was designed to examine the effects of 24-hour continuous warming and N
 addition on soil respiration and explore the underlying mechanisms in a
 semi-arid grassland on the Mongolian Plateau, China. Across the
 five years and all plots, soil respiration was not changed under the
 continuous warming, but was decreased by 3.7% under the N addition. The
 suppression of soil respiration by N addition in the third year and later
 could be mainly due to the reductions in the forb-to-grass biomass ratios.
 Moreover, there were interactive effects between continuous warming and N
 addition on soil respiration. Continuous warming increased soil
 respiration by 5.8% in the ambient N plots, but reduced it by 6.3% in the
 enriched N plots. Soil respiration was unaffected by N addition in the
 ambient temperature plots yet decreased by 9.4% in the elevated
 temperature plots. Changes of soil moisture and the proportion of legume
 biomass in the community might be primarily responsible for the
 non-additive effects of continuous warming and N addition on soil
 respiration. This study provides empirical evidence for the positive
 climate warming‒soil C feedback in the ambient N condition. However, N
 deposition reverses the positive warming‒soil C feedback into a negative
 feedback, leading to decreased C loss from soils under a warming climate.
 Incorporating our findings into C-cycling models could reduce the
 uncertainties of model projections for land C sink and global C cycling
 under multifactorial global change scenarios.
description: <b>2.1. Study
 site</b> The
 study site is located in a semi-arid grassland in Duolun County (42°02ʹ N,
 116°17ʹ E, 1324 m a.s.l.), Inner Mongolia, China. Long-term (1953–2019)
 mean annual temperature and precipitation are 2.4°C and 382 mm,
 respectively (China Meteorological Data Sharing Service System). The sandy
 soil is classified as chestnut soils (Chinese classification) and Xeric
 Haplocalcids according to the US soil taxonomy classification system (Soil
 Survey Staff, 1999). Plant community at the study site is dominated by
 perennial species such as <i>Agropyron cristatum</i>,<i>
 Artemisia frigida</i>, <i>Artemisia
 pubescens</i>,<i> Lespedeza davurica</i>,<i>
 </i>and<i> Stipa krylovii</i> (Zheng et al.,
 2021). <b>2.2.
 Experimental design and manipulations</b>
 A
 randomized complete block design was used in this experiment with four
 treatments including a control (C; no warming and without N addition),
 24-hour continuous warming (W), N addition (N), and continuous warming
 plus N addition (WN). There were six replicates (block) for each treatment
 (see also Xia et al., 2009). Twenty-four 3-m × 4-m plots were arranged in
 4 × 6 matrix, with a 3-m buffer zone between any two adjacent
 plots. The
 continuous warmed plots were heated continuously using MSR–2420 infrared
 radiators (Kalglo Electronics Inc., Bethlehem, PA, USA) suspended 2.25 m
 above the ground from 23 April to 15 November in 2006 and from 15 March to
 15 November during 2007–2010. In the unwarmed plots, one ‘dummy’ heater
 with the same shape and size as infrared radiators was suspended 2.25 m
 high to mimic the shading effects of infrared radiators. The level of N
 addition was set at 10 g N m<sup>-2</sup>
 year<sup>-1</sup> as ecosystem responses to N enrichment in
 this area could reach the saturation point at a rate of 10.5 g N
 application m<sup>-2</sup> year<sup>-1 </sup>(Bai
 et al., 2010). NH<sub>4</sub>NO<sub>3</sub> was
 applied in the enriched N plots once on 19 July in each of the five
 experimental years from 2006 to 2010. <b>2.3. Soil
 microclimate and plant biomass
 measurements</b> In
 each growing season from May to October, soil temperature (°C) at 10-cm
 depth was measured three times per month using a portable temperature
 probe (LI-8100-201) attached to a LI-8100 Soil CO<sub>2</sub>
 Flux System (Li-Cor Inc., Lincoln, Nebraska, USA). Soil volumetric water
 content (% V/V) at 10-cm depth was measured weekly with a Diviner-2000
 Portable Soil Moisture Probe (Sentek Pty Ltd, Balmain, Australia). Both
 the measurements of soil temperature and moisture were conducted adjacent
 to collars used for soil respiration measurements on clear and sunny days
 between 09:00 and 12:00 a.m. (local time). <b>2.4. Soil
 respiration measurements</b> Two
 5-cm tall polyvinyl chloride collars (10 cm in diameter) were permanently
 inserted 3 cm into the soil at two opposite corners of each plot. A
 CO<sub>2</sub> flux chamber attached to the LI-8100 Soil
 CO<sub>2</sub> Flux System was put on each collar for 90 secs
 to measure soil respiration, and then moved to the next collar. To
 eliminate influences of plant respiration, the aboveground parts of living
 plants inside collars were cut at least one day prior to the measurement.
 The aboveground cut materials were left inside the collars. Soil
 respiration was measured three times per month on clear and sunny days
 between 09:00 and 12:00 a.m. (local time).