DOI: 10.5061/dryad.66t1g1k22
title: Divergent terrestrial responses of soil N2O emissions to different levels of elevated CO2 and temperature

description: Understanding soil nitrous oxide (N2O) emissions responses from
 terrestrial ecosystems to future CO2 enrichment and warming is critical
 for the development of mitigation and adaptation policies. The continuous
 increase of elevated CO2 (EC) and elevated temperature (ET) effects on N2O
 emissions are not fully known. We synthesized 209 measurements from 70
 published studies and carried out a meta-analysis to examine individual
 and interactive effects of EC and ET on N2O emissions from grasslands,
 croplands, and forests. On average, a significant increase of 23% in N2O
 emission was observed under EC across all case studies. A sharp increase
 in the N2O emissions was noticed in the response to EC under less than 150
 ppm enrichment levels. Nonetheless, a significant but weak negative
 response of N2O emission to EC was examined. Compared with non-significant
 responses of grassland and forest, EC resulted in the highest increase
 (38%) in N2O emission of cropland. The extent of ET effect on N2O
 emissions was nonsignificant and there was no significant difference in
 N2O emissions responses among three terrestrial systems. ET only promoted
 the N2O emissions in the forest about 32% when ET was less than 2°C. The
 interactive effect of EC and ET on N2O emissions was significantly
 synergistic, ergo, showed a greater increase than the sum of effects
 caused by EC and ET alone. Our findings suggest that future studies on
 soil N2O need more long-term experiments with multifactor combination to
 better predict ecosystem responses.
description: We made an extensive search of articles published in
 peer-reviewed journals before October 2019. All published data were
 extracted from the “Web of Science” and “Google Scholar”. Every
 combination of searching keywords including “CO2”,
 “temperature”,“warming”, “N2O emissions” or “N2O fluxes”, “nitrous oxide
 emissions” was used for data extraction. Any further synthesis cited in
 papers of review, meta-analysis or synthesis were also considered. The
 following selection criteria were used to avoid bias in selecting
 publications: (1) experiments must be conducted in growth chambers,
 open-top chambers, free-air CO2 enrichment facilities, and greenhouses.
 Measurements from the soil incubation performed in laboratory studies or
 modeling results were excluded (2) data of soil N2O flux must be present
 in experimental results. Because effect size (natural log of the response
 ratio, Equation 1) could not be calculated for those observations with
 zero or negative values under either control or treatments, we excluded
 the observation if means soil N2O emission were less than zero; (3)
 ambient and elevated values of CO2 or temperature must have been provided;
 (4) means and replication number had to be reported for the treatments of
 ambient CO2, EC, ambient temperature and ET. A total of 209 observations
 from 1990 to 2019 on N2O response to EC and ET from 70 peer-reviewed
 articles were collected, including 86 effect sizes from 36 ET experiments,
 106 effect sizes from 43 EC experiments, and 17 effect sizes from 8
 combinations of co-elevated temperature and CO2 experiments. The dataset
 also included site location, experimental duration, N fertilization and
 land use types (e.g., grassland, forest and cropland). Observations from
 different years within the same study were considered and its average was
 considered as one observation averagely. Units were converted to μg N m-2
 day-1 for all observations. The CO2 concentration increase in the EC
 treatment ranged from 35 to 400 ppm (254 ppm on average). The temperature
 increase in the ET treatment ranged from 0.39°C to 4°C.
 Based on observations, land use types were categorized into: (1)
 grassland; (2) forest; and (3) cropland. Croplands were mainly planted
 wheat, maize and other food crops. The annual nitrogen application rate of
 croplands was ranged from 10 to 40 g m-2. Tillage mainly included
 traditional tillage and conservation tillage. Grasslands cover plants
 included a variety of herbs and shrubs. Most grasslands use nitrogen
 fertilization less than 10 g m-2 per year except for a few experiments
 with different levels of nitrogen application. And forests were mainly
 planted with perennial trees. Compared with croplands, grasslands and
 forests do not have much human cultivation activities. To address whether
 the response of soil N2O emissions to EC and ET might be affected by the
 change in the magnitude of the experimental factors, we categorized the EC
 into three groups: (0, 150), (150, 300) and 132(300, 450). Whereas, ET was
 categorized into two groups: ≤2°C and ＞2°C.  
description: See attached README. Methods contain a more detailed description
 of the data.