We used Web of Science (http://apps.webofknowledge.com/ ), Google Scholar (http://scholar.google.com/ ), and China National Knowledge Infrastructure (www.cnki.net/ ) for an exhaustive search of articles published before March 2018. The keywords and phrases used for literature research were as follows: (i) “nitrogen addition,” “nitrogen amendment,” “nitrogen enrichment,” “nitrogen fertilizer,” “nitrogen elevated,” or “nitrogen deposition”; (ii) “glucosidase,” “cellobiosidase,” “xylosidase,” “peroxidase,” “phenol oxidase,” “polyphenol oxidase,” “lignin modifying enzymes,” or “cellulase”; (iii) “soil carbon”; and (iv) “terrestrial,” “soil,” or “land”.
To be included in our data set, articles had to meet several requirements. First, we only considered experiments that lasted at least 1 year. Second, control and N addition treatments had to be applied at the same experimental site; that is, the microclimate, vegetation, and soil types were similar between treatments. Third, SDs and replicates had to be reported or could be derived from the results. Fourth, details on N addition methods (rate, frequency, form, and duration) had to be provided. We identified 40 studies that met these criteria, and 9 of these studies reported soil C data from the matching studies (see Supplementary Materials and Methods and data S1).
For each study, we recorded LME activity and cellulase activity (see Supplementary Materials and Methods and table S1), site location (longitude and latitude) and climatic variables (MAP and MAT), elevation, BND, vegetation and soil types, and N addition methods (rate, duration, frequency, and form of N addition). If these data were not reported, we contacted the corresponding author for more information. Otherwise, we obtained MAT and MAP from the WorldClim database (www.worldclim.org/ ), BND from the Global N deposition database (http://webmap.ornl.gov/ ). We classified vegetation types according to the Whittaker Biome Diagram (38 ), and soil types according to the Food and Agriculture Organization taxonomy (www.fao.org/soils-portal/soil-survey/soil-classification/usda-soil-taxonomy/en ). Where available, we also tabulated plant productivity, soil pH, soil C/N, microbial abundance, soil texture, and the size of the recalcitrant C pool (see Supplementary Materials and Methods and data S2 and S3). When results were presented graphically, we used Engauge Digitizer 4.1 (http://digitizer.sourceforge.net ) to digitize the data.
To be included in our data set, articles had to meet several requirements. First, we only considered experiments that lasted at least 1 year. Second, control and N addition treatments had to be applied at the same experimental site; that is, the microclimate, vegetation, and soil types were similar between treatments. Third, SDs and replicates had to be reported or could be derived from the results. Fourth, details on N addition methods (rate, frequency, form, and duration) had to be provided. We identified 40 studies that met these criteria, and 9 of these studies reported soil C data from the matching studies (see Supplementary Materials and Methods and data S1).
For each study, we recorded LME activity and cellulase activity (see Supplementary Materials and Methods and table S1), site location (longitude and latitude) and climatic variables (MAP and MAT), elevation, BND, vegetation and soil types, and N addition methods (rate, duration, frequency, and form of N addition). If these data were not reported, we contacted the corresponding author for more information. Otherwise, we obtained MAT and MAP from the WorldClim database (
