Data collection
Data were collected from peer-reviewed journal
articles. These articles were
searched by ISI Web of Science, ScienceDirect, Springer, Wiley, and
Google Scholar. The search terms were “(nitrogen addition OR nitrogen
deposition OR nitrogen fertilization) AND (microbial biomass OR fungal
biomass OR bacterial biomass).” If these papers were selected for
further analysis, they must meet the following criteria: (1) Only
original research papers were included. (2) Experiments were conducted
in terrestrial ecosystems. Laboratory incubation and agro-system
experiment were excluded, as it is hard to estimate the background N
deposition level and ecological factors. (3) Data only related to
control and N addition treatment were extracted. Data with treatments of
warming, CO2, water or phosphorous addition plus N
addition were excluded. (4) Means, sample sizes, and standard deviations
or standard errors of soil microbial characteristics of both control and
N fertilized treatments were extracted. (5) For N types,
NH4Cl,
(NH4)2SO4 and
(NH4)3PO4 were
classified as NH4+-N, while
NaNO3, KNO3 and
Ca(NO3)2 were classified as
NO3--N. N fertilization rate was
measured as N per unit area per year (kg N ha-1yr-1).
Soil microbial characteristics, such as total microbial biomass, fungal
biomass, bacterial biomass, actinomycete biomass, saprophytic fungal
biomass, arbuscular mycorrhizal (AM) fungal biomass, gram-positive
(G+) bacterial biomass, gram-negative
(G−) bacterial biomass, as well as fungi to bacteria
(F/B) ratio and G+ bacteria to G−bacteria (G+/G−) ratio, were
extracted. Data of field experiment such as N application rate (kg N
ha-1 yr-1) and duration (yr) were
also extracted. Information of sample site, such as ecosystem types,
background N deposition level (kg N ha-1yr-1), mean annual temperature (MAT, °C) and mean
annual precipitation (MAP, mm yr−1) were separately
recorded as well. If the data of background N deposition of sample sites
were missing, we used the “Nitrogen deposition onto the United States
and Western Europe” dataset to estimate the background level (Hollandet al. 2005) and extracted the information fromhttp://nadp.slh.wisc.edu/. If
the sample site was located in China, we extracted the information from
“National Science & Technology Infrastructure” fromhttp://www.cnern.org.cn. If the
MAT and MAP were not reported in the papers, we extracted the
information from the database athttp://www.worldclim.org/ by
latitude and longitude. Finally, our database included 1585 paired
observations (Supplementary Online Material 1 ) from 178
articles (Supplementary Online Material 2 ) were classified into
six ecotypes: desert, forest (including broadleaved and coniferous
forest), grassland, tundra, shrub, and wetland around the world
(Supplementary Online sMaterial 3 ).