INTRODUCTION
Atmospheric nitrogen (N) deposition has increased markedly since the
middle of the 20th century. The enhanced N has been
acknowledged as a global and increasing threat to biomass, biodiversity
(Sala et al. 2000) and ecosystem function (Phoenix et al.2012). For plants, N is a driving force of plant diversity (Zonget al. 2019) and ecosystem functioning (Dias et al. 2014).
For soil microbial community, N deposition may increase (Liu & Zhang
2019) or decrease soil microbial biomass (Zhou et al. 2019) and
diversity (Wang et al. 2018a). Many short- and long-term projects
have been executed to investigate the effects of increased N on soil
microorganisms in terrestrial ecosystem, such as the Dinghushan
Biosphere Reserve (DHSBR) Experiment (Wang et al. 2018b),
Changbaishan Forest Ecosystem Research Station (CBFERS), and Inner
Mongolian Grassland Experiment (Li et al. 2017) in China, Harvard
Forest Experiment in the USA (Turlapati et al. 2013), and
Nitrogen Saturation Experiment (NITREX) in Europe (Moldan et al.2018). Accumulative N fertilization has been shown to cause positive
(Tahovská et al. 2020) or negative (Zhang et al. 2018)
effects on soil microbial biomass. In their reports, however, ammonium
nitrate (NH4NO3) was usually selected as
the N fertilizer (Chen et al. 2018; Liu & Zhang 2019; Luanet al. 2019), followed by urea
(CO(NH2)2) (Li et al. 2017),
while ammonium (NH4+-N, e.g.,
NH4Cl,
(NH4)2SO4, and
(NH4)3PO4) and nitrate
(NO3−-N, e.g., NaNO3,
KNO3, and Ca(NO3)2) were
less used (Liu et al. 2016; Zhang et al. 2017). Actually,
the anthropogenic N from atmosphere is a set of mixture, mainly in the
form of CO(NH2)2,
NH4+-N, and
NO3−-N (Cornell 2011).
CO(NH2)2 is usually used as organic N
fertilizer in agricultural activities. The
NH4+-N is closely associated with
intensive livestock husbandry and ammonia (NH3) emission
(Zhou et al. 2019). The source of
NO3−-N is nitrogen oxide
(NOX), which are closely associated with industrial
activities, such as fossil fuel combustion (Liu et al. 2013).
Some studies have shown that the addition of different N types induced
different soil and microbial characteristics changes. For soil
characteristics, the application of
NO3−-N fertilization did not
significantly affect soil pH, but
NH4+-N application significantly
decreased its pH (Paredes et al. 2011).
NH4+-N and
NO3−-N also revealed different effects
on soil organic matter decomposition (Yang et al. 2014; Khaliliet al. 2016). For microbial biomass,
CO(NH2)2 addition significantly
increased soil microbial biomass (+12.1%), while
NH4NO3 did not significantly affect soil
microbial biomass (Treseder 2008). For enzymatic activities,
NH4NO3 deposition inhibited microbial
enzymatic activities, but CO(NH2)2deposition promoted the processes (Du et al. 2014).N -acetyl-glucosaminidase, cellobiohydrolase, phosphatase, and
phenol oxidase activities increased after
NH4+-N addition. By contrast,
NO3−-N fertilization did not
significantly affect the activities of α -glucosidase,
cellobiohydrolase, β -xylosidase, and phosphatase (Currey et
al. 2010). Therefore, it is essential to systematically compare the
effects of different N fertilizers on ecosystem, especially under the
same dose.
The responses of soil microbial biomass to N addition may be affected by
the ecological factors of experiment site as well. For example, soil
microbial biomass to N addition at high altitude and latitude regions
was different from other regions (Fu & Shen 2017). N addition rates and
latitude might directly and negatively affect the effects of N addition
on N resorption efficiency (You et al. 2018). The responses of
soil microorganisms to N addition were also affected by experimental
duration, precipitation and soil type (Jia et al. 2020). Thus, it
is also important to compare the effects of various N types on soil
microorganisms under different ecological factors.
In terrestrial ecosystems, soil microorganisms are critical drivers of
plant diversity and play important roles in ecosystem function (Liet al. 2019). Here, we focused on soil microbial biomass and
analyzed the impact of different types of N on them with meta-analysis.
Based on previous findings, we hypothesized that: (1) soil microbial
biomass would increase greater after
CO(NH2)2 addition compare with other N
types. (2) Soil microbial biomass would increase significantly after
NH4+-N addition as
NH4+-N is thought to be the essential
component for amino acid. (3) The effect of
NO3−-N on soil microbial biomass would
be less significant as NO3−-N is more
mobile and subjected to leaching, especially in the area with greater
precipitation. (4) The effects of NH4NO3addition on soil microbial biomass would be less than
NH4+-N but greater than
NO3−-N. (5) Soil microbial biomass
with lower background N deposition level might be more sensitive to N
addition.