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.