Soil microbes make up a significant portion of the genetic diversity and play a critical role in belowground carbon (C) cycling in terrestrial ecosystems. Soil microbial diversity and organic C are often tightly coupled in C cycling processes; however, this coupling can be weakened or broken by rapid global change. A global meta‐analysis was performed with 1148 paired comparisons extracted from 229 articles published between January 1998 and December 2021 to determine how nitrogen (N) fertilization affects the relationship between soil C content and microbial diversity in terrestrial ecosystems. We found that N fertilization decreased soil bacterial (−11%) and fungal diversity (−17%), but increased soil organic C (SOC) (+19%), microbial biomass C (MBC) (+17%), and dissolved organic C (DOC) (+25%) across different ecosystems. Organic N (urea) fertilization had a greater effect on SOC, MBC, DOC, and bacterial and fungal diversity than inorganic N fertilization. Most importantly, soil microbial diversity decreased with increasing SOC, MBC, and DOC, and the absolute values of the correlation coefficients decreased with increasing N fertilization rate and duration, suggesting that N fertilization weakened the linkage between soil C and microbial diversity. The weakened linkage might negatively impact essential ecosystem services under high rates of N fertilization; this understanding is important for mitigating the negative impact of global N enrichment on soil C cycling.

Nitrogen fertilization weakens the linkage between soil carbon and microbial diversity: A global meta‐analysis

Grassland restoration across the world increases soil organic carbon (SOC) sequestration which is critical for global C cycling and CO2 removal from the atmosphere. However, the relative importance of plant- and microbially-derived C for SOC is still an open question for temperate grasslands. Here, amino sugars and lignin phenols were used as biomarkers to investigate the relative microbial and plant residue contribution to SOC in a 30-year (1-, 5-, 10-, 15-, 25-, 30-year) restoration chronosequence of temperate grassland. The contribution of microbially-derived C (from 4.9 to 13 g kg−1) to SOC was much greater than that of plant-derived C (from 1.3 to 2.3 g kg−1). At the early stage of restoration (<15 years), grassland soils accumulated more C in the form of plant-derived C. In contrast, grassland soils at the late stage of restoration (>15 years) accumulated more microbially-derived C, and less from plant residues. These findings highlight the dominance of microbial contribution to SOC stabilization compared with plant residues. The contribution of bacteria-derived C to SOC gradually increased from 29% to 50% with progress of grassland restoration, while the contribution of fungal C to SOC decreased from 30% to 21%. Consequently, microbial residue contribution to SOC shifts from fungal and bacterial to mainly bacterial residues during grassland restoration. This shift may be due to the faster bacterial growth and a increasing living biomass during grassland restoration, leading to higher accumulation of bacterial residues. Correlation analysis and random forest models showed that belowground plant biomass, soil pH, and living microbial biomass were the main factors regulating plant-derived C. The microbially-derived C in SOC, however, was dependent on living microbial biomass, soil pH and dissolved organic C. Concluding, grassland restoration increased soil C sequestration primarily by microbial necromass (mainly bacterial necromass), and is affected by abiotic and biotic factors, as well as plant C input. 

Increasing contribution of microbial residues to soil organic carbon in grassland restoration chronosequence

Global atmospheric nitrogen deposition significantly affects the nutrient cycling and C–N–P stoichiometry in ecosystems. Herein, a global meta-analysis was conducted based on 898 pairwise observations to analyze the impact of nitrogen addition on plant-soil-enzyme C–N–P stoichiometry and microbial nutrient limitation in different ecosystem types (cropland, grassland, and forest), nitrogen addition intensity (0–5, 5–10, and >10 g N m−2 yr−1) and duration (0–5, 5–10, and >10yr). Results showed that nitrogen addition significantly decreased plant C:N (shoot: 16.5%, root: 27.1%, litter: 16.5%), soil C:N (5.9%), enzyme C:P (1.2%), and enzyme N:P (5.1%), whereas significantly increased soil C:P (4.9%), enzyme C:N (7.1%), vector angle (4.4%), vector length (3.9%), and plant N:P (shoot: 24.1%, root: 23.8%, and litter: 13.5%). Furthermore, nitrogen addition mainly affected the enzyme C:N and vector length in grasslands. Additionally, the changes in C:N in plants, soil, and enzymes, and vector angle and length were higher at nitrogen addition intensity of >10 g N m−2 yr−1. The changes in C:N and C:P in plant and soil were higher at nitrogen addition duration of >10 yr. Finally, the N:P in shoot, soil and enzyme, and vector angle were strongly correlated with mean annual precipitation (MAP). In conclusion, nitrogen addition significantly reduced the C:N ratio in plants and soil and increased plant N:P, and microbial C and P limitation. These effects vary with the ecosystem type, MAP, and nitrogen addition intensity and duration. The results improve our understanding of the plant-soil-microbial nutrient cycling processes in terrestrial ecosystems under global nitrogen deposition. 

Impact of nitrogen addition on plant-soil-enzyme C–N–P stoichiometry and microbial nutrient limitation

A global meta-analysis of the impacts of exotic plant species invasion on plant diversity and soil properties

Bio-photoelectrochemical degradation, and photocatalysis process by the fabrication of copper oxide/zinc cadmium sulfide heterojunction nanocomposites: Mechanism, microbial community and antifungal analysis.

Soil microbial diversity is one of the key factors affecting the structure and function of the belowground ecosystem; yet, little is known about the response of microbial diversity to multiple global change factors. Here, we conducted a global meta-analysis based on data collected from 237 published papers to explore the effect of multiple global change factors (elevated carbon dioxide (eCO2), warming, elevated nitrogen addition (eN), wetting–drying cycle, drought, decreased precipitation (precipitation(−)), and increased precipitation (precipitation(+))) on microbial diversity (Shannon index) across different ecosystems (cropland, grassland, forest, shrubland, desert, wetland, and tundra). Global change decreased soil bacterial and fungal diversity by an average of 2.9% and 3.5%, respectively. For each global change factor, the effect sizes of precipitation(−), eN, wetting–drying cycle, and drought on soil microbial diversity were negative, whereas the effect sizes of eCO2, warming, and precipitation(+) were positive. This phenomenon was driven by changes in mean annual temperature (MAT) and edaphic factors (especially soil pH, bulk density and organic carbon content) rather than mean annual precipitation. Moreover, the effect size of soil microbial diversity linearly declined with increasing MAT, suggesting that microbial diversity was highly dependent on climate conditions at the global scale. In addition, two- and three-way interactions of global change factors aggravated the negative effects of individual effects. We suggest that it is essential to conduct long-term, multiple-factor experiments to assess the response of soil microbial diversity to global change because multiple global change factors often occur simultaneously.

Negative effects of multiple global change factors on soil microbial diversity

Linkage between soil ectoenzyme stoichiometry ratios and microbial diversity following the conversion of cropland into grassland

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Negative effects of multiple global change factors on soil microbial diversity

Linkage between soil ectoenzyme stoichiometry ratios and microbial diversity following the conversion of cropland into grassland