Soil microbial diversity underpins decomposition, nutrient cycling, and carbon sequestration, yet its responses to the concurrent global change drivers that co-occur in reality remain poorly resolved. A global meta-analysis by Yang et al. (2021; 237 papers, 1374 observations) found that global change factors collectively reduce soil bacterial and fungal diversity and that two- and three-way factor combinations aggravate the losses, and explicitly called for long-term, multiple-factor experiments to assess these interactive effects. Four linked questions are additionally open in the current literature: verification of a proposed double-asymmetric response of microbial diversity to altered precipitation; identification of the mechanisms underlying the effect of elevated CO2 on microbial diversity and of the disagreement over its sign; identification of microbial groups adapted to intensified wetting-drying cycles; and development of measures that mitigate diversity loss under continued nitrogen deposition. Every thread remains open: a second meta-analysis reports mainly additive interactions, the double-asymmetry hypothesis has received only one regional two-year test, the elevated-CO2 mechanism is untouched and even the response sign is contested, wetting-drying-adapted taxa are unidentified beyond single-site studies, and nitrogen mitigation remains undeveloped.

Background

Soil microorganisms — bacteria and fungi in particular — drive organic matter decomposition, nutrient cycling, and carbon sequestration, so the diversity of belowground communities is widely used as an indicator of soil health and functional capacity. 'Microbial diversity' here usually means alpha-diversity indices such as the Shannon index, computed from DNA sequences of taxonomic marker genes (16S rRNA for bacteria, ITS for fungi) obtained by high-throughput amplicon sequencing of soil samples. Human activity imposes several global change factors (GCFs) on soils simultaneously: elevated atmospheric CO2 (often manipulated in free-air CO2 enrichment, or FACE, experiments), warming, nitrogen addition (fertilizer or simulated deposition), increased and decreased precipitation, drought, and intensified wetting-drying cycles. Most knowledge of how these drivers affect microbial diversity came from single-factor, short-term experiments. Two global syntheses frame the open question. Zhou, Wang and Luo (2020) meta-analyzed 1235 observations and found that GCFs do not uniformly reduce microbial diversity — responses depend strongly on changes in soil pH, and rare species are more sensitive than common ones. Yang et al. (2021) then meta-analyzed 237 papers (1374 observations) with explicit multi-factor structure, using the log response ratio (the natural log of diversity under treatment divided by diversity under an untreated control) as effect size. They reported that GCFs collectively decrease bacterial diversity by about 2.9% and fungal diversity by about 3.5%; that decreased precipitation, nitrogen addition, wetting-drying cycles, and drought have negative effects while elevated CO2, warming, and increased precipitation have positive ones; that the negative effect of decreased precipitation outweighs the positive effect of increased precipitation — a pattern related to the double asymmetric model, the framework from precipitation-productivity research (Knapp, Ciais and Smith 2017) in which dry anomalies affect an ecosystem property more strongly than equivalent wet anomalies; and that two-way and three-way combinations of factors aggravate the negative effects of individual factors, where 'aggravate' means a non-additive (synergistic) interaction — the combined effect being more negative than the sum of individual effects — as opposed to additive or antagonistic (offsetting) combinations. Because these effects operated mainly indirectly through soil pH, bulk density, soil organic carbon, and mean annual temperature, and because the underlying data were predominantly from short-term studies, Yang et al. (2021) concluded, verbatim in their abstract, that 'it is essential to conduct long-term, multiple-factor experiments to assess the response of soil microbial diversity to global change'. The mechanistic and applied questions that complete the problem stated below — the mechanisms and sign of the elevated-CO2 effect, which taxa tolerate repeated wetting-drying cycles, and how nitrogen-driven diversity loss can be mitigated — remain open in the subsequent literature.

Problem Statement

Determine how soil microbial diversity responds to multiple concurrently acting global change factors — in direction, magnitude, and interaction — over realistic long timescales, and explain and mitigate the resulting diversity loss. Concretely, Yang et al. (2021) call for long-term, multiple-factor experiments to assess these responses, and the current literature leaves that experimental need and four linked mechanistic and applied questions open; the task comprises these five linked parts, all of which must be addressed: (i) whether two- and three-way combinations of elevated CO2, warming, nitrogen addition, and altered precipitation act non-additively (synergistically or antagonistically) on soil bacterial and fungal diversity and in which direction, based on the longest available multi-factor records with the duration limitation stated, since existing global syntheses rest on predominantly short-term and single-factor data and disagree about the sign of interactions; (ii) whether the diversity response to altered precipitation follows a double asymmetric model, that is, whether decreases in precipitation reduce diversity more strongly than equal increases raise it, using the available controlled symmetric precipitation manipulations; (iii) the mechanisms underlying the effect of elevated CO2 on microbial diversity, including, if the effect is positive, the relative roles of plant-mediated carbon inputs and changes in soil properties, and the source of the disagreement over its sign; (iv) which specific microbial groups are adapted to frequent wetting-drying cycles, and how community composition shifts under intensified hydrological cycling; and (v) which measures effectively mitigate the decline of microbial diversity under continued nitrogen deposition.

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