Arbuscular mycorrhizal fungi (AMF) play an important role on litter decomposition, which is the keyways for nutrients returning to natural ecosystems. Although having no known saprotrophic abilities like ectomycorizal and ericoid fungi, AMF can affect litter decomposition by releasing low molecular compounds such as sugar, amino acids and organic acids through extraradical mycorrhizal hyphae, or deposit dead hypha fragments as nutrient (e.g., carbon) input and to stimulate microbial decomposers, i.e., priming effect. Besides, interactions of AMF with saprotrophic microorganisms through ways such as niche competition, synergistic cooperation, specific microorganisms recruitment and by soil nutrient removal that can influence litter decomposition. Under different microenvironments, AMF display various nutrient acquisition strategies, which also affects this process. The magnitude and direction of AMF effects on litter decomposition depend on plant communities, litter quality, soil and litter nutrient dynamics, soil physical and chemical properties (mainly, physical structure, temperature, humidity, pH and nutrient content). The exacerbation of global changes (drought, chemical fertilization and soil acidification) is exerting pervasive impacts on ecosystem processes and functions mediated by AMF. In this review, we explore the current state of knowledge about the impacts of AMF on litter decomposition from microenvironment perspective. The summary of the potential influence of various microsite abiotic and biotic factors on this process in order to provide theoretical reference for comprehensive analysis of ecosystem functions of AMF is made.

Acid rain is generally believed to inhibit litter decomposition, but this inhibition effect often depends on local soil environmental factors such as nutrient and physicochemical property. Karst ecosystem is typically characterized by rich CaCO ₃ , which can potentially react with H ⁺ from acid rain. However, it is unknown whether the effect of acid rain on litter decomposition will be mitigated by this potential reaction. To investigate the effect of acid rain on decomposition, we undertook a field experiment on the decomposition of litters of Cunninghamia lanceolata needle and Cinnamomum camphora leaf during 1 year in a karst region in subtropical China. Contrary to hypothesis, litter decomposition was significantly decelerated by acid rain for the two litter types. For C. lanceolata litter, at the end of incubation, we noted that moderate acid rain resulted in the highest loss in mass (43%), while severe one showed the lowest loss (39%); for C. camphora litter, the mass loss was as follows: ambient treatment (66%) > moderate treatment (56%) > severe treatment (42%). Also microbial activities (including CO ₂ flux, fungal biomass and microbial degrading enzymes) showed similar variation patterns among acid rain treatments to mass loss. These responses indicated that the deceleration effect of acid rain on litter decomposition did not depend on calcareous soil as well as litter quality. Together, these results suggest that acid rain had a predominant controlling of litter decomposition over soil and litter qualities, and this information may be useful for predicting functional responses of karst ecosystem to environmental factors.

Arbuscular mycorrhizal fungi (AMF) can preferentially absorb the released ammonium (NH₄⁺) over nitrate (NO₃⁻) during litter decomposition. However, the impact of AMF’s absorption of NH₄⁺ on litter nitrogen (N) decomposition is still unclear. In this study, we investigated the effects of AMF uptake for NH₄⁺ on litter N metabolic characteristics by enriching NH₄⁺ via AMF suppression and nitrification inhibition in a subtropical Cinnamomum camphora forest. The results showed that AMF suppression and nitrification inhibition significantly decelerated litter decomposition in the early stage due to the repression of NH₄⁺ in extracellular enzyme activity. In the late stage, when soil NH₄⁺ content was low, in contrast, they promoted litter decomposition by increasing the extracellular enzyme activities. Nitrification inhibition mainly promoted the utilization of plant-derived N by promoting the degradation of the amide I, amide II, and III bands by increasing protease activity, and it promoted ammonification by increasing urease activities, whereas it reduced the utilization of microbial-derived N by decreasing chitinase activity. On the contrary, AMF suppression, which significantly reduced the ammonification rate and increased the nitrification rate, only facilitated the degradation of the amide II band. Moreover, it intensified the microbial-derived N decomposition by increasing chitinase activity. The degradation of the amide I and II bands still relied on the priming effects of AMF on soil saprotrophs. This was likely driven by AMF-mediated phosphorus (P) mineralization. Nutrient acquiring, especially P by phosphatase, were the main factors in predicting litter decomposition and protein degradation. Thus, AMF could relieve the end-product repression of locally enriched NH₄⁺ in extracellular enzyme activity and promote early-stage litter decomposition. However, the promotive effects of AMF on litter protein degradation and NH₄⁺ release rely on P mineralization. Our results demonstrated that AMF could alleviate the N limitation for net primary production via accelerating litter N decomposition and reducing N loss. Moreover, they could restrict the decomposition of recalcitrant components by competing with saprotrophs for nutrients. Both pathways will contribute to C sequestration in forest ecosystems, which advances our understanding of AMF’s contribution to nutrient cycling and ecosystem processes in subtropical forests.

Soil fauna play an important role in litter decomposition and affect the “home-field advantage” (HFA) of litter decomposition. However, how this effect is modulated by the microenvironment needs further investigation. We conducted a reciprocal transplant experiment of litter decomposition using different mesh-size litterbags across litter and soil layers in subtropical coniferous (Pinus massoniana) and broad-leaved (Quercus variabilis) forests. Our results revealed a pronounced HFA in P. massoniana. P. massoniana litter decomposed faster in its home habitat by 40.6% in the litter layer and 10.2% in the soil layer in coarse mesh bags and by 21.8% in the litter layer and 21.4% in the soil layer in fine mesh bags. However, Q. variabilis litter showed faster decomposition in its home soil layer by 10.8% and 4.3% for coarse and fine mesh bags, whereas in the litter layer it decomposed faster in the away habitat by 16.7% and 20.6% for coarse and fine mesh bags, respectively. Higher soil mesofauna abundance and microbial activities in the coniferous forest compared to the broad-leaved forest drive the observed HFA of litter decomposition. Especially in the litter layer, the abundance of mesofauna was 89.8% higher in the coniferous forest. Coarse mesh bags generally facilitated a higher decomposition rate across litter and soil layers, likely due to a better interaction between soil mesofauna and extracellular enzyme activity. The HFA index exhibited distinct seasonal fluctuations, peaking in October for coarse mesh bags and in April for fine mesh bags within the litter layer, while soil layer peaks occurred in August and April. Notably, an increase in Acarina abundance strongly correlated with enhanced decomposition and HFA effects in the litter layer during October. This study revealed the sensitivity of HFA to the soil layer and soil fauna and underscores the complex role of the microclimate in shaping interactions among soil microorganisms, litter quality, and mesofauna, thereby enriching our understanding of litter decomposition dynamics in forest ecosystems.