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.