Changes in soil ammonium-to-nitrate ratio and nutritional symbionts enhance Phyllostachys edulis suppression of heterogeneous competitors in shade

Positive interspecific interactions in resource capture and soil property modification benefit the establishment of mixed plantations, contingent on environmental conditions. However, primary species frequently reject companion tree species frequently arises in forest management practices. To understand this contradiction, we investigated the effects of five planting patterns involving two local companion tree species (Phoebe chekiangensis and Torreya grandis) and Phyllostachys edulis on the soil nitrogen (N) and phosphorus (P) content, enzyme activities, net N mineralization rates, microbial composition, fungal functional groups, and the AMO gene abundance under both shade and full light conditions using a pot experiment. Under the full light condition, altered soil properties were predominantly influenced by interspecific interaction compared to intraspecific interactions, leading to an overall enhancement in nutrient availability and enzymatic activities. Under the shade condition, mixing with P. edulis similarly facilitated increased NH₄⁺-N content, ammonification rate, and urease activity. However, P. edulis significantly decreased ammonia monooxygenase and its encoding gene, as evidence by the unbalanced ratio of net ammonification to net nitrification (2.2-fold) under the shade condition, resulting in high NH₄⁺-N/NO₃^–-N ratios. Additionally, the shade condition induced soil microbial communities in the mixture to resemble those of monocultured dominant species (P. edulis). Our findings also indicated that specific plant-fungal associations in companion species were disrupted when intercropped with P. edulis. Therefore, this study highlights how the performance of companion species, linked to soil NH₄⁺-N/NO₃^–-N ratios and plant-fungal associations, leads to their suppression by P. edulis under shade conditions.