Despite multiple pieces of evidence that root-derived carbon (C) can profoundly regulate mineral nitrogen (N) cycling, it is still not elucidated whether tree roots differentially modulate the production and retention of ammonium (NH₄⁺) versus nitrate (NO₃⁻) through the rhizosphere effect (RE). Using the ¹⁵N isotope labeling technique, we investigated how plant roots regulated the production and retention of NH₄⁺ and NO₃⁻ via rhizosphere processes and thus affected soil N availability in two alpine coniferous forests. Meanwhile, the activities of enzymes associated with N cycling and soil physicochemical properties in the rhizosphere and bulk soils were measured to explore the underlying mechanism. The results showed that roots induced positive REs of gross mineralization, microbial NH₄⁺ immobilization, and dissimilatory nitrate reduction to ammonium (DNRA) to improve rhizosphere NH₄⁺ availability. These positive REs can be attributed to higher microbial biomass C and N contents and higher activities of N cycling-associated enzymes that are fueled by root-derived C inputs. In contrast, the REs on NO₃⁻ production were negative, which could be due to a higher soil C:N ratio and greater microbial NH₄⁺ immobilization in the rhizosphere soil than those in the bulk soil, thus leading to relatively low NO₃⁻ availability in the rhizosphere. Collectively, our results provide field-based empirical evidence that plant roots can stimulate NH₄⁺ production and immobilization, whereas they can limit NO₃⁻ production to achieve a high rhizosphere NH₄⁺ supply in alpine coniferous forests. These findings provide comprehensive insights into how plants sustain nutrient supply and growth via mediating soil microbial N processes in the rhizosphere.

尽管有多项证据表明根源性碳 (C) 可以显着调节矿质氮 (N) 循环，但仍然没有阐明树根是否差异调节铵 (NH ₄ ⁺ ) 与硝酸盐 (NO ₃ ^- )的产生和保留。) 通过根际效应 (RE)。使用¹⁵ N 同位素标记技术，我们研究了植物根系如何调节 NH ₄ ⁺和 NO ₃ ^-的产生和保留通过根际过程，从而影响了两个高山针叶林中土壤氮的有效性。同时，测量了根际和大块土壤中与氮循环和土壤理化性质相关的酶的活性，以探索其潜在机制。结果表明，根诱导了总矿化、微生物 NH ₄ ⁺固定化和异化硝酸盐还原成铵 (DNRA) 的积极 RE，以提高根际 NH ₄ ⁺可用性。这些积极的 RE 可归因于更高的微生物生物量 C 和 N 含量以及更高的 N 循环相关酶的活性，这些酶由根来源的 C 输入提供燃料。相比之下，NO ₃上的 REs ^-产量为负，这可能是由于根际土壤中更高的土壤 C:N 比和更大的微生物 NH ₄ ⁺固定比散装土壤中的更高，从而导致根际中的NO ₃ ^-可用性相对较低。总的来说，我们的结果提供了基于实地的经验证据，表明植物根部可以刺激 NH ₄ ^{+ 的}产生和固定，而它们可以限制 NO ₃ ^{- 的}产生，从而在高山针叶林中实现高根际 NH ₄ ⁺供应。这些发现为植物如何通过调节根际土壤微生物氮过程来维持养分供应和生长提供了全面的见解。