Roots regulate microbial N processes to achieve an efficient NH4+ supply in the rhizosphere of alpine coniferous forests
2021
Zhu, Xiaomin | Liu, Dongyan | Yin, Huajun
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.
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