Rapid positive response of young trees growth to warming reverses nitrogen loss from subtropical soil

<ol> <li>Global warming is widely expected to alter nitrogen (N) cycling in terrestrial ecosystems by accelerating N transformations in soils. However, it is unclear how warming will affect plant–soil N cycling in subtropical ecosystems.</li> <li>Here, we measured the N transformations including net ammoniation, nitrification, nitrous oxide emissions, and nitrate in soil solution throughout the plant–soil continuum with two years of experimental soil warming (+5 °C) in a young subtropical Chinese fir mesocosm. Seasonal variations of soil and plant (foliage and root) N concentrations and isotopes (δ¹⁵N), foliar water use efficiency, and arbuscular mycorrhizal colonization rate were measured.</li> <li>Soil warming significantly increased net ammoniation and nitrification of the soil, together with the transient positive response observed in inorganic N of the soil. Warming increased nitrate N fluxes in soil solution and nitrous oxide emissions in the first year but not in the second year, suggesting N losses through leaching and gaseous in the initial period of warming. Warming primarily induced enrichment of ¹⁵N in foliage relative to the soil, which was attributed to the trade-offs of persistent increases in plant N uptake caused by enhanced tree growth and a decrease in N losses with continuous warming.</li> <li>Warming significantly increased arbuscular mycorrhizal colonization and foliar water use efficiency throughout the warming period. These results suggest that young trees' growth and N uptake can rapidly acclimate to warming by mechanisms including increases in plant water use efficiency and mycorrhizal colonization.</li> <li>Our findings highlight that warming accelerates the plant–soil N cycle and promotes young trees' growth and N uptake, which in turn reduces soil N lost from this subtropical ecosystem. Therefore, our study suggests that the competition for N between plants and microbes governs whether subtropical forests are opened or closed N cycle systems under climate warming. We highlight that young trees can still maintain their high productivity in facing future climate warming since warming can improve plant N uptake and reduce N loss from subtropical ecosystems. </li> </ol>