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

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. 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. 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. 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. 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.