Slower Soil Nitrogen Transformations Under Longer-Term Warming and the Consequences for Plant Growth

Climate warming is expected to stimulate soil nitrogen (N) transformations, thereby increasing the supply of available N to support plant growth and carbon (C) assimilation. However, it remains uncertain whether this stimulatory effect on soil N transformations can persist over time to sustain plant growth and N uptake. This study combines a global meta-analysis with a local warming experiment with 7 years to shed light on how soil N transformations respond to long-term warming and the underlying mechanisms, and how these transformations affect plant growth. We found that longer-term warming (≥ 6 years) did not stimulate gross soil N transformation rates, with gross nitrification and NO₃ ^- immobilization rates being significantly inhibited. These responses of gross N mineralization and nitrification rates were primarily explained by biotic rather than abiotic factors. Altered substrate supply through N mineralization and nitrification influenced the responses of NH₄ ⁺ and NO₃ ^- immobilizations. Interestingly, despite the slower rate of soil N transformations, plant growth and N uptake (mainly NH₄ ⁺ uptake) increased under warming, as the inhibition of NH₄ ⁺ consumption by nitrifiers left more NH₄ ⁺ for plants. The findings break the traditional views that warming has a positive effect on soil N cycling and thus contribute to plant N uptake and growth. The slower soil N transformations under longer-term warming and the changes in plant N uptake preference imply a coordination between soil and plant N cycling. Land management practices and C-N cycling simulations should incorporate this soil-plant N coordination and the slower soil N transformations under longer-term warming.