Temporal Precipitation Variation and Leaf Stoichiometric Changes Mediate the Dynamics of Tree Growth Responses to Nitrogen Addition Over Time

Nitrogen (N) addition can stimulate tree growth; however, the strength of this growth effect usually changes over time and the factors underlying these responses are not fully understood. Based on a decade-long N addition experiment (by adding 0, 20, 50, and 100 kg N ha⁻¹ yr⁻¹) in a boreal forest, we studied responses of tree growth to N addition over time and explored the potential role of temporal precipitation variation and plant stoichiometric changes in mediating this. We found positive growth responses to N addition but this effect changed nonlinearly over time. Annual precipitation was positively related to growth under high-level N addition; hence, a hump-shape temporal pattern in precipitation contributed to the nonlinear tree growth responses. After precipitation effects were accounted for, the positive growth responses to N addition peaked in the seventh year and then declined for all levels of N. Later reductions in growth responses could partly be attributed to increased leaf N:phosphorus (P) ratio over time, especially at higher N addition rates. We also found an increase in soil acid phosphatase, the ratio of labile to occluded soil P fraction, and a decreased ratio in leaf N to P resorption efficiency with increasing N addition rates during the late stage of this experiment, suggesting increased P demand. Collectively, our results imply that changes in plant nutrient stoichiometry with cumulative N input may limit the N stimulation on tree growth over time, while temporal precipitation variation appears unlikely to modulate this effect under the atmospheric N deposition.