Aijun Xing

Soil stores the largest pool of actively cycling carbon (C) on land, and its dynamics under global changes are critical for projecting climate feedbacks. Global nitrogen (N) deposition has more than doubled since the 1860s; however, the temporal dynamics of soil C emissions under prolonged N deposition are poorly understood. Here, using a global time-series dataset compiled from 38 long-term experiments in forests and grasslands, we find that the response of soil respiration (Rs) to N addition systematically shifts from a transient increase persisting for approximately 2 y to a sustained suppression over time (mean effect = −6%; 95% CI = [−10%, −3%]). Rs in forests and grasslands followed consistent nonlinear trajectories, characterized by a rapid initial decline in the magnitude of the response, which progressively leveled off. Notably, suppression of Rs emerged when cumulative N inputs exceeded a threshold of ~100 kg N ha ⁻¹ . Consequently, the suppression was both stronger and occurred more rapidly under high-level N additions (>50 kg N ha ⁻¹ y ⁻¹ ). These temporal dynamics were associated with N-induced soil acidification and related changes in microbial properties. Overall, these findings indicate that accurate prediction of land–atmosphere C fluxes under global change requires models to capture the reported temporal dynamics.

Nitrogen (N) in the litter and soil inorganic N can have contrasting effects on litter decomposition, such that the positive effects of litter N on the rate of decomposition will likely decrease with increased soil inorganic N due to possible suppression of lignin oxidase. To test this, we reciprocally buried litter collected from multiple treatments in an N addition experiment (0, 20, 50, and 100 kg N ha⁻¹ yr⁻¹). With increasing N addition, we found that the concentration of N in the litter and the availability of inorganic N in the soil increased linearly. Contrary to our expectations, the litter N effects on decomposition changed nonlinearly with the N addition, first decreasing with the N addition rates but then reversed at the highest N addition treatment. This reversal at higher N rates is mainly due to a change in soil decomposing microbes with N addition. We examined the activity of extracellular enzymes in soil and found that phenol oxidase activity was not affected by N addition, but cellulase activity increased linearly with N addition rates. We further found that the relative abundance of bacterial functional genes involved in cellulose and the abundance of soft‐rot fungi that decay cellulose increased with N addition. Our study contrasts the assumption that increased soil inorganic N suppresses lignin oxidation and instead reveals an increase in cellulolytic groups and activity of cellulase; this shift in decomposing microbes mediates the litter N effects on decomposition and suggests that more slowly decomposing fractions might be less affected.

Plant biomass and its allocation are fundamental for understanding biospheric matter production. However, the impacts of atmospheric phosphorus (P) deposition on species-specific biomass and its allocation in global terrestrial plants remain unclear. By synthesizing 5548 observations of plant biomass and its allocation related to P addition worldwide, we find that P addition increases plant biomass by an average of 35% globally. This increase varies across plant functional groups, with stronger responses in deciduous (45%), C₃ (36%), and N₂-fixing plants (54%) than in evergreen (28%), C₄ (19%), and non-N₂-fixing plants (31%), respectively. Plants possessing traits indicative of an acquisitive strategy, such as higher nutrient concentrations and specific leaf area, faster photosynthetic rates and shorter leaf lifespan, are particularly responsive to P addition. Furthermore, P addition promotes a greater allocation of biomass to aboveground than belowground organs, resulting in a 5% decrease in root-to-shoot ratio. Our findings provide global-scale quantifications of how P addition regulates biomass accumulation and allocation strategies in terrestrial plants, offering critical insights for predicting the response of terrestrial carbon storage to rising atmospheric P deposition.

Summary: Leaf dark respiration (R_dark), an important yet rarely quantified component of carbon cycling in forest ecosystems, is often simulated from leaf traits such as the maximum carboxylation capacity (V_cmax), leaf mass per area (LMA), nitrogen (N) and phosphorus (P) concentrations, in terrestrial biosphere models. However, the validity of these relationships across forest types remains to be thoroughly assessed. Here, we analyzed R_dark variability and its associations with V_cmax and other leaf traits across three temperate, subtropical and tropical forests in China, evaluating the effectiveness of leaf spectroscopy as a superior monitoring alternative. We found that leaf magnesium and calcium concentrations were more significant in explaining cross‐site R_dark than commonly used traits like LMA, N and P concentrations, but univariate trait–R_dark relationships were always weak (r² ≤ 0.15) and forest‐specific. Although multivariate relationships of leaf traits improved the model performance, leaf spectroscopy outperformed trait–R_dark relationships, accurately predicted cross‐site R_dark (r² = 0.65) and pinpointed the factors contributing to R_dark variability. Our findings reveal a few novel traits with greater cross‐site scalability regarding R_dark, challenging the use of empirical trait–R_dark relationships in process models and emphasize the potential of leaf spectroscopy as a promising alternative for estimating R_dark, which could ultimately improve process modeling of terrestrial plant respiration.

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.

Precipitation changes exert a fundamental effect on the nitrogen (N) cycle in water‐limited grasslands. Soil microbes are essential drivers of N cycle, and the rates and their stabilities of interrelated N‐cycling processes are reflected by the abundance and diversity of N‐cycling genes. Yet, little is known about how altered precipitation affects the genes involved in the entire N‐cycling pathways. By combining a 6‐year precipitation manipulation experiment (−30%, ambient, +30%, +50%) with metagenomic sequencing, we investigated the responses of N‐cycling gene abundance and diversity to altered precipitation at two soil depths (0–10 and 30–50 cm). We found that increased precipitation enhanced the abundance of numerous key genes, leading to an acceleration of N turnover, but decreased the diversity of ammonium assimilation genes. Decreased precipitation did not reduce abundance or diversity of N‐cycling genes. Most N‐cycling genes showed generally consistent responses to altered precipitation in the topsoil (0–10 cm) and subsoil (30–50 cm), albeit with clear distinctions in both abundance and diversity by soil depth. These precipitation‐specific responses and depth‐dependent variabilities of functional genes were attributed to the distinct taxonomic composition of each N‐cycling gene. Furthermore, we quantified gross N transformation rates and found that they were well predicted by the abundance of most N‐cycling genes (e.g. genes involved in ammonium assimilation and nitrification). Our study sheds new light on the soil N cycle under precipitation alterations from the perspective of individual gene abundance and diversity and shows that future increases in precipitation could accelerate soil N turnover in arid and semi‐arid lands. Read the free Plain Language Summary for this article on the Journal blog.

Increased nitrogen (N) inputs are widely recognised to reduce soil respiration (Rs), but how N deposition affects the temporal dynamics of Rs remains unclear. Using a decade‐long fertilisation experiment in a boreal larch forest (Larix gmelini) in northeast China, we found that the effects of N additions on Rs showed a temporal shift from a positive effect in the short‐term (increased by 8% on average in the first year) to a negative effect over the longer term (decreased by 21% on average in the 11th year). The rates of decrease in Rs for the higher N levels were almost twice as high as those of the low N level. Our results suggest that the reduction in Rs in response to increased N input is accelerated by high‐level N additions, and experimental high N applications are likely to overestimate the contribution of N deposition to soil carbon sequestration in a boreal forest.