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Nitrogen (N) deposition-induced soil acidification has become a global problem. However, the response patterns of soil acidification to N addition and the underlying mechanisms remain far from clear. Here, we conducted a meta-analysis of 106 studies to reveal global patterns of soil acidification in responses to N addition. We found that N addition significantly reduced soil pH by 0.26 on average globally. However, the responses of soil pH varied with ecosystem types, N addition rate, N fertilization forms, and experimental durations. Soil pH decreased most in grassland, whereas boreal forest was not observed a decrease to N addition in soil acidification. Soil pH decreased linearly with N addition rates. Addition of urea and NH4NO3 contributed more to soil acidification than NH4-form fertilizer. When experimental duration was longer than 20 years, N addition effects on soil acidification diminished. Environmental factors such as initial soil pH, soil carbon and nitrogen content, precipitation, and temperature all influenced the responses of soil pH. Base cations of Ca2+, Mg2+ and K+ were critical important in buffering against N-induced soil acidification at the early stage. However, N addition has shifted global soils into the Al3+ buffering phase. Overall, this study indicates that acidification in global soils is very sensitive to N deposition, which is greatly modified by biotic and abiotic factors. Global soils are now at a buffering transition from base cations (Ca2+, Mg2+ and K+) to non-base cations (Mn2+ and Al3+). This calls our attention to care about the limitation of base cations and the toxic impact of non-base cations for terrestrial ecosystems with N deposition.

Afforestation has been proposed as an effective method of carbon (C) sequestration; however, the magnitude and direction of soil carbon accumulation following afforestation and its regulation by soil nitrogen (N) dynamics are still not well understood. We synthesized the results from 292 sites and carried out a meta-analysis to evaluate the dynamics of soil C and N stocks following afforestation. Changes in soil C and N stocks were significantly correlated and had a similar temporal pattern. Significant C and N stock increases were found 30 and 50 yr after afforestation, respectively. Before these time points, C and N stocks were either depleted or unchanged. Carbon stock increased following afforestation on cropland and pasture, and in tropical, subtropical and boreal zones. The soil N stock increased in the subtropical zone. The soil C stock increased after afforestation with hardwoods such as Eucalyptus, but did not change after afforestation with softwoods such as pine. Soil N stocks increased and decreased, respectively, after afforestation with hardwoods (excluding Eucalyptus) and pine. These results indicate that soil C and N stocks both increase with time after afforestation, and that C sequestration through afforestation depends on prior land use, climate and the tree species planted.

Widely recognized as a significant carbon sink, North American forests have experienced a history of recovery and are facing an uncertain future. This growing carbon sink is dictated by recovery from land-use change, with growth trajectory modified by environmental change. To address both processes, we compiled a forest inventory dataset from North America to quantify aboveground biomass growth with stand age across forest types and climate gradients. Here we show, the biomass grows from 90 Mg ha –1 (2000–2016) to 105 Mg ha –1 (2020 s), 128 Mg ha –1 (2050 s), and 146 Mg ha –1 (2080 s) under climate change scenarios with no further disturbances. Climate change modifies the forest recovery trajectory to some extent, but the overall growth is limited, showing signs of biomass saturation. The future (2080s) biomass will only sequester at most 22% more carbon than the current level. Given such a strong sink has limited growth potential, our ground-based analysis suggests policy changes to sustain the carbon sink. The recovery of North American forests is likely to impact their capacity as a carbon sink. Here, Zhu et al. show a growth in aboveground biomass in various climate change scenarios, with these forests expected to sequester no more than 22% more carbon than current levels by the 2080s.

With continuous nitrogen (N) enrichment and sulfur (S) deposition, soil acidification has accelerated and become a global environmental issue. However, a full understanding of the general pattern of ecosystem belowground processes in response to soil acidification due to the impacting factors remains elusive. We conducted a meta-analysis of soil acidification impacts on belowground functions using 304 observations from 49 independent studies, mainly including soil cations, soil nutrient, respiration, root and microbial biomass. Our results show that acid addition significantly reduced soil pH by 0.24 on average, with less pH decrease in forest than non-forest ecosystems. The response ratio of soil pH was positively correlated with site precipitation and temperature, but negatively with initial soil pH. Soil base cations (Ca2+, Mg2+, Na+) decreased while non-base cations (Al3+, Fe3+) increased with soil acidification. Soil respiration, fine root biomass, microbial biomass carbon and nitrogen were significantly reduced by 14.7%, 19.1%, 9.6% and 12.1%, respectively, under acid addition. These indicate that soil carbon processes are sensitive to soil acidification. Overall, our meta-analysis suggests a strong negative impact of soil acidification on belowground functions, with the potential to suppress soil carbon emission. It also arouses our attention to the toxic effects of soil ions on terrestrial ecosystems.

Effective soil organic carbon (SOC) management can mitigate the impact of climate warming. However, the response of different SOC fractions to warming in abandoned croplands remains unclear. Here, categorizing SOC into particulate and mineral-associated organic carbon (POC and MAOC) with physical fractionation, we investigate the responses of POC and MAOC content and temperature sensitivity (Q10) to warming through a 3-year in situ warming experiment (+1.6 °C) in abandoned croplands across 12 sites in China (latitude: 22.33–46.58°N). Our results indicate that POC content remains unchanged while MAOC content significantly increases under warming. POC and MAOC content changes are mainly influenced by root biomass and microbial necromass carbon changes, respectively. The Q10 of MAOC is significantly lower than that of POC regardless of the warming or control treatment, suggesting that MAOC represents the most persistent and least vulnerable carbon fraction within SOC. Collectively, the sequestration of stable soil carbon can be enhanced in abandoned croplands under short-term warming.

Global nitrogen (N) deposition generally reduces ecosystem stability. However, less is known about the responses of ecosystem stability and its driving mechanisms under different N addition gradients. We conducted a four-year N addition experiment in an alpine meadow, using six levels of N addition rates (0, 2, 4, 8, 16, 32 g N m⁻² year⁻¹) to examine the effects of N addition on plant community biomass stability and the underlying mechanisms. We found that the stability of ecosystem aboveground net primary productivity (ANPP) decreased linearly with increasing N addition rates, even though it had no effect on plant rates. The most remarkable finding is that the main mechanism underlying ecosystem stability shifted with N addition rates. The decrease of common species stability contributed most to the reduction of plant community biomass stability under low N addition rates (N0–N4), whereas the decrease of species asynchrony contributed most to the reducing plant community biomass stability under high N addition rates (N8–N32). Our results indicate that species diversity was not a significant predictor of plant community biomass stability in this alpine meadow, which challenges the traditional knowledge. This study highlights the shifts of main mechanism regulating plant community biomass stability under different N addition rates, and suggests that continuous nitrogen deposition in the future may reduce ecosystem stability and potentially impeding the sustainable provision of ecosystem functions and services.

The continually increasing nitrogen (N) deposition is expected to increase ecosystem aboveground net primary production (ANPP) until it exceeds plant N demand, causing a nonlinear response and N saturation for ANPP. However, the nonlinear response of ANPP to N addition gradient and the N saturation threshold have not been comprehensively quantified yet for terrestrial ecosystems. In this study, we compiled a global dataset of 44 experimental studies with at least three levels of N treatment. Nitrogen response efficiency (NRE, ANPP response per unit N addition) and the difference in NRE between N levels (ΔNRE) were quantified to test the nonlinearity in ANPP response. We found a universal response pattern of N saturation for ANPP with N addition gradient across all the studies and in different ecosystems. An averaged N saturation threshold for ANPP nonlinearity was found at the N addition rates of 5-6 g m−2 yr−1. The extent to which ANPP approaches N saturation varied with ecosystem type, N addition rate and environmental factors. ANPP in grasslands had lower NRE than those in forests and wetlands. Plant NRE decreased with reduced soil C:N ratio, and was the highest at intermediate levels of rainfall and temperature. These findings suggest that ANPP in grassland or the ecosystems with low soil C:N ratio (or low and high rainfall or temperature) is easier to be saturated with N enrichment. Overall, these results indicate that the beneficial effect of N deposition on plant productivity likely diminishes with continuous N enrichment when N loading surpasses the N saturation threshold for ANPP nonlinearity.

The traditional paradigm is that plant growth at high latitudes is generally nitrogen (N) limited, whereas phosphorus (P) limitation occurs at low latitudes. However, this latitudinal pattern of nutrient limitation is not empirically tested and the underlying mechanisms are far from clear. Here we performed a coordinated experiment of N and/or P addition at three forest sites in China, a subtropical forest, a warm‐temperate forest and a cold‐temperate forest. By measuring relative growth rate (RGR) and leaf nutrient traits among different tree size groups, we assessed how they vary with nutrient addition and tree sizes and uncovered the likely mechanisms underlying these observed responses. Our results revealed that P addition enhanced the RGR of small trees (DBH < 15 cm) by 41% in subtropical forest and 114% in warm‐temperate forest, but reduced it by 57% in cold‐temperate forest. Moreover, small tree RGR increased linearly with soil available P at subtropical and warm‐temperate sites, while it followed a quadratic relationship with soil available N:P ratio at a cold‐temperate site. N addition only affected small tree RGR at the cold‐temperate site. In contrast, the RGR of large trees (DBH > 15 cm) was not impacted by any nutrient addition treatment or soil nutrient variations at any site. Leaf P concentration and resorption efficiency in both small and large trees mostly showed a linear response to soil available P at subtropical and warm‐temperate sites, while leaf N:P ratio in small trees elevated linearly with soil available N:P ratio at a cold‐temperate site. Overall, this study presents robust experimental evidence that growth in small trees, not large trees, is primarily limited by P in subtropical and warm‐temperate forests, but is co‐limited by N and P in cold‐temperate forests. This size‐dependent nutrient limitation highlights the importance of considering tree size classes when assessing nutrient limitation in forest. A plain language summary is available for this article. Plain Language Summary

This article is a Commentary on Knapp et al., 214: 41–47.

Ecosystem gross primary productivity (GPP) is the largest carbon flux between the atmosphere and biosphere and is strongly influenced by soil moisture. However, the response and acclimation of GPP to soil moisture remain poorly understood, leading to large uncertainties in characterizing the impact of soil moisture on GPP in Earth system models. Here we analyze the GPP-soil moisture response curves at 143 sites from the global FLUXNET. We find that GPP at 108 sites exhibits hump-shaped response curves with increasing soil moisture, and an apparent optimum soil moisture ( SM opt GPP , at which GPP reaches the maximum) exists widely with large variability among sites and biomes around the globe. Variation in SM opt GPP is mostly explained by local water availability, with drier ecosystems having lower SM opt GPP than wetter ecosystems, reflecting the water acclimation of SM opt GPP . This acclimation is further supported by a field experiment that only manipulates water and keeps other factors constant, which shows a downward shift in SM opt GPP after long-term water deficit, and thus a lower soil water requirement to maximize GPP. These results provide compelling evidence for the widespread SM opt GPP and its acclimation, shedding new light on understanding and predicting carbon-climate feedbacks. This study provides compelling evidence for the widespread optimum soil moisture for ecosystem photosynthesis and its acclimation to local water conditions, shedding new light on understanding and predicting carbon-climate feedbacks.