Simple additive simulation overestimates real influence: altered nitrogen and rainfall modulate the effect of warming on soil carbon fluxes

Effects of future climate and land use changes on runoff in tropical regions of China

Climate change and human activities are the primary drivers influencing changes in runoff dynamics. However, current understanding of future hydrological processes under scenarios of gradual climate change and escalating human activities remains uncertain, particularly in tropical regions affected by deforestation. Based on this, we employed the SWAT model coupled with the near future (2021-2040) and middle future (2041-2060) global climate models (GCMs) under four shared socioeconomic pathways (SSP1-2.6 (SSP1 + RCP2.6), SSP2-4.5 (SSP2 + RCP4.5), SSP3-7.0 (SSP3 + RCP7.0), and SSP5-8.5 (SSP5 + RCP8.5)) from the CMIP6 and the CA-Markov model to evaluate the runoff response to future environmental changes in the Dingan River Basin (DRB). The quantification of the impacts of climate change and land use change on future runoff changes was conducted. The results revealed a non-significant increasing trend in precipitation during the historical period (1999-2018). Furthermore, all three future scenarios (SSP1-2.6, SSP3-7.0, and SSP5-8.5) exhibited an upward trend in precipitation from 2021 to 2060. Notably, the SSP5-8.5 scenario demonstrated a highly significant increase (P < 0.01), while the SSP2-4.5 scenario displayed a non-significant decreasing trend. The future precipitation pattern exhibits a decrease during spring and winter, while showing an increase in summer and autumn. The temperature exhibited a significant increase (P < 0.05) across the four future scenarios, with amplitudes of 0.24 °C/(10 years), 0.36 °C/(10 years), 0.36 °C/(10 years), and 0.50 °C/(10 years) for SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 respectively. The future trend of land use change entails a continuous increase in cultivated land and a corresponding decrease in artificial forest land. By 2032, the area of cultivated land is projected to witness a growth of 4.10%, while artificial forest coverage will experience a decline of 4.45%. Furthermore, by 2046, the extent of cultivated land is anticipated to expand by 4.41%, accompanied by a reduction in artificial forest cover amounting to 5.39%. The average annual runoff during the historical period was 53.35 m³/s, and the Mann-Kendall (MK) trend test showed that it exhibited a non-significant increasing trend. Compared with the historical period, the comprehensive impact of climate change and land use will cause changes in the runoff by 0.49%, 1.98%, - 3.13%, and 3.65% for the scenarios of SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 in the near future, and - 3.24%, 1.30%, - 3.75% and 18.24% in the middle future respectively. The intra-annual variations in future runoff suggest an earlier peak and a more concentrated distribution of runoff during the wet season (May to October). Compared to historical periods, the total runoff in the wet season under SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios increased by 6.53%, 8.91%, 7.17%, and 7.39%, respectively. The research findings offer significant insights into the future hydrological processes in tropical regions, while also serving as a valuable reference for watershed water resource management and disaster control.

Cooling has stimulated soil carbon storage in forest ecosystems

The interactive effect of soil cooling and nitrogen (N) addition can accurately simulate climatic and anthropogenic effects on terrestrial and other land-based ecosystems, but direct empirical measurements on the effects of cooling and N addition on soil carbon (C) and N are lacking. Hence, transplanting soils into colder regions was used to evaluate the effects of cooling and N addition on soil C and N. We used PVCs of 30 cm in height and 8 cm in diameter to extract soil samples. Soil C and N were significantly (P < 0.05) increased by transplanting soils into colder regions. In contrast, cooling has insignificantly (P > 0.05) increased the soil dissolved organic C (DOC) and dissolved organic (DON), but the effect was negatively significant on soil pH compared to the C/N ratio. Similarly, N addition significantly increased the measured soil N stock. However, the effect was negatively significant on soil pH (P < 0.05) compared to the C/N ratio (P > 0.05). Nevertheless, the interaction of cooling and N addition did not affect the soil C and N storage. A similar effect was observed on the soil DOC and DON. The results presented here illustrate that transplanting soils into colder regions and N deposition has perfectly simulated the effects of climate-forcing factors on soil C and N storage in terrestrial and other land-based ecosystems. Accordingly, this study suggests that low temperatures have stimulated the accumulation of the measured soil organic and dissolved properties, but the effect is less consequential when low temperature interacts with N addition in high-elevation areas where ecosystem structures and functions are limited by temperature and may serve as a baseline for future research on land feedbacks to the climate system.

Long-term warming increased carbon sequestration capacity in a humid subtropical forest

Tropical and subtropical forests play a crucial role in global carbon (C) pools, and their responses to warming can significantly impact C-climate feedback and predictions of future global warming. Despite earth system models projecting reductions in land C storage with warming, the magnitude of this response varies greatly between models, particularly in tropical and subtropical regions. Here, we conducted a field ecosystem-level warming experiment in a subtropical forest in southern China, by translocating mesocosms (ecosystem composed of soils and plants) across 600 m elevation gradients with temperature gradients of 2.1°C (moderate warming), to explore the response of ecosystem C dynamics of the subtropical forest to continuous 6-year warming. Compared with the control, the ecosystem C stock decreased by 3.8% under the first year of 2.1°C warming; but increased by 13.4% by the sixth year of 2.1°C warming. The increased ecosystem C stock by the sixth year of warming was mainly attributed to a combination of sustained increased plant C stock due to the maintenance of a high plant growth rate and unchanged soil C stock. The unchanged soil C stock was driven by compensating and offsetting thermal adaptation of soil microorganisms (unresponsive soil respiration and enzyme activity, and more stable microbial community), increased plant C input, and inhibitory C loss (decreased C leaching and inhibited temperature sensitivity of soil respiration) from soil drying. These results suggest that the humid subtropical forest C pool would not necessarily diminish consistently under future long-term warming. We highlight that differential and asynchronous responses of plant and soil C processes over relatively long-term periods should be considered when predicting the effects of climate warming on ecosystem C dynamics of subtropical forests.

Global cross-biome patterns of soil respiration responses to individual and interactive effects of nitrogen addition, altered precipitation, and warming

Anthropogenic activities have increased atmospheric N, precipitation, and temperature events in terrestrial ecosystems globally, with N deposition increasing by 3- to 5-fold during the previous century. Despite decades of scientific research, no consensus has been achieved on the impact of climate conditions on soil respiration (Rs). Here, we reconstructed 110 published studies across diverse biomes, magnitudes, and driving variables to evaluate how Rs responds to N addition, altered precipitation (both enhanced and reduced precipitation or precipitation changes), and warming. Our findings show that N addition significantly increased Rs by 44 % in forests and decreased it by 19 % and 26 % in croplands and grasslands, respectively (P < 0.05). In forests and croplands, altered precipitation significantly increased Rs by 51 % and 17 % (all, P < 0.05), respectively, whereas impacts on grassland were insignificant. In comparison, warming stimulated Rs by 62 % in forests but inhibited it by 10 % in croplands (all, P < 0.05), whereas impacts on grassland were again insignificant. In addition, across all biomes, the responses of Rs to altered precipitation and warming followed a Gaussian response, increasing up to a threshold of 1800 mm and 25 °C, respectively, above which respiration rates decreased with further increases in precipitation and temperature. Our work suggests that the dual interaction of warming × altered precipitation promotes belowground CO₂ emission, thus enhancing global warming. In general, the interactive effect of N addition × altered precipitation decreases Rs. Soil moisture was identified as a primary driver of Rs. Given these findings, we recommend future research on warming vs. changed precipitation to better forecast and understand the interaction between Rs and climate change.

Divergent responses of CO2 and CH4 fluxes to changes in the precipitation regime on the Tibetan Plateau: Evidence from soil enzyme activities and microbial communities

Carbon fluxes (CO₂ and CH₄) are important indicators of the response of alpine meadow ecosystems to global climate change. Alpine meadows on the Qinghai-Tibet Plateau are sensitive to climate change. Although the temporal allocation of precipitation can vary, its intensity is expected to increase, and its frequency is expected to decrease in the future. In this study, a manipulative field experiment was conducted to investigate how carbon fluxes are altered in response to moderate and severe changes in the precipitation regime. Fluctuations in CH₄ flux were large under a severely altered precipitation regime (range of -0.048-0.038 mg m^-2 h^-1). Severe changes in the precipitation regime significantly reduced soil CH₄ uptake by approximately 54.3%. This was probably affected by the decrease in the dissolved organic carbon concentration and changes in the microbial community (mainly Gammaproteobacteria), which were induced by variation in soil water conditions under various precipitation regimes. Under moderate changes in the precipitation regime, the average value of CO₂ fluxes (ecosystem respiration) was 698.21 ± 35.19 mg m^-2 h^-1, which was significantly decreased by 20.7% compared with the control. This likely stems from the suppression of enzyme activity (particularly α-1,4-glucosidase and β-1,4-glucosidase) and the alteration of microbial community structure in this treatment, which led to a decrease in organic matter breakdown and a reduction in the release of CO₂ to the atmosphere. However, CO₂ fluxes were slightly (i.e., not significantly) decreased under the severely altered precipitation regime. Such different responses of CO₂ flux are probably driven by differences in microbial strategies. This study not only increases our understanding of the mechanisms underlying the adaptation of alpine meadow ecosystems to global climate change but also provides new insight into the carbon source/sink functions of alpine meadows.

Differential effects of altered precipitation regimes on soil carbon cycles in arid versus humid terrestrial ecosystems

Changes in precipitation regimes have significant effects on soil carbon (C) cycles; however, these effects may vary in arid versus humid areas. Additionally, the corresponding details of soil C cycles in response to altered precipitation regimes have not been well documented. Here, a meta-analysis was performed using 845 pairwise observations (control vs. increased or decreased precipitation) from 214 published articles to quantify the responses of the input process of exogenous C, the contents of various forms of C in soil, and the soil-atmosphere C fluxes relative to increased or decreased precipitation. The results showed that the effects of altered precipitation regimes did not differ between rainfall and snowfall. Increased precipitation significantly enhanced the soil C inputs, pools and outputs by 18.17%, 18.50%, and 21.04%, respectively, while decreased precipitation led to a significant decline in these soil C parameters by 10.18%, 9.96%, and 17.98%, respectively. The effects of increased precipitation on soil C cycles were more significant in arid areas (where mean annual precipitation, MAP <500 mm), but the effects of decreased precipitation were more significant in humid areas (where MAP ≥500 mm), indicating that the original MAP partially determined the responses of the soil C cycles to altered precipitation regimes. This study implies that for the same of precipitation variation, soil C cycles respond at different magnitudes: not only should the direction (decrease vs. increase) be counted but also the region (arid vs. humid) should be considered. These results deepened our understanding on regional differentiation in soil C cycles under climate change scenarios.

Three-year-period nitrogen additions did not alter soil organic carbon content and lability in soil aggregates in a tropical forest

Soil immobilizes a considerable proportion of carbon (C) as organic matter in terrestrial ecosystems and is thus critical to stabilize the global climate system. Atmospheric nitrogen (N) deposition could influence soil C storage and stabilization, but how N deposition changes soil organic C (SOC) fractions and lability remains elusive. We investigated the effects of 3-year-period N inputs on SOC fractions and lability along three soil depths (0-10, 10-20, and 20-40 cm) in a tropical forest of southern China. Results showed that N additions did not significantly change contents of SOC fractions and the C lability, either in bulk or aggregate-based soils at any of the three depths, and it showed no significant interaction with soil aggregate or soil depth. The SOC content was 43.7 ± 1.5, 18.2 ± 1.0, and 10.7 ± 0.4 mg g^-1 at the three soil layers downwards, with the non-readily oxidizable SOC (NROC) contributing over 70% while the remaining SOC consisting of readily oxidizable SOC at each soil layer. Moreover, contents of SOC and NROC were consistently higher in small soil aggregates, but the C decrement with increasing size of soil aggregates declined along soil profile downwards. This scenario suggests that physical protection of the small soil aggregate is limited, but its greater specific surface area could obviously contribute to the SOC pattern among soil aggregates. These results indicate that the highly developed forests could be resistant to short-term N deposition, even with a high load, to maintain its SOC stabilization.

Interactive global change factors mitigate soil aggregation and carbon change in a semi-arid grassland

The ongoing global change is multi-faceted, but the interactive effects of multiple drivers on the persistence of soil carbon (C) are poorly understood. We examined the effects of warming, reactive nitrogen (N) inputs (12 g N m^-2  year^-1 ) and altered precipitation (+ or - 30% ambient) on soil aggregates and mineral-associated C in a 4 year manipulation experiment with a semi-arid grassland on China's Loess Plateau. Our results showed that in the absence of N inputs, precipitation additions significantly enhanced soil aggregation and promoted the coupling between aggregation and both soil fungal biomass and exchangeable Mg²⁺ . However, N inputs negated the promotional effects of increased precipitation, mainly through suppressing fungal growth and altering soil pH and clay-Mg²⁺ -OC bridging. Warming increased C content in the mineral-associated fraction, likely by increasing inputs of root-derived C, and reducing turnover of existing mineral-associated C due to suppression of fungal growth and soil respiration. Together, our results provide new insights into the potential mechanisms through which multiple global change factors control soil C persistence in arid and semi-arid grasslands. These findings suggest that the interactive effects among global change factors should be incorporated to predict the soil C dynamics under future global change scenarios.

Accumulation of soil carbon under elevated CO2 unaffected by warming and drought

Elevated atmospheric CO₂ concentration and climate change may substantially alter soil carbon (C) dynamics, which in turn may impact future climate through feedback cycles. However, only very few field experiments worldwide have combined elevated CO₂ (eCO₂ ) with both warming and changes in precipitation in order to study the potential combined effects of changes in these fundamental drivers of C cycling in ecosystems. We exposed a temperate heath/grassland to eCO₂ , warming, and drought, in all combinations for 8 years. At the end of the study, soil C stocks were on average 0.927 kg C/m² higher across all treatment combinations with eCO₂ compared to ambient CO₂ treatments (equal to an increase of 0.120 ± 0.043 kg C m^-2  year^-1 ), and showed no sign of slowed accumulation over time. However, if observed pretreatment differences in soil C are taken into account, the annual rate of increase caused by eCO₂ may be as high as 0.177 ± 0.070 kg C m^-2  year^-1 . Furthermore, the response to eCO₂ was not affected by simultaneous exposure to warming and drought. The robust increase in soil C under eCO₂ observed here, even when combined with other climate change factors, suggests that there is continued and strong potential for enhanced soil carbon sequestration in some ecosystems to mitigate increasing atmospheric CO₂ concentrations under future climate conditions. The feedback between land C and climate remains one of the largest sources of uncertainty in future climate projections, yet experimental data under simulated future climate, and especially including combined changes, are still scarce. Globally coordinated and distributed experiments with long-term measurements of changes in soil C in response to the three major climate change-related global changes, eCO₂ , warming, and changes in precipitation patterns, are, therefore, urgently needed.

Negative effects of climate change on upland grassland productivity and carbon fluxes are not attenuated by nitrogen status

Effects of climate change on managed grassland carbon (C) fluxes and biomass production are not well understood. In this study, we investigated the individual and interactive effects of experimental warming (+3 °C above ambient summer daily range of 9-12 °C), supplemental precipitation (333 mm +15%) and drought (333 mm -23%) on plant biomass, microbial biomass C (MBC), net ecosystem exchange (NEE) and dissolved organic C (DOC) flux in soil cores from two upland grasslands of different soil nitrogen (N) status (0.54% and 0.37%) in the UK. After one month of acclimation to ambient summer temperature and precipitation, five replicate cores of each treatment were subjected to three months of experimental warming, drought and supplemental precipitation, based on the projected regional summer climate by the end of the 21st Century, in a fully factorial design. NEE and DOC flux were measured throughout the experimental duration, alongside other environmental variables including soil temperature and moisture. Plant biomass and MBC were determined at the end of the experiment. Results showed that warming plus drought resulted in a significant decline in belowground plant biomass (-29 to -37%), aboveground plant biomass (-35 to -77%) and NEE (-13 to -29%), regardless of the N status of the soil. Supplemental precipitation could not reverse the negative effects of warming on the net ecosystem C uptake and plant biomass production. This was attributed to physiological stress imposed by warming which suggests that future summer climate will reduce the C sink capacity of the grasslands. Due to the low moisture retention observed in this study, and to verify our findings, it is recommended that future experiments aimed at measuring soil C dynamics under climate change should be carried out under field conditions. Longer term experiments are recommended to account for seasonal and annual variability, and adaptive changes in biota.