CO2 Fertilization Effect Research Articles

Projecting the impact of climate change and elevated CO2 concentration on rice irrigation water requirement in China

Climate change and elevated CO2 concentrations significantly affect rice growth and water consumption. Understanding the specific impacts of climate change and elevated CO2 concentrations on rice physiological phenology, crop water demand (ETC), and irrigation water requirement (IR) is of great significance for the sustainable utilization of water resources and food security. This is particularly true in China, the world's largest rice producer. In this study, with the help of two rice phenological models, the modified Penman-Monteith equation, and the paddy water balance model, we project the changes in rice phenological period, ETC, and IR in four main rice-producing regions of China in the period 2015–2100 based on the 11 GCM outputs. The results show that the rice growing period is shortened in most rice-producing regions, except for the parts of the middle and lower reaches of the Yangtze River. Meanwhile, the trend of ETC and IR of rice varies slightly among regions in the future scenario, with almost all regions decreasing yearly except for the middle and lower reaches of the Yangtze River, where the trend is increasing. The progressively increasing atmospheric CO2 concentration has a “fertilization effect” on the crop, which can reduce the water requirements of rice. In the SSP585 scenario, the “ CO2 fertilization effect” can reduce up to 8.87 × 108 m3 of ETC and 6.94 × 108 m3 of IR in the middle and lower reaches of the Yangtze River in the period of 2090s. This study provides beneficial references to understand the response of rice ETC and IR to future climate change and CO2 concentration elevation in China and highlights that the simulation in terms of crop irrigation must account for the “CO2 fertilization effect”.

VPD modifies CO2 fertilization effect on tomato plants via abscisic acid and jasmonic acid signaling pathways

Atmospheric CO2 concentration is elevated globally, which has “CO2 fertilization effects” and potentially improves plant photosynthesis, yield, and productivity. Despite the beneficial effect of CO2 fertilization being modulated by vapor pressure deficit (VPD), the underlying mechanism is highly uncertain. In the present study, the potential roles of hormones in determining CO2 fertilization effects under contrasting high and low VPD conditions were investigated by integrated physiological and transcriptomic analyses. Beneficial CO2 fertilization effects were offset under high VPD conditions and were constrained by plant water stress and photosynthetic CO2 utilization. High VPD induced a large passive water driving force, which disrupted the water balance and consequently caused plant water deficit. Leaf water potential, turgor pressure, and hydraulic conductance declined under high VPD stress. The physiological evidence combined with transcriptomic analyses demonstrated that abscisic acid (ABA) and jasmonic acid (JA) potentially acted as drought-signaling molecules in response to high VPD stress. Increased foliar ABA and JA content triggered stomatal closure to prevent excessive water loss under high VPD stress, which simultaneously increased the diffusion resistance for CO2 uptake from atmosphere to leaf intercellular space. High VPD also significantly increased mesophyll resistance for CO2 transport from stomatal cavity to fixation site inside chloroplast. The chloroplast “sink” CO2 availability was constrained by stomatal and mesophyll resistance under high VPD stress, despite the atmospheric “source” CO2 concentration being elevated. Thus, ABA- and JA-mediated drought-resistant mechanisms potentially modified the beneficial effect of CO2 fertilization on photosynthesis, plant growth, and yield productivity. This study provides valuable information for improving the utilization efficiency of CO2 fertilization and a better understanding of the physiological processes.

Trend in seasonal amplitude of northern net ecosystem production: Simulated results from IAP DGVM in CAS-ESM2

Net ecosystem production (NEP) over northern latitudes has an obvious seasonal variation, which dominantly drives the seasonal variation in atmospheric CO2. Investigating these carbon processes and understanding the underlying drivers is a key issue of climate research. This study focuses on the seasonal amplitude of northern NEP (40°–90°N) and investigates its trend in the period 1990–2014 by using the dynamic global vegetation model (IAP DGVM) in the second version of the Chinese Academy of Sciences Earth System Model (CAS-ESM2). Basing on a spin-up simulation, the authors conduct a control simulation to evaluate the modeled trend in the seasonal amplitude of northern NEP, and three sensitivity simulations to detect the contributions of the climate and atmospheric CO2. The results show that the modeled seasonal amplitude of northern NEP increases significantly from 1990 to 2014, with a trend of 9.69 TgC month−1 yr−1, which is mainly because of the increasing maximum NEP. The positive trend is largely reduced when CO2 fertilization effects and climatic effects are separately excluded. These significant reductions suggest important effects of atmospheric CO2 and climate change on the seasonal amplitude of northern NEP. Even though model uncertainties remain, the results favor further development of IAP DGVM in accurately simulating the terrestrial carbon cycle, and also provide an important reference for the application of CAS-ESM in further investigating carbon–climate interactions.摘要北方陆地净生态系统生产力(NEP)具有明显的季节变化特征, 这是大气CO2季节变化的关键驱动. 研究这些碳循环过程并理解潜在的驱动因素是气候研究的一个关键问题. 本文利用第二代中国科学院地球系统模式(CAS-ESM2)中的全球植被动态模型(IAP DGVM), 研究了1990−2014年北方NEP(40°−90°N)的季节振幅及其变化趋势. 在初始化试验的基础上, 本文开展了一个控制试验来评估模拟的北方NEP季节幅度的变化趋势, 同时开展了三个敏感性试验来研究气候和大气CO2的贡献. 结果表明: 1990−2014年, 模拟的北方NEP季节振幅显著增加, 趋势为9.69 万吨碳/月/年, 这主要是由于最大NEP增加所致. 当分别排除CO2施肥效应和气候效应时, 上述增加趋势大大减弱.这些显著的减少表明大气CO2和气候变化对北方NEP的季节性振幅有重要影响. 尽管模式存在不确定性, 但这些结果有利于进一步提升IAP DGVM对陆地碳循环的精确模拟, 也为CAS-ESM研究碳-气候相互作用的应用提供了重要参考.

Comparing stem growth of strip-bark and whole-bark growth morphologies in a subarctic conifer (Pinus banksiana), Yellowknife, Northwest Territories

Generally, organisms living near the periphery of their range experience increased environmental stress compared to individuals of the same species growing at the core of their geographic distribution. The greater environmental stress at the ecological limits of a species distribution normally leads to greater sensitivity to growth-limiting factors. For some tree species, this can lead to the development of unique growth morphologies such as strip-bark or partial cambial dieback. Previous studies have concluded that the growth differences induced by CO2 fertilization effects in high elevation strip-bark trees makes them unsuitable for climate reconstructions. However, comparisons of strip-bark and whole-bark growth morphologies at high latitude and low elevation sites are limited. This study compares the growth of strip-bark and whole-bark jack pine (Pinus banksiana Lamb.) trees near Yellowknife, Canada. Annual ring widths from trees of each morphological type were measured, and the results indicate the growth of the two populations are statistically different from one another prior to the late 19th century. After the late 19th century, growth of the two populations remains statistically different, but the average mean growth of strip-bark trees increases 44% after 1891 while average mean growth of whole-bark trees decreases nearly 11%. Climate sensitivity of the two populations was tested using monthly and seasonal climate indices from 1943 to 2015 and results indicate the two populations share similar responses to climate. The results of this research suggest these strip-bark and whole-bark trees share similar climate-growth relations at inter-annual timescales, however, their growth trends diverge at decadal to centennial timescales. The timing of cambial dieback was dated to a short time interval in the early 19th century which coincided with a period of cool summer temperatures across the northern hemisphere. Increased growth in strip-bark trees after ∼1891 could be related to CO2 fertilization effects, although further studies into intrinsic water use efficiency would be helpful in assessing this possibility more accurately.

Recent shift from dominant nitrogen to CO2 fertilization control on the growth of mature Qinghai spruce in China's Qilian Mountains

Terrestrial vegetation growth is stimulated by rising atmospheric CO2 concentration, a warmer climate, and increased soil nutrient availability. However, as plants age, progressive nutrient limitation is known to occur, especially in mature forests where soil nitrogen is deficient. Yet the long-term growth response of mature trees to rising CO2 accompanied by changing climate and nitrogen availability in semi-arid mountain regions is unclear. Here we used tree-ring widths and stable carbon (δ13C) and nitrogen (δ15N) isotopes to investigate the drivers of radial growth of mature Qinghai spruce (Picea crassifolia) in the central Qilian Mountains, northwest China, from 1840 through 2019. Tree growth benefited from improved nitrogen availability, chiefly via changes in bioavailable nitrogen pools modified by a favorable climate during 1930–1964. Enhanced intrinsic water-use efficiency (iWUE), driven by reduced stomatal conductance (gs) related to water deficit, lead to radial growth declines in 1985–2002. Recent acceleration of tree growth was largely attributed to a CO2 fertilization effect through enhanced iWUE during 2003–2019. Nitrogen availability was positively related to tree growth from the 1920s onward until greater CO2 fertilization ensued from 2000 onward. Hence, the negative effects of low nitrogen availability on growth could be mitigated or reversed by a high atmospheric CO2 concentration and warmer climate conditions. Our results suggest that mature spruce forests still harbor potential to increase ecosystem-level carbon sequestration and thereby partially mitigate climate warming. Such a nature-based solution in drought-prone forests would be achieved under warmer-wetter climate conditions in northwest China.

Does drought stress eliminate the benefit of elevated CO2 on soybean yield? Using an improved model to link crop and soil water relations

Crop simulation models are indispensable tools that facilitate studies to assess climate impacts and adaptation responses, but have not been adequately tested in terms of accurately predicting crop growth and water stress responses to high carbon dioxide concentrations [CO2]. The soybean model, GLYCIM, previously modified with a coupled leaf-level gas exchange – energy balance model, was integrated with a two-dimensional convective-diffusive root growth module which linked soil and leaf water potentials with the regulation of stomatal conductance. We evaluated the accuracy of this modified GLYCIM using experimental data from a Free-Air CO2 Enrichment (FACE) site, SoyFACE, and used the calibrated model to quantify current and projected elevated CO2 effects on soybean yields in the United States (US) Midwest. The model accurately matched SoyFACE observed data including canopy temperature (RMSE, 2.26 -2.42 °C; IA, 0.89-0.90), photosynthesis rate (RMSE, 6.33 -12.88 µmol m−2 s−1; IA, 0.82-0.92), seasonal LAI (RMSE, 0.96 -1.08 cm2 cm−2; IA, 0.91-0.93), biomass (RMSE, 2.06 -2.34 Mg ha−1; IA, 0.87-0.90), soil water contents (RMSE, 5.42 -7.82 %; IA, 0.80-0.85) and root length (RMSE, 2.12 -5.64 m plant−1; IA, 0.70-0.86). GLYCIM also reproduced the observed declining response of yield to CO2 fertilization with increasing drought. Using an ensemble of five climate model projections, GLYCIM showed that the CO2 fertilization effect on yield was highest in northern counties and declined in southern and the western counties by late 21st century in response to increased drought characterized by higher daily maximum temperature and decreased rainfall. This study highlights the need to continue to test and improve crop models and use geospatially referenced adaptive strategies to capitalize the benefits from elevated [CO2] to better counteract increasing drought.

Do rice growth and yield respond similarly to abrupt and gradual increase in atmospheric CO2?

Crops have been well studied at abruptly elevated CO2 (e[CO2]). In fact, atmospheric CO2 concentration is rising gradually, but its ecological effect is little known. Thus, rice growth and yield were investigated under gradual e[CO2] (GE) and abrupt e[CO2] (AE) using open-top chambers. Gradual e[CO2] involved an ambient CO2 (a[CO2]) + 40 μmol mol−1 per year in 2016 until a[CO2] + 200 μmol mol−1 in 2020, while AE maintained a[CO2] + 200 μmol mol−1 from 2016 to 2020. We found that steady-state photosynthetic rates responded similarly and increased significantly under GE and AE, however, photosynthetic induction time in dynamic photosynthesis was reduced by AE. Gradual e[CO2] had little effect on biomass before the grain filling stage, while AE significantly stimulated biomass because of the stronger tillering ability and faster photosynthetic induction rate. Neither e[CO2] increased biomass at maturity, however, a significant increase in panicle density was observed under AE. Surprisingly, rice yield was not promoted by both e[CO2], possibly resulting from the reduced carbon assimilation caused by accelerated phenology from grain filling to maturity. These results promote a new understanding of the CO2 fertilization effect with small and slow increases in CO2 concentration, closer to what happens in nature. This may partly challenge the classic view of elevated CO2 fertilization effects from AE.

Large latitudinal difference in soil nitrogen limitation on global vegetation response to elevated CO2

This study is the first to evaluate latitudinal characteristics of the coupled impacts of atmospheric-CO2 fertilization and soil nitrogen limitation on global plant photosynthesis for 2001–2019 by using a remote sensing-based light use efficiency model (DTECM-l-CN). It predicted that the global GPP should have increased by 0.40 Pg C a − 2 (PMK < 0.001) from 2001 to 2019. Leaf area index change contributed 55.0% to this predicted increase in the global GPP trend, followed by coupled CO2 and soil nitrogen effects (32.5%) and the effects from the climate forcing (12.5%). Without CO2 fertilization effects, the global GPP trend was predicted lower by 32.5%. With CO2 fertilization effects included but without a nitrogen constraint, the model would predict global GPP trend higher by 67.5%. These contrasting changes strongly imply the need to consider all these effects simultaneously to estimate future GPP. Comparing the annual mean CO2 fertilization factor with and without soil nitrogen stress showed that the fertilization factor under soil nitrogen stress was only 62.5% of that with no soil nitrogen stress.As a result of soil nitrogen stress, annual mean CO2 response metric (RCO2), indicating the CO2 fertilization effect, declined from 2002 to 2019. Soil nitrogen stress was found to attenuate the positive effect of CO2 fertilization on GPP trend with a large latitudinal difference, i.e., the tropics had a weaker CO2 fertilization effect than the extratropics. This study highlights that soil nitrogen has progressively become limiting on plant response to CO2 fertilization with a large latitudinal divergence.

Increasing sensitivity of dryland water use efficiency to soil water content due to rising atmospheric CO2

Examining the intricate interplay between ecosystem carbon-water coupling and soil moisture sensitivity serves as a crucial approach to effectively assess the dilemma arising from escalating global carbon emissions and concomitant water scarcity. Using the Lund–Potsdam–Jena Dynamic Global Vegetation Model (LPJ), this study investigated the potential effects of climate change and soil water content (SWC) on terrestrial ecosystem water use efficiency (WUE) across China from 1982 to 2060. The results revealed that: (1) WUE was higher in South China and Northeast China, but lower in Northwest China and it had shown a significant upward trend in the past 40 years, especially in Northwest China where grasslands were widely distributed. The increase in WUE was mainly closely related to the greening of vegetation. In the past 40 years, the area of net primary productivity (NPP), evapotranspiration (ET), and WUE showing an upward trend accounted for 85.85 %, 63.66 %, and 83.88 % of the total area of the country, respectively. Although ET also showed an increasing trend nationwide, the increase of NPP was more obvious; (2) The control experiment showed that WUE showed a significant increase trend in arid and semi-arid areas of Northwest China with the increase of CO2 concentration, while SWC showed a significant drying trend, but both WUE and SWC showed an increasing trend in humid areas. The sensitivity of WUE to SWC was enhanced in arid and semi-arid areas, and the effect of soil drought was partially offset by the increase of WUE; (3) Future climate projections also indicated that the CO2 fertilization effect will contribute to an increase in WUE while causing drier soil moisture conditions in the arid and semi-arid regions. Especially under the SSP5-8.5 scenario, CO2 fertilization in Northwest China contributed more than 14 % to WUE from 2015 to 2060, while the impact on SWC depletion exceeded 3 %. This highlights the potential implications of rising atmospheric CO2 concentration, as it may promote a significant rise in WUE and exacerbate the drying of soil moisture in these areas. These findings emphasize the need for careful attention and consideration in managing water resources in arid and semi-arid regions in the face of future climate change.

Carbon Sequestration Potential of Biomass Production along Highways in China.

In response to climate change, China is making great efforts to increase the green area for carbon sequestration. Road verges, as marginal land with favorable conditions for plant growth and ease of transportation, can be used for biomass production, but the biomass production and carbon sequestration potential have not been assessed. Here, we mapped the biomass production potential of road verges in China by combining a biomass model and Geographic Information System and then evaluated the effect of road runoff and CO2 fertilization on the production according to the runoff coefficient and vehicle emission inventory. Nationwide, road verges can produce 15.86 Mt C yr-1 of biomass. Road runoff contributes to a biomass production of 1.26 Mt C yr-1 through increasing soil water availability, which mainly occurs in arid regions. The CO2 fertilization effect by vehicle emission is considerable in Eastern and Southern China, contributing to a production of 0.09 Mt C yr-1. Life cycle assessment shows that major road verges in China have a carbon sequestration potential of 6.87 Mt C yr-1 currently. Our results revealed that road verges can make a significant contribution to carbon neutrality under proper management.

Unevenly distributed CO2 and its impacts on terrestrial carbon uptake under the changing land uses

Changes in land-use structure and pattern can affect both atmospheric CO2 concentrations and the terrestrial carbon budget. To explore the effects of non-uniformly distributed CO2 concentration on terrestrial carbon uptake under land-use changes, this study integrated global CO2 concentrations, Net Primary Productivity (NPP), and land-use data under historical period and SSP1-2.6, SSP2-4.5 and SSP5-8.5 scenarios from 1850 to 2100. Land-use intensity (LUI) and the CO2 correlation to NPP were calculated using partial correlation analysis by controlling LUI. The results showed that NPP growth over the forest was the highest among the land-use types, reaching 0.54 g C·m2, 2.06 g C·m2 and 4.64 g C·m2, respectively, under SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios. Among all the scenarios, the average correlation levels of atmospheric CO2 and NPP considering the LUI effect and controlling LUI ranged respectively from 0.34 to 0.68 and from 0.32 to 0.61 at a 5 % level of significance. It suggested that sensible land use planning might enhance the CO2 fertilization effect and that rises in CO2 concentrations could stimulate terrestrial carbon absorption. The findings add to the body of knowledge about the effects of atmospheric CO2 on terrestrial carbon uptake and serve as a scientific guide for protecting terrestrial carbon stocks and managing land use.

How climate change and international trade will shape the future global soybean security pattern

Soybean planting is highly concentrated and sensitive to climate change, which makes global soybean security vulnerable to production fluctuations in the main producing countries. As an important trade commodity, the impacts of climate change on soybean production in the main producing countries can ripple globally through international trade. However, little is known about the spatial distribution of soybean security under future climate at the global scale. This study couples an ensemble of multi-crop models and a global economic model (GTAP, Global Trade Analysis Project) to evaluate variations in soybean yield of the top ten soybean-producing countries in the 2050s (2046–2055) and the consequent impacts on global soybean supply and price under a range of future climate scenarios. Our results show that (1) The top ten soybean-producing countries, accounting for nearly 97% of global soybean production, suffer an average soybean yield reduction of 0.82% (specific to each country: −5.81%∼7.05%), 0.43% (−23.5%∼11.72%) and 5.22% (−17.88%∼18.73%) under Representative Concentration Pathway (RCP) 2.6, RCP4.5 and RCP 8.5, respectively, when considering the fertilization effect of CO2. A substantial soybean yield loss of 19.51% (−28.86%∼2.97%) is projected under RCP8.5 without accounting for the CO2 fertilization effect. (2) Climate-induced yield reduction leads to insufficient soybean supply and higher price worldwide through the transmission of international trade, with the largest supply decline of 14.20% in Bolivia and the largest price increase of 91.19% in India. Regionally, soybean net-importing countries are more vulnerable because of the ripple effect of the soybean supply chain. For example, despite a simulated increase in soybean yield of 2.97%–18.73%, China's soybean supply will decline by 0.61%–2.27% due to import shortages from the United States and Brazil where severe soybean yield losses are witnessed. (3) Liberal trade does dampen the negative impact of climate change on global soybean supply and price by reducing trade costs and facilitating trade flows, but in soybean export-oriented regions, trade liberalization can result in increased exports at the expense of domestic soybean supply (e.g., Canada and Russia). Our findings highlight the necessity of developing a liberal trade policy that benefits all regions of the world and improving domestic soybean productivity to adapt to climate change.

Relationship between CO2 Fertilization Effects, and Stand Age, Stand Type, and Site Conditions

The CO2 fertilization effect (CFE) plays a crucial role in the amelioration of climate change. Many physiological and environmental factors, such as stand age, stand type, and site conditions, may affect the extent of the CFE. However, the relationship between the CFE and these factors remains elusive. In this study, we used the emerging gross primary production (GPP) remote sensing products, with GPP predicted using eddy covariance–light use efficiency models (EC-LUE GPP) based on satellite near-infrared reflectance of vegetation (NIRv GPP) and assessed with a random forest model to explore the CFE trends with stand age in a coniferous forest and a broad-leaved forest in Heilongjiang Province, China. We additionally compared the differences among the CFEs under different site conditions. The CFEs in coniferous forests and broad-leaved forests both showed a rapid increase in stands of 10 to 20 years of age, followed by a decline after reaching a maximum, with the rate of decline reducing with age. Eventually, CFE remained stable in stands near 100 years of age. However, the CFE in coniferous forests exhibited more extended periods of rapid increase and a higher maximum than in broad-leaved forests. Moreover, in this study, we used the site class index (SCI) to grade site conditions. The results demonstrate that the CFE differed significantly under different levels of site conditions, and these differences gradually decreased with age. The site with the highest SCI had fewer environmental restrictions on the CFE, and consequently, the CFE rate of decline was faster. Our results are of significance in understanding the CFE and adapting to future changes in atmospheric CO2 concentration.

Using tree rings to detect a CO2 fertilization effect: a global review

Using tree rings to detect a CO2 fertilization effect: a global review

Effects of elevated CO2 concentration and experimental warming on morphological, physiological, and biochemical responses of winter wheat under soil water deficiency.

Global climate change and freshwater scarcity have become two major environmental issues that constrain the sustainable development of the world economy. Climate warming caused by increasing atmospheric CO2 concentration can change global/regional rainfall patterns, leading to uneven global seasonal precipitation distribution and frequent regional extreme drought events, resulting in a drastic reduction of available water resources during the critical crop reproduction period, thus causing many important food-producing regions to face severe water deficiency problems. Understanding the potential processes and mechanisms of crops in response to elevated CO2 concentration and temperature under soil water deficiency may further shed lights on the potential risks of climate change on the primary productivity and grain yield of agriculture. We examined the effects of elevated CO2 concentration (e[CO2]) and temperature (experimental warming) on plant biomass and leaf area, stomatal morphology and distribution, leaf gas exchange and mesophyll anatomy, rubisco activity and gene expression level of winter wheat grown at soil water deficiency with environmental growth chambers. We found that e[CO2] × water × warming sharply reduced plant biomass by 57% and leaf photosynthesis (P n) 50%, although elevated [CO2] could alleviated the stress from water × warming at the amount of gene expression in RbcL3 (128%) and RbcS2 (215%). At ambient [CO2], the combined stress of warming and water deficiency resulted in a significant decrease in biomass (52%), leaf area (50%), P n (71%), and G s (90%) of winter wheat. Furthermore, the total nonstructural carbohydrates were accumulated 10% and 27% and increased R d by 127% and 99% when subjected to water × warming and e[CO2] × water × warming. These results suggest that water × warming may cause irreversible damage in winter wheat and thus the effect of "CO2 fertilization effect" may be overestimated by the current process-based ecological model.

Greening rate in North Korea doubles South Korea

South and North Korea have experienced contrasting economic developments since the 1950s while sharing similar climates. Previous studies revealed an overall long-term greening trend across the Korean Peninsula based on greenness data from coarse-resolution satellite images. However, there has been no comprehensive comparison of the greenness patterns and the driving mechanisms between two countries due to the limitations of coarse-resolution satellite data. Here, we performed cross-calibration among Landsat sensors and adopted a phenology-based approach to generate Landsat annual maximum Normalized Difference Vegetation Index (NDVImax) time series for each pixel from 1986 to 2017. We found that over 1986–2017, the greening rate in North Korea was almost twice that of South Korea. Cropland in South Korea is the main source of the greening discrepancy. The expansion of agricultural facilities in the stable cropland area and urbanization in the cropland loss area of South Korea contributed 57% to the significant negative NDVImax trend, which was dominant over the forest NDVImax increase resulting from rising temperatures, CO2 fertilization effects and afforestation projects. However, in North Korea, CO2 fertilization effects in the stable cropland area and transition from grassland to cropland promoted an increase in NDVImax, despite decreasing NDVImax in forest areas due to deforestation. Our results highlight the need for delineating fine-scale land-use changes to advance our understanding of regional vegetation dynamics.

Unveiling the driver behind China’s greening trend: urban vs. rural areas

Urban and rural areas play an important role in the greenness change in China, despite exhibiting divergent landscape ecologies. Although recent studies have revealed an overall greening pattern in China, the relative contribution of urban and rural vegetation to nationwide greening trend and their driving mechanisms behind these changes remain poorly understood. Here, we first utilized a high-resolution land use/cover dataset (GlobeLand30) to establish a framework for distinguishing between urban and rural areas. We then assessed and compared the greenness changes in both urban and rural areas using multiple vegetation indices from 2000 to 2020. By employing Random Forest model and generalized linear model regression, we further investigated drivers behind the changes in urban and rural vegetation trends. Our results demonstrated a significant greening trend in China, and the greenness increased 13.71% from 2000 to 2020. Vegetation changes in both urban (+4.96%, 0.0011 yr−1) and rural areas (+14.25%, 0.0026 yr−1) have contributed positively to China’s greening trend, with their contribution being 11.3% and 88.7%, respectively. Urban core areas exhibited the largest trend magnitudes (0.0043 ± 0.0035 yr−1) among all the urban–rural subregions. Increased tree cover was identified as the primary driver of greening trends in both urban and rural areas, explaining 36% and 29% of the greening, respectively. However, the pathways of tree cover increase differed between urban and rural areas, with urban areas focusing on green space construction and rural areas implementing afforestation programs. In contrast, climate change and the CO2 fertilization effect had a greater contribution to the greening trend in rural areas than in urban areas. Our study demonstrates the positive role played by both urban and rural areas in China’s greening trends and elucidates the underlying mechanisms driving these changes, highlighting the need for differentiated strategies in urban and rural areas for future vegetation restoration.

Divergent trends in grassland degradation and desertification under land use and climate change in Central Asia from 2000 to 2020

Grassland degradation (within-state change) and desertification (state conversion from grasslands to deserts) are different states in the complex dynamic process of grassland deterioration. However, the dynamics and associated drivers of the two states have been rarely examined separately, which is critical for early warning and conservation policy making. Here, the grassland degradation and desertification in Central Asia (CA) were studied due to their evident vulnerability to climate extremes and human activities. We performed a two-step approach to identify the critical state zones first (i.e., grasslands, deserts, and the desertification zones) and then assess the vegetation dynamics within each zone. Finally, we quantified the roles of CO2 fertilization effect, climate change, climate variability, land use (LU), and anthropogenic climate change (ACC) on the vegetation dynamics in different state zones. The results showed that the areas of grasslands and sparse vegetation regions (including deserts and desertification zones) were stable from 2000 to 2020, but the trends of vegetation greenness were divergent with a reduction in grasslands and an increase in sparse vegetation regions. Furthermore, climate change and climate variability were the main driving force affecting grassland degradation in CA. Yet the vegetation greening in the desertification and desert zones was mainly driven by the rising CO2 and LU. Moreover, ACC degraded 20.63% of grasslands and sparsely vegetated lands in CA on the baseline of 2000. The results highlighted the risks of grassland degradation in CA. This study proposed a methodology to examine grassland degradation and desertification in detail and quantify the associated driving factors (especially ACC) at the pixel scale, which provided some insights on developing region-precise strategies for grassland conservation in CA and other drylands.

Modelling the terrestrial nitrogen and phosphorus cycle in the UVic ESCM

Abstract. Nitrogen (N) and phosphorus (P) biogeochemical dynamics are crucial for the regulation of the terrestrial carbon cycle. In Earth system models (ESMs) the implementation of nutrient limitations has been shown to improve the carbon cycle feedback representation and, hence, the fidelity of the response of land to simulated atmospheric CO2 rise. Here we aimed to implement a terrestrial N and P cycle in an Earth system model of intermediate complexity to improve projections of future CO2 fertilization feedbacks. The N cycle is an improved version of the Wania et al. (2012) N module, with enforcement of N mass conservation and the merger with a deep land-surface and wetland module that allows for the estimation of N2O and NO fluxes. The N cycle module estimates fluxes from three organic (litter, soil organic matter and vegetation) and two inorganic (NH4+ and NO3-) pools and accounts for inputs from biological N fixation and N deposition. The P cycle module contains the same organic pools with one inorganic P pool; it estimates influx of P from rock weathering and losses from leaching and occlusion. Two historical simulations are carried out for the different nutrient limitation setups of the model: carbon and nitrogen (CN), as well as carbon, nitrogen and phosphorus (CNP), with a baseline carbon-only simulation. The improved N cycle module now conserves mass, and the added fluxes (NO and N2O), along with the N and P pools, are within the range of other studies and literature. For the years 2001–2015 the nutrient limitation resulted in a reduction of gross primary productivity (GPP) from the carbon-only value of 143 to 130 Pg C yr−1 in the CN version and 127 Pg C yr−1 in the CNP version. This implies that the model efficiently represents a nutrient limitation over the CO2 fertilization effect. CNP simulation resulted in a reduction of 11 % of the mean GPP and a reduction of 23 % of the vegetation biomass compared to the baseline C simulation. These results are in better agreement with observations, particularly in tropical regions where P limitation is known to be important. In summary, the implementation of the N and P cycle has successfully enforced a nutrient limitation in the terrestrial system, which has now reduced the primary productivity and the capacity of land to take up atmospheric carbon, better matching observations.

Global evaluation of terrestrial biogeochemistry in the Energy Exascale Earth System Model (E3SM) and the role of the phosphorus cycle in the historical terrestrial carbon balance

Abstract. The importance of carbon (C)–nutrient interactions to the prediction of future C uptake has long been recognized. The Energy Exascale Earth System Model (E3SM) land model (ELM) version 1 is one of the few land surface models that include both N and P cycling and limitation (ELMv1-CNP). Here we provide a global-scale evaluation of ELMv1-CNP using the International Land Model Benchmarking (ILAMB) system. We show that ELMv1-CNP produces realistic estimates of present-day carbon pools and fluxes. Compared to simulations with optimal P availability, simulations with ELMv1-CNP produce better performance, particularly for simulated biomass, leaf area index (LAI), and global net C balance. We also show ELMv1-CNP-simulated N and P cycling is in good agreement with data-driven estimates. We compared the ELMv1-CNP-simulated response to CO2 enrichment with meta-analysis of observations from similar manipulation experiments. We show that ELMv1-CNP is able to capture the field-observed responses for photosynthesis, growth, and LAI. We investigated the role of P limitation in the historical balance and show that global C sources and sinks are significantly affected by P limitation, as the historical CO2 fertilization effect was reduced by 20 % and C emission due to land use and land cover change was 11 % lower when P limitation was considered. Our simulations suggest that the introduction of P cycle dynamics and C–N–P coupling will likely have substantial consequences for projections of future C uptake.