Carbon Cycling In Terrestrial Ecosystems Research Articles

Nitrogen addition mediates the effect of soil microbial diversity on microbial carbon use efficiency under long‐term tillage practices

AbstractTillage practices can influence soil microbial carbon use efficiency (CUE), which is critical for carbon cycling in terrestrial ecosystems. The effect of tillage practices could also be regulated by nitrogen (N) addition. However, the soil microbial mechanism relating to N fertilizer effect on microbial CUE under no‐tillage (zero‐tillage) is still unclear. We investigated how N fertilizer regulates the effect of tillage management on microbial CUE through changing microbial properties and further assessed the impact of microbial CUE on particulate (POC) and mineral‐associated organic matter carbon (MAOC). For this we used a 16‐year field experiment with no‐tillage (NT) and conventional tillage (CT), both of which combined with 105 (N1), 180 (N2), and 210 kg N ha−1 (N3) N application. We found that microbial CUE increased with increasing N application rate. NT increased microbial CUE compared with CT in the 0–10 cm. The bacterial and fungal diversities of NT were higher than CT and N application decreased their diversities in 0–10 cm. The partial least squares path model showed that bacterial and fungal diversity had a significant influence on microbial CUE. Furthermore, POC and MAOC under NT were higher than CT and they also increased with increasing N application rate. It suggested that increasing microbial CUE induced by N application had the potential to increase POC and MAOC. Overall, this study highlights that N addition can alter the effect of soil microbial diversity on CUE, which further improves our understanding to explain and predict the fractions of SOC (i.e., POC and MAOC) in tillage systems.

Response of Land Use and Net Primary Productivity to Coal Mining: A Case Study of Huainan City and Its Mining Areas

The terrestrial ecosystem carbon cycle is essential to the global carbon cycle. Mining activities have seriously damaged the terrestrial ecosystem and destroyed the carbon sequestration ability of vegetation, which is of great significance to studying the effect of coal mining on land structure change and carbon sink function in cities and mining areas. However, the existing research lacks the targeted analysis of the carbon sink level of the mining area combined with the mining data. Based on the coal-mining information, land-use data, and MODIS NPP data, this study analyzed the spatio-temporal change characteristics of land use and NPP in Huainan City and its mining areas from 2001 to 2020. The results showed that: (1) 22.5% of the land types in the mining area have changed, much higher than 3.2% in Huainan; 40.08 km2 of the cropland in the mining area has been transformed into waterbodies, seriously affecting regional food security. (2) NPP fluctuates with rainfall, has a weak correlation with temperature, and is restricted by coal-mining factors. The average NPP of most coal mines is significantly lower than that of non-mining areas. The NPP of Huainan City showed an overall growth trend of 2.20 g/(m2 × a), which was much higher than the average value of 0.43 g/(m2 × a) in the mining area. Especially in the Guqiao mine, the difference in NPPslope before and after mining was as high as 16.92 g/(m2 × a). (3) The probability integral method was used to estimate that 195.16 km2 of land in Huainan would be damaged by mining in 2020. The distribution of damage degree was negatively correlated with NPPslope, which meant the more serious the damage was, the less NPPslope was. This study revealed the characteristics of land-use change and NPP spatio-temporal response in resource-based cities and mining-disturbed areas. It quantitatively estimated the impact of mining activities on regional carbon sink function. It can provide theory and data support for mining areas to carry out ecological protection and restoration, improve the environmental service function of resource-based cities, and formulate sustainable development strategies.

Effects of global change and human disturbance on soil carbon cycling in boreal forest: A review

Increasing human demands for Earth's resources are hastening many environmental changes and creating a need to incorporate the routine monitoring of ecosystem functions into forest management. Under global change and anthropogenic disturbances, soil carbon (C) cycling in terrestrial ecosystems is undergoing substantial changes that result in the transformation between soil C sources and sinks. Therefore, the forest C budget requires an understanding of the underlying soil C dynamic under environmental disturbances. The present review focuses on the response and feedback of soil C cycling to global change (climate warming and nitrogen (N) deposition) and human disturbances (fire and logging) and detects the association of soil C cycling with soil C and N efflux and inflow in boreal forests. The effects of climate warming and N deposition on soil C cycling are complex, especially at short-term temporal scales. Climate warming can decay soil organic matter (SOM) to emit substantial amounts of CO2, and differing warming durations result in different effects on soil C loss, ranging from ca. 1 to 15 Mg C ha–1. Short-term soil warming mainly reduces the labile soil C pool and increases the decomposition of recalcitrant soil C compounds (e.g., lignin), whereas longer-term warming may limit soil C loss due to impoverished soil C substrate and microbial communities. Moderate N addition is conducive to enhancing soil C storage (ca. 2–22 Mg C ha–1), by increasing plant productivity including above- and belowground biomass; however, chronic N deposition or excess N addition can result in soil acidity, reducing N use efficiency and plant growth and further resulting in no changes or declines in soil C pool. Fire and logging lead to a large quantity of soil C loss via impaired plant productivity and increased organic matter degradation, exacerbating global warming. In particular, severe fire can cause a large amount of soil C loss, ca. 16–34 Mg C ha–1 in the data we reviewed. Meanwhile, the black C input induced by fire and the plant residual C input from the roots of logged trees can increase the proportion of recalcitrant soil C and enhance the stability of soil C pool. We also highlight the positive feedback of forest restoration to soil C storage after fire and logging disturbances, indicating that effective forest restoration projects (e.g., afforestation and natural forest recovery) are necessary to sequester soil C belowground. Additionally, combined with microbial technologies and metagenomics- and metabolomics-based approaches, soil microorganisms are proved crucial for driving soil C cycling via C capture and the N recycling of plants and soil. We, therefore, suggest that clarifying the relationship among plant, SOM, and microorganisms is essential to better evaluate soil C cycling and to predict how boreal forests respond to global change and human disturbances. Further work is needed to assess long-term soil C feedback from high-latitude forests to broader regions.

Calibration and validation of phenological models for Biome-BGCMuSo in the grasslands of Tibetan Plateau using remote sensing data

Modeling vegetation phenology is crucial to assessing how climate change impacts carbon cycles in terrestrial ecosystems. The process-based biogeochemical model Biome-BGCMuSo is widely used for simulating carbon and water storages and fluxes of grassland ecosystems. However, the lack of accurate phenological information, such as the start of the growing season (SOS), impedes better simulations of the biogeochemical processes in the Tibetan Plateau (TP). Here, based on the snow-free satellite-derived SOS and the end of the growing season (EOS) in the TP during 1982–2018, we calibrated and validated three phenological models for SOS (i.e., the Biome-BGC phenological (BBGC) model, the heatsum growing season index (HSGSI) model, and the alpine meadow prognostic phenological (AMPP) model) and five phenological models for EOS (i.e., BBGC, HSGSI, AMPP, the low temperature and photoperiod multiplicative model induced by photoperiod (TPMP) and temperature (TPMT)) using particle swarm optimization (PSO) algorithm. For SOS, all three phenological models with calibrated parameters performed similarly and all captured the change in SOS well along gradients of aridity. The performance of BBGC, HSGSI, and AMPP models were largely improved with the calibration. The AMPP model simulated SOS with the lowest estimation errors with the mean absolute error (MAE) of 18.67 days and the Kling-Gupta efficiency (KGE) of 0.47 in validation. For EOS, the calibrated HSGSI and AMPP models, with mean MAEs of 9.85 and 9.29 days, respectively, captured the change in EOS well along the gradients of aridity and performed better than other models. The calibration significantly improved the simulation performance of all five models. Therefore, the phenological models can be calibrated and validated at a large scale with snow-free satellite-derived phenological data. Our study recommends that calibration and validation for the phenological model play a vital role in accurately simulating SOS and EOS in the regional carbon cycle simulation.

Litter Mixing Alters Microbial Decomposer Community to Accelerate Tomato Root Litter Decomposition.

ABSTRACTMixing plant litters of multiple species can alter litter decomposition, a key driver of carbon and nutrient cycling in terrestrial ecosystems. Changes in microbial decomposer communities is proposed as one of the mechanisms explaining this litter-mixture effect, but the underlying mechanism is unclear. In a microcosm litterbag experiment, we found that, at the early stage of decomposition, litter mixing promoted tomato root litter decomposition, thus generating a synergistic nonadditive litter-mixture effect. The transplanting decomposer community experiment showed that changes in microbial decomposer communities contributed to the nonadditive litter-mixture effect on tomato root litter decomposition. Moreover, litter mixing altered the abundance and diversity of bacterial and fungal communities on tomato root litter. Litter mixing also stimulated several putative keystone operational taxonomic units (OTUs) in the microbial correlation network, such as Fusarium sp. fOTU761 and Microbacterium sp. bOTU6632. Then, we isolated and cultured representative isolates of these two taxa, named Fusarium sp. F13 and Microbacterium sp. B26. Subsequent in vitro tests found that F13, but not B26, had strong decomposing ability; moreover, these two isolates developed synergistic interaction, thus promoted litter decomposition in coculture. Addition of F13 or B26 both promoted the decomposing activity of the resident decomposer community on tomato root litter, confirming their importance for litter decomposition. Overall, litter mixing promoted tomato root litter decomposition through altering microbial decomposers, especially through stimulating certain putative keystone taxa.IMPORTANCE Microbial decomposer community plays a key role in litter decomposition, which is an important regulator of soil carbon and nutrient cycling. Though changes in decomposer communities has been proposed as one of the potential underlying mechanisms driving the litter-mixture effects, direct evidence is still lacking. Here, we demonstrated that litter mixing stimulated litter decomposition through altering microbial decomposers at the early stage of decomposition. Moreover, certain putative keystone taxa stimulated by litter mixing contributed to the nonadditive litter-mixture effect. In vitro culturing validated the role of these taxa in litter decomposition. This study also highlights the possibility of regulating litter decomposition through manipulating certain microbial taxa.

Adaptation of soil micro-food web to elemental limitation: evidence from the forest-steppe ecotone

Stoichiometric imbalance between resources and their decomposers can alter the activity and structure of soil microbial communities and play an important role in regulating nutrient and carbon cycling in terrestrial ecosystems. However, whether and how ecological networks of soil micro-food web change to cope with the stoichiometric imbalance has never been assessed so far. In this study, we tested how the soil micro-food web responded to elemental limitations along a forest-steppe ecotone. We measured several adaptation mechanisms including soil microbial stoichiometry, enzyme activities and the composition of soil microbial and nematode communities. The microbial investment in resource acquisition shifted from nutrient-to C-acquiring enzymes with decreasing soil C:N:P ratios along the forest-steppe ecotone. The shifts in element use efficiencies could be a compensatory way besides enzyme allocation, implemented by microbial communities to cope with stoichiometric imbalance in their substrates. The community structure of soil micro-food web also changed with a decrease in saprotrophic fungi and fungivorous nematodes from the steppe plots towards the forest plots. The co-occurrence networks were less complex and stable with decreasing soil C:N:P ratios, suggesting that trophic interactions were less diverse and C-limitation plays an important role in structuring ecological interactions in soil food webs. Our findings provide insights into how stoichiometric constraints may induce shifts in ecological networks and have important implications for integrating shifts in individual physiological metabolism as well as changes in community composition of soil biota and to better understand and predict soil biogeochemical cycling in terrestrial ecosystems.

Plant–microbial linkages regulate soil organic carbon dynamics under phosphorus application in a typical temperate grassland in northern China

Anthropogenic activities have increased ecosystem phosphorus (P) inputs and have affected terrestrial ecosystem carbon (C) cycles. However, the fate of soil organic C (SOC) under P addition remains elusive, and the potential mechanisms underlying plant-microbial linkages mediated SOC formation and decomposition are poorly understood. Here, we conducted a field P fertilization experiment to explore the effects on SOC dynamics in a typical temperate grassland in northern China. P addition increased soil P availability and pH, thereby stimulating plant nutrient uptake and growth, leading to higher C inputs to soils via plant biomass (shoots, litter and roots). However, P addition exacerbated microbial N limitation, altered microbial community composition (with a lower fungal to bacterial ratio), shifted microbial life-history strategies, which transitioned from the K-strategy (Acidobacteria, Chloroflexi, and Verrucomicrobia dominant) to the r- strategy (Proteobacteria, Bacteroidetes, and Firmicutes dominant), and modulated their functional characteristics. All of these factors regulated microbial substrate preferences and changed the C components that undergo decomposition. These changes ultimately accelerated the utilization of active C, hampered passive C decomposition, and helped to create a longer-term stable C sink in the grassland.

Assessing spatiotemporal sources of biogenic and anthropogenic sedimentary organic matter from the mainstream Haihe River, China: Using n-alkanes as indicators

Sedimentary organic matter (SOM) plays an important role in the transportation and transformation of various pollutants and the carbon cycle in aquatic and terrestrial ecosystems, especially for seagoing rivers. However, few studies have focused on the sources and factors of SOM in rivers under the significant pressure of high urbanization and industrialization. In this study, we adopted the molecular markers of n-alkanes and their proxies in the mainstream Haihe River to reveal the spatiotemporal distributions and biogenic and anthropogenic sources of SOM. The concentrations of Σ29n-alkanes, Σbiogenicn-alkanes, and Σanthropogenicn-alkanes were 4985.6 (127.5–26,296.6), 1872.1 (38.1–9216.9), and 3113.5 (89.4–18,259.7) ng·g−1 dw (dry weight), respectively. Hybrid sources of n-alkanes were found in this study. The composition distribution and proxies of n-alkanes showed that woody and herbaceous plants are the main sources of biogenic SOM, while incomplete fossil fuel burning and heavy oil emissions served as the main contributors to anthropogenic SOM in the mainstream Haihe River, especially through industrial activities. The average chain length of biogenic n-alkanes (ACLbio) was verified to quantify the relative contributions of biogenic sources of SOM and proxies; the average chain length of anthropogenic n-alkanes (ACLanthro), and the ratio of unit short‑carbon to unit long‑carbon anthropogenic n-alkanes (L/H) were verified to quantify the relative contributions of anthropogenic sources of SOM in the river system. Impacts from sedimentary geochemistry (such as total organic carbon (TOC) and grain size of sediments) on n-alkanes were explored, and the correlations of Σ29n-alkanes with TOC and grain size of the river sediment indicated that terrestrial organic matter was the main source of SOM, while emissions from incomplete combustion and intensive manufacturer activities should also not be neglected.

The Relationship between Soil Respiration and Plant Community Functional Traits in Ebinur Lake Basin

Soil respiration (Rs) plays an important role in the carbon cycle of terrestrial ecosystems. Understanding the impacts of plant community functional traits on Rs is a key prerequisite for accurate prediction of the future carbon balance of terrestrial ecosystems under climate change. This study examined the relationship of Rs with plants in the Ebinur Lake Basin in the arid desert region. Traditional statistical methods and geostatistical methods were used to study the spatial variation characteristics of Rs and to analyze the effects of plant community functional traits and environmental factors on the spatial heterogeneity of Rs. The variation in Rs in the arid desert area of the Ebinur Lake Basin showed a strong spatial dependence (C0/(C + C0) = 0.11) and a medium variation (α = 25.50, CV = 86.21%). Principal component analysis revealed that morphological traits of plants and soil water content had great contributions to PC1, soil nutrient had great contributions to PC2, and physiological traits of plants and soil temperature had large contributions to PC3. Multiple regression analysis showed that PC1, PC2, and PC3 can explain 83% of the spatial heterogeneity of Rs (F = 157.41, p < 0.01). In conclusion, maximum height, leaf width, leaf dry matter content, leaf thickness, and soil water content were the principal driving elements of soil respiration heterogeneity.

Microbial properties in tropical montane forest soils developed from contrasting parent material—An incubation experiment

AbstractBackgroundSoil microbes are key drivers of carbon (C) and nutrient cycling in terrestrial ecosystems, and their properties are influenced by the relationship between resource demand and availability.AimsOur objective was to investigate patterns of microbial properties and their controls to understand whether they differ between soils derived from geochemically contrasting parent material in tropical montane forests.MethodsWe measured microbial biomass C (MBCSoil), potential extracellular enzyme activity (pEEA), and assessed microbial investments in C and nutrient acquisition at the beginning and end of a 120‐day laboratory incubation experiment using soils developed from three geochemically contrasting parent material (i.e., mafic, mixed sediment, and felsic) and three soil depths (0–70 cm).ResultsWe found that MBCSoil and pEEA were highest in soils developed from the mafic parent material. Microbial investment in C acquisition was highest in soils developed from mixed sedimentary rocks and lowest in soils developed from the felsic parent material. We propose that our findings are related to the strength of contrasting mineral‐related C stabilization mechanisms and varying C quality. No predominant microbial investment in nitrogen (N) acquisition was observed, whereas investment in phosphorus (P) acquisition was highest in subsoils. We found lower microbial investment in C acquisition in subsoils indicating relatively high C availability, and that microbes in subsoils can substantially participate in C cycling and limit C storage if moisture and oxygen conditions are suitable. Geochemical soil properties and substrate quality were important controls on MBCSoil per unit soil organic C (MBCSOC), particularly after the exhaustion of labile and fast cycling C, that is, at the end of the incubation.ConclusionAlthough a laboratory incubation experiment cannot reflect real‐world conditions, it allowed us to understand how soil properties affect microbial properties. We conclude that parent material is an important driver of microbial properties in tropical montane forests despite the advanced weathering degree of soils.

Responses of soil respiration to the interaction of rainfall changes and nitrogen deposition: A review

Soil respiration (Rs), as a key process of carbon cycle in terrestrial ecosystems, has a direct impact on atmospheric CO2 concentration. How Rs responds to global change factors, such as rainfall changes and increased N deposition, has become a hot and difficult issue in the field of global change. Compared with the responses of Rs to the single factor of rainfall changes or increased N deposition, studying the response of Rs to the interaction of these two factors is more in line with natural environment, which can predict the future changes of soil carbon emission more accurately. At present, the related researches focused on different terrestrial ecosystems all over the world, and revealed the response mechanism from three aspects: soil, microorganism, and plant. Here, the research progress of soil respiration in response to the interaction of rainfall changes and increased N deposition in different terrestrial ecosystems was reviewed from the aspects of Rs and its components, factors related with soil properties, microorganisms and plant, and the deficiencies of current researches, and the research direction to be strengthened in the future were pointed out. Our review would provide a reference for further understanding the response law and the mechanism of soil respiration to the interaction between rainfall changes and increased N deposition.

Using an object-based machine learning ensemble approach to upscale evapotranspiration measured from eddy covariance towers in a subtropical wetland

Accurate prediction of evapotranspiration (ET) in wetlands is critical for understanding the coupling effects of water, carbon, and energy cycles in terrestrial ecosystems. Multiple years of eddy covariance (EC) tower ET measurements at five representative wetland ecosystems in the subtropical Big Cypress National Preserve (BCNP), Florida (USA) provide a unique opportunity to assess the performance of the Moderate Resolution Imaging Spectroradiometer (MODIS) ET operational product MOD16A2 and upscale tower measured ET to generate local/regional wetland ET maps. We developed an object-based machine learning ensemble approach to evaluate and map wetland ET by linking tower measured ET with key predictors from MODIS products and meteorological variables. The results showed MOD16A2 had poor performance in characterizing ET patterns and was unsatisfactory for estimating ET over four wetland communities where Nash-Sutcliffe model Efficiency (NSE) was less than 0.5. In contrast, the site-specific machine learning ensemble model had a high predictive power with a NSE larger than 0.75 across all EC sites. We mapped the ET rate for two distinctive seasons and quantified the prediction diversity to identify regions easier or more challenging to estimate from model-based analyses. An integration of MODIS products and other datasets through the machine learning upscaling paradigm is a promising tool for local wetland ET mapping to guide regional water resource management.

Transient Flooding and Soil Covering Interfere with Decomposition Dynamics of Populus euphratica Leaf Litter: Changes of Mass Loss and Stoichiometry of C, N, P, and K

Litter decomposition plays a critical role in carbon and nutrient cycling in terrestrial and aquatic ecosystems. However, the effects transient flooding and soil covering have on leaf litter decomposition remain unclear. The changes of litter mass loss and stoichiometric ratio of C:N (the ratio of carbon to nitrogen), C:K (the ratio of carbon to potassium), C:P (the ratio of carbon to phosphorus), N:P (the ratio of nitrogen to phosphorus), and N:K (the ratio of nitrogen to potassium) of fresh Populus euphratica (P. euphratica) leaves in surface, transient flooding, and soil covering treatments were studied using litterbags in a desert riparian forest in a field decomposition experiment for a period of 640 d. The results showed that there was a significant influence of disturbance type and incubation time on litter mass loss rate and stoichiometric ratios of C:N, C:K, C:P, N:P, and N:K of fresh P. euphratica leaves, but no significant influence of the interaction between disturbance type and incubation time on leaf litter mass loss. In three treatments, five sequential phases of leaf litter mass loss rate and different temporal change patterns of stoichiometric ratio were identified within 640 d. Transient flooding was shown to affect P. euphratica leaf litter mass loss phases compared to that in no-disturbance conditions, and especially promote leaf litter mass loss within 0–173 d of incubation time. It was also demonstrated that transient flooding and soil covering can influence leaf litter decomposition, which led to the leaf litter mass loss rate and the stoichiometric ratios of C:N, C:K, C:P, N:P, and N:K exhibiting varied patterns and phases in different treatments during decay.

Seasonal flooding wetland expansion would strongly affect soil and sediment organic carbon storage and carbon-nutrient stoichiometry

In the past few decades, many non-flooding uplands (NF) and permanent flooding waters (PF) have been turned into seasonal flooding wetlands (SF) at the global scale. This trend could severely threaten global climate system by changing carbon cycling in terrestrial and aquatic ecosystems. However, the effects of SF expansion on soil and sediment organic carbon (SOC) storage and carbon-nutrient stoichiometry are far from clearly understood. Therefore, we explored SOC storage and carbon-nutrient stoichiometry among adjacent NF, SF and PF using 817 samples at 0–100 cm depth increment at Poyang Lake and Shengjin Lake in the middle-lower Yangtze River floodplain, China. The SFs of the two lakes were both Carex lakeshore wetlands. The NF of Shengjin Lake was a near-natural forest, while the NF of Poyang Lake was a disturbed grassland. The results showed that SOC storage at SFs of Poyang Lake and Shengjin Lake was 75.61 and 98.01 Mg C/ha at 0–100 cm depth increment. The difference in SOC storage among nearby NF, SF and PF depended on depth and disturbance. SOC storage at SF was equivalent to that at near-natural NF, but was much higher than that at disturbed NF. SOC storage at SF was 12.62%–24.50% higher than that at PF at 0–30 cm depth increment, but was 15.16%–25.87% lower than that at PF at 0–100 cm depth increment. Edaphic carbon and nutrients followed allometric relationships at most sites and C increased faster than N and P along the depth gradients. Carbon-nutrient stoichiometric relationships at SF and PF were similar, and were more coupled than those at near-natural NF. This research illustrates the strong effects of seasonal flooding on SOC sequestration in terrestrial and aquatic ecosystems, and expands our understanding of carbon cycling in these two ecosystems.

Disentangling the relative effects of soil moisture and vapor pressure deficit on photosynthesis in dryland Central Asia

Soil moisture (SM) and vapor pressure deficit (VPD) are key factors affecting photosynthesis, and quantifying their effects on this process can help us to understand the mechanisms of carbon cycling in terrestrial ecosystems. However, the strong coupling of SM and VPD makes it difficult to quantify their relative importance in the ecosystem carbon cycle. In this study we decoupled the negative correlation between SM and VPD by sorting and binning reanalysed SM and VPD data, and based on this, we quantified the relationship between VPD and SM and sunlight-induced chlorophyll fluorescence (SIF) in dryland Central Asia (CA). We found that SM promoted and suppressed photosynthesis in CA, accounting for 94.08% and 5.92% of the vegetated areas in CA, respectively; VPD promoted and suppressed photosynthesis in CA accounted for 39.68% and 60.32% of the vegetation area, respectively. The effects of SM and VPD on the photosynthesis of different vegetation types were different: the photosynthesis of croplands and forests tended to increase and then decrease with increasing SM, and the photosynthesis of grasslands, shrublands and sparse vegetation tended to increase with increasing SM; the photosynthesis of croplands, grasslands and forests increase with VPD, while the opposite is true for shrublands and sparse vegetation. Relative effects on photosynthesis indicated that the areas where SM had a greater effect than VPD on photosynthesis accounted for 74.26% of the vegetation area, mainly in the central part of Kazakhstan and Kyrgyzstan, which was related to the fact that the main vegetation types in these regions were croplands, grasslands, forests and sparse vegetation. The areas with greater effects of VPD than SM on photosynthesis were mainly located in the northern part of Kazakhstan and most of Uzbekistan and Turkmenistan (25.74% of vegetated areas). Our study contributes to further understanding of the key processes involved in carbon cycling in terrestrial ecosystems.

Decreased ultraviolet radiation and decomposer biodiversity inhibit litter decomposition under continuous nitrogen inputs

Abstract Atmospheric nitrogen (N) deposition has altered biogeochemical cycles and ecosystem functioning. As a key process involved in carbon and nutrient cycles in terrestrial ecosystems, litter decomposition is sensitive to external N inputs. However, it remains unclear how the interactions of ultraviolet (UV) radiation, soil biodiversity (bacteria, fungi and invertebrates) and conventional drivers (e.g. litter chemistry and microbial activities) regulate the responses of litter decomposition to continuous N inputs. Based on a multilevel N addition experimental platform, we conducted a 2‐year litter decomposition experiment to examine the relative importance of N‐induced changes in biotic and abiotic factors in mediating changes in the decomposition rates of four litter types (three representative species and their mixture) along an experimental N gradient in a Tibetan alpine steppe. Our results showed that litter decomposition rates exhibited a consistent decrease in response to N enrichment among all species and their mixture. The slowed decomposition rates with increasing N addition were associated with N‐induced reductions in UV radiation and soil bacterial diversity. An additional UV radiation manipulative experiment further confirmed that photodegradation had strong effects on plant litter decomposition at our study site. These results demonstrated that N‐induced declines in UV radiation and soil bacterial diversity inhibited litter decomposition, challenging the traditional view that changes in litter chemistry and microbial activities determine the responses of litter decomposition to external N inputs. Read the free Plain Language Summary for this article on the Journal blog.

Global assessment of lagged and cumulative effects of drought on grassland gross primary production

Drought has large impacts on the vegetation growth of global terrestrial ecosystems, particularly grasslands. Extensive in-situ studies have shown that the impact of drought on vegetation growth has lagged and cumulative effects, but it is not well known how grassland productivity (gross primary production or GPP) responds to drought over time at large scales. Here, we investigated the spatiotemporal patterns of lagged and cumulative effects of drought on global grassland GPP using an OCO-2 solar-induced chlorophyll fluorescence (SIF) based GPP product (GOSIF GPP) and the Standardized Precipitation Evapotranspiration Index (SPEI). The results showed that globally, 88.37% of the grassland had a lagged response to drought, with a lagged time scale of mainly 1 month. With the increase in soil water availability, the magnitude of the lagged effect gradually weakened. Grasslands in semiarid and semi humid conditions that were adaptable to periodic drought responded fastest to drought. Globally, more than three quarters (78.55%) of the grassland was affected by the cumulative effect of drought. The accumulated months occurred at longer time scales (8 ∼ 10 months) for the region at 40°N–55°N, which is relatively arid. The cumulative effect in relatively arid areas was stronger than that in relatively humid areas, indicating that grasslands in more arid areas had stronger tolerance to drought. More importantly, we found that the cumulative effect of drought on grassland GPP was stronger than the lagged effect. This study highlights the great importance of considering the legacy effect, especially the cumulative effect, of drought on grassland productivity, which would advance our understanding of the impact of climate change on the carbon and water cycles of terrestrial ecosystems.

Land surface phenology detections from multi-source remote sensing indices capturing canopy photosynthesis phenology across major land cover types in the Northern Hemisphere

Land surface phenology, which records the start of growing season (SOS) and the end of growing season (EOS), plays an essential part in reflecting plant photosynthesis and the response of carbon cycle in terrestrial ecosystems to climate change. Significant advances have been made toward tracking vegetation responses to climate variability based on land surface phenology derived from satellite remote sensing information. However, the advantages and disadvantages of single remote sensing index in estimating land surface phenology across major land cover types has not been well documented, which hindered our ability to better understand the impact of climate variability on plant phenology at large scales. In our study, four remote sensing indices, including solar-induced chlorophyll fluorescence (SIF), leaf area index (LAI), normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) based on 66 eddy flux tower sites in the Northern Hemisphere during the period of 2007–2014, were integrated to estimate land surface phenology across five land cover types, including evergreen needle-leaf forests, deciduous broadleaf forests, mixed forests, grasslands and croplands. The phenology extracted from gross primary production (GPP) from eddy covariance measurements was treated as real canopy photosynthesis phenology to verify the estimates of phenology transitions based on remote sensing indices. Results showed that all four remote sensing indices can capture land surface phenology, but showed different ability within land cover types. In details, phenology derived from LAI and SIF in three types of forests appeared to have good relationships with canopy photosynthesis phenology based on GPP, while phenology based on EVI or NDVI was close to GPP based phenology at grasslands and croplands sites. Meanwhile, the integration of four remote sensing indices could estimate land surface phenology more comparable to canopy photosynthesis phenology than a single remote sensing index for most sites. Furthermore, SOS was affected primarily by shortwave radiation, while EOS was regulated by a combination of different climatic variables in the Northern Hemisphere. The integration of remote sensing indices phenology could improve the capacity of estimating phenology transitions, which help us to better understand the impacts of climatic variables on land surface phenology and vegetation dynamics in future climate change.

Rhizosphere effects on soil organic carbon processes in terrestrial ecosystems: A meta-analysis

Rhizosphere processes are one of the most important ways in which plants affect carbon (C) cycling in terrestrial ecosystems. However, how rhizosphere processes related to C cycling are regulated by microorganisms is still poorly understood. Here, using a meta-analysis based on data compiled from 110 published articles and our measured data, we quantified the magnitudes of the rhizosphere effects on soil organic C (SOC), microbial biomass C (MBC), respiration (Rs), microbial biomass and enzymes involved in C acquisition, and discovered the linkages between the rhizosphere effect on Rs and microbial characteristics. This study provided a global-scale assessment in which positive rhizosphere effects on SOC, MBC, and Rs were observed across terrestrial ecosystems worldwide. We also found that the positive rhizosphere effects on microbial biomass and enzyme activities were likely widespread phenomena in terrestrial ecosystems. The results of the structural equation model also indicated that the rhizosphere effects on SOC and total nitrogen had positive effects on the rhizosphere effect on Rs, but the rhizosphere effects on fungal and bacterial biomass showed negative effects. Our findings highlight the importance of microbial-mediated rhizosphere Rs in global SOC cycling and suggest that the consideration of the rhizosphere effects on C cycling processes in Earth system models may improve the accuracy of predicting global SOC dynamics.

Terrestrial Ecosystem Modeling with IBIS: Progress and Future Vision

Dynamic Global Vegetation Models (DGVM) are powerful tools for studying complicated ecosystem processes and global changes. This review article synthesizes the developments and applications of the Integrated Biosphere Simulator (IBIS), a DGVM, over the past two decades. IBIS has been used to evaluate carbon, nitrogen, and water cycling in terrestrial ecosystems, vegetation changes, land-atmosphere interactions, land-aquatic system integration, and climate change impacts. Here we summarize model development work since IBIS v2.5, covering hydrology (evapotranspiration, groundwater, lateral routing), vegetation dynamics (plant functional type, land cover change), plant physiology (phenology, photosynthesis, carbon allocation, growth), biogeochemistry (soil carbon and nitrogen processes, greenhouse gas emissions), impacts of natural disturbances (drought, insect damage, fire) and human induced land use changes, and computational improvements. We also summarize IBIS model applications around the world in evaluating ecosystem productivity, carbon and water budgets, water use efficiency, natural disturbance effects, and impacts of climate change and land use change on the carbon cycle. Based on this review, visions of future cross-scale, cross-landscape and cross-system model development and applications are discussed.