Carbon Cycling In Terrestrial Ecosystems Research Articles

Distribution of SOCD along different offshore distances in China's fresh-water lake-Chaohu under different habitats

Carbon storage in wetland ecosystems is an important part of the carbon cycle of terrestrial ecosystems and provides important ecosystem services. Chaohu Wetland is a typical freshwater lake wetland in China. In this study, soil and plant samples were collected every 500 m through three sample lines of different vegetation habitats (estuarine banks, woodlands and shrub beaches) and different offshore distances, revealing the spatial distribution characteristics of soil organic carbon density (SOCD) in Chaohu wetland. The overall SOCD of Chaohu wetland was low, with different habitats ranking as Woodland > Estuary and riverside > Shrub and beach. SOCD of different offshore distances had no obvious law, and the SOCD decreased significantly with soil depth. The plant biomass was significantly higher at the woodland habitat than at other habitats. Most of soil nutrient indicators were the highest at the woodland habitat, while the estuary-riverside habitat had the highest N and P contents. Soil and plant nutrients at different offshore distances had no obvious change patterns. The contents of soil K, Ca, Mg, and N were significantly positively correlated with SOCD, but soil bulk density and pH were significantly negatively correlated with SOCD, and vegetation P content was significantly negatively correlated with SOCD. The spatial pattern of SOCD changes in this lake coastal wetland was determined by the combined effects of plant nutrients, biomass, and soil physical and chemical properties. Our results indicate Chaohu wetlands may have been experiencing serious degradation. The SOCD of Chaohu wetland is lower than that of other wetlands in China, which is mainly affected by human activities. Different offshore distances and habitat heterogeneity are the main factors affecting the soil carbon cycle of the wetland.

Three-dimensional mapping of carbon, nitrogen, and phosphorus in soil microbial biomass and their stoichiometry at the global scale.

Soil microbial biomass and microbial stoichiometric ratios are important for understanding carbon and nutrient cycling in terrestrial ecosystems. Here, we compiled data from 12245 observations of soil microbial biomass from 1626 published studies to map global patterns of microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), microbial biomass phosphorus (MBP), and their stoichiometry using a random forest model. Concentrations of MBC, MBN, and MBP were most closely linked to soil organic carbon, while climatic factors were most important for stoichiometry in microbial biomass ratios. Modeled seasonal MBC concentrations peaked in summer in tundra and in boreal forests, but in autumn in subtropical and in tropical biomes. The global mean MBC/MBN, MBC/MBP, and MBN/MBP ratios were estimated to be 10, 48, and 6.7, respectively, at 0-30 cm soil depth. The highest concentrations, stocks, and microbial C/N/P ratios were found at high latitudes in tundra and boreal forests, probably due to the higher soil organic matter content, greater fungal abundance, and lower nutrient availability in colder than in warmer biomes. At 30-100 cm soil depth, concentrations of MBC, MBN, and MBP were highest in temperate forests. The MBC/MBP ratio showed greater flexibility at the global scale than did the MBC/MBN ratio, possibly reflecting physiological adaptations and microbial community shifts with latitude. The results of this study are important for understanding C, N, and P cycling at the global scale, as well as for developing soil C-cycling models including soil microbial C, N, and P as important parameters.

Soil Inorganic Carbon as a Potential Sink in Carbon Storage in Dryland Soils—A Review

Soil organic carbon (SOC) pool has been extensively studied in the carbon (C) cycling of terrestrial ecosystems. In dryland regions, however, soil inorganic carbon (SIC) has received increasing attention due to the high accumulation of SIC in arid soils contributed by its high temperature, low soil moisture, less vegetation, high salinity, and poor microbial activities. SIC storage in dryland soils is a complex process comprising multiple interactions of several factors such as climate, land use types, farm management practices, irrigation, inherent soil properties, soil biotic factors, etc. In addition, soil C studies in deeper layers of drylands have opened-up several study aspects on SIC storage. This review explains the mechanisms of SIC formation in dryland soils and critically discusses the SIC content in arid and semi-arid soils as compared to SOC. It also addresses the complex relationship between SIC and SOC in dryland soils. This review gives an overview of how climate change and anthropogenic management of soil might affect the SIC storage in dryland soils. Dryland soils could be an efficient sink in C sequestration through the formation of secondary carbonates. The review highlights the importance of an in-depth understanding of the C cycle in arid soils and emphasizes that SIC dynamics must be looked into broader perspective vis-à-vis C sequestration and climate change mitigation.

Nitrogen fertilization weakens the linkage between soil carbon and microbial diversity: A global meta-analysis.

Soil microbes make up a significant portion of the genetic diversity and play a critical role in belowground carbon (C) cycling in terrestrial ecosystems. Soil microbial diversity and organic C are often tightly coupled in C cycling processes; however, this coupling can be weakened or broken by rapid global change. A global meta-analysis was performed with 1148 paired comparisons extracted from 229 articles published between January 1998 and December 2021 to determine how nitrogen (N) fertilization affects the relationship between soil C content and microbial diversity in terrestrial ecosystems. We found that N fertilization decreased soil bacterial (-11%) and fungal diversity (-17%), but increased soil organic C (SOC) (+19%), microbial biomass C (MBC) (+17%), and dissolved organic C (DOC) (+25%) across different ecosystems. Organic N (urea) fertilization had a greater effect on SOC, MBC, DOC, and bacterial and fungal diversity than inorganic N fertilization. Most importantly, soil microbial diversity decreased with increasing SOC, MBC, and DOC, and the absolute values of the correlation coefficients decreased with increasing N fertilization rate and duration, suggesting that N fertilization weakened the linkage between soil C and microbial diversity. The weakened linkage might negatively impact essential ecosystem services under high rates of N fertilization; this understanding is important for mitigating the negative impact of global N enrichment on soil C cycling.

Changes in soil organic carbon stocks from 1980‐1990 and 2010‐2020 in the northwest arid zone of China

AbstractSoil is the largest carbon reservoir in terrestrial ecosystems, and thus minor changes in it can dramatically affect atmospheric CO2 concentrations. In the northwestern arid zone of China, the prediction of soil organic carbon (SOC) changes is often limited by the scarcity of soil samples and the scale and depth of research, which limit the understanding of carbon cycling processes in arid zone terrestrial ecosystems. Therefore, this study produced digital soil maps of SOC stocks (SOCS) for two periods (1980‐1990 and 2010‐2020) at a 90‐m resolution based on historical soil profile data and a random forest model. The results showed that the prediction accuracy for SOCS in the topsoil (0–30 cm) was superior to that of the subsoil (30–100 cm). Among them, the mean annual evapotranspiration, normalized difference vegetation index during the growing season, multi‐year mean temperature, and clay content were the main environmental factors affecting the spatial distribution of SOCS. In the past 30 years, the SOCS of the northwestern arid zone has decreased by 585.50 Tg, with a mean decline of 19.52 Tg C yr−1. The changes in SOCS caused by land‐use conversion and reductions in SOCS were further shown to be attributable to grassland desertification and agricultural reclamation. These findings are valuable for exploring the carbon cycle in terrestrial ecosystems in the context of global climate change and for achieving China's goal of carbon neutrality.

Dynamics of soil organic carbon and nitrogen and their relations to hydrothermal variability in dryland

Carbon (C) and nitrogen (N) cycles of terrestrial ecosystems play key roles in global climate change and ecosystem sustainability. In recent decades, climate change has threatened the nutrient balance of dryland ecosystems. However, its impact on soil organic carbon (SOC) and soil total nitrogen (STN) in drylands of China are still unclear. In this study, the structural equation model (SEM) was used to explain the relationship between environmental variables used by the best model and SOC or STN. Then Adaptive Boosting Regressor (AdaBoost), Gradient Boosting Regression (GBRT), Extreme gradient boosting Regression (XGBoost) and Random Forest Regression (RF) were used to establish the prediction model of SOC and STN based on soil samples along with environmental variables. The performance of these models was assessed based on a 10-fold cross-validation method using three statistical indicators. Finally, we predicted the SOC and STN of soil samples from 2000 to 2019 based on the best model. Overall, the RF model performed better at predicting SOC and STN in drylands than the other three prediction models (AdaBoost, GBRT, XGBoost). Climate factors were the main factors affecting SOC and STN in the study area. In the Alashan, a dryland in northern China, the precipitation in the growing season increased from 2000 to 2019, at a rate of 12.9 mm/decade. During the same period, the annual sunshine duration significantly decreased by 66 h/decade. Along with interannual hydrothermal variability, SOC showed a fluctuating upward trend at a rate of 0.04 g/kg/decade, while STN exhibited a fluctuating downward trend at 0.003 g/kg/decade from 2000 to 2019. Due to the effects of climate change, dryland were considered as potential sites for carbon sequestration. However, due to the annual hydrothermal variance causing dynamic annual changes, it was deemed unstable. Moreover, it would cause STN loss, which might reduce soil fertility. More attention should be paid to STN monitoring in dryland in the future.

Rhizosphere Effects along an Altitudinal Gradient of the Changbai Mountain, China

Rhizosphere effects (REs) play important roles in regulating carbon (C) and nutrient cycling in terrestrial ecosystems. However, little is known about the REs of mature trees in the field, especially at the ecosystem scale. This study aimed to explore the variation and patterns of REs in natural ecosystems. Here, combining soil monoliths with an adhering soil (shaking fine roots) method was adopted to sample paired rhizosphere soil and bulk soil along an altitudinal gradient. Based on the relative REs and the percentage of rhizosphere soil mass, the REs on soil C and net nitrogen mineralization rates (Cmin and net Nmin) at the ecosystem scale were estimated. Our results showed that the REs on soil processes, soil microbial biomass C and extracellular enzyme activities (β-glucosidase and N-acetyl-glucosaminidase activities), and soil chemical properties (total C, total N, inorganic N, extractable P, K, Ca, Mg, Fe, and Mn) were significantly positive across altitudinal sites, while soil pH was significantly negative. Although the relative REs on investigated variables varied significantly among altitudes, the relative REs did not show a clear trend with the increased altitudes. Across altitudes, the mean magnitude of ecosystem-level REs on Cmin and net Nmin were 19% (ranging from 4% to 48%) and 16% (ranging from 3% to 34%), respectively. Furthermore, the magnitude of ecosystem-level rhizosphere effects increased linearly with the increased altitudes. The altitudinal patterns of ecosystem-level RE mainly depend on the percentage of rhizosphere soil mass. In conclusion, our results provided a set of new evidence for the REs, and highlighted the need to incorporate REs into land C and N models.

Fine root morphology and chemical responses to N addition depend on root function and soil depth in a Korean pine plantation in Northeast China

Fine roots play a crucial regulatory role in the subsurface carbon (C) and nitrogen (N) cycles of terrestrial ecosystems and are potentially influenced by atmospheric N deposition. However, there is still a lot of uncertainty regarding the effect of long-term N deposition on fine root traits in forest ecosystem. Our study aims to research the responses to N addition on fine root biomass, morphology and stoichiometry at different soil layers in a temperate Korean pine (Pinus koraiensis Sieb. et Zucc.) plantation. N addition experiments were initiated in Korean pine plantations for 3 consecutive years. Twenty plots (four treatments with five replicates) were assigned: Control, Low-N, Medium-N and High-N (0, 20, 40 and 80 kg N ha−1 yr−1). The coring method was applied to quantify absorptive and transport roots morphological and chemical traits and biomass along soil profiles (0–20 cm and 20–40 cm) and to assess the responses of these root traits to a gradient of N additions. N addition increased the specific root length (SRL), root N and P concentrations, and decreased the root tissue density (RTD) and C/N ratio of absorptive roots in the two soil layers. N addition increased the diameter, decreased RTD and the C/N ratio of transport roots in the two soil layers. N addition increased absorptive and transport roots biomass, and decreased the ratio of absorptive root biomass to transport root biomass. N addition significantly increased shallow root biomass and changed the vertical distribution pattern. N deposition increased the nutrient absorption and transportation efficiency of Korean pine by adjusting root morphology and chemistry. N deposition increased the amount of C allocated to fine roots. N deposition perhaps delayed the decomposition of roots and improved the C and nutrient retention in soil. In general, adjusting fine root functional traits to acquire water and nutrients is an effective strategy for plants to adapt to the environmental changes.

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.