Soil Water-soluble Organic Carbon Research Articles

Responses of Soil CO2 Emission and Tree Productivity to Nitrogen and Phosphorus Additions in a Nitrogen-Rich Subtropical Chinese Fir Plantation

Nitrogen (N) and phosphorus (P) nutrients have been regularly applied to improve productivity in intensively managed and short-rotation forest plantations in subtropical China. Under the constraint of the national policy of “carbon neutrality”, it is necessary to determine the rational fertilization options by considering both forest productivity and soil CO2 emissions. Past worldwide studies have shown varied responses of forest soil heterotrophic respiration and CO2 emissions to N and P additions. This study designed six treatments with N additions (high level: 15 g N/m2, HN), P (low: 5 g P/m2, LP; high: 15 g P/m2, HP), and their interactions (HNLP and HNHP) to explore the effects of N and P additions on soil CO2 emissions in a P-limited and N-rich Chinese fir plantation (Cunninghamia lanceolata), and we identified the underlying controls using the structural equation model (SEM). The results indicated that LP, HNLP, and HNHP treatments significantly increased soil CO2 emissions in the first four months after treatment and the effects leveled since then. The balance between N and P inputs affected the responses of soil CO2 emissions to P additions. A low P addition significantly increased tree productivity, but the promoting effect gradually declined and was no longer significant after 3 years. Other treatments did not significantly affect tree productivity. The SEM analysis revealed that the promoting effects of P additions on CO2 emission were mainly due to their effects on increasing soil water-soluble organic carbon content and reducing microbial biomass nitrogen content. Considering both soil respiration and tree productivity, this study suggested that LP treatment can effectively balance the N and P nutrients and, in the meantime, maintain relatively low greenhouse gas emissions; thus a low P application level is suggested for N-rich Chinese fir plantations.

Response mechanism of interaction between Rhododendron hainanense and microorganisms to heat stress

Heat stress is a major limitation to Rhododendron cultivation in tropical and subtropical regions. Microbial community plays an important role in plant resistance to heat stress, however the microbial mechanism is poorly understood. We conducted a pot culture experiment on Rhododendron hainanense to reveal the mechanism of root microorganisms and soil microorganisms in response to heat stress, using an artificial climate simulation. The experimental control (CK) was set as day (25 ℃, 14 h) /night (20 ℃, 10 h), treatments were medium temperature stress (MTS, 35/30 ℃), and severe temperature stress (STS, 40/35 ℃). The result showed that compared with CK, MTS treatment considerably increased malondialdehyde content, and STS treatment significantly increased proline, and both heat stress treatments significantly decreased chlorophyll content. In comparison to CK, MTS considerably reduced soil nitrate nitrogen, soil microbial biomass carbon, and soil water-soluble organic carbon content. STS dramatically increased soil available nitrogen, soil water-soluble organic carbon, and soil microbial biomass carbon. Heat stress also significantly affected soil microbial community and root endophytic microbial community. Soil microbial and root bacterial community significantly correlated with soil nitrate nitrogen content, and root endophytic fungi significantly correlated with proline and soilwater-soluble organic carbon content. Therefore, soil microorganisms impacted soil nitrogen availability under heat stress, root endophytic bacteria impacted the ability of R. hainanense to absorb nitrogen, and root endophytic fungi controlled the physiological traits of R. hainanense by modifying Pro content to increase the resistance of R. hainanense to heat stress.

Mixed grass species differ in rhizosphere microbial community structure and function response to drought compared to monocultures

Natural grassland succession is the interactions among different grass species and various environmental factors. The dynamics of soil microbial communities along succession stages has received tremendous attention. The effects of the coexistence of different grass species on the structure and function of rhizosphere soil microbial communities, however, are poorly understood, especially under drought stress conditions. Here, we investigated how different grass planting patterns and soil water content affect rhizosphere microbial community structure and function. Two typical grass species (Bothriochloa ischaemum and Stipa bungeana) on the Loess Plateau were planted in three planting systems: B. ischaemum monoculture, S. bungeana monoculture, and a mixed culture of the two species under two water regimes (optimal water: 80% field capacity [FC]; drought stress: 60% FC). Rhizosphere soil samples were collected to evaluate the rhizosphere microbial community structure (via phospholipid fatty acid [PLFA] biomarkers) and function (via six enzyme activities). The mixed culture of grass species noticeably shifted the rhizosphere microbial community structure. It caused the rhizosphere gram-positive:gram-negative PLFA ratio and fungal:bacterial PLFA ratio to increase under optimum water conditions. In comparison, drought stress caused total PLFA, bacterial PLFA, and gram-positive PLFA biomass to increase. Drought stress caused rhizosphere-specific enzyme activity to increase under monoculture treatment and rhizosphere-specific enzyme activity to decrease under mixed culture treatment. Soil water-soluble organic carbon, pH, and water-soluble organic phosphorus were the most important factors driving changes to rhizosphere microbial community composition. In comparison, pH was the predominant factor driving variation in rhizosphere specific enzyme activities. Thus, the coexistence of grass species altered the structure and function of the rhizosphere microbial community through changes in rhizosphere pH and water-soluble nutrients. In conclusion, changes to rhizodeposits under a mixed culture of grass species likely affect the structure and function of the rhizosphere microbial community. The results of the present study improve our understanding of the impacts of grass species interaction on soil biogeochemical processes under future global drought stress scenarios.

Variation Characteristics of Soil Organic Carbon Storage and Fractions with Stand Age in North Subtropical Quercus acutissima Carruth. Forest in China

Soil labile organic carbon sensitively reflects subtle changes in the soil carbon pool and is an important aspect of forest soil carbon pool research. However, little is known regarding soil labile organic carbon storage and its dynamic changes during the development of Quercus acutissima Carruth. forests. Consequently, we investigated the dynamic changes in soil organic carbon and its labile organic carbon fraction stocks at soil depths of 0–10 cm, 10–20 cm, and 20–40 cm along a 17-year-old, 26-year-old, and 65-year-old chronosequence in Quercus acutissima forests. We found that stand age significantly impacted particulate organic carbon (POC), light fraction organic carbon (LFOC), and soil organic carbon (SOC). The POC, LFOC, and SOC contents at different soil depths exhibited an increasing trend with stand age, which could be described by simple linear regression. However, there was no noteworthy difference in the soil water-soluble organic carbon (WSOC) content between different stand ages. Moreover, the 17-year-old, stand had higher POC, LFOC, and WSOC to SOC ratios. Soil nutrients significantly affected organic carbon and fractions, which revealed that POC, LFOC, WSOC, and SOC were remarkably positively correlated with alkaline hydrolysis nitrogen (AN) and available phosphorus (AP) (p < 0.05). Furthermore, WSOC, POC, LFOC, and SOC were significantly positively correlated with available potassium (AK) (p < 0.05). POC, LFOC, and SOC storage in the 0–40 cm soil layer increased with stand development, while WSOC storage decreased at 65a. In addition, LFOC stocks accounted for the highest proportion of organic carbon stocks. Our results indicated that the development of Quercus acutissima forests was a process of carbon sink; however, the soil organic carbon activity was high, and the soil structure was unstable during the early development stage.

Research of the influences of utilization of sediment and exogenous materials on the changes of SDOM and WSOC of soil

As mankind pays attention to environmental protection and reuse of resources, more and more exogenous materials are being utilized for soil improvement, such as straw, sediment, etc. However, further research is needed on the mechanism of how sediments and other exogenous materials change soil properties. In this study, the changes of soil water soluble organic carbon (WSOC) and soil dissolved organic matter (SDOM) in the process of resource utilization of sediment and other exogenous materials were investigated. Different exogenous substances such as river sediment (RS), lake sediments (LS), straw (SS), algae (AS), Myriophyllum verticillatum L. submerged plants (MS) and livestock manure (FS) were added to the soil, and fluorescence spectroscopy combined with parallel factor analysis was used to explore the characteristics of WSOC changes and SDOM sources in the treated soil. The results showed that the SDOM fluorescence intensity changes show the total fluorescence intensity of the three groups of SS, FS, and MS was significantly higher (P < 0.01) than the control group. After 50 days of culture, the content of WSOC in the 6 groups was: MS (72.2 mg/kg) > LS (58.2 mg/kg) > FS (45.6 mg/kg) > RS (44.8 mg/kg) > AS (41.4 mg/kg) > SS (39.4 mg/kg) > CK (32.7 mg/kg). the MS group increased the most, reaching a percentage of 220%. The parallel factor analysis of SDOM three-dimensional fluorescence spectra showed that SDOM in soil mainly contains three components, groups contain protein-like, humus-like and tyrosine-like amino acids. In the initial stage, protein-like substances are the main components, and after 50 days, humus-like substances become the main components of SDOM in each treated soil. Among them, the FS group has the highest humus-like content. According to the fluorescence intensity, the SS, FS and MS groups have significant effects on increasing the content of soil water-soluble organic matter, and the SS group has the strongest effect. In summary, sediment and straw, algae, Myriophyllum verticillatum L., livestock manure can change the content of SDOM and WSOC in the soil can change the content of SDOM and WSOC in the soil, enhance the degree of soil vitality, increase the vitality of the soil, and improve the structure, composition, and quality of the soil. This research provided theoretical guidance and technical support for the resource utilization of sediment, straw, algae, submerged plants and livestock manure.

林冠氮添加和林下植被去除对杉木林土壤有机碳组分的影响

为探讨氮添加和林下植被管理对杉木人工林土壤有机碳组分的影响，以福建沙县官庄国有林场杉木人工林为对象，设置对照（CK）、林冠氮添加（CN）、林下植被去除（UR）、林冠氮添加和林下植被去除（CNUR）4个处理的野外控制实验，研究林冠氮添加和林下植被去除对土壤总有机碳、惰性有机碳、易氧化有机碳、颗粒有机碳、微生物生物量碳和水溶性有机碳的影响。结果表明：5年CN添加处理显著降低易氧化有机碳（10—20 cm）和微生物生物量碳（20—40 cm）含量，增加表层土壤颗粒有机碳占总有机碳的比例。UR处理对土壤有机碳组分的作用不显著，而CNUR处理显著降低表层土壤水溶性有机碳含量及其比例。土壤各有机碳组分均与土壤含水量、可溶性有机氮、微生物生物量氮和铵态氮呈显著正相关。研究表明，土壤活性有机碳比惰性有机碳对林冠氮添加（5年）的响应更敏感，且表现为中下层土壤响应大于表层土壤，短期氮添加能促进土壤活性有机碳的分解，而林下植被去除在短时间内可能通过改变土壤含水量和可利用氮减缓有机碳的分解与转化，从而补偿由于氮添加引起的土壤活性有机碳下降，未来需要通过长期氮添加实验进一步研究土壤有机碳动态变化的响应机制。;Increased nitrogen (N) deposition due to anthropogenic activities has been recognized to have important effects on forest carbon dynamic and plant diversity. However, most N manipulative experiments from the understory have suggested that external N addition can influence soil organic carbon pool, very little is known about the effects of canopy N addition on soil organic carbon fractions. To investigate these effects, a field experiment was carried out with four different treatments including control (CK), canopy N addition (CN, 25 kg N hm^-2a^-1, understory removal (UR), and canopy N addition plus understory removal (CNUR, 25 kg N hm^-2a^-1) in a Chinese fir plantation at Shaxian State Forest Farm, Fujian Province. The experiment was started in June 2014 and was a random block design with four replicates. After 5-years treatment, soil samples were collected by a stainlessness core (3.5 cm diameter, 60 cm length) and separated into three depths of 0-10 cm, 10-20 cm, and 20-40 cm. Then, total organic carbon (TOC), recalcitrant organic carbon (ROC), readily oxidizable organic carbon (ROOC), particular organic carbon (POC), microbial biomass carbon (MBC), and water-soluble organic carbon (WSOC) were determined. The results showed that soil TOC and ROC contents in the CN treatment had no significant changes, but the contents of soil ROOC at the 10-20 cm layer and MBC at the 20-40 cm layer, and the proportion of WSOC to TOC in surface soil layer significantly decreased and increased, respectively, compared to CK treatment. Moreover, single UR treatment did not influence soil organic carbon components, while CNUR treatment significantly decreased soil WSOC contents and proportions to TOC. Pearson analysis showed that both soil labile and recalcitrant organic carbons were positively correlated with soil moisture, water-soluble organic nitrogen, microbial biomass nitrogen, and ammonium nitrogen. These results indicated that soil labile organic carbon (LOC) was more sensitive to the short-term CN (5 years) compared to ROC, and the responses of subsoil (10-40 cm) were larger than top soil (0-10 cm). CN could promote soil LOC decomposition and consequently have a negative effect on the accumulation of soil organic carbon. However, UR was likely to offset the decrease of soil LOC induced by CN in the short time since UR would change the soil water and available nitrogen to decline the decomposition and transformation of soil organic carbon. All these findings suggest that the effects of CN and UR on soil organic carbon fractions and dynamics are complex, which requires a much longer-term manipulative experiment to understand the responding mechanism of soil carbon dynamics in the future.

Rationalizing the Use of Agricultural Organic Waste: Effects on Soil Dissolved Organic Carbon in Primary Saline-alkali Soil

ABSTRACT Poor soil fertility conditions limit agricultural production and development in saline-alkali areas. At the same time, in order to solve the problem of environmental pollution caused by agricultural wastes, agricultural wastes are used for organic fertilization of saline-alkali soils. In this study, the amount and structure of soil dissolved organic carbon after the application of different organic wastes was assessed. The results showed that the content of dissolved organic carbon in each component increased significantly (p < .05), while treatment GS recorded the highest content of dissolved organic carbon in each organic carbon component. The UV-visible spectrum of soil dissolved organic carbon recorded different results after the application of organic materials. For water-soluble organic carbon (WSOC), treatment CS was most effective in increasing the molecular weight of soil WSOC while treatment GS was most beneficial in improving the hydrophobicity and aromaticity of soil WSOC. For thermal hydrolyzed organic carbon (THOC), treatment GS was most effective for increasing the molecular weight and hydrophobicity, while treatment SM was most beneficial for increasing aromaticity. For hydrochloric acid and hydrolyzed organic carbon (HHOC), treatment SM was most effective in increasing molecular weight, while treatment FG and GS were most beneficial in improving the hydrophobicity and aromaticity of HHOC. For Alkali hydrolysis of organic carbon (AHOC), treatment GS was most beneficial for increasing the molecular weight, aromaticity, and hydrophobicity of AHOC. Amongst all the treatments the organic material treatments increased the aromaticity, hydrophobicity, molecular weight, and humification degree of soil soluble organic carbon compared with CK.

Altitudinal changes of soil organic carbon fractions of evergreen broadleaved forests in Guanshan Mountain, Jiangxi, China

We investigated soil total organic carbon (TOC), recalcitrant organic carbon (ROC), and labile organic carbon (LOC) of evergreen broadleaved forests at different altitudes (400, 600, 800, 1000 and 1200 m) in Guanshan National Nature Reserve, Jiangxi Province, with the aim to understand their altitudinal distribution. The results showed that soil TOC, ROC and LOC contents were the highest in the surface layer and decreased with soil depth. With the increases of altitude, contents of soil TOC, ROC, readily oxidizable organic carbon (ROOC), microbial biomass carbon (MBC), and particulate organic carbon (POC, 0-20 cm depth) increased with a peak at 1000 m and then decreased, whereas soil water-soluble organic carbon (WSOC) contents and POC contents in 20-40 cm layer did not change. In 0-10 cm soil layer, the proportions of ROC to TOC at 800 and 1200 m were significantly higher than those at 400 and 1000 m, while the proportions of LOC to TOC were the highest at 400 m. The proportions of ROC and LOC to TOC in 10-40 cm layer showed a low-high-low tendency along the altitude, with peaks at 1000 and 600 m, respectively. Soil organic carbon fractions were positively correlated with soil moisture, microbial biomass nitrogen, and soluble organic nitrogen. A positive correlation was observed between LOC and ammonium concentration. Our results suggested that altitude significantly affected the distribution of soil organic fractions, with soil ROC, ROOC and MBC being more sensitive to altitudinal changes. Soil ROC and LOC at high altitude were prone to decomposition and transformation under conditions with sufficient water and nitrogen, which reduced soil carbon stability. It was essential to study the dyna-mics of soil organic carbon in high altitude forests under global warming scenarios.

Effects of Management Measures on Soil Water-soluble Carbon and Nitrogen and Their Three-Dimensional Fluorescence Characteristics of Pinus tabulaeformis Plantations on Loess Plateau

The Pinus tabulaeformis plantation on the Loess Plateau was selected as the research site to study the changes of soil water-soluble carbon and nitrogen content and components of soil dissolved organic matter (SDOM) under different management measures (litter removal, young Pinus tabulaeformis forest, shrubs, grassland). The uncleared Pinus tabulaeformis plantation was used as the control. Results show that the soil water-soluble organic carbon content of shrubs and young Pinus tabulaeformis forests was significantly higher than that in other management measures. The content of soil water-soluble organic carbon decreased with an increase in soil depth, whereas soil water-soluble organic carbon/soil organic carbon (WSOC/SOC) increased. The change of soil water-soluble nitrogen content corresponded to that of water-soluble carbon. The content of soil water-soluble nitrogen content in the young Pinus tabulaeformis forest was higher than that of other management measures, and generally decreased with an increase in depth. Different management measures have significant effects on SDOM components. Based on three-dimensional fluorescence spectroscopy combined with parallel factor (EEM-PARAFAC) analysis, four components of SDOM are identified, which are mainly divided into categories, humus, protein-like, and soluble microbial metabolites in three major categories, among which the main component of the SDOM is the rich acid humus, and the highest content in the soil of the young forest of Pinus tabulaeformis is rich acid. The results suggested that management measures had certain effects on soil water-soluble carbon and nitrogen content as well as SDOM components, especially in young Pinus tabulaeformis forests and shrubs. The young Pinus tabulaeformis forest and shrubs increased the content of soil water-soluble organic carbon and water-soluble nitrogen, enhanced their ability to migrate and transform in the soil, and changed the structural composition of soil organic matter, hence enhancing the degree of humification and further improving soil quality.

Phosphorus affects enzymatic activity and chemical properties of cotton soil

Pot experiments were conducted in 2017 with two cotton cultivars (CCRI 79 and LMY 28) and three phosphorus (P) levels: 3, 8 and 12 mg P&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt;/kg as P&lt;sub&gt;0&lt;/sub&gt;, P&lt;sub&gt;1&lt;/sub&gt; and P&lt;sub&gt;2&lt;/sub&gt;, respectively. In this study, the soil water-soluble organic carbon content increased as the soil available P (AP) increased, while there were no significant variations for soil total organic matter content among the three AP levels. The activities of invertase, cellulase and urease in cotton soil decreased significantly in the P0. There were positive correlations between invertase and cellulose activities with soil organic carbon and inorganic-nitrogen (N); these correlated negatively with soil C/N ratio and AP level. In addition, high soil AP can raise soil AP and enhance alkaline phosphatase activity, which had a significant negative relationship with the soil C/P ratio. Urease activity had a significant positive relationship with soil NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;-N, C/P and N/P, as well as a negative correlation with soil C/N. Moreover, soil NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;-N and NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;–&lt;/sup&gt;-N in the P&lt;sub&gt;1&lt;/sub&gt; and P&lt;sub&gt;2&lt;/sub&gt; were lower than in the P&lt;sub&gt;0&lt;/sub&gt;, which might be an effect of high AP on soil N availability.

The fate of litter-derived dissolved organic carbon in forest soils: results from an incubation experiment

Despite being a crucial component of nutrient cycling and soil carbon (C) dynamics in forest ecosystems, there is too little information from past studies to discern whether dissolved organic carbon (DOC) exchanges with soil organic carbon or passes unaltered through soils. In this study, we added 13C-labelled litter-derived DOC into different depth soil columns in a 180-day incubation experiment to determine the fate of DOC in soils, and to monitor the changes in DOC composition when it percolates through the soil. The results showed that δ13C values increased in soil microbes, which indicated that some litter-derived DOC was immobilized by soil microbial communities. Approximately 76% of litter-derived DOC was retained in the soil (60% in topsoil and 16% in midsoil). Meanwhile, 18%, 4%, and 3% of litter-derived DOC were mineralized into CO2 in topsoil, midsoil and subsoil respectively. Only 0.04% of litter-derived DOC leached from the soil column (0–60 cm). These results indicated that DOC was mainly retained on soil, and a small portion was mineralized by microorganisms, with minimal leaching. The composition of water soluble soil organic carbon (WSOC) and leachate DOC (LDOC) were similar between the control and treatment. This indicated that the composition of WSOC and LDOC was more similar to soil C than the added DOC, which supports the previously hypothesized dynamic exchange model. These findings provide new insight by showing that most litter-derived DOC is sequestered in forest soils.

Effects of Cinnamomum camphora leaf litter cover on soil carbon and nitrogen cycles under different water conditions

Litter, as an important carbon stock, plays a significant role in carbon and nitrogen cycle in forest soil. To examine the effects of Cinnamomum camphora leaf litter on soil carbon and nitrogen cycles, the contents of carbon and nitrogen were investigated in C. camphora leaf litter-covered forest soils with the water content of 10%, 20% and 30% under laboratory-controlled condition. The results showed that leaf litter cover significantly increased both soil CO2 emission rates and water soluble organic carbon (WSOC) content, but decreased soil nitrate nitrogen content under the three water conditions, indicating that leaf litter cover could improve soil respiration and carbon mineralization but inhibit soil nitration. Leaf litter cover significantly enhanced the content of soil ammonium with the water content of 10%, but reduced those in soils with water content of 20% and 30%, revealing that the effects of leaf litter cover on soil ammonium nitrogen contents was dependent on soil water content. The content of some monoterpenes in leaf litter had a significantly positive correlation with soil CO2 emission rate and ammonium content, but a prominently negative relation with soil WSOC and nitrate content, implying that the effects of leaf litter cover on soil carbon and nitrogen cycles might be attributed to monoterpenes in leaf litter.

Vermicompost Improves Tomato Yield and Quality and the Biochemical Properties of Soils with Different Tomato Planting History in a Greenhouse Study.

A greenhouse pot test was conducted to study the impacts of replacing mineral fertilizer with organic fertilizers for one full growing period on soil fertility, tomato yield and quality using soils with different tomato planting history. Four types of fertilization regimes were compared: (1) conventional fertilizer with urea, (2) chicken manure compost, (3) vermicompost, and (4) no fertilizer. The effects on plant growth, yield and fruit quality and soil properties (including microbial biomass carbon and nitrogen, -N, -N, soil water-soluble organic carbon, soil pH and electrical conductivity) were investigated in samples collected from the experimental soils at different tomato growth stages. The main results showed that: (1) vermicompost and chicken manure compost more effectively promoted plant growth, including stem diameter and plant height compared with other fertilizer treatments, in all three types of soil; (2) vermicompost improved fruit quality in each type of soil, and increased the sugar/acid ratio, and decreased nitrate concentration in fresh fruit compared with the CK treatment; (3) vermicompost led to greater improvements in fruit yield (74%), vitamin C (47%), and soluble sugar (71%) in soils with no tomato planting history compared with those in soils with long tomato planting history; and (4) vermicompost led to greater improvements in soil quality than chicken manure compost, including higher pH (averaged 7.37 vs. averaged 7.23) and lower soil electrical conductivity (averaged 204.1 vs. averaged 234.6 μS/cm) at the end of experiment in each type of soil. We conclude that vermicompost can be recommended as a fertilizer to improve tomato fruit quality and yield and soil quality, particularly for soils with no tomato planting history.

Climate warming impacts on soil organic carbon fractions and aggregate stability in a Tibetan alpine meadow

Alpine meadows in the Tibetan Plateau contain a large amount of soil organic carbon (SOC), which is highly vulnerable to climate change and has thus been a research priority for scientists in recent decades. However, how climate warming influences the composition and stability of SOC remains unclear. In this study, a warming experiment (2010–2015) using open-top chambers was established (with unwarmed control [CK], winter warming [WW], and year-round warming [YW]) to investigate the effects of warming on the contents of SOC and its fractions, chemical composition of SOC, and water stability of aggregates in an alpine meadow located at the Damxung Grassland Station in the northern Tibetan Plateau. Experimental warming had no apparent effect (p > 0.05) on SOC content (17.8 ± 1.96, 17.8 ± 1.13 and 17.8 ± 1.09 g kg−1 under CK, WW and YW, respectively) in the 0–20 cm soil layer. However, warming significantly (p < 0.05) increased soil water-soluble organic carbon (C) content by 46.2% and 69.2%under WW and YW, respectively, and affected SOC chemical composition with decreasing phenol C (by 2.6% under WW, p > 0.05; and 8.4% under YW, p < 0.05) and increasing carboxyl C (by 11.6% under WW, p < 0.05; and 5.0% under YW, p > 0.05). Warming decreased (p > 0.05) the proportions of macroaggregates (2–0.25 mm) and free microaggregates (0.25–0.053 mm), whereas warming significantly increased the proportion of non-aggregated silt- and clay-sized fractions (<0.053 mm) by 41.0% and 55.7% under WW and YW, respectively. The variation in the aggregate size distribution resulted in the decline of the mean weight diameter and geometric mean diameter of water-stable aggregates by 5.1% (p > 0.05) and 8.5% (p < 0.05) under WW and by 8.5% and 6.6% (both p < 0.05) under YW, respectively. Of importance, the organic C content in free microaggregates, which provides greater physical protection to stabilize SOC, decreased by 10.9% (p > 0.05) under WW and by 22.4% (p < 0.05) under YW. The organic C contents of particulate organic matter and silt- and clay-sized fractions inside free microaggregates also significantly decreased under YW by 49.4% and 16.9%, respectively. However, the organic C content in non-aggregated silt- and clay-sized fractions significantly increased under WW and YW by 52.9% and 46.8%, respectively. The results suggest that short-term climate warming did not affect total SOC stocks, whereas it had a positive effect on WSOC, carboxyl C and non-aggregated silt- and clay-associated C and exhibited a negative effect on phenol C, free microaggregates-associated C and water stability of aggregates. These variables may act as sensitive indicators of climate warming in the Tibetan alpine meadows, which will in turn affect grassland ecosystem C fluxes in response to further climate change. The findings of the present study help improve our understanding of the responses of terrestrial ecosystem C cycling to future climate change.

Arsenic in Bangladesh Soils and Its Relationship with Water Soluble Soil Organic Carbon

An experiment was done to see the effect of water soluble organic carbon in soil on the release of arsenic (As). Fifty soil samples, rich in organic carbon were collected from five districts of Bangladesh namely Comilla, Jessore, Satkhira, Gopalganj and Bagerhat. Water soluble organic carbon (WSOC) of the analyzed samples ranged between 15 to 466 mg/L with an average of about 155 mg/L whereas the arsenic concentrations varies between 1.4 to 5.9 mg/L with an average of 4.2 mg/L. The arsenic contents were much higher in soils where peat deposits are prominent (Gopalganj). There has been a log-log relationship between WSOC and soil, as for all the soils, the R2 ranging from 0.355 to 0.811. Correlation between water soluble organic carbon and soil arsenic showed a positive and significant relation between the two parameters indicating the fact that organic or peat deposits contributes to the release of arsenic in Bangladesh environments.

Spartina alterniflora invasion alters soil microbial community composition and microbial respiration following invasion chronosequence in a coastal wetland of China.

The role of exotic plants in regulating soil microbial community structure and activity following invasion chronosequence remains unclear. We investigated soil microbial community structure and microbial respiration following Spartina alterniflora invasion in a chronosequence of 6-, 10-, 17-, and 20-year-old by comparing with bare flat in a coastal wetland of China. S. alterniflora invasion significantly increased soil moisture and salinity, the concentrations of soil water-soluble organic carbon and microbial biomass carbon (MBC), the quantities of total and various types of phospholipid fatty acids (PLFAs), the fungal:bacterial PLFAs ratio and cumulative microbial respiration compared with bare flat. The highest MBC, gram-negative bacterial and saturated straight-chain PLFAs were found in 10-year-old S. alterniflora soil, while the greatest total PLFAs, bacterial and gram-positive bacterial PLFAs were found in 10- and 17-year-old S. alterniflora soils. The monounsaturated:branched PLFAs ratio declined, and cumulative microbial respiration on a per-unit-PLFAs increased following S. alterniflora invasion in the chronosequence. Our results suggest that S. alterniflora invasion significantly increased the biomass of soil various microbial groups and microbial respiration compared to bare flat soil by increasing soil available substrate, and modifying soil physiochemical properties. Soil microbial community reached the most enriched condition in the 10-year-old S. alterniflora community.

Sample storage-induced changes in the quantity and quality of soil labile organic carbon.

Effects of sample storage methods on the quantity and quality of labile soil organic carbon are not fully understood even though their effects on basic soil properties have been extensively studied. We studied the effects of air-drying and frozen storage on cold and hot water soluble organic carbon (WSOC). Cold- and hot-WSOC in air-dried and frozen-stored soils were linearly correlated with those in fresh soils, indicating that storage proportionally altered the extractability of soil organic carbon. Air-drying but not frozen storage increased the concentrations of cold-WSOC and carbohydrate in cold-WSOC, while both increased polyphenol concentrations. In contrast, only polyphenol concentration in hot-WSOC was increased by air-drying and frozen storage, suggesting that hot-WSOC was less affected by sample storage. The biodegradability of cold- but not hot-WSOC was increased by air-drying, while both air-drying and frozen storage increased humification index and changed specific UV absorbance of both cold- and hot-WSOC, indicating shifts in the quality of soil WSOC. Our results suggest that storage methods affect the quantity and quality of WSOC but not comparisons between samples, frozen storage is better than air-drying if samples have to be stored, and storage should be avoided whenever possible when studying the quantity and quality of both cold- and hot-WSOC.

Changes in microbial community structure and functioning of a semiarid soil due to the use of anaerobic digestate derived composts and rosemary plants

We studied the changes in structure and functioning of the microbial community in a degraded agricultural soil after the addition of two composts, obtained from cattle manure or pig slurry anaerobic digestate, and the use of rosemary plants for restoring soil quality.The composts were added at low or high doses to soil samples (30 or 60tha−1, respectively), which were kept in microcosms for 6months. Some soil microcosms were treated with inorganic fertiliser and other non-treated soils were used as microbiological controls. Rosemary plants, used both for their ability to grow in semi-arid regions and for the capacity of their root system to protect soil from erosion, were planted in half of the entire microcosm set up. At different times (0–180days) microbial abundance and dehydrogenase activity were measured in the various experimental treatments. Total and water-soluble soil organic carbon and nitrogen contents were assessed at 0 and 180days. With an increase in carbon and nitrogen soil content, a rise in microbial abundance was also observed in the presence of both composts. However, microbial activity was significantly influenced by the presence of rosemary, without considering the allochthonous carbon and nitrogen input. Microbial community structure and diversity were also assessed by Fluorescence In Situ Hybridization in the different treatments. The highest values for microbial community biodiversity were found in the co-presence of rosemary and at low concentrations of both composts.The overall results suggest that the use of composts together with plant species suited to Mediterranean areas seems to be an appropriate strategy for restoring soil quality and the ecosystem services provided by microorganisms.

Can δ13C abundance, water-soluble carbon, and light fraction carbon be potential indicators of soil organic carbon dynamics in Zoigê wetland?

Zoigê alpine wetland located at the Qinghai–Tibet Plateau, China, is the largest alpine wetland in the world and very sensitive to global climate changes. However, little is known of the dynamics of soil water soluble organic carbon (WSOC), light fraction organic carbon (LFOC), soil organic carbon (SOC) and its indicators in this alpine wetland. In this study, four wetland sites and one degraded wetland site in the Zoigê alpine wetland were selected to investigate the relationships of stable isotope and labile organic carbon dynamics with groundwater elevation or water table level. We also examine if the δ13C abundance, WSOC, and LFOC can be used as potential indicators of SOC dynamics in these wetland sites. The results showed that the plant dominant species in the degraded wetland site were different from those in the non-degraded wetland sites. Moreover, the declining water table did not significantly lower foliar δ13C, C and N concentrations and C/N ratio of dominant plant species, but led to a significant decrease of soil SOC, WSOC and LFOC concentrations and an increase of soil δ13C. In the wetland sites, SOC was significantly correlated with soil δ13C, WSOC, and LFOC. Our regression analysis showed that LFOC was the best potential indicator of SOC dynamics with WSOC and soil δ13C being less useful as indicators, respectively. In addition, the absolute slope coefficients of the regression models increased as the soil depth increased, which indicated that the SOC turnover increased with soil depth.

Dissolved Organic Carbon in Headwater Streams and Riparian Soil Organic Carbon along an Altitudinal Gradient in the Wuyi Mountains, China

Stream water dissolved organic carbon (DOC) correlates positively with soil organic carbon (SOC) in many biomes. Does this relationship hold in a small geographic region when variations of temperature, precipitation and vegetation are driven by a significant altitudinal gradient? We examined the spatial connectivity between concentrations of DOC in headwater stream and contents of riparian SOC and water-soluble soil organic carbon (WSOC), riparian soil C:N ratio, and temperature in four vegetation types along an altitudinal gradient in the Wuyi Mountains, China. Our analyses showed that annual mean concentrations of headwater stream DOC were lower in alpine meadow (AM) than in subtropical evergreen broadleaf forest (EBF), coniferous forest (CF), and subalpine dwarf forest (SDF). Headwater stream DOC concentrations were negatively correlated with riparian SOC as well as WSOC contents, and were unrelated to riparian soil C:N ratio. Our findings suggest that DOC concentrations in headwater streams are affected by different factors at regional and local scales. The dilution effect of higher precipitation and adsorption of soil DOC to higher soil clay plus silt content at higher elevation may play an important role in causing lower DOC concentrations in AM stream of the Wuyi Mountains. Our results suggest that upscaling and downscaling of the drivers of DOC export from forested watersheds when exploring the response of carbon flux to climatic change or other drivers must done with caution.