Soil Dissolved Organic Carbon Research Articles

Characteristics of Paddy Soil Organic Carbon Mineralization and Influencing Factors Under Different Water Conditions and Microbial Biomass Levels

Paddy soil often undergoes frequent dry-wet alternation. The change in water status not only affects the physical and chemical properties of the soil, but also changes the structure and diversity of the soil microbial communities, which in turn determines the rate of soil organic carbon mineralization. However, the effects of different water conditions and soil microbial biomass levels on the process of soil organic carbon mineralization and its mechanisms are still unclear. Therefore, this study took typical subtropical paddy soil as the research object, applied a laboratory incubation experiment with two water treatments of dry-wet and continuous flooding, and reduced the soil microbial biomass through chloroform fumigation, thereby obtaining high and low soil microbial biomass carbon contents, to elucidate the influencing mechanisms of microbial biomass and water conditions on organic carbon mineralization in paddy soil. The results showed that during the first 30 d of incubation, the dry-wet treatment was in a non-flooded stage and its cumulative CO2 emissions were significantly lower than those of the continuous flooded treatment. After 30 d, the dry-wet treatment entered the flooded stage. The difference in the cumulative CO2 emissions of the soils with a high microbial biomass carbon content between the dry-wet and continuous flooding treatments gradually decreased, and there was no significant difference on day 78. In the soil with a low microbial biomass carbon content, the cumulative CO2 emissions of the dry-wet treatment on day 78 was still significantly lower than that of the continuous flooded treatment. The soils with a low microbial biomass carbon content showed a faster CO2 emission rate at the beginning of the incubation period (first 20 d), which was 1.1-6.1 times greater than that of the high microbial biomass carbon soils owing to their high soil dissolved organic carbon (DOC) content, and the CO2 emission rate then gradually decreased until it was below that of the soil with a high microbial biomass carbon content. The soil organic carbon mineralization rate became stable later in the incubation period (days 45-78). The stable mineralization rate of the high microbial biomass carbon soil was 20%-30% higher than that of the low microbial biomass carbon soil. The multiple regression analysis results showed that the decrease in the soil DOC content (ΔDOC) and the increase in the Fe2+ content (ΔFe2+) significantly affected the change in cumulative CO2 emissions (ΔCO2) under continuous flooding conditions, but had no effect on ΔCO2 during the flooding stage of the dry-wet treatment. The correlation analysis showed that the daily CO2 emission rate of soils with high microbial biomass carbon was significantly positively correlated with glucosidase activity under dry-wet treatment and significantly negatively correlated with acetylglucosaminidase (NAG) and peroxidase activities under continuous flooding treatment. In the low microbial biomass carbon soils, the daily CO2 emission rate of the continuous flooding treatment was negatively correlated with the NAG activity, but showed no correlation with enzyme activities under dry-wet management. In summary, the cumulative CO2 emissions of dry-wet treatment were lower than those of continuous flooding treatment, and the difference was significant in soils with low microbial biomass carbon. The size of the soil microbial biomass determined the level of the stable soil organic carbon mineralization rate. The amount of soluble organic carbon and iron reduction affected the soil CO2 emissions under continuous flooding conditions, and the soil water conditions affected the daily CO2 emission rate and its key influencing enzymes. This study provides data and theoretical support for the carbon cycle and carbon sequestration potential in paddy soil.

Polyethylene microplastics increase cadmium uptake in lettuce (Lactuca sativa L.) by altering the soil microenvironment

Little research has focused on the combined pollution of microplastics (MPs) and heavy metals in soil, especially the mechanism of their interaction. We conducted a 45-day microcosm experiment to test the hypothesis that polyethylene (PE) MPs and cadmium (Cd) had a joint toxicity to lettuce fitness. The effects of MPs at different addition ratios on Cd bioavailability and soil properties were also investigated in the microenvironment of three levels of Cd-contaminated soils. The results showed that the 10% MPs had an adverse impact on the plant biomass and significantly decreased soil pH and cation exchange capacity (CEC), but significantly increased soil dissolved organic carbon (DOC). The presence of MPs increased the soil Cd bioavailability and plant Cd concentrations and accumulations across all three levels of Cd-contaminated soils, which potentially aggregated the combined toxicity. The amounts of the bacterial 16SRNA and the fungal ITSRNA genes displayed a hormesis effect in response to the MP addition ratios while the abundance of Cd resistance genes cadA and czcA increased across all three Cd levels. The regression path analysis indicated that MPs affected shoot Cd concentrations by altering soil properties, which directly and indirectly contributed to the alteration mechanism, while the soil pH, DOC, and Cd bioavailability played core roles. The results suggest that the co-exposure of PE MPs in heavy metal-contaminated soil may therefore increase the toxicity, uptake, accumulation, and bioavailability of heavy metals by altering the properties of the soil microenvironment, which deserves further research.

Intercropping and N application enhance soil dissolved organic carbon concentration with complicated chemical composition

Soil dissolved organic carbon (DOC) plays a critical role in migration and transformation of soil organic carbon. For revealing effects of intercropping and N application on dynamics of DOC concentration and chemical structure, a multi-year located field experiment with cropping patterns (maize monocropping and maize-potato intercropping) subjected to four N levels (0 kg ha−1, 125 kg ha−1, 250 kg ha−1, 375 kg ha−1) was conducted. The results showed that N application raised DOC concentration while DOC accumulation, had a quadratic relationship with N application rate during the entire growth period of maize. At the same N level, intercropping increased the concentration and accumulation of DOC compared with monocropping. Meanwhile N application promoted the contents of amines and aromatic compounds and average molecular weight of DOC molecules, which reached the highest value at rate of 125 kg N ha-1. Compared with monocropping at the same N level, there was a tendency to increase the content of amines and aromatic compounds and the average molecular weight of DOC in intercropping, thus improving the degree of humification and aromatization of soil DOC. Soil temperature, water content and NH4+-N were the main factors mediating DOC concentration, while DOC concentration and soil temperature significantly affected the chemical structure of DOC. The structural equation model showed intercropping and N application elevated soil DOC concentration by increasing soil temperature, litter and root biomass, and the improvement of DOC concentration and soil temperature further enhanced the average molecular weight of DOC, resulting in accumulation of DOC.

Nutrient availability is a dominant predictor of soil bacterial and fungal community composition after nitrogen addition in subtropical acidic forests.

Nutrient addition to forest ecosystems significantly influences belowground microbial diversity, community structure, and ecosystem functioning. Nitrogen (N) addition in forests is common in China, especially in the southeast region. However, the influence of N addition on belowground soil microbial community diversity in subtropical forests remains unclear. In May 2018, we randomly selected 12 experimental plots in a Pinus taiwanensis forest within the Daiyun Mountain Nature Reserve, Fujian Province, China, and subjected them to N addition treatments for one year. We investigated the responses of the soil microbial communities and identified the major elements that influenced microbial community composition in the experimental plots. The present study included three N treatments, i.e., the control (CT), low N addition (LN, 40 kg N ha-1 yr-1), and high N addition (HN, 80 kg N ha-1 yr-1), and two depths, 0-10 cm (topsoil) and 10-20 cm (subsoil), which were all sampled in the growing season (May) of 2019. Soil microbial diversity and community composition in the topsoil and subsoil were investigated using high-throughput sequencing of bacterial 16S rDNA genes and fungal internal transcribed spacer sequences. According to our results, 1) soil dissolved organic carbon (DOC) significantly decreased after HN addition, and available nitrogen (AN) significantly declined after LN addition, 2) bacterial α-diversity in the subsoil significantly decreased with HN addition, which was affected significantly by the interaction between N addition and soil layer, and 3) soil DOC, rather than pH, was the dominant environmental factor influencing soil bacterial community composition, while AN and MBN were the best predictors of soil fungal community structure dynamics. Moreover, N addition influence both diversity and community composition of soil bacteria more than those of fungi in the subtropical forests. The results of the present study provide further evidence to support shifts in soil microbial community structure in acidic subtropical forests in response to increasing N deposition.

Greenhouse Gas Emissions from Forest Soils Reduced by Straw Biochar and Nitrapyrin Applications

Forestlands are widely distributed in the dominantly agricultural landscape in western Canada, and they play important ecological functions; such forestlands (e.g., shelterbelts) accumulate soil organic matter and may receive a substantial amount of nitrogen in the form of surface and subsurface runoff from adjacent croplands and become a significant source of emissions of greenhouse gases (GHGs) such as CO2, N2O, and CH4. Biochar and nitrapyrin applications could potentially mitigate GHG emissions, but their co-application in forest soils has not been studied. We investigated the effect of the application of biochars produced at low (300 °C; BC300) and high temperatures (700 °C; BC700) using canola (Brassica napus L.) straw and the effect of their co-application with nitrapyrin on GHG emissions and soil properties in a 35-day laboratory incubation experiment using forest soils collected from five shelterbelt sites. Results showed no significant interaction effect of biochar and nitrapyrin on the global warming potential (GWP) of the GHG emissions, and the GWP was 15.8% lower in the soil with nitrapyrin than without nitrapyrin application treatments. The GWP was significantly enhanced by BC300 addition due to a 26.9% and 627.1% increase in cumulative CO2 and N2O emissions, respectively, over the 35-day incubation. The GWP significantly decreased by BC700 addition due to a 27.1% decrease in cumulative CO2 emissions. However, biochar addition did not affect CH4 emissions, while nitrapyrin decreased CH4 uptake by 50.5%. With BC300 addition, soil-dissolved organic carbon and microbial biomass carbon increased by 26.5% and 33.9%, respectively, as compared to no biochar addition (CK). Soil pH increased by 0.16 and 0.37 units after the addition of BC300 and BC700, respectively. Overall, the effect of biochar and nitrapyrin was independent in mitigating GHG emissions and was related to the type of biochar applied and changes in soil properties.

Nitrogen fertilization overweighs intercropping in promotion of dissolved organic carbon concentration and complexity in potato-cropped soil

Intercropping promoted land productivity and be performed worldwide. This study aimed to assess the effect of intercropping on soil dissolved organic carbon (DOC) and its nitrogen (N) regulation. The potato was monocropped and intercropped with maize under four N levels (0, 62.5, 125 and 187.5 kg ha−1 for N0, N1, N2 and N3, respectively) in a fixed plot experiment from 2014 in subtropical China. Soil DOC concentration was determined and seasonal and interannual dynamics were analyzed from 2016 to 2018; Absorption values of specified wavelength were selected to characterize the annul DOC composition. Finally, the driving factors of DOC variation were explored using structural equation modeling (SEM). Soil DOC concentration and accumulation under N-applied treatments were higher than N0 and the increase peaked at N1. Compared with monocropping, intercropping significantly increased DOC accumulation by 20.29% only at N0. N application significantly improved the contents of amines and aromatic compounds and the average molecular weight of DOC, thus improving the degree of humification and aromatization of soil DOC. The SEM showed that the chemical composition of DOC was complicated with increasing soil temperature, aboveground and root biomass, while DOC concentration and chemical composition significantly affected the accumulation of DOC. Agricultural diversified cropping affects the dynamics and composition of soil DOC, which is intensively regulated by N fertilization. This study expands the insight into the variable DOC concentration and composition and its role in soil organic carbon turnover under the varying fertilization and cropping patterns.

Effect of land use and cover change on soil organic carbon fractions and enzymatic activities in lakeshore wetland of north shore of Lake Chaohu

为了揭示湖滨带土地利用与覆被改变对土壤有机碳库及生态功能的影响，本文选取了巢湖湖滨带（北岸）9个典型样方，分析和比较了表层（0~30 cm）土壤有机碳组分特征以及相关酶的活性.结果表明，巢湖湖滨带不同采样点土壤总有机碳（TOC）含量变化范围为2.88~11.2 g/kg，平均含量为9.12 g/kg，其中原生芦苇（Phragmites australis）湿地土壤TOC含量最高（11.2 g/kg），而芦苇群落消失后形成的荒滩土壤TOC含量最低，仅为2.88 g/kg.表征湖滨带湿地缓冲性能的土壤阳离子交换量（CEC）也以原生芦苇湿地土壤为最高，并与TOC含量呈现明显正相关.湖滨带表层土壤溶解性有机碳（DOC）和易氧化有机碳（EOC）含量变化范围分别为150~370 mg/kg和1.7~5.2 g/kg，其变化幅度明显高于TOC，其中DOC含量各采样点差异最为显著.除多酚氧化酶外，次生水柳林（Homonoia riparia Lour.）表层土壤几种酶的活性较原生芦苇湿地土壤皆有所上升，特别是蔗糖酶活性增加幅度最为明显.受人类活动干扰较大的湖滨绿地公园和人工草滩土壤过氧化氢酶、脲酶和蔗糖酶酶活性普遍显著低于原生芦苇湿地.除多酚氧化酶外，土壤中几种酶活性与土壤有机碳组分EOC和DOC含量均呈现显著的正相关，其中蔗糖酶活性与DOC含量之间相关系数最高（r=0.907），其相关性均达到极显著水平.土壤溶解性有机碳和蔗糖酶可以作为表征因土地利用与覆被变化导致湖滨带湿地退化以及生态恢复效果的敏感性指标.;In order to explore the effect of land use and cover change on lakeshore soil organic carbon sequestration and it ecological function, nine typical sample plots with different plantations along the north shore of Lake Chaohu were investigated, and the surface soils (0-30 cm) organic carbon fractions and the enzymes activities were analyzed and compared. The results showed that soil total organic carbon (TOC) was 2.88-11.2 g/kg, with an average of 9.12 g/kg. The soil under native Phragmites australis reed wetland showed the highest TOC content, and the TOC content in the wasteland formed after the disappearance of P. australis reed community was the lowest with the value of only 2.88 g/kg. The cation exchange capacity (CEC), which can be used to characterize the buffer performance of lakeside zone, was highest in the native P.australis reed wetland soil and positively correlated with TOC. Soil labile organic carbon fractions content, including dissolved organic carbon (DOC) and easily oxidized organic carbon (EOC) from the lakeshore wetland, ranged from 150 to 370 mg/kg and from 1.7 to 5.2 g/kg, respectively. The range of change in soil DOC and EOC between different sampling sites was larger than that in TOC. Especially, the difference of DOC content between different vegetations was the most significant. The activities of several determined enzymes in the surface soil of secondary Homonoia riparia Lour. Forest, except polyphenol oxidase, were higher than those in native P. australis reed wetland soil. Especially, the invertase activity increased most obviously. However, lakeshore urban green park and artificial grassland showed significantly lower activities of catalase, urease and invertaseas compared with native P. australis wetland. Correlation analysis showed that there existed significant relationships between soil enzyme activities except polyphenol oxidase and soil DOC and EOC. Especially, the correlations of invertase activity to the content of DOC reached remarkably significant level with the highest coefficient (r=0.907). Based on the above results, it is concluded that the DOC and invertase as the sensitive indicators, could well characterize the degradation process and ecological restoration effect on lakeshore wetland buffering function due to land use and cover change.

氮沉降对杉木人工林土壤可溶性有机质数量和结构的影响

氮沉降作为现在乃至未来气候变化的趋势之一，其可能深刻影响土壤可溶性有机质的数量和结构。选取我国中亚热带杉木人工林不同深度土壤（0-10 cm和10-20 cm）进行氮沉降试验，利用光谱技术研究氮沉降对土壤可溶性有机质数量和结构的影响。试验设对照（CT，0 kg hm^-2 a^-1）、高氮（HN：80 kg hm^-2 a^-1）、低氮（LN：40 kg hm^-2 a^-1）3种处理。结果表明：（1）在0-10 cm和10-20 cm土层，HN、LN处理的土壤可溶性有机碳和可溶性有机氮含量显著高于CT。（2）在0-10 cm和10-20 cm土层，1月时HN、LN处理的芳香性指数和腐殖化程度都显著高于CT，而4月时HN、LN处理的芳香性指数和腐殖化程度都显著低于CT。除了氮含量的直接影响外，RDA （冗余分析）表明，两土层中土壤含水量、pH和土壤有机碳是驱动氮沉降对土壤DOM数量和结构的重要环境因子。因此，氮沉降对土壤DOM的影响是复杂的，未来尤其应该注重氮沉降对生态系统影响的季节模式。;Nitrogen deposition, in relation with cliamte change, is anticipated to increase in the following decades. It is a potential factor that might profoundly affect ecosystem functioning. Enhanced nitrogen deposition might result in stoichiometric imbalance or other changed soil environment, and thereby affect plant growth and soil processes. Soil DOM (dissolved organic matter) plays an important role in ecosystem biogeochemical cycles. It is central to the questions of carbon sequestration in soils and nutrient availability to microorganisms. For example, in the early stage of plant litter decomposition, non-structural compounds are lost to soils mainly by the vehicle of DOM, which in return forms mineral-stabilized soil organic matter. Soil DOM is thus often considered to be a senstitive indicator of the interaction among plant, soil and microorganisms. Although many experiments on nitrogen deposition have been reported, inconsistency still remains about how soil DOM will respond to nitrogen deposition, especially in subtropical China.. Natural evergreen broad-leaved trees make up the typical vegetation in the subtropical zone of China, but large areas of natural forests have been transformed to Cunninghamia lanceolata plantations. Therefore, it is critical that we should explore the effects of nitrogen deposition on the dynamic changes of soil DOM in these plantations. In this study, we conducted a mesocosm field experiment in a Cunninghamia lanceolata plantations that stimulated nitrogen deposition. DOM from soils of 0-10 cm and 10-20 cm depth were examined by spectroscopic characterization, to explore the effect of nitrogen deposition on the quantity and structure of soil DOM. Three treatments (three replicates per treatment) were assigned to 9 plots: CT (control), HN (80 kg hm^-2 a^-1) and LN (40 kg hm^-2 a^-1). The results showed that (1) compared with CT, nitrogen deposition significantly increased the quantity of soil DOC (dissolved organic carbon) and soil DON (dissolved organic nitrogen) for both 0-10 cm and 10-20 cm soil layers. (2) The aromaticity index and humification index of DOM from both 0-10 cm and 10-20 cm soil layers were significantly enhanced by nitrogen deposition treaments in January, whereas an opposite pattern were observed in April. Besides the direct impact of nitrogen content, RDA (redundancy analysis) revealed that soil moisture, pH and soil organic carbon content were also main drivers regulating soil DOM. Our study suggests that nitrogen deposition might regulate soil DOM in either a direct way or an indirect way. More attention should be paid on the seasonal pattern of the influence of nitrogen deposition on ecosytem functioning.

DA-6 and GLDA Enhanced Pancium virgatum L. to Phytoextract Cd from Contaminated Soils

To reveal the effects of diethyl aminoethyl hexanoate (DA-6) combined with L-Glutamic acid, N,N-diacetic acid (GLDA) on the remediation of Cd contaminated soils by Panicum virgatum L., different GLDA application methods with total application doses of 600, 1200, and 2400 kg·hm-2equally divided 1, 2, and 4 times, respectively, were studied on the basis of DA-6 concentrations of 0 and 10 μmol·L-1, and application intervals (2 and 4 times) were 30 and 15 days, respectively, over 60 days. Soil pH, dissolved organic carbon (DOC), biomass and Cd contents, Cd extraction, total Cd, and available Cd (DTPA-Cd) were analyzed. The results showed that first, the combination of DA-6 and GLDA could improve soil pH and DOC, and the pH and DOC increased significantly with increased GLDA application. Secondly, the combination of DA-6 and GLDA could significantly improve the biomass and Cd contents of Panicum virgatum L., and for DA-6, the amounts and times of GLDA application were the most important factors affecting the biomass and Cd contents. Third, the highest amount of Cd extracted was 22.18g·hm-2 at 1200 kg·hm-2 GLDA applied 4 times and 10μmol·L-1 DA-6, which was 1.93 times higher than CK (wherein no DA-6 and GLDA were applied) and 1.23 times higher than 10D0 (in which only DA-6 was applied). Finally, the lowest soil total Cd was 0.529mg·kg-1 at 2400 kg·hm-2 GLDA applied 2 times and 10 μmol·L-1 DA-6, which was 21.04% lower than CK and 18.23% lower than 10D0. Therefore, the combination of DA-6 and GLDA can further strengthen the recovery of Cd contaminated farmlands via Panicum virgatum L..

Aspen Soils Retain More Dissolved Organic Carbon Than Conifer Soils in a Sorption Experiment

The effect tree species have on soil organic carbon (SOC) has been hotly debated but, so far, few clear patterns have emerged. One example of a differing tree species effect on SOC are aspen forests in North America, which have been found to have more stable SOC than adjacent conifer forest stands. An important source for the formation of stable organo-mineral complexes in soil is dissolved organic carbon (DOC). DOC concentrations in mineral soil are often higher under the thick O-horizons of conifer forests than under aspen forests, but this does not correspond to more stable mineral SOC. This suggests that, instead of DOC concentration, DOC quality could be driving the observed differences in SOC. Therefore, we quantified the retention of contrasting forest detritus DOC in soils. Using a batch sorption experiment approach, we compared the retention of detritus leachates from four sources – aspen leaves (AL), aspen roots (AR), conifer (subalpine fir) needles (CN), and conifer (subalpine fir) roots (CR) – on soils sampled under aspen and conifer (subalpine fir and Douglas fir) overstories. The calculated sorption isotherms showed a higher retention of AL DOC than AR DOC, as indicated by all four sorption parameters – k and n (curve-fitting parameters), null point concentration (NPC; net sorption = net desorption), and endpoint (EP, retention at the highest initial DOC concentration). Leachates from CN and CR showed very similar retention behavior, and between the two species the retention of root leachates was more similar than the retention of foliage leachates. Soils sampled from aspen forests showed higher affinity for new DOC than conifer soils [higher sorption rate (n), lower NPC, and higher EP] regardless of the DOC source. The findings suggest that the higher DOC sorption on aspen soils might be a major driver for more stable SOC under aspen stands in North America.

Five-year soil warming changes soil C and N dynamics in a single rice paddy field in Japan

Soil temperature is an important determinant of carbon (C) and nitrogen (N) cycling in terrestrial ecosystems, but its effects on soil organic carbon (SOC) and total nitrogen (TN) dynamics as well as rice biomass in rice paddy ecosystems are not fully understood. We conducted a five-year soil warming experiment in a single-cropping paddy field in Japan. Soil temperatures were elevated by approximate 2 °C with heating wires during the rice growing season and by approximate 1 °C with nighttime thermal blankets during the fallow season. Soil samples were collected in autumn after rice harvest and in spring after fallow each year, and anaerobically incubated at 30 °C for four weeks to determine soil C decomposition and N mineralization potentials. The SOC and TN contents, rice biomass, dissolved organic carbon (DOC) and microbial biomass carbon (MBC) concentrations were measured in the study. Soil warming did not significantly enhance rice aboveground and root biomasses, but it significantly decreased SOC and TN contents and thus decreased soil C decomposition and N mineralization potentials due to depletion of available C and N. Moreover, soil warming significantly decreased DOC concentration but significantly increased MBC concentration. The ratios of C decomposition potential to N mineralization potential, decomposition potential to SOC, and N mineralization to TN were not affected by soil warming. There were significant seasonal and annual variations in SOC, C decomposition and N mineralization potentials, soil DOC and MBC under each temperature treatments. Our study implied that soil warming can decrease soil C and N stocks in paddy ecosystem probably via stimulating microbial activities and accelerating the depletion of DOC. This study further highlights the importance of long-term in situ observation of C and N dynamics and their availabilities in rice paddy ecosystems under increasing global warming scenarios.

Responses of phenanthrene degradation to the changes in bioavailability and microbial community structure in soils amended with biochars pyrolyzed at low and high temperatures

This study investigated the impact of wheat straw biochars pyrolyzed at temperatures of 100–700 ℃ (BC100-BC700) on biodegradation of phenanthrene in soils. During a 42-day experiment, biochar amendment reduced the biodegradation ratio of phenanthrene in soils by no change-77.0%. The biodegradation ratio decreased with the increase of pyrolysis temperature from 100 to 400 ℃ and then increased with the increase of pyrolysis temperature from 400 to 700 ℃, exhibiting a U-shape. Meanwhile, desorbing fraction of phenanthrene extracted by n-butanol declined with increasing pyrolysis temperature. Biochar-derived dissolved organic carbon (DOC) obviously influenced the soil DOC contents which were negatively correlated with the total relative abundances of dominant polycyclic aromatic hydrocarbon (PAH)-degraders. These results indicated that in soils amended with biochars pyrolyzed at low temperatures (i.e. 100–400 ℃), both the reduced bioavailability of phenanthrene and the reduced PAH-degrader abundance resulted in decreasing phenanthrene degradation with pyrolysis temperature. In soils amended with biochars pyrolyzed at high temperatures (i.e. 500–700 ℃; HT-biochars), two possible reasons contribute to increasing phenanthrene degradation with pyrolysis temperature: (1) high sorbed-phenanthrene concentration due to large specific surface area and high aromaticity of the biochars, and (2) the increased dominant PAH-degrader abundance for the removal of sorbed-phenanthrene due to the impact of HT-biochars on soil properties (mainly on DOC content).

Effects of Intercropping of Brassica chinenesis L. and Tagetes patula L. on the Growth and Cadmium Accumulation of Plants

A pot experiment was conducted to reveal the effects of intercropping a low-cadmium (Cd) accumulating cultivar and a Cd hyperaccumulator on the safe utilization and phytoextraction of Cd-polluted soils. Two cultivars of Brassica chinensis L. (the low-Cd accumulating cultivar Huajun, and the common cultivar Hanlü), were intercropped with four cultivars of Tagetes patula L. (Dwarf Red, Dwarf Yellow, Tall Red, and Tall Yellow). We examined the biomass, photosynthetic characteristics, and Cd accumulation in the plants and available Cd content and dissolved organic carbon (DOC) content in the soils. The results show that under the intercropping treatments, the biomass of B. chinensis decreased significantly and those of T. patula increased significantly, compared with the monoculture treatments. When intercropped with T. patula, the net photosynthetic rate, stomatal conductance, and transpiration rate in the leaves of B. chinensis decreased significantly, compared with the monoculture treatments. When Huajun was intercropped with Dwarf Red, the shoot Cd content of Huajun significantly decreased by 14.5%, and that of Dwarf Red increased significantly by 36.5% compared with the monoculture. Under the other intercropping treatments, the shoot Cd content of B. chinensis increased significantly, or showed no significant change, and that of T. patula showed no significant change. Under the intercropping treatments, the total amount of Cd in the shoot of B. chinensis decreased significantly, and that of T. patula increased significantly, compared with the monoculture. There were no significant differences in the Cd extraction ratios between the intercropping treatments and the monoculture of T. patula. The shoot Cd content of B. chinensis was significantly correlated with soil available Cd content and DOC content (P<0.01 and P<0.05, respectively). In conclusion, the intercropping treatment of Huajun and Dwarf Red significantly reduced shoot Cd content in B. chinensis and increased that in T. patula, and it did not affect the Cd extraction ratio. This is suitable for the safe utilization and phytoextraction of Cd-polluted soils.

Poorly crystalline iron and aluminium oxides contribute to the carbon saturation and sorption of dissolved organic carbon in the soil

AbstractSoil carbon (C) saturation implies an upper limit to a soil's capacity to store C depending on the contents of silt + clay and poorly crystalline Fe and Al oxides. We hypothesized that the poorly crystalline Fe and Al oxides in silt + clay fraction increased the C saturation and thus reduced the capacity of the soil to sorb additional C input. To test the hypothesis, we studied the sorption of dissolved organic carbon (DOC) on silt + clay fractions (&lt;53 µm) of highly weathered oxic soils, collected from three different land uses (i.e., improved pasture, cropping and forest). Soils with high carbon saturation desorbed 38% more C than soils with low C saturation upon addition of DOC, whereas adsorption of DOC was only observed at higher concentration (&gt;15 g kg−1). While high Al oxide concentration significantly increased both the saturation and desorption of DOC, the high Fe oxide concentration significantly increased the desorption of DOC, supporting the proposition that both oxides have influence on the DOC sorption in soil. Our findings provide a new insight into the chemical control of stabilization and destabilization of DOC in soil.

Soil dissolved organic carbon in terrestrial ecosystems: Global budget, spatial distribution and controls

AbstractAimsSoil dissolved organic carbon (DOC) is a primary form of labile carbon in terrestrial ecosystems, and therefore plays a vital role in soil carbon cycling. This study aims to quantify the budgets of soil DOC at biome and global levels and to examine the variations in soil DOC and their environmental controls.LocationGlobal.Time period1981–2019.MethodsWe compiled a global dataset and analysed the concentration and distribution of DOC across 10 biomes.ResultsLarge variations in DOC are found among biomes across space and the soil DOC concentration declines exponentially along soil depths. Tundra has the highest soil DOC concentration in 0–30 cm soils [453.75 (95% confidence interval: 324.95–633.5) mg/kg], whereas tropical and temperate forests have relatively lower DOC concentrations, ranging from 30.20 (24.78–36.80) to 54.54 (49.77–59.77) mg/kg. DOC generally accounts for &lt; 1% of total organic carbon in soils, and DOC in 0–30 cm contributes more than half of the total DOC in the 0–100 cm soil profile. Furthermore, variations in DOC are primarily controlled by soil texture, moisture, and total organic carbon.Main conclusionsA global synthesis is combined with an empirical model to extrapolate the DOC concentration along soil profiles across the globe, and global budgets of DOC are estimated as 7.20 Pg C in the top 0–30 cm and 12.97 Pg C in the 0–100 cm soil profile, respectively, with a considerable variation among biomes. The strong soil texture control but weak total organic carbon (TOC) control on DOC variations suggest that the investigation of physical protection of soil organic carbon might need to expand to consider the labile C in soils. The global maps of DOC concentration serve as a benchmark for validating land surface models in estimating carbon storage in soils.

Dissolved organic carbon sorption dynamics in tidal marsh soils

AbstractCoastal wetlands are significant sources of dissolved organic carbon (DOC) to adjacent waters and, consequently, exert a strong influence on the quantity and quality of DOC exported to the coastal oceans. Our understanding of the factors that control the exchange of DOC at the tidal marsh‐estuarine interface, however, remains limited. We hypothesize that tidal marsh soils act as a regulator and that their physical characteristics, such as organic carbon content and mineral phase composition, are key controls on DOC exchange between soil surfaces and both surface and interstitial waters. To test this hypothesis, we generated traditional Langmuir sorption isotherms using anaerobic batch incubations of four tidal wetland soils, representing a range of soil organic carbon content (1.77% ± 0.12% to 36.2% ± 2.2%) and salinity regimes (freshwater to mixoeuhaline), across four salinity treatments. Results suggest that the maximum soil sorption capacity and DOC binding affinity increase and decrease with greater salinity, respectively, though the enhancement of maximum soil sorption capacity is somewhat mitigated in soils richer in poorly crystalline iron minerals. Initial natively sorbed organic carbon showed a significant positive correlation with soil specific surface area and K showed a moderate yet significant positive correlation with poorly crystalline iron mineral content. Taken together, these results point to a strong mineralogical control on tidal marsh sorption dynamics and a complex physicochemical response of those dynamics to salinity in tidal marsh soils.

Dissolved organic carbon enhances both soil N2O production and uptake

Nitrous oxide (N2O) is a potent greenhouse gas with high global warming potential. Soil acts as sources and sinks of N2O. The mechanism of soil N2O uptake is important for both the net emission of N2O and the N2O concentration in the atmosphere. The use of organic matter as an amendment is an effective management practice for soil fertility worldwide, whereas, its effects on soil N2O uptake remain unclear. To understand the responses of N2O uptake with organic carbon incorporation. A laboratory incubation using 15N2O dilution tracing technology with and without acetylene (C2H2) was conducted. The result showed that gross rate of soil N2O production in Glucose treatment (3.33 g kg−1 glucose incorporated with soil) with and without C2H2 were 497.9 and 639.2 μg N kg−1 soil h−1, which were significantly higher than that in CK (no carbon incorporation) and Straw (3.33 g kg−1 straw incorporated with soil) treatments (25.3 and 29.8 μg N kg−1 soil h−1 and 42.0 and 52.5 μg N kg−1 soil h−1, respectively). The gross rates of N2O uptake was significantly increased by Glucose with 221.2 μg N kg−1 soil h−1, whereas, in CK and Straw treatments, they were 21.6 and 28.3 μg N kg−1 soil h−1. However, the gross rates of N2O uptake were decreased to 2.2, 3.2 and 6.2 μg N kg−1 soil h−1 by C2H2 for CK, Straw and Glucose treatments, which indicated that denitrification could be a key pathway of soil N2O uptake. We also found that soil dissolved organic carbon (DOC) content was an important factor that positively correlated with the gross rates of N2O production and uptake. These results indicate that soil DOC enhances both soil N2O production and uptake, and denitrification is an important pathway of soil N2O uptake.

Acid rain mediated nitrogen and sulfur deposition alters soil nitrogen, phosphorus and carbon fractions in a subtropical paddy

Agricultural ecosystems are globally important sinks of carbon and other nutrient elements. In China, acid rain events affect about 0.62 million km2, representing about 6.4% of total land area; however, the impacts of acid rain mediated nitrogen (N) and sulfur (S) depositions on soil organic carbon (SOC) fractions and nutrient stocks in paddy soils and implications for yield production under climate change are unclear. We conducted a field experiment during two annual crop seasons to determine the effects of simulated acid rain on SOC fractions and nutrients in a subtropical paddy in China. Acid rain treatments comprised solutions of HNO3 + H2SO4 to simulate N and S deposition at pH levels of 4.5, 3.5, and 2.5. The results showed that content of soil C fractions varied with acid rain pH. Acid rain led to increased SOC concentrations and decreased the ratios of soil labile organic carbon (LOC): SOC and dissolved organic carbon (DOC): SOC concentrations, independently of crop season and growth stage. Soil salinity was positively associated with SOC suggesting that higher levels of salinity inhibit C decomposition favoring SOC accumulation. Treatment effects of acid rain on soil microbial C and N depended on crop growth stage. Concentration of Fe2+ was positively correlated with DOC in early and late paddy soils under acid rain, possibly as a result of Fe2+ retention by DOC, when Fe3+ is reduced to Fe2+. Acid rain led to increases in soil TN, TP, available N, NH4+ concentration, and SOC, and decreases in ratios of DOC: SOC, indicating decreases in soil biological activity and mineralization processes. Increases in dead rice plant biomass under acid rain were consistent with the increase in soil N and P concentrations, due to reduced nutrient uptake, and higher levels of total SOC.

Effects of composited organic mobilizing agents and their application periods on cadmium absorption of Sorghum bicolor L. in a Cd-contaminated soil

Organic mobilizing agents have been advocated for phytoremediation of heavy metals contaminated soils, while the effects of application period of such agents remain unclear. A pot experiment was conducted, with two composited organic agents (oxalic acid or citric acid + dissolved organic fertilizer (OA + DOF and CA + DOF)) and four application periods (seeding, jointing, flag leaf and heading stages) of sorghum (Sorghum bicolor L.), to investigate their impacts on Cd bioavailability in soil. Results indicated that application of the two composited agents increased soil dissolved organic carbon (DOC) and DTPA extractable Cd by 7.31–49.13%, Cd contents in roots and shoots by 21.49–72.10%, bioaccumulation factor (BCF) and translocation factor (TF) of shoots by 4.44–71.99%, while reduced soil pH by 0.25–0.53 units, respectively. Most of these indices increased with the application periods, and largely peaked with their application during the flag leaf to heading stages. Meanwhile, the maximum sorghum biomass (132.84 g pot−1) and Cd bioaccumulation quantity (BCQ, 0.71 mg pot−1) in shoots were obtained for the CA + DOF applied at the heading. The DTPA extractable Cd was closely related to soil pH and DOC. Similar close relationships were observed between the Cd contents in shoots and soil DTPA extractable Cd, pH and DOC. The BCQ of Cd was positively related to the shoots biomass rather than their Cd contents. Therefore, the sorghum combined with the CA + DOF may be advocated as an alternative phytoremediation mode in Cd-contaminated soils, and the mobilizing agent should be primarily applied at the heading stage.

Climatic temperature controls the geographical patterns of coastal marshes greenhouse gases emissions over China

Coastal marshes contribute greatly to atmospheric greenhouse gases (GHGs) and have thus attracted considerable attention at regional and global scales. However, the relative importance of climates and soil variables to the geographical variation of GHG emissions at a large geographical scale remains poorly understood. We thus investigated the spatial patterns of GHG (CO2, CH4 and N2O) emissions from coastal marshes along a latitudinal gradient in China and identified the climatic and soil variables controlling the emissions. Emissions of CO2, CH4 and N2O were estimated at rates of 12.8–310 mg CO2 m–2 h−1, 23.6–986 µg CH4 m–2 h−1 and 1.58–110 µg N2O m–2 h−1, respectively. The emissions of CH4, N2O and CO2 increased from high to low latitudes and were strongly correlated with climatic temperature. These emissions also varied significantly between temperate and subtropical climates and between winter and summer. Soil Eh, total organic carbon (TOC), dissolved organic carbon (DOC), total nitrogen, microbial biomass carbon (MBC) and nitrogen (MBN), bacterial abundance were the key regulators of CH4 emissions. Soil Eh, TOC, DOC, MBC and MBN were the crucial variables controlling N2O emissions. CO2 emissions across the coastal marshes were controlled mainly by soil pH, TOC, DOC, MBC, MBN and bacterial abundance. Although GHG emissions were strongly correlated with climatic temperature, 28–67% of the variations in GHG emissions were attributed to soil variables. The relative amplitudes (%) of the increases in GHG emissions, estimated on the basis of projected annual increases of 2 or 4 °C by 2100, were larger in temperate than subtropical coastal marshes, indicating that the coastal marshes with lower temperature were more sensitive to global warming. The annual emissions of CO2, CH4 and N2O estimated from the Chinese coastal marshes accounted for 6.6, 0.62 and 0.70‰ of the annual emissions from global terrestrial ecosystems, respectively. These results suggest that coastal marshes will be important to future global warming and will play a critical role in global GHG emissions.