Increase In Dissolved Organic Carbon Research Articles

An Increase in Stream Water DOC Concentrations May Not Necessarily Imply an Increase in DOC Fluxes in Areas Affected by Acid Deposition and Climate Change—An Example from Central European Catchments

Over a period of 30 years (1993–2022), headwater catchments in the Slavkov Forest (Czech Republic) exhibited a robust increase in stream water DOC (dissolved organic carbon) concentrations following a significant reduction in acidic atmospheric deposition. Sulfur deposition decreased from 34 kg ha−1 yr−1 in 1993 to 2.6 kg ha−1 yr−1 in 2022. Three Norway-spruce-dominated research sites—Černý Potok (CEP), a 15.2 ha peatbog catchment, Lysina (LYS), a 27.3 ha granitic catchment, and Pluhův Bor (PLB), a 21.6 ha serpentinite catchment, were investigated. The three–year average DOC concentration increased from 48.2 mg L−1 (1993–1995) to 68.3 mg L−1 (2020–2022) at CEP (0.69 mg L−1 yr−1). LYS showed an increase from 16.9 mg L−1 to 25.4 mg L−1 (0.30 mg L−1 yr−1 annually). The largest increase was recorded at PLB, with an increase from 15.7 mg L−1 to 36.7 mg L−1 (0.89 mg L−1 yr−1). A decline in ionic strength was identified as the main driver of the DOC increase. The annual runoff declined significantly at CEP and LYS from 465 mm to 331 mm as a result of rising air temperatures and reduced precipitation between 2014 and 2022. PLB (average of 266 mm) did not show a statistically significant decline. Recently, PLB experienced significant deforestation that likely lowered transpiration and thus increased catchment runoff. As a result, DOC fluxes did not change significantly at CEP (average 210 kg ha−1 yr−1) and LYS (90 kg ha −1 yr−1). However, PLB’s DOC flux more than doubled, increasing from 44 to 106 kg ha−1 yr−1. Drivers connected with global change, such as increasing temperatures, or potential chemical drivers, such as reductions in Al concentrations and pH changes, were not able to explain the observed changes in DOC concentra tions and fluxes.

Quantitative analysis of dissolved carbon sources in the farmland artificial ditch drainage-Lake UlanSuhai continuum in the Hetao Irrigation District's, Inner Mongolia

Study regionAgricultural Irrigation and Drainage system - Ulansuhai Lake Continuum, located in the Hetao Irrigation District, Inner Mongolia, China. Study focusIn this study, carbon isotope tracing technology was applied to analyze the sources and migration-transformation characteristics of Dissolved Organic Carbon (DOC) and Dissolved Inorganic Carbon (DIC) in the continuous system of the agricultural irrigation and drainage system - Ulansuhai Lake in the Hetao Irrigation Area of Inner Mongolia during typical irrigation periods. New hydrological insights for the regionDuring typical irrigation periods, DOC and DIC increase simultaneously with the increase in salinity, and dissolved carbon dioxide in the irrigation and drainage channels escapes. The main sources of DOC in farmland drainage water are terrestrial C3 plants (94.00 %) and plankton (6.00 %). The primary sources of DOC in the lake are farmland drainage (98.19 %) and macroaquatic plants (1.81 %). The main sources of DIC in farmland drainage water are irrigation water from the Yellow River (64.73 %) and the equilibrium dissolution process between carbonates and soil CO₂ (35.27 %); while the primary sources of DIC in the lake are farmland drainage (93.10 %) and air-water exchange (6.90 %). Crop types in the irrigated area, soil salinization, rock weathering, metabolic processes, and the hydrological connectivity induced by irrigation from the Yellow River jointly control the sources, migration, and transformation of carbon within the continuum.

Four decades of changing dissolved organic matter quality and stoichiometry in a Swedish forest stream

Dissolved organic matter (DOM) concentrations have risen by a factor of two or more across much of Europe and North America during recent decades. These increases have affected the carbon cycle, light regime, drinking water treatability, and the energy and nutrient budgets of lakes and streams. However, while trends in DOM quantity are well characterised, information on how/whether qualitative properties of DOM have changed are scarce. Here, we describe over 40 years of monitoring data from a forested headwater stream in the Gårdsjön experimental catchment, southwest Sweden, which provides a unique record of biogeochemical change, including optical and stoichiometric DOM quality metrics, spanning the entire period of recovery from acidification. For the period 1980–2020 we find a 71% reduction in decadal mean sulphate concentrations, and a similar reduction in inorganic aluminium concentrations, alongside a 64% increase in dissolved organic carbon (DOC) concentrations. Over the same period, colour (absorbance at 420 nm) increased almost twice as much as DOC, whereas dissolved organic nitrogen (DON) increased by only one third as much. These results demonstrate a shift in stream water composition, with DOM becoming dominated by highly coloured, complex, nitrogen-poor compounds. This material is likely more resistant to biological degradation, but more susceptible to photochemical degradation. Changes in DOM stoichiometry could lead to intensified nitrogen and/or phosphorus limitation in surface waters, while increased colour/DOC ratios could intensify light-limitation of primary production beyond that expected from DOC increases alone. We observed increases in organic matter associated metals (iron 117%, organically complexed aluminium 85%) that exceeded the increase in DOC, consistent with their increased mobilisation by more aromatic organic matter. All observed changes are consistent with recovery from acidification being the primary driver of change, implying that past acidification, and ongoing recovery, have profoundly affected terrestrial and aquatic biogeochemistry, ecology and the carbon cycle.

Precipitation patterns impact soil aggregates and organic carbon of an alpine wetland on the Qinghai-Tibetan Plateau

Global climate change is predicted to influence both precipitation amount and frequency in many regions. However, little is known about how precipitation changes affect soil aggregates and soil organic carbon (SOC) in alpine wetlands. Plots of an alpine wetland on the Qinghai-Tibetan Plateau were subjected to two precipitation frequencies (high vs. low, i.e., 1 and 3 times of nature rainfall intervals) crossed with two precipitation amounts (high vs. low, i.e., 100 % and 70 % of the amount of natural rainfall). Two years later, we investigated changes of soil aggregates (silt + clay, < 53 μm; microaggregates, 53∼250 μm; and macroaggregates, >250 μm) and SOC in three soil depths, i.e., 0–15 cm, 15–30 cm and 30–50 cm. Averaged across the three soil depths, the content of SOC and dissolved organic carbon (DOC) in the simulated natural precipitation (high amount, high frequency treatment) were 127.22 g kg−1 and 0.19 g kg−1, respectively. Reducing precipitation amount or frequency led to a 19.6 %-26.2 % increase in SOC, and 28.7 %-44.4 % increase in DOC. Microbial biomass carbon in natural precipitation was 1.33 g kg−1 across the three soil depths, and its response to precipitation varied with soil depths. The fraction of macroaggregates in the 0–15 cm soil depth was 63.5 % in the simulated natural precipitation treatment, and decreased by 18.6 %–21.9 % in response to reduced precipitation amount or frequency. Aggregate fractions in the 15–30 cm soil depth did not respond significantly to the changes of precipitation patterns. Reducing precipitation frequency or amount decreased the contribution of macroaggregate-associated organic carbon to SOC compared to the simulated natural precipitation, but increased the contribution of microaggregate-associated organic carbon, especially in the 0–15 cm soil depth. Structural equation models revealed that SOC was regulated by soil aggregate composition and microbial biomass carbon. The buffering of the surface soil weakened the effects of precipitation changes on aggregates and aggregate-associated organic carbon in the deep soil. These results suggest that reducing precipitation frequency or amount can decrease the fraction of macroaggregates in the surface soil, and weaken the physical protection of macroaggregates for SOC. Thus, changes in precipitation patterns may have a significant impact on SOC storage and stability in alpine wetlands through their influence on soil aggregate composition.

Biomass of Shoots and Roots of Multicomponent Grasslands and Their Impact on Soil Carbon Accumulation in Arenosol Rich in Stones

To prevent the degradation of light-textured soils, it is advisable to use them for grasslands. These soil management systems help with the faster accumulation of soil organic carbon (SOC), thereby improving the soil’s properties and reducing carbon emissions from agricultural land. In this experiment, we studied the distribution of multi-component perennial grass roots in the Arenosol profile and their impact on SOC sequestration in temperate climate zones. Our research aimed to identify differences in root biomass at depths of 0–15 cm, 15–30 cm, and 30–50 cm and to assess their correlation with SOC and dissolved organic carbon (DOC) in the soil. The roots, shoots, and soil samples of fertilized and unfertilized grasslands were collected at the flowering stage and after the final grass harvest two years in a row. Our findings revealed that, in sandy loam Arenosol rich in stones, 12.4–15.9 Mg ha−1 of root biomass was accumulated at 0–50 cm of soil depth. The application of NPK fertilizers did not significantly affect grass root biomass, but significantly affected shoot biomass. Most roots (84–88%) were concentrated in the 0–15 cm layer. On average, 5.10–6.62 Mg ha−1 of organic carbon (OC) was stored in the roots of perennial grasses within 0–50 cm of soil depth. We found that the SOC content in the 0–50 cm soil layer correlated more strongly (r = 0.62, p &lt; 0.001) with C accumulated in the roots of the corresponding layer than with shoot biomass (r = 0.41, p = 0.04). However, a significant correlation was found between DOC and shoot biomass (r = 0.68, p &lt; 0.001) and between DOC and the biomass of residues (r = 0.71, p &lt; 0.001), explaining the significant increase in DOC in the 30–50 cm soil layer and indicating the leaching of mobile soil organic matter (SOM) substances from the above-ground biomass using fertilizers.

Divergent controlling factors of freeze–thaw-induced changes in dissolved organic carbon and microbial biomass carbon between topsoil and subsoil of cold alpine grasslands

Freeze-thaw (FT) processes in cold regions significantly influence the mineralization and sequestration of soil organic carbon (SOC), particularly in its active pools, such as dissolved organic carbon (DOC) and microbial biomass carbon (MBC). However, manipulation experiments often amplify the effects of natural FT processes and overlook soils below 20 cm depth. To investigate carbon responses to seasonal FT events across soil depths (0–80 cm) and identify controlling factors, field sampling was conducted before freezing and after thawing in alpine grassland soils on the Qinghai-Tibet Plateau. Results are as follows: 1) After soil thawing, the entire soil profile showed insignificant changes in SOC (−4.46%) and MBC (3.21%), but DOC (45.27%) and DOC/SOC (160.08%) increased significantly, and strong associations were observed in DOC changes between adjacent soil layers. 2) FT indicators (number and temperature amplitude of daily FT cycles, frozen temperature, and frozen days) influenced SOC changes differently in the topsoil (0–10 cm) and subsoil (20–30 cm). Contrary to the impact of frozen temperature, other FT indicators largely facilitated the production of DOC0–10 and DOC20–30. The effects of FT indicators on MBC were nonlinear. 3) Structural equation models indicated that average soil water governed FT-induced changes in both DOC0–10 and DOC20–30. The varying effects of porosity and pH from topsoil to subsoil, along with the weakening relationship between changes in DOC and MBC, favored a stronger increase in DOC20–30. Increases in soil water and DOC after thawing stimulated the increment of MBC0–10, while porosity (positively) and sand content (negatively) regulated the MBC20–30 change. It indicates that, influenced by soil structure and biogeochemical environment, DOC and MBC in grassland soils are more sensitive to natural FT processes than SOC. Nevertheless, changes in FT patterns due to long-term climate change may cumulatively affect SOC.

Phosphorus immobilization/release behavior of lanthanum-modified bentonite amended sediment under the dual effects of pH and dissolved organic carbon

Lanthanum modified bentonite (LMB) is typical P-inactivating agent that has been applied in over 200 lakes. Dissolved organic carbon (DOC) and high pH restrict the phosphorus (P) immobilization performance of LMB. However, the P immobilization/release behaviors of LMB-amended sediment when suspended to overlying water with high pH and DOC have not yet been studied. In the present work, batch adsorption and long-term incubation experiments were performed to study the combined effects of pH and DOC on the P control by LMB. The results showed that the coexistence of low concentration of DOC or preloading with some DOC had a negligible effect on P binding by LMB. In the presence of DOC, the P adsorption was more pronounced at pH 7.5 and was measurably less at pH 9.5. Additionally, the pH value was the key factor that decided the P removal at low DOC concentration. The increase in pH and DOC could significantly promote the release of sediment P with a higher EPC0. Under such condition, a higher LMB dosage was needed to effectively control the P releasing from sediment. In sediment/water system with intermittent resuspension, the alkaline conditions greatly facilitated the release of sediment P and DOC, which increased from 0.087 to 0.581 mg/L, and from 11.05 to 26.56 mg/L, respectively. Under the dual effect of pH and DOC, the P-immobilization performance of LMB was weakened, and a tailor-made scheme became essential for determining the optimum dosage. The desorption experiments verified that the previously loaded phosphorus on LMB was hard to be released even under high pH and DOC conditions, with an accumulative desorption rate of less than 2%. Accordingly, to achieve the best P controlling efficiency, the application strategies depending on LMB should avoid the high DOC loading period such as the rainy season and algal blooms.

Optimizing nitrogen and phosphorus application to improve soil organic carbon and alfalfa hay yield in alfalfa fields.

Soil organic carbon (SOC) is the principal factor contributing to enhanced soil fertility and also functions as the major carbon sink within terrestrial ecosystems. Applying fertilizer is a crucial agricultural practice that enhances SOC and promotes crop yields. Nevertheless, the response of SOC, active organic carbon fraction and hay yield to nitrogen and phosphorus application is still unclear. The objective of this study was to investigate the impact of nitrogen-phosphorus interactions on SOC, active organic carbon fractions and hay yield in alfalfa fields. A two-factor randomized group design was employed in this study, with two nitrogen levels of 0 kg·ha-1 (N0) and 120 kg·ha-1 (N1) and four phosphorus levels of 0 kg·ha-1 (P0), 50 kg·ha-1 (P1), 100 kg·ha-1 (P2) and 150 kg·ha-1 (P3). The results showed that the nitrogen and phosphorus treatments increased SOC, easily oxidized organic carbon (EOC), dissolved organic carbon (DOC), particulate organic carbon (POC), microbial biomass carbon (MBC) and hay yield in alfalfa fields, and increased with the duration of fertilizer application, reaching a maximum under N1P2 or N1P3 treatments. The increases in SOC, EOC, DOC, POC, MBC content and hay yield in the 0-60 cm soil layer of the alfalfa field were 9.11%-21.85%, 1.07%-25.01%, 6.94%-22.03%, 10.36%-44.15%, 26.46%-62.61% and 5.51%-23.25% for the nitrogen and phosphorus treatments, respectively. The vertical distribution of SOC, EOC, DOC and POC contents under all nitrogen and phosphorus treatments was highest in the 0-20 cm soil layer and tended to decrease with increasing depth of the soil layer. The MBC content was highest in the 10-30 cm soil layer. DOC/SOC, MBC/SOC (excluding N0P1 treatment) and POC/SOC were all higher in the 0-40 cm soil layer of the alfalfa field compared to the N0P0 treatment, indicating that the nitrogen and phosphorus treatments effectively improved soil fertility, while EOC/SOC and DOC/SOC were both lower in the 40-60 cm soil layer than in the N0P0 treatment, indicating that the nitrogen and phosphorus treatments improved soil carbon sequestration potential. The soil layer between 0-30 cm exhibited the highest sensitivity index for MBC, whereas the soil layer between 30-60 cm had the highest sensitivity index for POC. This suggests that the indication for changes in SOC due to nitrogen and phosphorus treatment shifted from MBC to POC as the soil depth increased. Meanwhile, except the 20-30 cm layer of soil in the N0P1 treatment and the 20-50 cm layer in the N1P0 treatment, all fertilizers enhanced the soil Carbon management index (CMI) to varying degrees. Structural equation modeling shows that nitrogen and phosphorus indirectly affect SOC content by changing the content of the active organic carbon fraction, and that SOC is primarily impacted by POC and MBC. The comprehensive assessment indicated that the N1P2 treatment was the optimal fertilizer application pattern. In summary, the nitrogen and phosphorus treatments improved soil fertility in the 0-40 cm soil layer and soil carbon sequestration potential in the 40-60 cm soil layer of alfalfa fields. In agroecosystems, a recommended application rate of 120 kg·ha-1 for nitrogen and 100 kg·ha-1 for phosphorus is the most effective in increasing SOC content, soil carbon pool potential and alfalfa hay yield.

Recent, widespread nitrate decreases may be linked to persistent dissolved organic carbon increases in headwater streams recovering from past acidic deposition

Long-term monitoring of water quality responses to natural and anthropogenic perturbation of watersheds informs policies for managing natural resources. Dissolved organic carbon (DOC) and nitrate (NO3−) in streams draining forested landscapes provide valuable information on ecosystem function due to their biogeochemical reactivity and solubility in water. Here we evaluate a 20-year record (2001−2021) of biweekly stream-water samples (n > 3000) and continuous discharge in three forested catchments in the Adirondack region of New York to investigate and interpret long-term trends in DOC and NO3− concentrations. Results from the intensively monitored catchments were compared with data from synoptic surveys of streams throughout the Adirondack region. A weighted regressions on time, discharge, and season (WRTDS) model, used to estimate daily flow-normalized concentrations, determined that DOC increased by ~30 to 50 % while NO3− decreased by ~50 to 70 % over the study period. The large amount of data from catchments with different soil properties permitted us to assess the relative effects of hydrology, season, and land cover factors on temporal trends in DOC and NO3− concentrations. We found weak evidence of climatic forcing of long-term increases in DOC, and instead contend that declining ionic strength in precipitation linked to declining anthropogenic acid deposition is driving DOC trends in stream waters. Nitrate concentrations were more variable but clearly decreased in recent years possibly related to declining N deposition. The recent increase in DOC:NO3− in all catchments indicates a major shift in stream stoichiometry that reflects changes in ecosystem functioning that may have important biogeochemical implications for terrestrial as well as aquatic ecosystems.

Chemical Fertilization Alters Soil Carbon in Paddy Soil through the Interaction of Labile Organic Carbon and Phosphorus Fractions

The influence of long-term chemical fertilization in paddy soils is based on the interaction between labile carbon and phosphorus fractions and the manner in which this influences soil organic carbon (SOC). Four soil depths (0–30 cm) were analyzed in this study. Easily oxidized organic carbon components, such as permanganate oxidized carbon (POXC) and dissolved organic carbon (DOC), and other physicochemical soil factors were evaluated. The correlation and principal component analyses were used to examine the relationship between soil depth and the parameter dataset. The results showed that Fe-P concentrations were greater in the 0–5 cm soil layer. DOC, inorganic phosphate fraction, and other soil physiochemical characteristics interacted more strongly with SOC in the 0–5 cm soil layer, compared to interactions in the 10–15 cm layer, influencing soil acidity. An increase in DOC in the 0–5 cm soil layer had a considerable effect on lowering SOC, consistent with P being positively correlated with POXC, but negatively with SOC and water-soluble carbon (WSC). The changes in SOC could be attributed to the relationship between DOC and inorganic phosphate fractions (such as Fe-P) under specific soil pH conditions. An increase in soil DOC could be caused by changes in the P fraction and pH. The DOC:Avai. P ratio could serve as a compromise for the C and P dynamic indicators. The soil depth interval is a critical element that influences these interactions. Agricultural policy and decision-making may be influenced by the P from chemical fertilization practices, considering the yields and environmental effects.

Differential Trends in Iron Concentrations of Boreal Streams Linked to Catchment Characteristics

AbstractIncreasing iron (Fe) concentrations have been reported for freshwaters across northern Europe over the last decades. This increase, together with elevated concentrations of dissolved organic carbon (DOC), leads to browning of freshwaters, which affects aquatic organisms, ecosystem functioning, biogeochemical cycles, and brings challenges to drinking water production. However, how such increasing trends in stream Fe concentrations reflect the contribution of different catchment sources remains poorly resolved. Here, we explored how catchment characteristics, that is, mires and coniferous soils, regulate spatial and temporal patterns of Fe in a boreal stream network. For this, we determined Fe speciation in riparian and mire soils, and studied temporal Fe dynamics in soil‐water and stream‐water over a span of 18 years. Positive Fe trends were found in the solution of the riparian soil, while no long‐term trend was observed in the mire. These differences were reflected in stream‐water, where three headwater streams dominated by coniferous cover also displayed positive Fe trends, whereas the mire dominated stream showed no trend. Surprisingly, the majority of higher order streams showed declining Fe trends, despite long‐term increases in DOC. In addition, we found that an extreme drought event led to a prolonged release of Fe and DOC from the riparian soils, that could have long‐term effects on stream Fe concentrations. Our results show that riparian forest soils can be major contributors to ongoing increases in freshwater Fe concentrations and that drought can further promote the release of Fe from organic soils.

Initial responses of soil chemical properties to simulated warming in Norway spruce (Picea abies (L.) H.Karst.) stands in the Western Carpathians

The present-day decline observed in Norway spruce stands in the Western Carpathians is particularly worrying given the large forest area of this species. Relatively little attention has been paid to the contribution of soil chemistry to the Norway spruce decline in the context of global warming. We conducted a field experiment to simulate warming for almost 500 days in the Western Carpathians, in a mature Norway spruce stand. Soil samples were collected approximately every 3 months from the organic and mineral topsoil horizons. Total carbon, total nitrogen, pH, dissolved organic carbon (DOC), total dissolved nitrogen (TDN), nitrate (NO3-N), and ammonium (NH4-N) nitrogen concentrations were estimated. Warming caused increase in DOC, TDN, NH4-N, and pH in organic horizon, as well as increased pH in mineral topsoil. A trend for increasing total carbon and total nitrogen concentrations in the warmed organic horizon was observed. Increased concentrations of DOC and TDN may be associated with decrease in SOM stability or increase in production of biomass of the understory vegetation or soil microbial activity. Increased pH in warmed soil may be associated with the increased NH4-N and may contribute to the decrease in soil ability to adsorb DOC. We conclude that soil warming negatively stimulates soil organic matter stability, resulting in higher concentrations of easy migrating labile forms, mostly organic acid anions. Observed changes may contribute to the deterioration of Norway spruce stands.

Effects of regional climate, hydrology and river impoundment on long-term patterns and characteristics of dissolved organic matter in semi-arid northern plains rivers

Diverse environmental and anthropogenic factors, such as the ongoing reservoir constructions may influence riverine dissolved organic matter (DOM) properties. This has important implications for river water quality, particularly when reservoirs are a source of drinking water. Simultaneous studies of multidecadal trends in dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) are scarce. We studied the patterns in DOC and DON concentration in two major rivers of the South Saskatchewan River (SSR) basin over a 42-year period (1978–2019). We also examined the impact of a large reservoir on riverine DOC properties. Contrary to many studies, we did not find a long-term increase in DOC and DON concentration, and DOC and DON patterns were not always synchronous. In an agriculture dominated watershed like the SSR basin, agricultural land use (e.g., nitrogen-fertilizer application) could influence DOC and DON concentration differently, potentially resulting in asynchronous patterns over time. River discharge was an important driver of DOM patterns. Regional precipitation in the lower SSR basin may also influence DOM patterns in locations where runoff contribution is greater. These regional factors explained greater variability in DOM compared to global scale indices (e.g., Pacific decadal oscillation) due to their direct control on DOM. A travel time corrected approach to account for the lengthy reservoir turnover time showed that a large reservoir caused a reduction in allochthonous DOC characteristics through photodegradation and perhaps, an increase in autochthonous characteristics. Our results illustrate: 1) the increase in DOM concentrations seen in the northern hemisphere is not present in semi-arid prairie rivers, 2) Controls on different DOM components could be different, and 3) large reservoirs may modify riverine DOC composition due to longer water residence time.

Insights into the role of prechlorination in algae-laden raw water distribution process: Algal organic matter and microcystin-LR release, extracellular polymeric substances (EPS) aggregation, and pipeline biofilm communities.

Prechlorination routinely applied for the treatment of algae-laden raw water has received extensive attention due to its influence on water quality and aquatic microbes. In this study, prechlorination experiments with different doses were conducted in sets of model raw water distribution systems. With the elevated dose of chlorine and prolonged hydraulic retention time (HRT), the ratio of intact algal cells decreased, and the stability of water enhanced. Dissolved organic carbon (DOC) and nitrogen (DON) increased when chlorine dose elevated from 0 to 0.5mg/L but decreased with elevations from 0.5 to 2.0mg/L, while UV254 showed a monotonically increasing tendency. DOC, DON and extracellular microcystin-LR increase initially and decrease thereafter with the prolonged HRT. Notably, the effects of prechlorination on extracellular polymeric substances aggregation behavior on pipe walls and microbial community composition was revealed, providing more profound understanding of the community dynamics in this engineered system. This study helped optimize strategies to improve the stability and efficiency of pretreatment of algae-laden water.

Warming increases the relative change in the turnover rate of decadally cycling soil carbon in microbial biomass carbon and soil respiration

Decadally cycling soil carbon (dSOC) is the main component of the terrestrial soil carbon (C) pool. The response of dSOC to warming largely determines the feedback between climate warming and the C cycle. However, there is a lack of investigations about the effect of warming on the relative change in turnover rate (RCT) of dSOC and annually cycling SOC (aSOC) in dissolved organic carbon (DOC), microbial biomass carbon (MBC) and CO2. We clarified this issue by incubating two C3-C4 vegetation switch soils (23 years switch, HA soil and 55 years switch, GG soil) at 20°C and 30°C in the recently improved continuous airflow CO2 trapping system for 1 year. Warming increased the contribution of dSOC (C3-C) by 21℅ (soil HA) and 8℅ (soil GG) in MBC, and 38℅ (soil HA) and 15℅ (soil GG) in CO2, while only 2%–3℅ increase in DOC at the final stage of the incubation. Furthermore, warming increased the RCT in MBC and CO2 by 5.3- and 4.1-fold, respectively, but had no significant influence on the RCT in DOC, indicating that soil microbes may be an important engine to accelerate dSOC-derived CO2 emission in a warming world.

Effect of temperature during the hydrothermal carbonization of sewage sludge on the aerobic treatment of the produced process waters

The treatment of the highly contaminated process water produced during the hydrothermal carbonization (HTC) of waste activated sludge is of major concern for the full-scale implementation of the HTC process. So far, no satisfying treatment strategies have been elaborated and the biodegradability under aerobic conditions has hardly been studied. To fill these gaps, aerobic tests were first carried out in batches with HTC process waters produced at 190 °C, 218 °C and 249 °C, and two parallel sequencing batch reactors (SBR) were operated to treat process waters produced at 190 °C and 217 °C. Both experiments show that the HTC temperature has only a little effect on the elimination of dissolved organic carbon (DOC). In the aerobic batch tests, DOC removal were 80.5–81.9 %. In the SBR, 28 % of the initial DOC was found to be recalcitrant, and 25–28 % of the initial nitrogen. In the SBR experiments, the nitrification was also monitored, and nitrification inhibition test were conducted on both process waters obtained at 190 °C and 217 °C. Nitrification the initial SBR reactors was only possible after dilution of the process waters, which indicates the presence of inhibiting substances. The inhibition tests validated those observations, and showed that process waters derived at 217 °C had a higher inhibition potential. This study demonstrates that aerobically treated HTC process waters are still too polluted to be discharged in a wastewater treatment plant: model calculations showed an increase in effluent DOC of 8.3 mg C/L.

Studies on DNA fragmentation and cell integrity of rapidly hydroxyl radical inactivated Microcystis aeruginosa

Microcystis aeruginosa (M. aeruginosa) blooms occur frequently in freshwater lakes and reservoirs, posing a great threaten to drinking water safety. When using conventional oxidants remedy toxic algae, there is an unavoidable shortage leading to cell rupture and release of intracellular organic matter (IOM), thus causing more serious hazard to water safety. In this study, with the synergistic effect of strong ionisation discharge and hydrodynamic cavitation, hydroxyl radical (•OH) were generated and injected into cells to inactivate M. aeruginosa within 3 s. •OH-inactivated M. aeruginosa was intact without rupture and exhibited a 39 % increase in dissolved organic carbon (DOC) due to the degradation of the components of the cell wall and extracellular matrix. For comparison, ClO2 inactivated M. aeruginosa after 10 min of exposure, which induced cell rupture and a 113 % increase in DOC due to the release of IOM. Meanwhile, in •OH-inactivated M. aeruginosa, intracellular •OH was increased to 2 times according to the detection by fluorescence probe and the DNA oxidative damage biomarker 8-hydroxy-2′-deoxyguanosine increased to 1.4 times according to the enzyme-linked immunosorbent assay, confirming the direct action of •OH on the DNA. And •OH induced DNA fragmentation in cells of M. aeruginosa was identified by comet assay and TUNEL assay, finding a 4636 % increase in the DNA tail length, and 96.71 % of the cells showed DNA phosphatediester bond cleavage. In summary, the main reason for the rapid •OH-inactivation of M. aeruginosa is DNA fragmentation, which may represent a new method for the safely and efficient control of harmful algal blooms.

Contrasting mobility of arsenic and copper in a mining soil: A comparative column leaching and pot testing approach.

The remediation of legacy metal(loid) contaminated soils in-situ relies on the addition of [organic] amendments to reduce the mobility and bioavailability of metal(loid)s, improve soil geochemical parameters and restore vegetation growth. Two vermicomposts of food and animal manure waste origin (V1 and V2) were amended to an arsenic (As) and copper (Cu) contaminated mine soil (≤1500mgkg-1). Leaching columns and pot experiments evaluated copper and arsenic in soil pore waters, as well as pH, dissolved organic carbon (DOC) and phosphate (PO43-) concentrations. The uptake of As and Cu to ryegrass was also measured via the pot experiment, whilst recovered biochars from the column leaching test were measured for metal sorption at the termination of leaching. Vermicompost amendment to soil facilitated ryegrass growth which was entirely absent from the untreated soil in the pot test. All amendment combinations raised pore water pH by ∼4 units. Copper concentrations in pore waters from columns and pots showed steep reductions (∼1mgL-1), as a result of V1 & V2 compared to untreated soil (∼500mgL-1). Combined with an increase in DOC and PO43-, As was mobilised an order of magnitude by V1. Biochar furthest reduced Cu in pore waters from the columns to <0.1mgL-1, as a result of surface sorption. The results of this study indicate that biochar can restrict the mobility of Cu from a contaminated mine soil after other amendment interventions have been used to promote revegetation. However, the case of As, biochar cannot counter the profound impact of vermicompost on arsenic mobility.

Biomass, community composition and N:P recycling ratios of zooplankton in northern high‐latitude lakes with contrasting levels of N deposition and dissolved organic carbon

Abstract Global changes are causing decreases in inorganic nitrogen (N) concentrations, increases in coloured dissolved organic carbon (DOC) concentrations, and decreases in dissolved inorganic N to total phosphorus ratios (DIN:TP) in northern lakes. The effects of these changes on phytoplankton and zooplankton biomass and the N:P recycling ratio of zooplankton remain unresolved. In 33 Swedish headwater lakes across subarctic‐to‐boreal gradients with different levels of N deposition (low N in the north [Västerbotten, boreal; Abisko, subarctic] vs. high N in the south [Värmland, boreal; Jämtland, subarctic]), we measured water chemistry, phytoplankton biomass (chlorophyll‐a [Chl‐a], Chl‐a:TP), seston mineral quality (C:P, N:P), as well as zooplankton biomass, community composition, and C:N:P stoichiometry. We estimated nutrient imbalances and the N:P recycling ratios of zooplankton using ecological stoichiometry models. There was a large‐scale gradient from low lake DIN and DIN:TP in the north to high DIN and DIN:TP in the south, with lower DIN:TP in lakes coinciding with higher DOC within each region. Lower lake DIN was associated with lower phytoplankton biomass (lower Chl‐a:TP). Lower lake DIN:TP was associated with richer seston mineral quality (lower seston C:P and N:P) and higher zooplankton biomass. Zooplankton community composition differed in the north vs. south, with a dominance of N‐requiring calanoid copepods with high N:P in the north and P‐requiring cladocerans with low N:P in the south. Also, greater differences in zooplankton community composition were found between subarctic regions (with lower DOC) than between boreal regions (with higher DOC), suggesting that increases in lake DOC and associated declines in lake DIN:TP reduce differences in zooplankton community composition. The combination of lower lake DIN, higher lake DOC, and lower lake DIN:TP led to reduced zooplankton N:P recycling ratios, possibly by reducing seston N:P and/or by enhancing calanoid copepod dominance in the zooplankton community. Our findings suggest that the combination of declining N deposition and increasing lake browning in northern high‐latitude lakes will reduce phytoplankton biomass, but will concurrently enhance seston mineral quality and probably also zooplankton biomass and their recycling efficiency of P relative to N.

Cover crops and carbon stocks: How under-vine management influences SOC inputs and turnover in two vineyards

There is a growing awareness surrounding the importance of maintaining and increasing soil organic carbon (SOC, henceforth) stocks in vineyard systems. Increasing SOC positively influences numerous soil properties and has the added advantage of removing atmospheric CO2, thereby helping to mitigate the effects of climate change. Cover crops have long been used to influence soil properties in vineyard mid-rows, including increasing SOC content. Few studies, however, have quantified cover crop influence on SOC stocks and composition in the under-vine area, owing to a general reluctance to adopt under-vine cover crop management. This research aims to quantify SOC stocks and dissolved organic carbon (DOC) in soils from four treatments of under-vine management practice including two cover crop combinations, a straw mulch and herbicide-managed control across two vineyard sites established in 2014. We sampled soils under-vine to depths of 0–30 cm (stratified; 0–10 cm and 10–30 cm) and quantified both SOC concentrations and bulk density to ascertain SOC stocks. Further to this, we quantified water extractable organic carbon (WEOC) as a measure of the labile carbon stock, and measured heterotrophic respiration in a laboratory incubation as an indication of SOC turnover. We found that cover crop-managed soil under-vine sequesters up to 23% more soil organic carbon (SOC) as the traditional, herbicide practice over a five-year period of growth. Microbial activity increased by more than double in cover crop soils, owing to an increase in DOC and that there is evidence for more resistant C in cover crop soils. These results suggest that cover crop management under-vine is a viable solution to increase SOC stocks within vineyard systems. Taken together, the results of this study indicate that a shift from bare earth to cover crops in the under-vine region has the potential to sequester carbon in vineyard soils.