Soil Dissolved Organic Carbon Concentrations Research Articles

Erratum to: Application of Green Manure and Pig Manure to Cd-Contaminated Paddy Soil Increases the Risk of Cd Uptake by Rice and Cd Downward Migration into Groundwater: Field Micro-Plot Trials

Land application of organic manure, crop residue, and biosolid, an important means for the disposal and recycling of wastes, has been shown to significantly increase the amount of dissolved organic matter (DOM) in soil. However, limited information is available on the dynamics of DOM, the concentration is usually expressed by dissolved organic carbon (DOC), and its influence on Cd behaviors in paddy soil amended with and without organic materials during rice (kinmaze) growing season. In this study, in situ field experiments were conducted to investigate the dynamics of DOC in paddy soil amended with green manure (GM), pig manure (PM), and chemical fertilizer (F) and its effect on Cd mobility and bioavailability. The results showed that DOC concentrations in soil solutions extracted from different depths were higher in GM and PM plots than those in F plot, and DOC concentrations all declined with time and rice growth. DOC concentrations in the root zone soil for all treatments were higher than those in the non-root zone due to root exudation and the higher pH value. The temporal dynamics of DOC in the root zone were found to be correlated to rice growth stage, as DOC concentrations decreased in the initial stage (week 1 to 6) of rice seedling and then gradually increased and reached the highest levels with 30.42 mg DOC L−1 for GM, 28.88 mg DOC L−1 for PM, and 19.19 mg DOC L−1 for F at rice heading and flowering stage (week 10), hereafter decreased again until when the rice was harvested. However, soil DOC in the non-root zone exhibited a continuous decrease trend and remained at a relatively low level after week 10 with 15.36 mg DOC L−1 for GM, 15.31 mg DOC L−1 for PM, and 8.43 mg DOC L−1 for F. The dynamics of water soluble Cd displayed statistically significant positive relationship with DOC (r 0.01 = 0.765, n = 9) regardless of soil depth and root presence/absence, suggesting that DOC enhanced the mobility and transport of through the formation of Cd-DOC complexes. As a result, DOC could increase the potential uptake of Cd by rice as well as the downward Cd migration to deeper soil. In these experiments, the uptake of Cd by rice grown in the GM and PM plots reached 5.55 and 3.71 mg plot−1, respectively, which were much higher than that in the F plot with 1.88 mg plot−1. The amounts of Cd downward migration were 17.0 mg plot−1 for GM plot, 14.74 mg plot−1 for PM plot, and 4.13 mg plot−1 for F plot, respectively. It could be concluded that the application of green manure and pig manure to Cd-contaminated paddy soil will increase the risk of Cd uptake by rice and Cd downward migration into groundwater. For this reason, care should be taken when organic manures was applied to contaminated soil to remediate or alleviate soil pollution and maintain soil fertility as well as provide nutrients for plant growth.

Variable respiration rates of incubated permafrost soil extracts from the Kolyma River lowlands, north-east Siberia

ABSTRACTThawing permafrost supplies dissolved organic carbon (DOC) to aquatic systems; however, the magnitude, variability and fate of this DOC is not well constrained. Our objective was to examine DOC respiration from seasonally thawed and near-surface (<1.5 m) permafrost soils collected from five locations in the Kolyma River Basin, north-east Russia. We measured soil organic carbon (OC) content, water-soluble macronutrients (DOC, NH4, PO4) and the heterotrophic respiration potentials of soil extract DOC in five-day laboratory incubations. DOC concentrations ranged from 2.8 to 27.9 mg L−1 (n = 14). Carbon respiration was 0.03–0.47 mg C (n = 16) and 8.7–31.4%, total DOC (n = 14). While DOC concentration was a function of soil OC concentration, we did not find a relationship between C respiration and soil OC or DOC concentrations. Respiration was highest in the top active layer, but varied widely among sites, and lowest at the bottom of the active layer. Respiration from yedoma varied across sites (0.04–0...

Watering increased DOC concentration but decreased N2O emission from a mixed grassland soil under different defoliation regimes

Changes in grazing regime and climatic conditions (such as precipitation) may affect soil N2O emissions; however, such effects have been inconsistent in grasslands. This study investigated the impact of simulated grazing, increased precipitation and N addition on soil N2O emissions, as well as soil dissolved organic carbon (DOC) concentrations, from a mixed grassland site in southern Alberta, Canada. A batch incubation study was conducted on soils (0–10 cm) collected from long-term field plots exposed to defoliation at either high intensity at low frequency (HILF) or high intensity at high frequency (HIHF) or low intensity at high frequency (LIHF) throughout the growing season (May–August). A deferred control (DC) treatment was also included where defoliation only occurred once at year end. All defoliation treatments formed a full factorial experiment with the watering treatment (ambient moisture (AM) or watering (W)) in the field. Soil N2O production was also quantified under an N addition treatment (no N and addition of 20 mg N kg−1 (NH4)2SO4) in the laboratory incubation experiment. Defoliation had no influence (p > 0.05) on soil DOC concentrations and cumulative N2O emissions. Watering increased DOC concentrations by 72–234 % but decreased N2O emissions by 33–60 %. Increments in N2O emissions after N addition were greater in the W (111–163 %) than the AM (64–67 %) treatment. Our results suggest that while defoliation regimes did not affect soil N2O emissions, watering may induce greater N2O loss in the more arid soil and N addition may further increase N2O loss under high precipitation within soils of the dry mixed grassland ecosystem.

Combined Effect of Sodicity and Organic Matter on Soil Properties under Long‐Term Irrigation with Treated Wastewater

Core Ideas We observed increasing sodicity hazard and a decreasing saturated soil hydraulic conductivity from treated wastewater irrigation. All of the data on sodicity hazard could be incorporated into a single Gapon constant for calcium‐sodium exchange that depends on the ratio of soil DOC concentration to calcite content. All the data on Ks could be incorporated in a single power‐law relationship involving the ratio of ESP to soil DOC concentration. The increasing reuse of treated wastewater (WW) for irrigation brings with it a need to reconsider irrigation water quality criteria because of the expected lower quality of WW. In particular, the impacts of higher sodium and dissolved organic carbon (DOC) concentrations on soil permeability must be evaluated in practical field settings over long periods of WW reuse. Here we report the long‐term impact of WW reuse for irrigation on soils at three different semiarid‐zone field sites under row crop or orchard agriculture. The soils contain about 60% clay, dominated by smectite, and present an order of magnitude variation in calcite content (1–11%). In two of the sites, parcels irrigated with freshwater (FW) are available for comparison. Our results show an increasing sodicity hazard and a decreasing saturated soil hydraulic conductivity (Ks) from WW irrigation, although the depth profiles of soil chemical and physical properties were highly site‐specific. Despite this spatial variability, all of the data on sodicity hazard, represented by the relationship between exchangeable sodium percentage (ESP) and the soil sodium adsorption ratio (SAR), could be incorporated into a single Gapon constant for calcium–sodium exchange whose values depended uniformly on the ratio of soil DOC concentration to calcite content. Moreover, all of the data on Ks, for both FW and WW irrigation, could be incorporated into a single power‐law relationship involving the ratio of ESP to soil DOC. These two relationships unify complex interactions between sodicity and organic matter (OM) that influence soil permeability to yield simple correlations with predictive power. The main detrimental effect of WW application was related to sodicity hazard. Therefore, effort should be invested in reducing the SAR of WW for irrigation. This could be achieved, for example, by mixing WW with FW, including desalinized water, when and if available.

Effects of simulated warming on soil DOC and DON concentrations in the alpine forest of western Sichuan based on altitudinal gradient experiment.

In order to understand the effects of climate warming on the process of soil carbon and nitrogen in the alpine forest, intact soil columns were collected in the alpine fir forest, and dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations in both soil organic layer (OL) and mineral soil layer (ML) placed at different altitudes were measured from May 2010 to April 2011 (the first incubation period) and from May 2011 to April 2012 (the second incubation period). The results indicated that both DOC and DON concentrations in OL and ML va-ried greatly during the sampling period. The DOC concentration in soil columns at the altitudes of 3300 and 3000 m increased 6.8 and 26.6 mg·kg-1 compared with that at 3600 m, respectively. The highest DOC concentrations in soil columns at the altitudes of 3600, 3300 and 3000 m were 408.0, 317.9 and 448.2 mg·kg-1, respectively, which were all detected in LG (late growth pe-riod) during the first incubation period, while the lowest concentrations were 33.1, 32.4 and 36.5 mg·kg-1, respectively, which were all detected in MG (mid growth period) during the second incubation period. Moreover, the DON concentration in soil columns at the altitudes of 3300 and 3000 m increased 2.3 and 30.4 mg·kg-1 respectively compared with that at 3600 m. The highest DON concentrations at the altitudes of 3600 and 3300 m were 65.9 and 64.6 mg·kg-1, which were both detected in MG in the first incubation period, while the lowest concentrations were 31.9 and 37.1 mg·kg-1, both detected in ET (early thawing period) in the second incubation period. Simulated warming increased the concentrations of DOC and DON in the OL, but decreased the concentration of DOC in ML and the ratio of DOC to DON in OL and ML. The ratio of DOC to DON was significantly positively related with DOC concentration and negatively related with DON concentration in OL, whereas the ratio of DOC to DON was significantly negatively related with DOC concentration and positively with DON concentration in ML. The results indicated that climate warming would increase the concentrations of DOC and DON in soils of the high-frigid forest by changing soil tempe-rature and freeze-thaw pattern, and in turn alter the process of soil carbon and nitrogen.

Increasing soil aeration reduces mitigation efficacy of dicyandiamide when targeted at ruminant urine-derived N2O emissions

Soil aeration effects on the efficacy of dicyandiamide (DCD) at reducing reactive nitrous oxide (N2O) fluxes from ruminant urine patches have not been assessed. We tested the null hypothesis that increasing soil aeration and dissolved organic carbon (DOC) levels, following ruminant urine deposition, would reduce DCD efficacy, as determined from measurements of N2O fluxes. Soil aeration was controlled (−1 or −10 kPa, the former being wetter and less aerated) within a factorial experiment that also included DCD and ruminant urine (700 kg N ha−1) treatments, destructively sampled over 40 days. Increased soil aeration corresponded with a decrease in DCD efficacy. However, contrary to the null hypothesis, increasing soil DOC concentrations following urine application did not correspond with greater DCD degradation rates. In fact, at −1 kPa applying urine increased the DCD half-life from 23.5 (±3.0) to 53.3 (±7.5) days, while at −10 kPa the DCD half-life was not affected by urine application (14.9 [±2.9] and 16.7 [±2.7] days, with or without urine, respectively). The efficacy of DCD at mitigating urine-induced cumulative N2O fluxes was 95% at −1 kPa and 57% at −10 kPa. This study confirms that the degradation of DCD is slower, and thus its efficacy is better, in wet soils compared with well-aerated soils.

Seasonal variations in labile soil organic matter fractions in permafrost soils with different vegetation types in the central Qinghai–Tibet Plateau

Labile soil organic matter (SOM) plays a crucial role in nutrient and carbon cycling, particularly in permafrost ecosystems. Understanding its variation is therefore very important. In the present study, we evaluated the seasonal patterns of labile SOM from April 2013 to March 2014 under alpine swamp meadow (ASM), meadow (AM), steppe (AS) and desert (AD) vegetation in permafrost regions of the China's Qinghai–Tibet Plateau. The fractions (0 to 10cm depth) included dissolved organic carbon (DOC), light-fraction carbon (LFC) and nitrogen (LFN), and microbial biomass carbon (MBC) and nitrogen (MBN). These fractions showed dramatic seasonal patterns in ASM and AM soils, but were relatively stable in AD soil. Soil DOC concentrations in the ASM, AM, and AD soils increased from April to May 2013, then increased again from July to August 2013 and from February to March 2014. The LFC and LFN concentrations in all four vegetation types were higher from June to August 2013. The highest MBC and MBN concentrations in the ASM, AM, and AS soils all occurred in the summer and the ASM soil showed a second peak in October or November 2013. Seasonal changes in climatic factors, vegetation types, and permafrost features were great causes of labile SOM variations in this study. Throughout the entire sampling period, the ASM soil generally had the highest labile SOM, followed by the AM, AS, and AD soils; thus, the ASM soil is the best system conserving soil nutrient (especially labile fractions) and microbial activity. Correlation analysis indicated that these fractions were not related to soil moisture and temperature in AS or AD soils, but soil temperature and moisture were significantly related to MBC and MBN in AM soil and DOC in ASM soil. Thus, the response of the labile SOM fractions in this high-altitude permafrost soils to climate change depended strongly on vegetation types.

Effects of Flooded Rice Cultivation on Soil Organic Carbon and Active Organic Carbon Content: A Microcosm Experiment

Information on carbon (C) dynamics and allocation in plant–soil system is essential to understand the terrestrial C cycle. Our aims were to determine the effects of rice growth on soil organic carbon (SOC), dissolved organic carbon (DOC) and microbial biomass carbon (MBC) content. For this a pot microcosm experiment in growth chamber was conducted with soils planted or unplanted with rice. The experiment lasted for 80 days. The SOC content increased both in unplanted and rice-planted soils over the incubation period. The increasing amount of SOC in rice-planted soil was larger than that of in unplanted soil, indicating that growing rice on bulk soil enhanced the function of soil as a carbon poo1. The DOC enhanced significantly (P < 0.05) compared to the zero time (Day 0 (CK)) both in planted soils and unplanted soils during 80 days of incubation. The DOC concentrations in planted soils were much larger than that of unplanted soils, suggesting the release of soluble root exudates from rice roots. However, MBC declined both in unplanted soil and rice-planted soil after incubation for 80 days, compared with the start of experiment. The results suggest that MBC dynamics in rice soil are largely controlled by organic substances released from rice roots.

Phosphorus and carbon competitive sorption–desorption and associated non-point loss respond to natural rainfall events

Long-term application of fertilizer or manure can increase the potential for P loss risk to ground and surface waters due to C and P competitive sorption-desorption. The aim of this study was to investigate the effect of long-term agricultural fertilizer application on dissolved organic carbon (DOC) and dissolved total phosphorus (DTP) loss. The study was conducted at the Yanting Agro-Ecological Experimental Station in Sichuan Province, People's Republic of China, during the 2012 rainy season. The results show that the variations in C and P leaching effects in fertilized soils exposed to natural rainfall events. As expected, application of inorganic and organic fertilizers increases DOC and DTP concentrations in soil and decreases the C:P ratio. Similarly, application of inorganic and organic fertilizers results in greater C and P leaching than that seen in unfertilized soils. The DOC flux was higher in subsurface runoff than in overland flow. In contrast, overland flow was the main pathway for P transport; subsurface runoff accounted for a smaller proportion of the total P transport. The increase of DOC and DTP was higher after use of organic manures than after treatment with inorganic fertilizers. DOC derived from surface-applied organic manures was found to leach at higher rates than that seen for DTP derived from the same source. However, organic manure-derived DOC was found to transport from soil prior to P, when they are surface-applied, demonstrating a higher sorption affinity of P over DOC compared to inorganic fertilization. Therefore, we should pay more attention to the P mobilized through long-term fertilization and enhance the P uptake due to C and P competitive sorption-desorption, and avoid potential leaching loss of P during rainfall process. (C) 2014 Elsevier B.V. All rights reserved.

Linking variability in soil solution dissolved organic carbon to climate, soil type, and vegetation type

AbstractLateral transport of carbon plays an important role in linking the carbon cycles of terrestrial and aquatic ecosystems. There is, however, a lack of information on the factors controlling one of the main C sources of this lateral flux, i.e., the concentration of dissolved organic carbon (DOC) in soil solution across large spatial scales and under different soil, vegetation, and climate conditions. We compiled a database on DOC in soil solution down to 80 cm and analyzed it with the aim, first, to quantify the differences in DOC concentrations among terrestrial ecosystems, climate zones, soil, and vegetation types at global scale and second, to identify potential determinants of the site‐to‐site variability of DOC concentration in soil solution across European broadleaved and coniferous forests. We found that DOC concentrations were 75% lower in mineral than in organic soil, and temperate sites showed higher DOC concentrations than boreal and tropical sites. The majority of the variation (R2 = 0.67–0.99) in DOC concentrations in mineral European forest soils correlates with NH4+, C/N, Al, and Fe as the most important predictors. Overall, our results show that the magnitude (23% lower in broadleaved than in coniferous forests) and the controlling factors of DOC in soil solution differ between forest types, with site productivity being more important in broadleaved forests and water balance in coniferous stands.

Green Manure and Overlapped Rhizosphere Effects on Pb Chemical Forms in Soil and Plant Uptake in Maize/Canola Intercrop Systems: A Rhizobox Study

To investigate the effects of single and mixed cultures of maize and canola, green manure addition, and distance from the rhizosphere on chemical forms of soil lead (Pb) and its uptake by plants, a greenhouse experiment was conducted using rhizobox systems. After plant harvest, the soils from different parts of the rhizoboxes were sampled and analyzed for dissolved organic carbon (DOC) concentrations and pH values. A sequential extraction procedure was also carried out to differentiate the chemical forms of Pb in the soil samples. Results showed that root activity significantly affected soil DOC concentration, pH and Pb chemical forms, while the effects generally ceased from the rhizosphere through the bulk soil. The pH value increased and DOC concentration decreased with distance from the roots. Green manure addition enhanced DOC concentration and exchangeable, carbonate-associated, and organic matter-bound Pb fractions. Green manure addition reduced the Pb concentration and uptake of maize but increased those of canola. The concentration of Pb in the roots of maize and canola was increased in intercropping compared with monocropping systems. The conclusion is that chemistry and bioavailability of soil Pb are interactively influenced by overlapped rhizospheres as compared to individual rhizospheres, use of green manure, and distance from the root surfaces.

Dissolved Organic Carbon Dynamics and Controls of Planted Slash Pine Forest Soil in Subtropical Region in Southern China

Soil dissolved organic carbon (DOC) is an active fraction of the soil organic carbon (SOC) pool and links terrestrial and aquatic systems. The degradation of DOC can affect carbon cycling, nutrient dynamics and energy supply to microorganism, and consequently change biogeochemical processes. This study investigated the vertical and seasonal variability of soil DOC concentrations and its controls in a 23-year-old planted slash pine (Pinus elliottii) forest at Qianyanzhou Forest Experimental Station (QFES) in Southern China. Soil solutions were collected at bimonthly intervals at depths of 10, 20 and 30 cm by a mechanical-vacuum extractor from November 2007 to March 2009, and at monthly intervals at depths of 10, 30 and 50cm from April 2009 to October 2010. The DOC concentrations were determined with a total organic carbon (TOC) analyzer. Mean (±standard deviation) DOC concentrations at depths of 10cm, 20cm, 30cm and 50cm were 12.4±4.4, 10.6±6.3, 8.7±2.6, and 8.0±5.9 mg L-1, respectively. Both seasonal and spring means of DOC concentration showed a decreasing trend with increasing depth, while there was no clear trend for the summer, autumn, or winter seasons. DOC concentrations during spring, summer, autumn and winter ranged from 4.8 to 21.5, 4.9 to 26.2, 5.4 to 17.1, 4.9 to 14.6 mg L-1, respectively, their mean DOC concentrations were 10.2, 10.5, 10.8 and 8.3 mg L-1, respectively. No consistent pattern of seasonal variability of DOC concentrations at different depths was observed. No obvious relationship between organic carbon content of forest litter and DOC concentration was found. There was a positive linear relationship between SOC and DOC concentration (R2=0.19, p<0.01), which showed that SOC was one of the main controls of DOC. A positive exponential relationship existed between soil temperature at 5 cm and DOC concentrations at 10 cm depth in slash pine, masson pine (Pinus massoniana) and Chinese fir (Cunninghamia lanceolate) planted forests (R2=0.12, p<0.01). DOC concentrations showed a negative linear relationship with soil moisture at all depths in slash pine forest (R2=0.15, p<0.001), and DOC concentrations at depth of 10 cm demonstrated a negative exponential relationship with soil moisture at 5 cm depth in three planted forests (R2=0.13, p<0.001). Precipitation in sampling months and mean seasonal DOC concentration were not correlated. However, a more detailed analysis of precipitation events at different times before sampling and seasonal DOC concentration showed that the timing of precipitation events prior to sampling had different effects on seasonal DOC concentrations at different depths. Our study highlights the importance of DOC dynamics for the carbon cycle in planted slash pine forest and it provides evidence for evaluating the effects of ecological restoration in subtropical red soil region.

Does dissolved organic carbon regulate biological methane oxidation in semiarid soils?

In humid ecosystems, the rate of methane (CH4 ) oxidation by soil-dwelling methane-oxidizing bacteria (MOB) is controlled by soil texture and soil water holding capacity, both of which limit the diffusion of atmospheric CH4 into the soil. However, it remains unclear whether these same mechanisms control CH4 oxidation in more arid soils. This study was designed to measure the proximate controls of potential CH4 oxidation in semiarid soils during different seasons. Using a unique and well-constrained 3-million-year-old semiarid substrate age gradient, we were able to hold state factors constant while exploring the relationship between seasonal potential CH4 oxidation rates and soil texture, soil water holding capacity, and dissolved organic carbon (DOC). We measured unexpectedly higher rates of potential CH4 oxidation in the wet season than the dry season. Although other studies have attributed low CH4 oxidation rates in dry soils to desiccation of MOB, we present several lines of evidence that this may be inaccurate. We found that soil DOC concentration explained CH4 oxidation rates better than soil physical factors that regulate the diffusion of CH4 from the atmosphere into the soil. We show evidence that MOB facultatively incorporated isotopically labeled glucose into their cells, and MOB utilized glucose in a pattern among our study sites that was similar to wet-season CH4 oxidation rates. This evidence suggests that DOC, which is utilized by MOB in other environments with varying effects on CH4 oxidation rates, may be an important regulator of CH4 oxidation rates in semiarid soils. Our collective understanding of the facultative use of DOC by MOB is still in its infancy, but our results suggest it may be an important factor controlling CH4 oxidation in soils from dry ecosystems.

Changes in Labile Organic Carbon Fractions and Soil Enzyme Activities after Marshland Reclamation and Restoration in the Sanjiang Plain in Northeast China

The extensive reclamation of marshland into cropland has tremendously impacted the ecological environment of the Sanjiang Plain in northeast China. To understand the impacts of marshland reclamation and restoration on soil properties, we investigated the labile organic carbon fractions and the soil enzyme activities in an undisturbed marshland, a cultivated marshland and three marshlands that had been restored for 3, 6 and 12 years. Soil samples collected from the different management systems at a depth of 0-20 cm in July 2009 were analyzed for soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC) and easily degradable organic carbon. In addition, the activities of the invertase, β-glucosidase, urease and acid phosphatase were determined. These enzymes are involved in C, N and P cycling, respectively. Long-term cultivation resulted in decreased SOC, DOC, MBC, microbial quotient and C (invertase, β-glucosidase) and N-transforming (urease) enzyme activities compared with undisturbed marshland. After marshland restoration, the MBC and DOC concentrations and the soil invertase, β-glucosidase and urease activities increased. Soil DOC and MBC concentrations are probably the main factors responsible for the different invertase, β-glucosidase and urease activities. In addition, marshland restoration caused a significant increase in the microbial quotient, which reflects enhanced efficiency of organic substrate use by microbial biomass. Our observations demonstrated that soil quality recovered following marshland restoration. DOC, MBC and invertase, β-glucosidase and urease activities were sensitive for discriminating soil ecosystems under the different types of land use. Thus, these parameters should be considered to be indicators for detecting changes in soil quality and environmental impacts in marshlands.

Dissolved organic carbon and its controls of urban soil in Hefei, Eastern China

Soil dissolved organic carbon (DOC) plays an important role in biogeochemical cycling and energy transformation in the ecosystem. The objectives of the study are to assess the impacts of land use types and planation modes on carbon dynamics and to determine the major soil factors influencing the DOC concentration in urban soils. The spatial pattern of DOC was studied with soil samples collected from a suburban natural forest park and urban green-lands (including campus green-land, park greenland, residential green-land, roadside green-land and factory green-land) in Hefei, Eastern China. The results showed that green-land types significantly impact on the DOC concentration (P<0.05) and that DOC concentration in suburban forest (44.28 mg kg -1 ) is obviously higher than that in the urban site (27.45 mg kg -1 ). The soil under arbor-lawn mode had the highest DOC concentration with an average of 44.96 mg kg -1 at the 0 to 30 cm soil depth. Land use change resulted in an obvious variation in soil DOC concentration (0 to 30 cm) across the environmental gradient. Results from regression analysis indicate that soil DOC concentration is significantly and positively correlated with soil moisture, soil NH4 + -N, microbial biomass nitrogen (MBN), dissolved organic nitrogen (DON), and negatively correlated with soil pH, bulk density, electric conductivity and total phosphorus. It is suggested that the expansion of the urban area may lead to the loss of DOC in the soil.

Does prescribed burning on peat soils influence DOC concentrations in soil and runoff waters? Results from a 10 year chronosequence

Summary Prescribed burning of moorland, heathland and blanket bog vegetation on peatlands in the UK is a contentious issue. Given the large carbon store in these peatlands, concern has been raised over land management and its effect on the carbon dynamics of peat ecosystems. In particular the spatial and temporal link between burning and concentrations of dissolved organic carbon (DOC) in waters draining these catchments has received particular attention. This study investigates water colour and DOC concentrations in soil pore water and runoff water at the plot-scale over a chronosequence of burn ages. Results from this study show that there is an elevated water colour in the few years immediately following burning but that this is not matched by a rise in DOC concentration. Therefore we propose that burning appears to affect the composition of the DOC rather than the absolute DOC concentration. This study also highlights that in some cases the use of water colour as a proxy for DOC concentration should be treated with caution.

Dissolved organic carbon (DOC) concentrations in UK soils and the influence of soil, vegetation type and seasonality

Given the lack of studies which measured dissolved organic carbon (DOC) over long periods, especially in non-forest habitat, the aim of this study was to expand the existing datasets with data of mainly non-forest sites that were representative of the major soil and habitat types in the UK. A further aim was to predict DOC concentrations from a number of biotic and abiotic explanatory variables such as rainfall, temperature, vegetation type and soil type in a multivariate way.Pore water was sampled using Rhizon or Prenart samplers at two to three week intervals for 1year. DOC, pH, organic carbon, carbon/nitrogen (C:N) ratios of soils and slope were measured and data on vegetation, soil type, temperature and precipitation were obtained.The majority of the variation in DOC concentrations between the UK sites could be explained by simple empirical models that included annual precipitation, and soil C:N ratio with precipitation being negatively related to DOC concentrations and C:N ratio being positively related to DOC concentrations.Our study adds significantly to the data reporting DOC concentrations in soils, especially in grasslands, heathlands and moorlands. Broad climatic and site factors have been identified as key factors influencing DOC concentrations.

Three representative UK moorland soils show differences in decadal release of dissolved organic carbon in response to environmental change

Abstract. Moorland carbon reserves in organo-mineral soils may be crucial to predicting landscape-scale variability in soil carbon losses, an important component of which is dissolved organic carbon (DOC). Surface water DOC trends are subject to a range of scaling, transport and biotic processes that disconnect them from signals in the catchment's soils. Long-term soil datasets are vital to identify changes in DOC release at source and soil C depletion. Here we show, that moorland soil solution DOC concentrations at three key UK Environmental Change Network sites increased between 1993–2007 in both surface- and sub- soil of a freely-draining Podzol (48 % and 215 % increases in O and Bs horizons, respectively), declined in a gleyed Podzol and showed no change in a Peat. Our principal findings were that: (1) considerable heterogeneity in DOC response appears to exist between different soils that is not apparent from the more consistent observed trends for streamwaters, and (2) freely-draining organo-mineral Podzol showed increasing DOC concentrations, countering the current scientific focus on soil C destabilization in peats. We discuss how the key solubility controls on DOC associated with coupled physico-chemical factors of ionic strength, acid deposition recovery, soil hydrology and temperature cannot readily be separated. Yet, despite evidence that all sites are recovering from acidification the soil-specific responses to environmental change have caused divergence in soil DOC concentration trends. The study shows that the properties of soils govern their specific response to an approximately common set of broad environmental drivers. Key soil properties are indicated to be drainage, sulphate and DOC sorption capacity. Soil properties need representation in process-models to understand and predict the role of soils in catchment to global C budgets. Catchment hydrological (i.e. transport) controls may, at present, be governing the more ubiquitous rises in river DOC concentration trends, but soil (i.e. source) controls provide the key to prediction of future C loss to waters and the atmosphere.

Controls on fluxes and export of dissolved organic carbon in grasslands with contrasting soil types

Export of dissolved organic carbon (DOC) from grassland ecosystems can be an important C flux which directly affects ecosystem C balance since DOC is leached from the soil to the groundwater. DOC fluxes and their controlling factors were investigated on two grassland sites with similar climatic conditions but different soil types (Vertisol vs. Arenosol) for a 2.5-year period. Parts of both grasslands were disturbed by deep ploughing during afforestation. Contrary to what was expected, ploughing did not increase DOC export but surface soil DOC concentrations decreased by 28% (Vertisol) and 14% (Arenosol). DOC flux from the soil profile was negatively influences by the clay content of the soil with seven times larger DOC export in the clay-poor Arenosol (55 kg C ha−1 a−1) than in the clay-rich Vertisol (8 kg C ha−1 a−1). At the Arenosol site, highest DOC concentrations were measured in late summer, whereas in the Vertisol there was a time lag of several months between surface and subsoil DOC with highest subsoil DOC concentrations during winter season. DOC export was not correlated with soil organic carbon stocks. Large differences in 14C concentrations of 22–40 pMC between soil organic carbon and DOC in the subsoil indicated that both C pools are largely decoupled. We conclude that DOC export at both sites is not controlled by the vegetation but by physicochemical parameters such as the adsorption capacity of soil minerals and the water balance of the ecosystem. Only in the acidic sandy Arenosol DOC export was a significant C flux of about 8% of net ecosystem production.

Influence of drought‐induced acidification on the mobility of dissolved organic carbon in peat soils

AbstractA strong relationship between dissolved organic carbon (DOC) and sulphate (SO42−) dynamics under drought conditions has been revealed from analysis of a 10‐year time series (1993–2002). Soil solution from a blanket peat at 10 cm depth and stream water were collected at biweekly and weekly intervals, respectively, by the Environmental Change Network at Moor House‐Upper Teesdale National Nature Reserve in the North Pennine uplands of Britain. DOC concentrations in soil solution and stream water were closely coupled, displaying a strong seasonal cycle with lowest concentrations in early spring and highest in late summer/early autumn. Soil solution DOC correlated strongly with seasonal variations in soil temperature at the same depth 4‐weeks prior to sampling. Deviation from this relationship was seen, however, in years with significant water table drawdown (&gt;−25 cm), such that DOC concentrations were up to 60% lower than expected. Periods of drought also resulted in the release of SO42−, because of the oxidation of inorganic/organic sulphur stored in the peat, which was accompanied by a decrease in pH and increase in ionic strength. As both pH and ionic strength are known to control the solubility of DOC, inclusion of a function to account for DOC suppression because of drought‐induced acidification accounted for more of the variability of DOC in soil solution (R2=0.81) than temperature alone (R2=0.58). This statistical model of peat soil solution DOC at 10 cm depth was extended to reproduce 74% of the variation in stream DOC over this period. Analysis of annual budgets showed that the soil was the main source of SO42−during droughts, while atmospheric deposition was the main source in other years. Mass balance calculations also showed that most of the DOC originated from the peat. The DOC flux was also lower in the drought years of 1994 and 1995, reflecting low DOC concentrations in soil and stream water. The analysis presented in this paper suggests that lower concentrations of DOC in both soil and stream waters during drought years can be explained in terms of drought‐induced acidification. As future climate change scenarios suggest an increase in the magnitude and frequency of drought events, these results imply potential for a related increase in DOC suppression by episodic acidification.