Soil Water Dissolved Organic Carbon Concentrations Research Articles

Dissolved organic carbon leaching from montane grasslands under contrasting climate, soil and management conditions

Grasslands are thought to be more vulnerable than many other ecosystems to climate change since their soils are characterized by high organic carbon contents and warming in montane regions is twice the global average rate. Despite these expected vulnerabilities, little is known about how climate change and management influence dissolved organic carbon (DOC) losses from montane grasslands and how relevant these losses are compared to other ecosystem carbon fluxes. In this study, 36 large (1 m2, 140 cm height) grassland lysimeters were filled with undisturbed soil monoliths and operated at three different sites located along an elevational gradient from 860 to 600 m a.s.l. From 2012 to 2014, changes in soil DOC concentrations and DOC leaching losses were quantified under different climate, soil and management (intensive vs. extensive; i.e., differing in the frequency of cutting and manuring events) conditions. The annual DOC leaching losses ranged between 6.6 and 27.5 kg C ha−1 year−1, which was only a minor (< 3%) component of the net ecosystem carbon exchange. DOC leaching losses were not affected by management intensity but were positively correlated with soil organic carbon in the top soil. Climate warming (~ + 2 °C) significantly increased DOC leaching rates for intensive (+ 43%) and extensive management (+ 58%), but only if simultaneous reductions in leachate were moderate. The DOC concentrations in soil water significantly decreased with soil depth. In 10, 30 and 50 cm DOC concentration were positively correlated with aboveground biomass production, indicating that plants play a crucial role in topsoil DOC dynamics. However, this relationship did not occur for DOC in soil water collected at 140 cm, suggesting that adsorption and degradation processes in the subsoil ultimately determine the dynamics of DOC losses below the rooting zone.

Effect of land cover and hydro‐meteorological controls on soil water DOC concentrations in a high‐elevation tropical environment

AbstractPáramo soils store high amounts of organic carbon. However, the effects of climate change and changes in land cover and use (LC/LU) in this high‐elevation tropical ecosystem may cause a decrease in their carbon storage capacity. Therefore, better understanding of the factors influencing the Páramo soils' carbon storage and export is urgently needed. To fill this knowledge gap, we investigated the differences in dissolved organic carbon (DOC) content in the soil water of four LC/LU types (tussock grass, natural forest, pine plantations, and pasture) and the factors controlling its variability in the Quinuas Ecohydrological Observatory in south Ecuador. Weekly measurements of soil water DOC concentrations, meteorological variables, soil water content, and temperature from various depths and slope positions were monitored within the soils' organic and mineral horizons between October 2014 and January 2017. These data were used to generate regression trees and random forest statistical models to identify the factors controlling soil water DOC concentrations. From high to low concentrations, natural forest depict the highest DOC concentrations followed by pasture, tussock grass, and pine forest. For all LC/LU types, DOC concentrations increase with decreasing soil moisture. Our results also show that LC/LU is the most important predictor of soil water DOC concentrations, followed by sampling depth and soil moisture. Interestingly, atmospheric variables and antecedent evapotranspiration and precipitation conditions show only little influence on DOC concentrations during the monitoring period. Our findings provide unique information that can help improve the management of soil and water resources in the Páramo and other peat dominated ecosystems elsewhere.

Decreasing soil water Ca2+ reduces DOC adsorption in mineral soils: Implications for long-term DOC trends in an upland forested catchment in southern Ontario, Canada

Positive trends in dissolved organic carbon (DOC) concentration have been observed in surface waters throughout North America and northern Europe. Although adsorption in mineral soils is an important driver of DOC in upland streams, little is known about the potential for changes in DOC adsorption to contribute to these trends. We hypothesized that long-term declines in soil water Ca2+ levels, in response to declining acid deposition, might influence DOC adsorption and that this could contribute to long-term DOC trends in an upland forested catchment in south-central Ontario, Canada. Between 1987 and 2009, DOC concentrations increased significantly (p<0.05) in stream water and B horizon soil water (2 out of 3 sites). The null point DOC concentration (DOCnp), which is a measure of the soil water DOC concentration at equilibrium with the soil, ranged from 1.27 to 3.75mgL−1 in B horizon soils. This was similar to the mean DOC concentrations of B horizon soil water (2.04–6.30mgL−1) and stream water (2.20mgL−1), indicating that soil and stream water DOC concentrations are controlled by equilibrium processes at the soil–water interface. Adsorption experiments using variable Ca2+ concentrations demonstrated that as Ca2+ decreased the DOCnp increased (1.96 to 4.74mgL−1), which was consistent with the observed negative correlation between DOC and Ca2+ in B horizon soil water (p<0.05; r2=0.21). Additional adsorption experiments showed that Na+ had no effect on DOC adsorption (p>0.05), indicating that changes in DOC adsorption might be related to cation bridging. We conclude that declines in soil water Ca2+ concentration can contribute to increasing DOC trends in upland streams by reducing DOC adsorption in mineral soils.

Decreased DOC concentrations in soil water in forested areas in southern Sweden during 1987–2008

During the last two decades, there is a common trend of increasing concentrations of dissolved organic carbon (DOC) in streams and lakes in Europe, Canada and the US. Different processes have been proposed to explain this trend and recently a unifying hypothesis was presented, concluding that declining sulphur deposition and recovery from acidification, is the single most important factor for the long-term DOC concentration trends in surface waters. If this recovery hypothesis is correct, the soil water DOC concentrations should increase as well. However, long-term soil water data from Sweden and Norway indicate that there are either decreasing or indifferent DOC concentrations, while positive DOC trends have been found in the Czech Republic. Based on the soil water data from two Swedish integrated monitoring sites and geochemical modelling, it has been shown that depending on changes in pH, ionic strength and soil Al pools, the DOC solubility might be positive, negative or indifferent. In this study, we test the acidification recovery hypothesis on long-term soil water data (25 and 50 cm soil depth) from 68 forest covered sites in southern Sweden, showing clear signs of recovery from acidification. The main aim was to identify potential drivers for the DOC solubility in soil solution by comparing trends in DOC concentrations with observed changes in pH, ionic strength and concentrations of Al n+. As in earlier Swedish and Norwegian studies, the DOC concentrations in soil water decreased or showed no trend. The generally small increases in pH (median < 0.3 pH units) during the investigation period seem to be counterbalanced by the reduced ionic strength and diminished Al concentrations, increasing the organic matter coagulation. Hence, opposite to the conclusion for surface waters, the solubility of organic matter seems to decrease in uphill soils, as a result of the acidification recovery.

Regulation of stream water dissolved organic carbon (DOC) concentrations during snowmelt; the role of discharge, winter climate and memory effects

Abstract. Using a 15 year stream record from a northern boreal catchment, we demonstrate that the inter-annual variation in dissolved organic carbon (DOC) concentrations during snowmelt was related to discharge, winter climate and previous DOC export. A short and intense snowmelt gave higher stream water DOC concentrations, as did long winters, while a high previous DOC export during the antecedent summer and autumn resulted in lower concentrations during the following spring. By removing the effect of discharge we could detect that the length of winter affected the modeled soil water DOC concentrations during the following snowmelt period, which in turn affected the concentrations in the stream. Winter climate explained more of the stream water DOC variations than previous DOC export during the antecedent summer and autumn.

Dissolved carbon and nitrogen leaching following variable logging-debris retention and competing-vegetation control in Douglas-fir plantations of western Oregon and Washington

We examined the effect of logging-debris retention and competing-vegetation control (CVC, initial or annual applications) on dissolved organic carbon (DOC), dissolved organic nitrogen, and nitrate-N leaching to determine the relative potential of these practices to contribute to soil C and N loss at two contrasting sites. Annual CVC resulted in higher soil water nitrate-N concentration and flux, with the magnitude and duration of the effect greatest at the high-N site. Most of the increase in nitrate-N at the low-N site occurred in treatments where logging debris was retained. Dissolved organic nitrogen increased at the high-N site in March of each year following annual CVC, but the contribution of this increase to total N concentration was small (2%–4% of total N flux). There was no effect of logging-debris retention or CVC treatment on soil water DOC concentrations, indicating that DOC inputs from logging debris and competing vegetation were either retained or consumed in the mineral soil. The estimated increase in leaching flux of dissolved C and N associated with the treatments was low relative to total soil pools, making it unlikely that loss of these elements via leaching will negatively affect future soil productivity at these sites.

Dissolved Organic Carbon Concentrations in Throughfall and Soil Waters at Level II Monitoring Plots in Norway: Short- and Long-Term Variations

We investigated concentrations of dissolved organic carbon (DOC) in throughfall and soil solutions at 5, 15 and 40-cm depth in 16 Norway spruce and two Scots pine plots throughout Norway between 1996 and 2006. Average DOC concentrations ranged from 2.3 to 23.1 mg/l and from 1.1 to 53.5 mg/l in throughfall water and soil solutions, respectively. Concentrations of DOC in throughfall and soil waters varied seasonally at most plots with peaks in the growing season. By contrast to recently reported positive long-term trends in DOC concentrations in surface waters between 1986 and 2003, soil water data from 1996 to 2006 showed largely negative trends in DOC concentrations and no significant trends in throughfall. However, regression analysis for individual sites, particularly at 5- and 15-cm soil depths, showed that DOC concentrations in soil water were significantly and negatively related to non-marine sulphate (SO4) and chloride (Cl−). The lack of a long-term increase in DOC in soil water in the period May 1996–December 2006 may be due to the relatively small changes in the deposition of SO4 and Cl− in this period.

Effects of managed burning upon dissolved organic carbon (DOC) in soil water and runoff water following a managed burn of a UK blanket bog

Summary The burning of heather and grass to maintain a mosaic of different aged vegetation stands is a widespread management practice in the uplands of the UK. However, there is concern that burning also releases dissolved organic carbon (DOC) into drinking water supplies. This study was based on a long term experiment examining different rotational burning cycles and grazing intensities on upland vegetation. The study aimed to understand the effect these management practices have on water quality both in the long term and in the short term following a managed burn. The study has found that: 1. At the end of a burn cycle, DOC concentrations in soil water or runoff water are not significantly affected by burning treatment. However colour (absorbance at 400 nm) was found to be significantly lower on 20 year burn plots than unburnt controls. 2. In the weeks following the burn there were peaks in DOC concentration and colour in the soil water of burnt plots compared to unburnt controls but these peaks were short lived and neither DOC concentrations nor colour were significantly elevated 1 year after burning. 3. The composition of the DOC in soil water and runoff water is not affected by burning treatment; rather, the variation in data is controlled by time of sampling and season. 4. Values for three carbon parameters (absorbance at 400 nm, DOC concentration and specific absorbance) are significantly lower in runoff water than soil water. 5. Grazing does not significantly affect carbon parameters in soil water at the end of a burn cycle however grazing effects can be seen in runoff water at the end of a 10-year burn cycle.

Connecting organic carbon in stream water and soils in a peatland catchment

We investigated the spatial and temporal connectivity between concentrations of organic carbon in stream water and the soil carbon pool in a small (1.3 km2) upland catchment in NE Scotland by comparing downstream changes in dissolved organic carbon (DOC) with spatial changes in the soil carbon pool. Stream water DOC concentrations increased downstream to reach a maximum in the center of the catchment and then decreased in the lower part of the catchment, a pattern that was linked to spatial changes in the soil carbon pool. Subcatchment analysis of the soil carbon pool and DOC export showed that the strong relationship between percent of peat coverage and stream water DOC in the upper part of the catchment did not hold farther downstream, where freely draining mineral soils become spatially more important. Temporal variations in stream water and soil water DOC concentrations in the central part of the catchment were studied using samples collected from two depths (0.20–0.45 m and 0.65–0.95 m) in the two most important soil types there (histosols and fluvisols). Stream water DOC concentrations were only related to soil water DOC in shallow peat. DOC concentration in shallow peat and stream water were both positively correlated with temperature. Multivariate regression analysis showed that 55% of the variation in stream water DOC concentrations could be explained by a model including stream temperature, discharge, and soil solution DOC concentrations in the shallow histosols. We conclude that the linkage between stream water DOC concentrations and the soil C pool in the upper 1.5 km of the stream reach are likely to be driven by temperature‐related DOC production in near‐surface histosols. Farther downstream, this relationship between organic carbon in the soil and stream becomes weaker as other processes (e.g., lower inputs from minerals soils and allochthonous within‐stream processing of DOC) become more important.

Dissolved Organic Matter concentration and composition in the forests and streams of Olympic National Park, WA

Dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) character were investigated in soil water (15 and 40 cm) and streams at eleven sites in Olympic National Park. In addition, the effect of added nitrogen on soil water DOM concentration and composition was tested. Forested plots covering a gradient of precipitation, climate, slope, and aspect in Olympic National Park were fertilized with the addition of 20, 10 and zero (control) kg urea-N ha−1 y−1. Seven sites had the two different fertilizer treatments and control plots, while the additional four sites had no fertilizer treatments. Soil water DOC concentrations ranged from 0.5 mg C/L to 54.1 mg C/L, with an average value of 14.1 mg C/L. Streams had low DOC concentrations ranging from 0.2 mg C/L to 4.4 mg C/L, with an average value of 1.2 mg C/L. DOM composition was examined with regard to molar ratios, H:C, O:C and N:C, index of unsaturation, average carbon oxidation state, and specific absorbance. Fertilizer had no consistent effect on either DOM concentration or composition across the study sites. Soil depth influenced both DOM concentration and composition. Shallow soil water DOM had greater concentrations, higher specific absorbance, a higher degree of unsaturation, and had lower molar ratios compared to deep soil water samples. Overall, changes in DOM stoichiometry and specific absorbance as a function of soil depth were consistent despite the diversity of the forested study sites sampled.

A model of dissolved organic carbon concentrations in soil and stream waters

AbstractThe release of dissolved organic carbon (DOC) into drainage waters was modelled using the hydrology of the Birkenes model of stream water chemistry and estimated parameters for the formation and decay of soluble organic matter in the soil. The model was first tested against soil water DOC concentrations over a three year period in the Loch Dee catchment in southwest Scotland. It predicted annual cycles in DOC at one site, but underestimated maximum DOC in the first year, overestimated in the following year, and predicted values very similar to the measured values in the third year. With small modifications to the parameters controlling organic matter addition and decay it successfully predicted DOC variations at two other soil water sites with smaller mean DOC.Further tests were made against short‐term DOC variations in a stream draining a subcatchment of Loch Ard in west‐central Scotland, using the same decay and addition parameters. Prediction of temporal variation was good, although predicted concentrations were about 25 per cent less than measured values. Considerable potential for the use of fully optimized models of DOC production in soil and stream waters is identified on the basis of these results.