Runoff Dissolved Organic Carbon Research Articles

Straw mulching alters the composition and loss of dissolved organic matter in farmland surface runoff by inhibiting the fragmentation of soil small macroaggregates

Straw mulching is a widespread practice for reducing the soil carbon loss caused by erosion. However, the effects of straw mulching on dissolved organic matter (DOM) runoff loss from black soil are not well studied. How straw mulching affects the composition and loss of runoff DOM by changing soil aggregates remains largely unclear. Here, a straw mulching treatment was compared to a no mulching treatment (as a control) on sloping farmland with black soil erosion in Northeast China. We divided the soil into large macroaggregates (>2 mm), small macroaggregates (0.25–2 mm), and microaggregates (<0.25 mm). After five rain events, the effects of straw mulching on the concentration (characterized by dissolved organic carbon (DOC)) and composition (analyzed by fluorescence spectroscopy) of runoff and soil aggregate DOM were studied. The results showed that straw mulching reduced the runoff amount by 54.7%. Therefore, although straw mulching increased the average DOC concentration in runoff, it reduced the total runoff DOM loss by 48.3%. The composition of runoff DOM is similar to that of soil, as both contain humic-like acid and protein-like components. With straw mulching treatment, the protein-like components in small macroaggregates accumulated and the protein-like components in runoff declined with rain events. Fluorescence spectroscopy technology may help in understanding the hydrological paths of rain events by capturing the dynamic changes of runoff and soil DOM characteristics. A variation partitioning analysis (VPA) indicated that the DOM concentration and composition of microaggregates explained 68.2% of the change in runoff DOM from no mulching plots, while the change in runoff DOM from straw mulching plots was dominated by small macroaggregates at a rate of 55.1%. Taken together, our results demonstrated that straw mulching reduces the fragmentation of small macroaggregates and the loss of microaggregates, thus effecting DOM compositions in soil and reducing the DOM loss in runoff. These results provide a theoretical basis for reducing carbon loss in sloping farmland.

Effects of cornstalk mulching and rainfall intensity on the quantity and quality of dissolved organic carbon in runoff: A field rainfall simulation at the plot scale

AbstractUnderscoring how crop mulching changes the dissolved organic carbon (DOC) in runoff is essential to understanding the sustainable application of cornstalk mulching. This study examined the runoff DOC quantity and quality in plots with and without cornstalk mulching under various rainfall intensities in a representative steep fallow land of southwestern China. Field rainfall simulations within a 60 min duration were conducted on the triplet plots (1.5 m × 1 m for a single plot) with bare soils under 30‐ and 90‐mm h−1 intensities and with cornstalk mulching (air‐dried whole plants with 60% coverage and weight 1.65 kg per plot) under 90 mm h−1 intensity, respectively. The concentration and loss of runoff DOC and its quality variables, including SUVA254 (aromaticity of DOC), C:C (proportion of colored humic substances in DOC), and E4:E6 (fulvic acid to humic acid in DOC), are determined. Results showed that runoff DOC concentration and loss were both significantly higher at 90 mm h−1 intensity than at 30 mm h−1 intensity (p &lt; 0.05), simultaneously, they were 52.6% and 53.2% higher in mulched plots than in bare plots (p &lt; 0.05), respectively. Runoff SUVA254 and C:C in bare plots were 90% and 88.9% higher in 30 mm h−1 intensity than in 90 mm h−1 intensity (p &lt; 0.05), respectively. Whereas, E4:E6 of runoff was 12.3% higher in mulched plots than that in bare plots (p &lt; 0.05). Our findings imply that cornstalk mulching enhances the export of runoff DOC with a decline in aromaticity and the molecular weight of humic substances.

Dynamics of dissolved organic carbon in runoff discharge under different rainfall patterns in a representative agricultural catchment

Rainfall-induced runoff transported plentiful terrestrial dissolved organic carbon (DOC) to downstream waters, significantly regulating the biogeochemical cycling of carbon in downstream waters. However, the dynamics of DOC exporting from agricultural catchments in response to different rainfall patterns remain largely unknown. This study aimed to understand the effects of rainfall patterns on the runoff DOC quantity and quality in a representative agricultural catchment in Southwest China. Six rainfall events producing runoff, including three moderate rainfalls, two heavy rainfalls, and one rainstorm, were recorded to investigate the shifts in runoff DOC concentration and its quality indicators, i.e., SUVA254 (indicating the aromaticity content), C:C ratio (denoting the proportion of colored substances to uncolored substances), and E4:E6 ratio (denoting the fulvic acid to humic acid of DOC). The results showed that DOC concentration in runoff discharge was highest in the moderate and heavy rainfalls occurring at the early rainy season, while was much lower in the rainstorm. Exported DOC flux reached 1.82 kg hm-2 h−1 in the rainstorm, which was 3.95–60.67 times that in the other rainfall events. SUVA254 and C:C ratio fluctuated softly during the moderate rainfalls and rainstorm, and changed strongly during the heavy rainfall with longest antecedent dry day. The moderate and heavy rainfalls with longer antecedent dry day and the rainstorm delivered DOC with higher SUVA254 and C:C ratio than the other rainfall events did. Discharge was positively correlated with DOC concentration during rainfall across all rainfall patterns, while was negatively correlated with SUVA254 and C:C ratio. Forward stepwise regression analysis showed that DOC flux per hour was only positively linearly correlated with rainfall intensity. Our results were valuable for understanding the biogeochemical cycling of DOC in analogous catchments worldwide.

Isotope composition of dissolved organic carbon in runoff and peat leachates from a Central European wetland: Temporal and spatial variability in DOC sources

In the near future, climate change will increasingly affect export of carbon (C) from peatlands. Terminal mineralization of dissolved organic carbon (DOC) is a source of greenhouse gases (carbon dioxide, CO2, and methane, CH4) for the atmosphere, and may contribute to climatic warming. Quantitative understanding of sources, dispersion pathways, and degradation of DOC produced in peat bogs is important for predictions of global change dynamics. Here we explore the potential of combining C isotope ratios of isotopically stratified, 210Pb-dated peat deposits and C isotope ratios of DOC exported via runoff to determine the depth and age of the predominant DOC source. In a poorly drained, Sphagnum-derived peat bog in the Czech Republic, δ13C values of bulk peat increased downcore to a depth of 10 cm. The C isotope signatures of fresh DOC and bulk peat differ only insignificantly, because C isotope changes in maturating peat require multiple microbial modifications of the organic matter taking place over relatively long time periods. Based on 41 samplings of runoff, 70% of exported DOC originated from peat substrate 1 to 10 years old, 2–10 cm deep. Recently photosynthesized DOC (<1 year old) contributed about 5% to runoff DOC, whereas approximately 25% of exported DOC was derived from peat layers older than 10 years. Because biogenic methane at the study site contained isotopically extremely light C (δ13C values of −64.2 to −53.2‰), it would seem reasonable to expect residual DOC following partial decomposition to become isotopically heavier. A short-term laboratory incubation of wet peat samples from four depths at mid-summer temperatures indicated more complicated δ13C systematics. The initial limited amount of DOC in the leachate was isotopically heavy, compared to bulk peat, but after 18 days δ13CDOC became lower, converging to the C isotope signature of bulk peat. Such δ13C systematics can be explained by multi-step isotope effects associated with different reaction rates during substrate decomposition. Limitations of using the combination of within-bog δ13C systematics and 210Pb dating of recent peat accretion are discussed. Horizontal heterogeneity in substrate C isotope signatures is one of the main sources of uncertainty of this approach to DOC source apportionment in peaty soils.

Atmospheric deposition coupled terrestrial export of organic carbon in Ganga River (India): linking cross-domain carbon transfer to river DOC

The atmosphere–land–water connectivity of dissolved organic carbon (DOC) is not altogether accounted in major rivers of India despite recent researches highlighting its importance. We studied the coupled effect of atmospheric deposition (AD) and local land use on DOC buildup in Ganga River for a period of 6 years. The AD-OC input increased consistently over time and there was over 1.5- to 1.8-fold increase in 2012 relative to 2007. Microbial activity and water soluble organic carbon (WSOC) in sub-catchment and DOC in land surface runoff increased consistently over time along the gradient of AD input. The river DOC showed positive correlation (R 2 = 0.24–0.84; p < 0.001) with AD-OC and runoff DOC and WSOC showed positive correlation (R 2 = 0.96; p < 0.001) with soil microbial activity. Principal component analysis segregated study sites into four groups demarcating source relationships. Our study, that forms the first report on atmosphere–land–water transfer of organic carbon in Ganga River, suggests that future climate models should include region-specific time series data on changing state of atmosphere– land–water connectivity and associated shift in carbon balance of major rivers for more accurately predicting the climate change drivers.

Precipitation and net ecosystem exchange are the most important drivers of DOC flux in upland boreal catchments

According to recent studies, dissolved organic carbon (DOC) concentrations in rivers throughout the boreal zone are increasing. However, the mechanistic explanation of this phenomenon is not yet well known. We studied how the short and long-term changes in precipitation, soil temperature, soil water content, and net ecosystem exchange (NEE) are reflected to DOC concentrations and runoff DOC fluxes in two small forested upland catchments in Southern Finland. We used continuous eddy covariance measurements above the forest and runoff flow measurements from the catchment areas conducted over a 15 year long time period to study the correlation between NEE, gross photosynthetic production, total ecosystem respiration, litter production, and runoff DOC. In addition, we looked for the most important environmental variables in explaining the interannual changes in runoff DOC by using multiple linear regression. Finally, we studied the temporal connection between runoff DOC concentrations, precipitation, soil water content, and NEE by using wavelet coherence analysis technique. Our results indicate that the DOC concentrations have increased over the last 15 years. The DOC flux was to a large extent determined by the amount of precipitation, but the previous year's NEE and litter production had also a small but significant effect on runoff DOC fluxes.

Effect of soil erosion on dissolved organic carbon redistribution in subtropical red soil under rainfall simulation

Water erosion governs soil carbon reserves and distribution across the watershed or ecosystem. The dynamics of dissolved organic carbon (DOC) under water erosion in red agricultural soil is not clear. To determine the effect of tillage management and water erosion on vertical and lateral transportation of soil organic carbon (SOC) and DOC production under distinct rainfall intensities in the hilly red soil region of southern China, a chisel tillage plot with low rainfall intensity (CT-L) and two no-tillage plots with high (NT-H) and low rainfall intensity (NT-L) studies were conducted. Soil samples were collected from 0–5, 5–10, 10–20, and 20–40cm soil layers from triplicate soil blocks pre- and post-rainfall for determining concentration of SOC and DOC. Runoff samples were collected at every 6min for determining concentration of DOC and sediments during rainfall simulations on runoff plots (2m×5m) with various intensities. No fertilizer was applied in any plots. Results clearly show that runoff volumes, sediments and SOC entrained with sediment, and laterally mobilized DOC were significantly larger on NT-H compared to other plots, coinciding with changes in rainfall intensity; and the extent of roughness of the plot surface (CT vs. NT) was the variation in runoff DOC concentration. During the simulated rainfall events, DOC exports average 0.76, 0.64, and 0.27gCm−2h−1; SOC exports average 3.52, 1.08, and 0.07gm−2h−1 in the NT-H, NT-L, and CT-L soils, respectively. The maximum export of DOC was obtained under a high intensity rainfall plot, which lagged behind maximum runoff volumes, sediments, and SOC losses with sediment. Export of DOC was proportional to SOC content of soil loss. The least DOC losses in surface runoff and SOC losses with sediment were observed in CT-L plots. Vertical DOC mobilization achieved its maximum with low intensity rainfall under CT treatment. The DOC did not accumulate at the soil surface and was distributed mainly in the second and third soil horizons. The distribution of DOC content down the soil profile increased compared to pre-rainfall, except for subplots E at NT-H and NT-L. Results indicate that rainfall significantly increased DOC content in experimental plots. The SOC content of sediment leaving the erosion zone was significantly correlated with overland flow volume and soil loss. These observations lead to the conclusion that soil erosion is an important factor controlling the export of dissolved organic carbon.

Sources and flowpaths of dissolved organic carbon during storms in two forested watersheds of the Precambrian Shield

Dissolved organic carbon (DOC) concentrations and export were studied in two small catchments in central Ontario to examine DOC sources and to assess the hypothesis that organic matter adjacent to the stream is a significant contributor of DOC during storms. Different DOC dynamics and exports were observed according to the depth of the riparian water table. In Harp 4-21, riparian flowpaths were predominantly through A and upper B soil horizons and riparian soils contributed between 73 and 84% of the stream DOC export during an autumn storm. In Harp 3A, riparian flowpaths were predominantly through lower B horizons. Consequently, riparian soils were less important and hillslopes contributed more than 50% of the stream DOC export in subcatchments without wetlands during storms. Wetlands and adjacent soils contributed significantly to DOC export in Harp 3A; 8% of the total catchment area exported 32 to 46% of the storm runoff DOC. DOC export dynamics in wetlands and riparian soils were distinctly different. In wetlands, transport was affected by leaching and flushing of DOC at the wetland surface leading to lower DOC concentrations with successive storms. In riparian soils, groundwater flowpaths were more important and stronger positive relationships between discharge and DOC concentration were observed. Precipitation, throughfall and stemflow were minor sources of stream DOC during storms and contributed less than 20% of the total export.