Soil Organic Carbon Transport Research Articles

Redox oscillations destabilize and mobilize colloidal soil organic carbon

The onset of oxic-anoxic events in soil induces biogeochemical processes that may have profound influences on the fate and transport of soil organic carbon (OC). An 18-wk long laboratory soil column study was conducted to investigate the influence of static oxic (SO), static anoxic (SA), or redox oscillation (RO) events on the release of OC. Column leachate samples were collected after 9-wk and 18-wk of incubations and separated into dissolved (<2.3 nm), natural nanoparticle (2.3–100 nm), fine colloid (100–450 nm), and particulate (> 450 nm) fractions that were characterized through spectrofluorometric analyses. The concentration of released OC from different treatment columns followed the order of SA > RO > SO. After 9-wk, total OC concentrations in the leachate samples were increased by 4-fold and 54-fold in the SO and SA columns compared to the time-zero control columns, respectively. However, after 18-wk, the released amount of OC doubled in SO but decreased by 50 % in SA columns compared to the 9-wk incubation samples. The RO columns had intermediate OC concentrations between the SO and SA treatments. The RO events further led to widely varied dynamics in the release and molecular composition of the size-fractionated OC compared to static conditions, indicating the effects of redox oscillation on the organo-mineral association in soil. The wide variations in the aromaticity of OC released after the 1st and 2nd RO events further support the notion that alternating redox processes regulate OC cycling differently than the SO or SA condition. The observed increase in fine colloid and particulate OC fractions (i.e., >100 nm) from 7 % to 40 % between 1st and 2nd RO suggests the clustering of nanoaggregates and/or formation of colloidal size aggregates. The composition of the released OC as influenced by redox fluctuations provides a baseline for the size continuum of soil OC and its potential ecological and environmental roles.

Drivers of water erosion-induced lateral soil carbon loss on the Tibetan Plateau

The lateral transport of soil organic carbon (SOC) induced by soil loss has global significance for understanding the terrestrial carbon budget. On the Tibetan Plateau, this is regulated by a complex interplay of environmental factors including soil properties, terrain attributes, climate variables, and vegetation conditions. However, it remains unclear how water erosion-induced SOC detachment responds to ongoing environmental changes. In this study, we used remote sensing, ground observations, and grid datasets to investigate the spatial distribution of lateral SOC loss and its drivers across the Tibetan Plateau from 2001 to 2017. The potential mean annual soil carbon erosion rate of 208.97 kg C ha−1 yr−1 was equivalent to an annual potential SOC removal flux of 40.72 Tg C over the study period. Approximately 95% of lateral SOC loss occurred in grasslands and forests. Soil properties such as bulk density and pH accounted for 31% of the relative influence on lateral SOC transfer, followed by terrain attributes such as terrain ruggedness index and altitude (25%). Climate and vegetation accounted for 22% of the relative influence on lateral SOC transfer, respectively. However, the most influential environmental indicator was the annual mean temperature. The interconnections of soil, terrain, climate, and vegetation in determining lateral SOC loss were identified by a path analysis. There is a need to include environmental drivers when assessing soil carbon dynamics to better inform decision makers developing strategies for managing carbon budgets.

Assessing branched tetraether lipids as tracers of soil organic carbon transport through the Carminowe Creek catchment (southwest England)

Abstract. Soils represent the largest reservoir of organic carbon (OC) on land. Upon mobilization, this OC is either returned to the atmosphere as carbon dioxide (CO2) or transported and ultimately locked into (marine) sediments, where it will act as a long-term sink of atmospheric CO2. These fluxes of soil OC are, however, difficult to evaluate, mostly due to the lack of a soil-specific tracer. In this study, a suite of branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are membrane lipids of soil bacteria, is tested as specific tracers for soil OC from source (soils under arable land, ley, grassland, and woodland) to sink (Loe Pool sediments) in a small catchment located in southwest England (i.e. Carminowe Creek draining into Loe Pool). The analysis of brGDGTs in catchment soils reveals that their distribution is not significantly different across different land use types (p&gt;0.05) and thus does not allow land-use-specific soil contributions to Loe Pool sediments to be traced. Furthermore, the significantly higher contribution of 6-methyl brGDGT isomers in creek sediments (isomerization ratio (IR) = 0.48±0.10, mean ± standard deviation (SD); p&lt;0.05) compared to that in catchment soils (IR = 0.28±0.11) indicates that the initial soil signal is substantially altered by brGDGT produced in situ. Similarly, the riverine brGDGT signal appears to be overwritten by lacustrine brGDGTs in the lake sedimentary record, indicated by remarkably lower methylation of branched tetraethers (MBT5ME′=0.46±0.02 in creek bed sediments and 0.38±0.01 in lake core sediments; p&lt;0.05) and a higher degree of cyclization (DC = 0.23±0.02 in creek bed sediments and 0.32±0.08 in lake core sediments). Thus, in this small catchment, brGDGTs do not allow us to trace soil OC transport. Nevertheless, the downcore changes in the degree of cyclization and the abundance of isoprenoid GDGTs produced by methanogens in the Loe Pool sediment do reflect local environmental conditions over the past 100 years and have recorded the eutrophication history of the lake.

Relative influence of forest and cropland on fluvial transport of soil organic carbon and nitrogen in the Nen River basin, northeastern China

Riverine transport of carbon and nitrogen strongly impacts terrestrial and aquatic ecosystem functions but is itself affected by changes in land cover and land use. China’s Nen River basin has experienced widespread conversion from forest to cropland in the past century, making it representative of similar global land use changes. We assessed organic carbon and nitrogen transport in this basin during 2018 and 2019, using water samples collected at one-hour intervals during the twelve flood events in two sub-basins (the Yalu and Yin rivers) dominated by forests and cropland, respectively. We also assessed dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations from 19 sampling sites distributed throughout the Nen River basin, in combination with hydrological records, to investigate the impacts of land cover on riverine carbon and nitrogen fluxes. The forested Yalu river basin showed significantly higher DOC content and lower TDN content than Yin river basin. Riverine DOC concentrations during non-flood periods in the forested Yalu river basin were significantly higher than in the cultivated Yin river basin, but riverine TDN concentrations were lower. Throughout the Nen River basin and its sub-basins, riverine TDN content was proportional to cropland area. During the twelve flood events, riverine DOC and TDN varied widely over time in the Yin and Yalu river basins, possibly due to dilution effects of water originating from different land covers within the basins. Our results demonstrate that land cover is the principal cause of changes in riverine DOC and TDN content in this region.

Relationships of the hydraulic flow characteristics with the transport of soil organic carbon and sediment loss in the Loess Plateau

Erosive power is characterized by the hydraulic forces of moving water and determines the water transport capacity of soil organic carbon (SOC) and sediment. Understanding the relationships of hydraulic flow characteristics (flow velocity and depth, shear stress and stream power) with the transport of sediments and SOC will improve the understanding of the transport mechanism of sediment and SOC, which in turn will improve SOC prediction. To address this issue, two loess soils (one silt loam and one silty clay) were selected, and 32 simulated rainfall experiments were conducted in a 1m by 5m soil pan at a varying slopes (10°, 15°, 20°, and 25°) and two rainfall intensities (90mmh−1 and 120mmh−1). The results showed that the flow velocities had significant positive linear relationships with the SOC concentrations (P<0.001) under the rainfall intensity of 90mmh−1 and that the flow velocities also had close relationships with the suspension transport of clay particles in the fine-textured soil and the rolling transport of light large aggregates in the coarse-textured soil. Under the rainfall intensity of 120mmh−1, the runoff depth was positively correlated with the SOC concentrations due to the suspension transport of the clay particles and was negatively correlated with the sediment concentration. The slope had a greater effect on the sediment and SOC loss of the coarse-textured soil than those of the fine-textured soil. Additionally, the stream power was a better descriptor of the sediment loss in the loess soils than shear stress. Overall, both the soil texture and rainfall intensity changed the relationships of the hydraulic flow characteristics with the sediment and SOC loss. Finally, the results of our study will provide important knowledge for improving or building hydraulic-based SOC and sediment loss models.

Forms and Fluxes of Soil Organic Carbon Transport via Overland Flow, Interflow, and Soil Erosion

Core Ideas Interflow is another crucial route of soil organic carbon lateral transport. Overland flow, interflow, and sediment contributed 5, 24, and 71%, respectively, of annual loss loads of soil organic carbon lateral transport. Dissolved organic carbon is another important component for SOC lateral transport. The contributions of hydrological pathways (including overland flow, interflow, and soil erosion) to lateral soil organic carbon (SOC) transport have remained unclear until now. Hillslopes were monitored using free‐draining lysimeters (8 m by 4 m) to quantify dissolved organic carbon (DOC) losses due to overland flow and interflow and the total organic carbon (TOC), water extractable organic carbon (WEOC), particulate organic carbon (POC), and mineral‐associated organic carbon (MOC) fractions in sediments from sloping croplands containing Regosols in Southwest China. The average annual DOC losses due to overland flow and interflow were 158.8 ± 33.0 and 750.4 ± 79.3 mg C m–2, respectively, and the TOC lost with sediment was 2201.0 ± 429.2 mg C m‒2. Overland flow, interflow, and sediment accounted for 5, 24, and 71%, respectively, of the annual SOC losses. The average annual DOC, POC, and MOC loss fluxes were 918.6 ± 115.3, 375.2 ± 94.4, and 1816.4 ± 331.8 mg C m–2, respectively. The MOC contents in the sediments were positively correlated with rainfall, and the DOC concentrations in the interflow water were negatively correlated with rainfall. In conclusion, soil erosion is the dominant hydrological route for lateral SOC transport, and interflow is another crucial route that is usually underestimated. Soil organic C is mainly lost in the forms of MOC and DOC, which is an important component of water ecosystems. Therefore, the mitigation of SOC losses would be more effective if soil erosion and interflow conservation practices were adopted together, particularly for Regosols on hillslopes.

The use of a raindrop aggregate destruction device to evaluate sediment and soil organic carbon transport

Abstract. Raindrop impact and subsequent aggregate breakdown can potentially change the movement behaviour of soil fractions and thus alter their transport distances when compared against non-impacted aggregates. In a given water layer, the transport distances of eroded soil fractions, and thus that of the associated substances across landscapes, such as soil organic carbon (SOC) and phosphorous, are determined by the settling velocities of the eroded soil fractions. However, using mineral size distribution to represent the settling velocities of soil fractions, as often applied in current erosion models, would ignore the potential influence of aggregation on the settling behaviour of soil fractions. The destructive effects of raindrops impacting onto aggregates are also often neglected in current soil erosion models. Therefore, the objective of this study is to develop a proxy method to effectively simulate aggregate breakdown under raindrop impact, and further identify the settling velocity of eroded sediment and the associated SOC. Two agricultural soils with different sandy and silty loam textures were subjected to rainfall using a raindrop aggregate destruction device (RADD). The aggregates sustained after raindrop impact were fractionated by a settling tube into six different classes according to their respective settling velocities. The same mass amount of bulk soil of each soil type was also dispersed and sieved into the same six classes, to form a comparison in size distribution. The SOC content was measured for each settled and dispersed class. Our results show the following: (1) for an aggregated soil, applying dispersed mineral grain size distribution, rather than its actual aggregate distribution, to soil erosion models would lead to a biased estimation on the redistribution of eroded sediment and SOC; (2) the RADD designed in this study effectively captures the effects of raindrop impact on aggregate destruction and is thus able to simulate the quasi-natural sediment spatial redistribution; (3) further RADD tests with more soils under standard rainfall combined with local rainfalls are required to optimize the method.

A laboratory experiment on the behaviour of soil-derived core and intact polar GDGTs in aquatic environments

Abstract. We have performed incubation experiments in order to examine the behaviour of soil-derived branched glycerol dialkyl glycerol tetraether (brGDGT) membrane lipids upon entering an aquatic environment and to evaluate the processes that potentially take place during their fluvial transport from land to sea. We incubated a soil from the Rakaia River catchment on the South Island of New Zealand using Rakaia River water and ocean water collected near the river mouth as inocula for a period of up to 152 days. The concentrations, as well as the relative distribution of brGDGTs derived from intact polar ("living"; IPL) lipids and core ("fossil"; CL) lipids remained unaltered over the course of the experiment. Although the stability of the brGDGTs may be a consequence of the higher than natural soil : water ratio used in the laboratory experiment, the substantial increase (27–72%) in the total pool of isoprenoid GDGTs (isoGDGTs) in all incubation setups, including the control using distilled water, indicates that entering an aquatic environment does influence the behaviour of soil-derived GDGTs. However, the availability of water appears to be more important than its properties. As a consequence of increasing isoGDGT concentrations, a decrease in Branched and Isoprenoid Tetraether (BIT) index values – a proxy for the relative input of fluvially discharged soil material into a marine system – became evident after an incubation period of 30 days, with a maximum final decrease of 0.88 to 0.74 in the experiment with river water. The relative distribution within the isoGDGT pool shows changes with time, suggesting that isoGDGT producers may either have different rates of membrane adaptation or production/degradation, or that preferential release from the soil matrix or a shift in source organism(s) may take place. While the apparent stability of soil brGDGTs during this incubation experiment reinforces their potential as tracers for land–sea transport of soil organic carbon and their use in paleoclimate reconstructions, the distributional differences between GDGTs in river water and nearby soil, as well as in river and ocean water, indicate that further research is needed to pinpoint the sources of GDGTs that are ultimately discharged to the oceans and are subsequently archived in continental margin sediments.

Atmospheric (210)Pb as a tracer for soil organic carbon transport in a coniferous forest.

Core soils and falling litter samples were collected in a Japanese cypress forest (Chamaecyparis obtusa) to determine the litter-fed (210)Pbex and organic carbon transfer from the forest canopy to soil and their subsequent distribution. Of the canopy residing (210)Pbex pool, litterfall annually transports 53% to the forest floor while it adds 117 g m(-2) per year of organic carbon to the forest soil, implying that litterfall dynamics can influence the distribution of (210)Pbex and soil organic carbon (SOC). (210)Pbex and SOC showed identical profile shapes and strong correlation in spatial as well as along the soil depth, indicating that both are affected by a similar process. Given the ubiquitous natural source of (210)Pbex, it is plausible to infer that radiolead can be a possible tracer to study the SOC redistribution at regional and global scales.

Temporal variability of soil organic carbon transport in the Enxoé agricultural watershed

The temporal variability of particulate (POC) and dissolved (DOC) organic carbon concentrations was analyzed in the Enxoe temporary river, southern Portugal, between September, 2010 and August, 2013. The overall aim was to study the variability of those elements during storm events, and determine their origin and the main transfer mechanisms to the river. Twenty-one flood events were observed. An empirical model was used to describe changes in solute concentrations, and the magnitude and rotational patterns of the hysteretic loops during flood events. POC and DOC concentrations varied between 0.49–88.93 and 0.25–25.75 mg L−1, respectively. POC and DOC annual yields varied between 0.06–2.15 and 0.03–1.47 t km−2, respectively. Flood events had greater effect in POC than in DOC variability. POC had mostly a terrestrial origin, with exports being related to soil erosion and runoff. POC revealed a flushing behavior during the entire monitored period, and clockwise or anticlockwise trajectory loops whenever the predominant origin of the exports was in river bed deposits or arable lands, respectively. DOC had also a terrestrial origin, but it revealed a contrasting dilution behavior and, in general, anticlockwise hysteresis loops. DOC showed a delay in the arrival of solutes to the river, consistent with mass flow through subsurface flow. DOC exports were thus associated with soil weathering and crop mineralization. This work highlights the main processes involved in POC and DOC loads in a temporary river during flood events, with a precise quantification of those elements.

Tracing soil organic carbon in the lower Amazon River and its tributaries using GDGT distributions and bulk organic matter properties

In order to trace the transport of soil organic carbon (OC) in the lower Amazon basin, we investigated the distributions of crenarchaeol and branched glycerol dialkyl glycerol tetraethers (GDGTs) by analyzing riverbed sediments and river suspended particulate matter (SPM) collected in the Solimões-Amazon River mainstem and its tributaries. The Branched and Isoprenoid Tetraether (BIT) index, a proxy for river-transported soil OC into the ocean, was determined from the distributions of these GDGTs. The GDGT-derived parameters were compared with other bulk geochemical data (i.e. C:N ratio and stable carbon isotopic composition). The GDGT-derived and bulk geochemical data indicate that riverine SPM and riverbed sediments in the lower Amazon River and its tributaries are a mixture of C3 plant-derived soil OC and aquatic-derived OC. The branched GDGTs in the SPM and riverbed sediments did not predominantly originate from the high Andes soils (>2500m in altitude) as was suggested previously. However, further constraint on the soil source area of branched GDGTs was hampered due to the deficiency of soil data from the lower montane forest areas in the Andes. Our study also revealed seasonal and interannual variation in GDGT composition as well as soil OC discharge, which was closely related to the hydrological cycle. By way of a simple binary mixing model using the flux-weighted BIT values at Óbidos, the last gauging station in the Amazon River, we estimated that 70–80% of the POC pool in the river was derived of soil OC. However, care should be taken to use the BIT index since it showed a non-conservative behaviour along the river continuum due to the aquatic production of crenarchaeol. Further investigation using a continuous sampling strategy following the full hydrological cycle is required to fully understand how soil-derived GDGT signals are transformed in large tropical river systems through their transport pathway to the ocean.

Empirical relationships for soil organic carbon transport from agricultural watersheds in Ohio

AbstractImproved quantification is needed for long‐term soil organic carbon (SOC) transport in runoff at watershed scales. Coshocton wheel samplers were used to collect runoff samples from no‐till and chisel‐till watersheds in corn (Zea mays) and soybean (Glycine max) rotations over 13 years. Samples were analyzed for SOC, N, P, K, and soil losses. The SOC losses, ranging from 0 to 357 kg ha−1 event−1, were correlated (r2 = 0·80–0·94) in power law relationships with N, P, K, soil loss, and runoff. Two events occurring in corn when soybean and cover crop residue were present in no‐till had combined SOC transport of 460 kg ha−1, nearly double the no‐till losses of a previous 11‐year period and 20 times higher than chisel‐till in the same events. Infrequent, extreme transport events that are not well characterized empirically, particularly in no‐till, can strongly influence hydrologic C transport from agriculture watersheds. Copyright © 2007 John Wiley &amp; Sons, Ltd.