The transport of aggregates associated with soil organic carbon under the rain‐induced overland flow on the Chinese Loess Plateau

Abstract

Organic carbon (OC) is easily enriched in sediment particles of different sizes due to aggregate breakdown and selective transport for sheet erosion. However, the transport of aggregate‐associated OC has not been thoroughly investigated. To address this issue, 27 simulated rainfall experiments were conducted in a 1 m × 0.35 m box on slope gradients of 15°, 10°, and 15°and under three rainfall intensities of 45 mm h−1, 90 mm h−1 and 120 mm h−1. The results showed that OC was obviously enriched in sediment particles of different sizes under sheet erosion. The soil organic carbon (SOC) concentrations of each aggregate size class in sediments were different from those in the original soil, especially when the rainfall intensity or slope was sufficiently low, such as 45 mm h–1 or 5°, respectively. Under a slope of 5°, the SOC enrichment ratios (ERocs) of small macroaggregates and microaggregates were high but decreased over time. As rainfall intensity increased, OC became enriched in increasingly fine sediment particles. Under a rainfall intensity of 45 mm h–1, the ERocs of the different aggregate size classes were always high throughout the entire erosion process. Under a rainfall intensity of > 45 mm h–1 and slope of > 5°, the ERocs of the different aggregate size classes were close to 1.0, especially those of clay and silt. Therefore, the high ERocs in sediments resulted from the first transport of effective clay. Among total SOC loss, the proportion of OC loss caused by the transport of microaggregates and silt plus clay‐sized particles was greater than 50%. We also found that low stream power and low water depth were two requirements for the high ERocs in aggregates. Stream power was closely related to sediment particle distribution. Flow velocity was significantly and positively related to the percentage of OC‐enriched macroaggregates in the sediments (P > 0.01). Our study will provide important information for understanding the fate of SOC and building physical‐based SOC transport models. © 2019 John Wiley & Sons, Ltd.

Effects of interrill erosion on the distribution of soil organic and inorganic carbon in different sized particles of Mediterranean Calcisols

“…Despite the important influence of the transport of aggregates by erosion on associated carbon concentrations in different aggregate size classes this topic has not been thoroughly investigated. Liu et al (2019) reported that the SOC concentrations of each aggregate size class in sediments were different from those in the original soil and SOC was enriched in sediment particles of different sizes under sheet erosion.…”

Hydraulic mechanisms of the uneven enrichment of soil organic carbon in sediments during rain-induced overland flow

“…To the contrary, the stream power increased and quickly became stable with time, corresponding with the sediment loss rate. In this study, the high flow velocities have a positive exponential correlation with the transport number of >0.05 mm aggregates with high density and low OC concentration [ 16 , 17 ]. This is because the erosion is controlled by transport limitation.…”

“…At this time, the OC mineralization potential of the sediments that can be deposited in low location may be large [ 48 , 49 ]. Under high runoff erosive power, that is, high flow velocity and stream power, the high sediment erosion rate ( Qs ) weakens the effect of aggregate stripping on uneven OC enrichment in sediments [ 16 ]. The OC mineralization potential of deposited sediments may be low because of the high soil erosion rate.…”

“…This uneven enrichment is due to the macroaggregates being broken down into microaggregates and even smaller particles with different organic carbon (OC) concentrations [ 18 ]. During this process, raindrop impact produces sediments with finer sizes than the original soil [ 19 ] and changes the OC concentration for all size classes of particles [ 16 , 20 , 21 ]. Given that light particles with high OC concentration are easily transported, large amounts of low-density OC are unevenly enriched in sediments and can be decomposed by microorganisms easily [ 11 , 12 , 14 , 17 ].…”

“…Previous studies have found that because of aggregate breakdown and selective transport of SOC fractions, the SOC concentration in sediments varies during erosion [13][14][15]. When the runoff erosive power is low enough, SOC can also be unevenly enriched between different size classes of sediment particles in soils with high SOC and aggregate contents [16,17]. This uneven enrichment is due to the macroaggregates being broken down into microaggregates and even smaller particles with different organic carbon (OC) concentrations [18].…”