Effects of overland flow and soil erodibility on soil organic carbon loss in red soil sloping land

Abstract: Clarifying soil organic carbon (SOC) loss plays a vital role in studying SOC sequestration and regional C cycling. In order to investigate effects of overland flow and soil erodibility on SOC loss, field simulated rainfall experiments were conducted at Soil and Water Conservation monitoring station (111°22′ E, 27°03′ N) located in the Shuangqing district in Shaoyang City of Hunan Province, in the hilly red soil region of southern China. Rainfall events were performed in 2012 on 4 chisel tillage plots with rainfall intensities of 100, 80, 40 and 30 mm/h (CT-1, CT-2, CT-3 and CT-4) and 2 no tillage plots with rainfall intensities of 80 and 30 mm/h (NT-1 and NT-2), respectively. Field plots (5 m length × 2 m width) were designed at a sloping land with a slope of 10°, and all the rainfall events lasted 60 min since the runoff began. Runoff and sediment samples were collected every 6 min for measuring runoff volume, sediment weight, and concentration of runoff-dissolved organic C and sediment-bound organic C. The results showed that sediment exports averaged 2.400, 1.400, 0.960, 0.290, 0.200 and 0.014 g/(m2·s); Sediment-bound organic C exports averaged 0.009, 0.007, 0.010, 0.002, 0.003 and 0.0001 g/(m2·s) in CT-1, CT-2, NT-1, CT-3, NT-2, and CT-4, respectively. The sediment and associated SOC loss in high-intensity rainfall events were significantly (P<0.05) higher compared to low rainfall intensity events, coinciding with changes in runoff. However, runoff-dissolved organic C loss rate presented the higher values in CT-1, CT-2, and CT-3 than the other plots, different from change in runoff rate. Runoff rate of all the plots increased first and then within 12 min. The sediment loss rate of CT-1, CT-2, and NT-1 firstly increased and then decreased within 30 min, subsequently kept a stable value with rainfall duration; The sediment loss rate of CT-3, CT-4, and NT-2 was steady during the rainfall event. Sediment-bound organic C loss rate of CT-1, CT-2, and CT-3 sharply increased and then decreased within 30 min, subsequently stayed a stable value with rainfall duration; The change of sediment-bound organic C loss rate in CT-3, CT-4, and NT-2 was relatively steady. Runoff-dissolved organic C loss rate of CT-1 appeared two peak value at 25 min and 45 min, respectively, and both CT-2 and CT-3 appeared the peak value at 30 min. Runoff-dissolved organic C loss rate of NT-1, and NT-2 kept a stable value within 35 min and then appeared the peak value at 45 min, while runoff-dissolved organic C loss rate of CT-3 was steady during the whole rainfall event. This indicated that the loss process of sediment-dissolved organic C was similar to that of runoff except for the peak values appeared during 10-30 min in high-intensity rainfalls, and the sediment-bound organic C loss rate firstly increased and subsequently kept a relatively stable value. The results above demonstrated the magnitude of runoff affected SOC loss; Loss process of runoff had an obvious effect on that of sediment-bound organic C, but was not highly correlated with that of runoff-dissolved organic C. A positive linear relationship between SOC loss rate and runoff rate was shown. The regression analysis reveals variability of runoff could explain 80% of the variance in SOC loss, and runoff rate had a larger effect on sediment-bound organic C than runoff-dissolved organic C. In addition, the results showed that the soil erodibility had a nonlinear effect on SOC loss. Greater soil erodibility indicator could result in higher SOC loss rate, but the growth rate of SOC loss gradually decreased with the increase of soil erodibility indicator, which indicated the effect of soil erodibilty on SOC loss was limited. These observations led to the conclusion that runoff and soil erodibility were two significant impact factors of SOC loss.