Soil erosion and carbon dynamics under simulated rainfall

Soil erosion affects soil organic carbon (SOC) dynamics by redistributing SOC within the watershed or ecosystem and by altering the C release rate into the atmosphere. Although the mechanism of this process is poorly understood it has the potential to influence global climate. Crosby loam soil (fine, mixed, mesic, Aeric Ochraqualfs) was used in a laboratory rainfall simulation to quantify erosion-induced CO2 evolution from the soil surface, distribution of SOC in various aggregate fractions, and SOC preferential transport. The soil was placed in five plots (0.15 X 0.3 X I m) positioned at different slopes (8, 7, 1, 0.5, and 0%) and connected in a cascade fashion so that the runoff from one plot was diverted to the top of the next plot. This setup allowed collection of runoff samples at various distances along the slope while the soil was detached, transported, sorted, and deposited. The soil was subjected to simulated rainfall at 80 rum h(-1) for 1.5 h. Runoff samples were collected from each of the plots every 10 min. and separated on four aggregate fractions. Core samples were extracted and placed in incubation chambers in such a way that only the original undisturbed soil surface was exposed. Evolution of CO2 from the core samples and runoff sediment was monitored and total SOC content determined. Sediment balance at different locations on the slope varied from 1307 g m(-2) to 980 g m(-2). SOC loss occurred through increased erosion-induced mineralization and preferential transport by runoff. CO2 emission from the deposition areas (17.0 g C m(-2) during a 60-day period) was 26% higher than at the control site, whereas the emissions from the eroded site and the control site were the same. Increased CO2 release from deposited sediments was related to their aggregate composition. Sediment consisted largely of aggregates 0.053 to 1 mm in diameter, increasing (15 to 50% compared with original soil) with SOC content. The carbon mineralization rate (0.07 g C kg(-1) soil d(-1)) ranging from 0.053 to 0.25 mm fractions, was 2 to 4 times greater than that from larger (0.25-1) and smaller (<0.053 mm) fractions during the initial 20 days of incubation. This was attributed not only to increased labile SOC content in the medium aggregate fractions but also to the substantial amount of medium size aggregates formed by the breakup of larger aggregates, thus providing the bacteria access to otherwise physically protected organic compounds. The enrichment ratio of SOC in runoff varied from 1.0 to 1.7, increasing with slope length and decreasing with rainfall duration.