Soil erosion and nutrient loss. III.¤ Changes in the enrichment ratio of total nitrogen and organic carbon under rainfall detachment and entrainment

Abstract

Experiments to investigate the enrichment mechanisms of organic carbon, nitrogen, and fine particles in eroded sediment were carried out on 2 contrasting soils under simulated rainfall with and without runon. Sediment samples for concentration measurement were collected at different sampling intervals and fractionated into various size fractions using sets of sieves. The fractionated sediment samples and original (uneroded samples) were analysed for total nitrogen and organic carbon to obtain the enrichment ratio (E_R) for both elements.

For both soils, sediment concentration, total nitrogen and organic carbon (E_R), and concentration ratios of nitrogen and organic carbon changed considerably under the different dominant erosion mechanisms in the experiments. For experiments on both 0·1 and 3% slopes, sediment concentrations of the smaller fractions (<53 and 53–500 µm) were higher than those of the original soil and remained so throughout the duration of the experiment. At the higher slope (3%) and thus lower water depths, a process described as raindrop peeling apparently was more pronounced, since stable soil aggregates were exposed to raindrop impact. The concentration of the smaller size fraction increases with time resulting in an increase in total nitrogen and organic carbon relative to the original soil. This leads to an E_R greater than unity in the eroded sediment.

Rilling was observed in experiments with both rainfall and runon. During the relatively short period prior to the establishment of active rilling, the proportion of larger aggregates increased with time. After this proportion had stabilised, the concentrations of total nitrogen and organic carbon were similar to those in the original soil. Thus, E_R was close to unity, sometimes being slightly below unity.

The presence of significant depths of water greatly influenced both the breakdown of soil aggregates and the size or settling velocity distribution of the eroded sediment. In these circumstances, the size-selected process of deposition can lead to enrichment. With a negligible depth of water on the soil surface, raindrop peeling was the dominant mechanism, producing fine particles richer in organic carbon and total nitrogen than the original soil. As depth of water increases, the effect of the highly varied settling velocity particles on the size characteristics of eroded sediment becomes more evident, and the effect of raindrop stripping diminishes. With a thin layer of water on its surface, both of these mechanisms seem to be operative.