Soil erosion and nutrient loss. IV.

The effect of slope length on runoff and soil loss, and the loss and enrichment ratio of nitrogen from steep slopes planted to pineapple, were studied under natural field conditions over 3 sampling dates at Imbil, Gympie, Queensland. The experiments were conducted on 3 sites, with each site having 3 slope lengths of 9, 15 and 23 m, each replicated twice. The mean slope gradient was 32% with extreme variation from 24 to 37%. During sampling for soil loss measurement, subsamples were also collected, air-dried, weighed, and analysed for total nitrogen concentrations. Samples of the original soil (uneroded) were also taken and analysed and the concentration of total nitrogen in the original soil was used for enrichment ratio and nutrient nitrogen loss determination.
The runoff per unit area was not significantly affected by slope length. Total soil loss per unit area in each erosion event increased with increasing slope length. The relationship between soil loss ( Y ) and slope ( L ) is well fitted by the function:
The measured sediment concentrations (in the case of site 3) for different slope lengths were higher than those for the estimated sediment concentration at the transport limit assuming either uniform overland flow, or furrow or rill geometry. The estimated sediment concentration at the transport limit provided evidence for the occurrence and importance of rilling which increased soil loss per unit area. Furthermore, sediment concentration estimated at the transport limit assuming rill or furrow geometry was higher than uniform overland flow and increased as slope length increased from 9 to 23 m.
The enrichment ratio for total nitrogen decreased with increasing eroded sediment. The eroded sediment was found to contain a nitrogen concentration very similar to that of the original soil, resulting in the enrichment ratio, in some cases, being close to or slightly below unity. Total nitrogen loss per unit area showed a similar trend to total soil loss and it increased with increasing slope length. The relationship between total soil loss in the event (SL) and enrichment ratio ( E R) for total nitrogen for all slope lengths is represented by the logarithmic function