SOIL-EROSION AND NUTRIENT LOSS .4. EFFECT OF SLOPE LENGTH ON RUNOFF, SEDIMENT YIELD, AND TOTAL NITROGEN LOSS FROM STEEP SLOPES IN PINEAPPLECULTIVATION

The effect of slope length on runoff and soil loss, and the loss and e nrichment ratio of nitrogen from steep slopes planted to pineapple, we re studied under natural field conditions over 3 sampling dates at Imb il, Gympie, Queensland. The experiments were conducted on 3 sites, wit h 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 als o collected, air-dried, weighed, and analysed for total nitrogen conce ntrations. 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 determinatio n. The runoff per unit area was not significantly affected by slope le ngth. Total soil loss per unit area in each erosion event increased wi th increasing slope length. The relationship between soil loss (Y) and slope (L) is well fitted by the function: Y = aL(b) The measured sedi ment concentrations (in the case of site 3) for different slope length s were higher than those for the estimated sediment concentration at t he 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 w hich increased soil loss per unit area. Furthermore, sediment concentr ation estimated at the transport limit assuming rill or furrow geometr y was higher than uniform overland flow and increased as slope length increased from 9 to 23 m. The enrichment ratio for total nitrogen decr eased with increasing eroded sediment. The eroded sediment was found t o contain a nitrogen concentration very similar to that of the origina l 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 slo pe 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 lnE(R) = a - b lnSL.