INFLUENCE OF LAND-USE ON LEAF LITTER DECOMPOSITION IN HEADWATER STREAMS OF THE NZOIA RIVER, KENYA

Abstract

Land-use plays a crucial role in affecting river function, particularly leaf litter decomposition. The study was aimed to investigate the influence of land-use on leaf litter decomposition in headwater streams of the Nzoia River, Kenya. The breakdown rate of three common leaf species; Syzygium cordatum, Vernonia myriantha and Eucalyptus globulus were examined in six headwater streams of Nzoia River Basin during the dry months of February- March 2020. Leaf decomposition rates were measured by placing 216 coarse- and fine-mesh litterbags in the six streams, three draining forested land and three draining agricultural land. For each stream, physical characteristics, physico-chemistry, leaf litter decomposition rates and macroinvertebrate community composition were measured. Significantly, there were no differences in physical characteristics (p > 0.05) between the two land-uses, except for canopy cover which was significantly higher (p = 0.044) in forested streams. Physico-chemistry differed significantly, with forested streams showing higher values (p < 0.05) for coarse particulate organic matter (p = 0.014) but lower for fine particulate organic matter (p = 0.053), total suspended solids (p = 0.014) and electrical conductivity (p = 0.043). Leaf decomposition rates differed significantly (p > 0.05), with forested streams showing faster rates by the end of 28 days. Macroinvertebrates from kick samples showed that collectors were the most abundant (46.3%) while shredders were the least abundant (7.4%). Order Ephemeroptera had the highest number of individuals in both forested and agricultural sites followed by Diptera and Trichoptera. In the coarse-mesh litterbags abundance of shredders was higher in forested than agricultural streams with Potamonaute sp. and Acanthiops sp. dominating in both land-use types, while Lepidostoma sp. dominated the forested streams. It is concluded that the decomposition of leaf litter in the headwater streams of Nzoia River has been adversely affected by agriculture with potential threats on nutrient cycling and energy transfer to higher trophic levels. The observed decomposition rates were greatly influenced by the changes in the physico-chemical characteristics of the streams. Restoration efforts, including conservation agriculture and re-afforestation, are recommended for Nzoia River basin to regain the basin’s capacity for high terrestrial and aquatic food production for the growing population in Kenya.