Effect of land use on ecosystem metabolism and decay rate of Eucalyptus saligna and Neuboutonia macrocalyx leaves in streams draining Upper Mara catchment, Kenya

Abstract

Replacement of native vegetation by exotic species of higher economic value has a potential ecological impact on detritus-based stream ecosystems. Litter processing and metabolism in streams can be used as an indicator of the functional status of streams. This study was conducted in the upper Mara catchment with a sole objective of determining the effect of land use change on leaf litter decay and metabolism in six first order streams namely: Sambambwet, Mosoriot and Chepkosiom that drain forested land use and Masese, Tenwek and Kapsebet that drain agricultural land. Litterbag experiment was used to determine leaf litter breakdown rates in the streams. The study involved collecting, drying and weighing 6 grams of Eucalyptus saligna (exotic) and Neuboutonia macrocalyx leaves (native). A total of 288 litterbags of both plant species were prepared. The litterbags were randomly exposed in the six streams on 27th May 2013 and later retrieved at intervals of 0, 1,3,7,14,28 42, and 56 days, dried and weighed. Macroinvertebrates associated with the litterbags were identified up to the family level. Stream ecosystem metabolism was conducted using the solute-addition experiment in Chepkosiom and Tenwek streams. Decay rates of Eucalyptus leaves were not significantly different between streams draining forested land use (-k = 0.039±0.009, pooled data) and streams draining agricultural land (-k = 0.045±0.009) (t = 0.404, df =16, p ≥ 0.05). Decay rates for Neuboutonia leaves were however, significantly higher in streams draining agricultural land (–k = 0.095±0.005) than in streams draining forested land use (-k=0.062±0.01) (t-2.89, df =143, p<0.05). Although macroinvertebrate composition did not differ significantly with land use, diversity was significantly higher in forested land use than agricultural one (t=4.527, df =18, p<0.05). Macroinvertebrate abundance significantly was higher in streams draining agricultural land (t=-3.244, df=18, p<0.05). Neuboutonia leachate enrichment had no effect on metabolism of both streams but Eucalyptus enrichment shifted Tenwek stream from heterotrophic to autotrophic inferring less secondary productivity. This study showed that agricultural land use and replacement of native riparian vegetation with exotic ones such as Eucalyptus has compromised leaf litter decay and metabolism in Upper Mara catchment streams.