Effects of land use on soil microbial community function in western Kenya highlands :

Abstract: Land degradation, agricultural intensification and deforestation may result in a loss of soil microbial community function, detrimental to resilience and to sustained productivity. In order to assess the effects of land use on microbial community function in a tropical soil the following hypotheses were considered: (1) functional capacity measured as substrate-utilization potential decreases with intensification of land use and (2) tree planting - i.e. as used in agroforestry - is a soil conservation and improvement measure which can restore physical, chemical and biological properties of degraded and overused soils. Samples from protected indigenous forest, forest plantations, agroforestry fields, conventional maize fields and eroded soil were taken on the slopes of Mount Elgon in the Rift Valley province of western Kenya. The agroforestry fields were earlier open agricultural fields. The microbial functional capacity was measured as substrate utilization of soil bacteria studied by using Biolog Ecoplates. Chemical and biological soil properties including pH, extractable P, total N, organic C, nitrate and microbial biomass C and N were also determined. The results follow a trend with eroded land and conventional maize fields on the low end of the scale and agroforestry, forest plantations and natural forest on the high end regarding pH values, total soil N and C concentration, microbial biomass C (MBC) and microbial biomass N (MCN). Extractable P shows higher levels in agroforestry fields than in indigenous forest, probably due to fertilization. Average well colour development (AWCD) in Biolog Ecoplates shows overall substrate consumption to be higher in land uses with higher tree cover and it is positively correlated with soil properties including pH, total N and C %, moisture, MBC and MBN. Significant differences were found between substrate utilization profiles in the various land uses - as observed in the PCA - indicating the existence of functionally different microbial communities. Additionally, more disparate catabolic responses appear between the samples of less conserved soils, which suggest a loss of functional stability. Results suggest that microbial functional capacity varies according to the land use and may be restored by increased tree cover and active soil management practices such as agroforestry.