Culturable soil microbial community structure, nutrient dynamics and greenhouse gas emissions in Maize-Banana Based agroforestry system in Kisii County, Kenya

Abstract

Microbial community structure are the characteristics of a group of microorganisms as measured by any metric of taxa or gene composition, diversity and abundance via a range of molecular or cultural techniques. Soil microbial community structure is an important biological component of soil function. Microbial community affect the belowground dynamics and fate of nutrients which can influence soil fertility. Soil microorganisms play a significant role in the efflux of greenhouse gases (GHG). To better comprehend agroforestry systems that can enhance soil nutrients and combat climate change, it is vital to ascertain the effects of agroforestry systems (AFS) on the soil microbial community structure. Declining soil fertility as a result of continuous cropping is a major problem facing farmers in Kisii County. The link between the microbial community and GHG emissions remains poorly understood due to the diversity and complexity of microorganisms in soils. Further, there is little understanding of the morphological and molecular characteristics of these microbes. There is paucity of information on the role of microbes on soil nutrient dynamics and greenhouse gas efflux in maize-banana based AFS in Kisii county. For smallholder farmers to adapt agroforestry systems for their potential to increase soil fertility and reduce GHC emissions, there is a dearth of simple methodologies. Therefore, the objective of this study was to assess soil microbial community structure, nutrient dynamics and greenhouse gas emissions in a maize-banana based agroforestry system in Kisii County. This study was conducted at Kenya Agricultural and Livestock Research Organization farm in Kisii County. This study was conducted on an established agroforestry experimental plots which were set using a completely randomized block design with maize and banana intercropped with agroforestry trees. Soil samples were taken randomly at 10 different spots per plot at a depth of 0-15cm. Soil microbial biomass was determined using the chloroform fumigation extraction and their population determined by direct counting. Microbes were isolated in pure cultures, characterized morphologically and molecularly, and their phylogenetic relationships determined. Total organic carbon(C), nitrogen(N), phosphorus(P), potassium(K), calcium(Ca) and magnesium(Mg) were determined. Soil GHG (carbon dioxide, methane, and nitrous oxide) measurements were done using closed chambers. Soil pH was determined using a combined pH meter. Morphological and molecular data was subjected to cluster analysis. Data was subjected to analysis of variance and means separated using Least Significant Differences (P ≤ 0.05) after testing for normality. Correlation analysis was carried out on soil microbial biomass, microbial populations, soil carbon, nutrients and GHG. Microbial biomass and population were significantly higher in intercrop treatment with maize-banana, Sesbania sesban (MBSS). Most of bacterial isolates were Gram-negative bacilli and cocii with a few Gram-positive bacilli belonging to thirteen genera, while fungal isolates belonged to seven genera. Bacteria isolates clustered into five orders and fungal isolates clustered into three orders. Morphological genograms clustered isolates in two groups at 75% similarity level. Molecularly, sequences of bacterial isolates had >97% similarity match with gene bank isolates except MBCC2 while all fungal isolates had >97% similarity match with Genebank isolates. Soils from agroforestry tree species combinations treatment MBSS had higher values of C, N, P, K, Ca and Mg levels. Plots with Calliandra calothyrsus agroforetry tree species had low values GHG emissions. Sesbania sesban has the ability to increase soil microbial biomass and population which in turn act to improve soil health through microbial organic matter decomposition and thus recommended for use in maize-banana based agroforestry system. The study recommends the use Calliandra