Effects of Longterm Application of Organic Residues and Inorganic Fertilisers on Soil Microbial Biomass, Diversity and Activities in Nyabeda, Siaya County, Kenya
Bolo, Peter Omondi
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Food insecurity is a global problem exacerbated by poor agricultural productivity due to poor soil health, soil infertility, and improper agricultural management practices. Conservation agriculture (CA) practices have been widely promoted and adopted in many regions, but little is known concerning their effects on soil health. This study focused on assessing the effect of long term use of mineral fertilizers, organic inputs and lime on soil microbes and their activities under different tillage and cropping systems. The specific objectives of the study were; (i) to determine the effects of long term use of mineral fertilizers, organic inputs and lime on soil microbial biomass, composition, abundance and enzymatic activities under different tillage and cropping systems; (ii) to determine how soil microbes influence soil aggregation under different tillage and cropping systems; (iii) to determine how tillage and cropping systems influence denitrification and net nitrogen mineralization in the soil. The study involved twelve treatments set up in randomized complete block design. Illumina Miseq sequencing was used to identify the microbial species present whose abundances were assessed using both Simpson’s and Shannon Wienner diversity indices. Aggregate stability was assessed using wet sieving technique; while denitrification potentials was determined using acetylene incubation technique; and net nitrogen mineralization assessed using In-situ resin core incubation methods. Data was analysed using R Project. Analysis of variance (ANOVA) was done and means separated using Tukey’s HSD test. Practicing reduced tillage significantly increased microbial populations, with Glomerales increasing by 50.3% relative to conventional tillage. Inorganic phosphorus application, respectively, significantly reduced arbuscular mycorrhizal fungi operational taxonomic units (OTUs), with Glomerales reducing by 29.8%. Addition of inorganic P fertiliser also significantly (p<0.001) reduced Alkaline phosphatase (ALP) potential activities by -38%, but significantly (p<0.001) increased aggregate Mean Weight Diameter by (16%) and microbial species richness by 34%; with relative abundances of cyanobacteria and rhizobia increasing by 64% and 68%, respectively. Maize-soybean intercropping significantly increased Glomerales (45.5%), with slight increases (not significant) in Microbial Biomass Carbon (MBC) (29%), Microbial Biomass Nitrogen (MBN) (32%), Acid Phosphatase activity (ACP) (12%), ALP (2%) and rhizobia (5%); but reduced cyanobacterial relative abundance (11%). However, aggregate stability significantly (P<0.001) increased under maize-soybean intercropping relative to rotation. Residue retention significantly (P<0.001) reduced ALP activity (-20%), but significantly (p<0.001) increased denitrification potentials (67%; 0-5 cm), MWD (8%; 5-15 cm), MBP (46%), net nitrogen mineralization (+83%), rhizobial relative abundance (80%), cyanobacterial relative abundance (60%) and Glomerales (41.9%). Liming slightly increased (though not significant) ACP (31.8%), ALP (28.9%), MBC (18.9%), MBN (7.8%) and MBP (12.4%). Denitrification potentials significantly (p<0.001) increased (61.0%) under reduced tillage than conventional tillage systems. In conclusion, practicing reduced tillage, residue retention and intercropping can increase overall soil microbial biomass, species abundance, extracellular phosphatase enzyme activities and aggregate stability. Application of inorganic phosphorus fertilisers, though needed for increased crop yields and nutrient enrichment, potentially reduces abundance arbuscular mycorrhizal fungi populations and phosphatase enzymes activities. Practicing reduced tillage without residue retention is an unstable agronomic system that potentially reduces soil microbial species richness and diversity.