Jackson Mutuku Kilonzi

Background: Emerging evidence indicates that maize genotypes regulate rhizosphere microbial processes through distinct interactions with microbiota, shaping soil organic matter mineralization (SOM) dynamics and N cycling pathways. Although both SOM mineralization and nitrification depend on genotype and N availability, the mechanistic link between genotype-driven carbon mineralization and nitrification remains poorly understood. Specifically, it remains unclear whether maize genotypes that stimulate greater SOM mineralization also promote higher net nitrification rates following N fertilization. We therefore investigated whether maize genotypes with a greater capacity to stimulate SOM mineralization exhibit higher net nitrification under contrasting N inputs. Results: We categorized tropical maize ( Zea mays ) genotypes that previously showed variation in SOM mineralization as high (DTPYC9-F143-5–4-1–2), medium (CML421 = P31DMR#1–55-2–3-2–1), and low ([CML312/CML444//[DTP2WC4H255-1–2-2-BB/LATA-F2-138–1-3–1-B]-1–3-2–3-B]-2–1-2) SOM mineralizers, and applied N fertilizer at 50 and 100 kg N ha ⁻¹ , representing low and high N regimes, respectively. Soil functional diversity and microbial activity were assessed using MicroResp™ assays, while qPCR was used to quantify the abundance of the ammonia monooxygenase ( amoA ) genes in archaea (AOA) and bacteria (AOB), and changes in the size of the bacterial community (16S rRNA gene copies). Nitrogen addition enhanced soil microbial biomass carbon (MBC) by 15%, basal activity by 8%, and net nitrification by 521% in the absence of maize plants, whereas the maize genotypes under a high N regime increased the basal activity by 5 – 25% and net nitrification by 52- 370%, depending on genotype. The presence of maize and the N regime strongly exerted selective pressure on the microbial community. Nitrogen rate, rather than maize genotype, strongly influenced the abundance of AOA, AOB, and 16S rRNA genes. Across the maize genotypes, nitrification and microbial activity were inversely related to the genotype-specific order of SOM mineralization. The clustering of microbial community-level physiological profiles (MCLPPs) depended on plant presence and N regime. NUE of the maize genotypes increased with increasing SOM mineralization associated with these genotypes. Conclusion: Collectively, our findings redefine the relationship between SOM mineralization and nitrification by showing that enhanced SOM mineralization is typically not associated with increased nitrification. Instead, we demonstrate that genotypic-specific regulation of microbial activity and N transformations depends on N input rates, underscoring that a maize genotype''s capacity to regulate nitrification is context-dependent. Thus, integrating genotype selection with optimized N management offers a promising strategy to enhance conservation and use efficiency, sustain biomass yield, and minimize N losses that harm the environment.

Weed infestation in potato field has become a major concern to smallholder farmers and environment, requiring sustainable intervention. This study aimed to determine (i) whether metribuzin application rate (0, 0.5, 1 (standard), 1.5, and 2 kg ai ha⁻¹) has effects on soil microbes and soil nutrients in the short term using potted soil and (ii) the influence of potato seed dormancy (short; Shangi variety and long; Unica variety) and planting technique (surface, ridge, or furrow planting) on the efficacy of weed management practice (metribuzin (480 g L⁻¹) herbicide and hoeing) in the field. Bacterial colony counts were recorded upon isolation from the potted soil. Soil nutrients were also analysed before and after metribuzin treatment. Data on crop growth (height and stem count), weed abundance, yield, costs, and revenues were recorded from a field experiment. Morphologically distinct bacterial strains were tested for Gram reaction and response to carbon utilization using analytical profile index kits (API 50 CH and 20 NE). In total, nine distinct bacterial strains were isolated and all were Gram positive, with variation in response to carbon substrates. Concentration of 2.0 kg a.i ha⁻¹ significantly (p ≤ 0.05) reduced bacterial count in the first day, followed by an increase in the subsequent incubation days, while 1.0 and 1.5 kg a.i ha⁻¹ concentrations had the highest colony development index and species richness. Significant effects of metribuzin on soil pH, total N, and total organic C in the short term were revealed. Notably, weedy plots had the highest Shannon weed index. Weeds reduced stem count and height growth by 57% and 62% respectively. Plots with Shangi had 9.8% lower weed count than those with Unica and hence higher yield was recorded from the former. While surface planting had the lowest weed control efficacy, crop growth attributes, and yield, furrow and ridge planting were not significantly different. Weed abundance and crop growth attributes in metribuzin and hand-hoed plots did not differ significantly, yet hoeing resulted in 5.30 t ha⁻¹ above that obtained from plots managed using metribuzin. Higher dry matter and tuber yield were observed from hoed plots with Shangi planted in furrow or ridge. Use of metribuzin resulted in higher net benefit ratio and marginal rate of return than hand hoeing. The results reveal that use of metribuzin early and later in the potato growing phase can minimize weed infestation, but the efficacy depends on planting method and seed dormancy.

Management of late blight relies on the use of fungicides and potato varieties which have been associated with loss of efficacy over time. Baseline survey and field studies were conducted to evaluate the effects of fungicide formulations and potato varieties in managing late blight and subsequently on net farm income improvement. Four fungicides formulations (Infinito® (Fluopicolide 62.5g L⁻¹ + propamocarb 625 g L⁻¹), Milraz® (Propineb 700 g kg⁻¹ + cymoxanil 60 g kg⁻¹), Mistress 72® (Mancozeb 640 g kg⁻¹ + cymoxanil 80 g kg⁻¹) and Ridomil® (Metalaxyl 40 g kg⁻¹ + mancozeb 640 g kg⁻¹)), and varieties (Kenya Mpya, Shangi and Dutch Robijn) were evaluated. Survey results revealed that the majority (78%) of the farmers selected varieties to grow based on market dynamics as compared to seed availability (18%), disease resistance (12%) and other factors (8%). The most widely and least used fungicides were Ridomil® (28.8%) and Infinito® (1.7%) respectively. Late blight significantly reduced crop growth (height and stem count) and yield in the unprotected plots in Shangi and Dutch Robijn, but no effect was observed in K. Mpya. Mistress® suppressed blight symptoms by 54%, while Milraz® (lowest effects) reduced the disease by (43%). In unprotected plots, disease severity on K. Mpya was below 1%, while in Shangi and Dutch Robijn varieties, it was 60 and 78% respectively. Reduction in disease by Mistress® resulted in higher yield (30.70 t ha⁻¹) as compared to the unprotected plots (9.25 t ha⁻¹). Maximum yield (28.56 t ha⁻¹) was obtained from K. Mpya, while Dutch Robijn, had the lowest observed yield (20.70 t ha⁻¹) in protected plots. Yet, the highest cost benefit ratio (CBR) was recorded in unprotected plots with K. Mpya. The smallest CBR (1.55) was observed on plots where Milraz® was applied on Dutch Robjn. Results of this study show that, the use of fungicides reduced late blight and improved yield of potato substantially, but the efficacy varied with fungicide formulation and potato variety. However, the use of resistant variety was effective in managing late blight hence a useful tool for minimizing fungicide applications. Therefore, use of resistant variety could enhance a healthy biodiversity and at the same time maximize food production.

Organophosphate compounds are widely used in agricultural activities to optimize food production. Contamination of field soil by these compounds may result in detrimental effects on soil biota. The aim of the present study was to isolate microorganisms from field soils and evaluate the strains on ability to degrade organophosphates as single isolate and as a consortium. Isolated strains were identified using both biochemical and molecular techniques. Results revealed that, out of the 46 isolated strains, three isolates herein referred to as S6, S36 and S37 showed an average diazinon degradation rate of 76.4%, 76.7% and 76.8% respectively, of the initial dose (50 ppm) within 11 days of incubation in mineral medium. Notably, isolates S36 and S37 were more effective than S6 in degrading diazinon by 40% in soil aliquot after 11 days and therefore were evaluated on biochemical reactions and molecular identification. The isolates showed variable biochemical characteristics. However, both isolates possessed catalase enzyme, but lacked oxidase enzyme. Molecular characterization showed that, the closest species for S36 and S37 were Priestia megaterium and P. arybattia, respectively, based on 16S rRNA gene similarity (> 99%). Combination of the strains increased diazinon degradation ability by 45% compared to single strain treatment. Chlorpyrifos was the most highly degraded organophosphate, compared to phorate and cadusafos. Therefore it is expected that the pesticide-degrading bacteria could be a solution to soil health improvement and contribution to the production of safe agricultural products.

Management of biotic stresses in agriculture solely depends on the use of pesticides globally. Little is currently known on the health risks attributed to pesticides in potato farming in Kenya. The level of exposure and safety precautions adopted by operators to avoid hazards attributed to handling of chemicals and develop an easy to use model of exposure in potato farming was investigated. A total of 197 potato farmers from Nyandarua County in Kenya were interviewed using a structured questionnaire. Data on pesticides handling parameters were collected. Results revealed that, most hazardous chemical (WHO class III) were highly (75%) used, whilst biologicals were unfamiliar to farmers. Majority of farmers were in agreement that, pesticides posed health risks (85%) and (90%) environmental impairment. Only 46% of the farmers were aware of pesticides safety, while 36% used personal protective equipments (PPEs) when handling pesticides. In addition, the percentage of farmers who could read and understand chemical label was 30%. Trained and educated farmers were more likely to follow pesticides safety precautions (88%). Poor disposal of chemical leftovers and empty containers was evident whereas most farmers (86%) had no standard storage facility for pesticides. Lack of training (70%) was pointed out as the major constrain. Merely, 10% experienced self-reported symptoms of acute poisoning. Estimation of exposure elucidated that, potato growers are at high risk to metalaxyl and mancozeb exposure. Therefore multi-institutional interventions would be crucial in providing sustainable solutions.