Zachary Getenga

Atrazine and terbuthylazine mineralization by an Arthrobacter sp. isolated from a sugarcane-cultivated soil in Kenya.

A tropical soil from a Kenyan sugarcane-cultivated field showed a very high capability to mineralize (14)C-ring-labeled atrazine. In laboratory experiments this soil mineralized about 90% of the applied atrazine within 98 d. The atrazine-degrading microbial community was enriched in liquid cultures containing atrazine as the sole N source and 100 mgL(-1) glucose as additional C source. From the enrichment culture a bacterial strain was isolated and identified by comparative sequence analysis of the 16S-rDNA as member of the genus Arthrobacter. The enriched mixed culture as well as the isolated strain, designated as Arthrobacter sp. strain GZK-1, could grow on atrazine and terbuthylazine as sole N-sources; Arthrobacter sp. GZK-1 mineralized (14)C-ring-labeled atrazine up to 88% to (14)CO(2) and (14)C-ring-labeled terbuthylazine up to 65% to (14)CO(2) in a liquid culture within 14 d. The enriched microbial consortium as well as the isolated strain could be a potential solution for the remediation of s-triazine polluted agricultural soils.

The major environmental factors that influence rapid disappearance of pesticides from tropical soils in Kenya

Results of field and laboratory studies on adsorption/desorption, leaching, dissipation, bound residue formation and metabolism of DDT, DDE, lindane, parathion, malathion, dimethoate and carbofuran in tropical soils in various regions in Kenya are summarized in this paper. Based on reported half‐lives of dissipation in temperate soils, DDT, DDE and lindane were found to dissipate much more rapidly in tropical soil conditions with half‐lives of dissipation of 64.5–245.6, 145 and 5–8 days, respectively. Carbofuran (t 1/2 = 66–115.5 days), malathion (t 1/2 = 36.7–770 days), parathion (t 1/2 = 48days) and dimethoate (t 1/2 = 72 days) were also less persistent. The major environmental factors, wind, rainfall, solar radiation intensity and soil moisture content that contributed to this rapid disappearance are presented, explaining also the influence of important soil characteristics such as pH, % organic carbon, texture and microbial activity on pesticide distribution and degradation in soil.

The Impact of Land Use Activities on Vegetation Cover and Water Quality in the Lake Victoria Watershed

The impact of land use activities on loss of vegetation cover and water quality was assessed in three selected sites within the Lake Victoria Basin using remote sensing technologies and standard water quality analysis techniques. The three study sites were: (i) Nzoia River Basin (Kenya), (ii) Nakivubo Wetland (Uganda) and (iii) Simiyu drainage basin (Tanzania). Lake Victoria is the second largest fresh water lake in the world and is served by a drainage basin area of over 193,000 km 2 , traversing five East African Community States; Kenya, Uganda, Tanzania, Rwanda and Burundi. This paper examines the impact of land use activities on vegetation cover and water quality based on remote sensing and Geographical Information Systems analysis combined with chemical and physical water analysis. The quality of the effluent generated by the industries found within the study sites and their effects on downstream discharge was also determined. Pesticide residues in soil and water samples were determined using analytical standard methods. Soils from some selected fields in Nzoia River basin showed high levels of compounds such as aldrin, dieldrin, endosulfan, DDT, and endrin which are together referred to as persistent organic pollutants (POPs). The physical and chemical analysis of water quality revealed high levels of phosphates and nitrates along the agricultural zones of River Nzoia Basin. The satellite images revealed that in all the three study sites land vegetation cover has continuously reduced in size. The extent of environmental degradation caused by agricultural, domestic and industrial wastes and how this affects loss of vegeta- tion cover and water quality is discussed.

Biodegradation of s-triazine herbicide atrazine by Enterobacter cloacae and Burkholderia cepacia sp. from long-term treated sugarcane-cultivated soils in Kenya

In this study soils from sugarcane-cultivated fields were screened for bacterial species capable of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) degradation due to long exposure of the soils to this herbicide. To enrich for atrazine degraders, Minimal Salt Medium containing atrazine as the sole N source and glucose as the C source was inoculated with soils impacted with this herbicide and incubated. Bacterial growth was monitored by measuring optical density. The degradation of atrazine was followed by measuring residual atrazine in liquid cultures over a given time period by high performance liquid chromatography. Bacterial strains isolated from the enrichment cultures were characterized by biochemical tests and identified by 16S rRNA gene sequencing. Two bacterial strains coded ISL 8 and ISL 15 isolated from two different fields were shown to have 94 and 96% 16S rRNA gene sequence similarity to Burkholderia cepacia respectively. Another bacterial sp., ISL 14 was closely related to Enterobacter cloacae with a 96% 16S rRNA gene sequence similarity. There was not much difference between the extents of atrazine degradation by the enrichment cultures with communities (79–82% applied amount) from which pure strains were isolated and the pure strains themselves in liquid cultures that showed a degradation of 53–83% of applied amount. The study showed existence of bacterial strains in different sugarcane-cultivated fields which can use atrazine as a nitrogen source. The bacterial strains isolated can be used to enhance the degradation of atrazine in contaminated soils where atrazine is still considered to be recalcitrant.

The dissipation of hexazinone in tropical soils under semi-controlled field conditions in Kenya

The dissipation of hexazinone (Velpar) in two tropical soil types in Kenya was studied under field and semi-controlled conditions for a period of 84 days. The dissipation was found to be very rapid and this could be attributed to adverse weather conditions including high initial rainfall as well as to low soil-organic-matter content, volatilization, surface run-off and biodegradation. The DT50 values of dissipation obtained by first order kinetics were 20 days and 21.3 days in clay and loam soil types, respectively. The influence of bargasse compost (1000 μg/g dry soil) was also studied and was found to enhance dissipation to some extent, giving DT50 values of 18 days and 18.3 days in clay and loam soil types, respectively.

Effect of fluctuating soil humidity on in situ bioavailability and degradation of atrazine.

This study elucidates the effect of fluctuating soil moisture on the co-metabolic degradation of atrazine (6-chloro-N(2)-ethyl-N(4)-isopropyl-1,3,5-triazine-2,4-diamine) in soil. Degradation experiments with (14)C-ring-labelled atrazine were carried out at (i) constant (CH) and (ii) fluctuating soil humidity (FH). Temperature was kept constant in all experiments. Experiments under constant soil moisture conditions were conducted at a water potential of -15 kPa and the sets which were run under fluctuating soil moisture conditions were subjected to eight drying-rewetting cycles where they were dried to a water potential of around -200 kPa and rewetted to -15 kPa. Mineralization was monitored continuously over a period of 56d. Every two weeks the pesticide residues in soil pore water (PW), the methanol-extractable pesticide residues, the non-extractable residues (NER), and the total cell counts were determined. In the soil with FH conditions, mineralization of atrazine as well as the formation of the intermediate product deisopropyl-2-hydroxyatrazine was increased compared to the soil with constant humidity. In general, we found a significant correlation between the formation of this metabolite and atrazine mineralization. The cell counts were not different in the two experimental variants. These results indicate that the microbial activity was not a limiting factor but the mineralization of atrazine was essentially controlled by the bioavailability of the parent compound and the degradation product deisopropyl-2-hydroxyatrazine.

Effects of carbon amendment on in situ atrazine degradation and total microbial biomass

This study elucidates the effects of carbon amendment on metabolic degradation of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and total microbial biomass in soil. Degradation of 14C-ring-labelled atrazine was monitored in laboratory incubations of soils supplemented with 0, 10, 100 and 1000 μg g−1 sucrose concentrations. An experiment to determine the effect of carbon amendment on total microbial biomass and soil respiration was carried out with different concentrations of sucrose and non-labelled atrazine. The soils were incubated at a constant temperature and constant soil moisture at water potential of −15 kPa and a soil density of 1.3 g cm−3. Mineralization of 14C-ring-labelled atrazine was monitored continuously over a period of 59 d in the first experiment. The CO2 production was monitored for 62 d in the second experiment and microbial biomass determined at the end of the incubation period. The addition of 1000 μg g−1 sucrose reduced atrazine mineralization to 43.5% compared to 51.7% of the applied amount for the treatment without sucrose. The addition of 1000 μg g−1 sucrose modified the transformation products to 1.08 μg g−1 deisopropylatrazine (DIA), 0.32 μg g−1 desethylatrazine (DEA) and 0.18 μg g−1 deisopropyl-2-hydroxyatrazine (OH-DIA). Treatment without sucrose resulted in formation of 0.64 μg g−1 hydroxyatrazine (HA), 0.28 μg g−1 DIA and 0.20 μg g−1 OH-DIA. Atrazine dealkylation was enhanced in treatments with 100 and 1000 μg g−1 of sucrose added. HA metabolite was formed in the control (no sucrose) and in the presence of 10 μg g−1 of sucrose, whereas DEA was only detected in treatment with 1000 μg g−1 sucrose. Results indicate that total microbial biomass increased significantly (P < 0.001) with the addition of 1000 μg g−1 sucrose.

Biodegradation of phenylurea herbicide diuron by microorganisms from long-term-treated sugarcane-cultivated soils in Kenya

The phenylurea herbicide diuron [N-(3,4-dichlorophenyl)-N,N-dimethylurea] is widely used alone or in a broad range of herbicide formulations. Its degradation in sugarcane-cultivated soils which have been impacted by the herbicide through repeated applications was studied. Liquid culture experiments with diuron as the only carbon source led to the isolation of different bacterial strains capable of degrading diuron. The bacterial species belonging to the genera Bacillus, Vagococcus, and Burkholderia, identified through biochemical and molecular characterization, degraded diuron to different extents. The isolated Bacillus cereus, Vagococcus fluvialis, Burkholderia ambifaria, and Bacillus spp1 degraded diuron by 21%, 25%, 22%, and 19% of the initially applied concentration of 40 mg L−1, respectively, after 35 days of incubation in liquid culture media. Small amounts of 3,4-dichloroaniline and the de-methylated metabolite N-(3,4-dichlorophenyl)-N-methylurea were detected in liquid culture media. The combination of V. fluvialis and B. ambifaria showed an enhanced degradation of up to 30% of the initially applied concentration of 40 mg L−1. Degradation by pure isolates was low (18–25%) compared to the capacities of diuron degradation shown by the bacterial communities (58–74%). This study showed the presence of diuron degraders in sugarcane-cultivated soils impacted by diuron due to repeated applications.

Study of atrazine degradation in soil from Kenyan sugarcane-cultivated fields in controlled laboratory conditions

A study to compare the extent of atrazine mineralization in soils from Kenyan sugarcane-cultivated fields with and without history of atrazine use was carried out in the laboratory under controlled conditions. The study was testing the hypothesis that repeated atrazine application to soil will not result in enhanced atrazine mineralization. The study was carried out with 14C-uniformly ring-labeled atrazine in a laboratory under controlled conditions. Atrazine mineralization to 14CO2 in soil with no history of atrazine use was negligible (0.16%) after 163 days of soil incubation. The three metabolites hydroxyatrazine, desisopropylatrazine, and desethylatrazine in the proportion of 17.7%, 1.3%, and 2.6%, respectively, were in the soil after 75 days. In the soil from the sugarcane-cultivated field with history of atrazine use, atrazine mineralization was 89.9% after 98 days. The same soil, amended with mature compost, showed a lag phase of eight days before rapid atrazine mineralization was observed.

First Comprehensive Study on Total Contents and Hot water Extractable Fraction of selected elements in 19 Medicinal Plants from Various Locations in Nyamira County, Kenya.

A large number of medicinal plants is traditionally known in Kenya and used for treatment of various diseases, for example diabetes, where metals are supposed to be involved in pathogenesis and therapy. Therefore, detailed investigation of the concentration of a large number of metals in medicinal plants is required for improved understanding and optimisation of the therapeutic role of metals and also to exclude potentially toxic effects. Our study focused on the determination of 30 selected elements in 19 medicinal plant species each collected from 3 sampling locations in Nyamira County, Kenya. The obtained comprehensive data set showed large variability and multivariate data analysis revealed that the differences in the elemental composition were stronger dependent on the plant species than on the sampling location. In addition, hot water extractions were performed to mimic the traditional preparation of medicine from the plants. It was found that the mean extraction efficiencies were below 20% except for B, Mg, P, K, Mn, Co, Ni, Cu, Zn, Rb, Mo, Cd and Tl, which are mostly essential elements apart from Cd and Tl. Sequential (ultra)filtration of the extracts was applied as novel approach for molecular size-fractionation of the extracted elemental species. The results indicate more than 50% low molecular weight species (<3kDa) for Mg, Mn, Co, Ni and Zn while predominantly larger size-fractions (>3kDa up to<5μm) were detected for V, Cu, Al and Fe.