Abstract

Pesticides protect crops, but may also affect non-target microorganisms important to soil functioning. This study evaluated Lambdashi (lambda-cyhalothrin), Temifos (chlorpyrifos) and Cyperforce (cypermethrin) at 0.07%, 0.35% and 0.70% in the rhizosphere of maize (Zea mays) grown in soil from the Rivers State University Demonstration Farm, Port Harcourt, Nigeria. In a controlled pot experiment, total heterotrophic bacteria (THB), coliforms, heterotrophic fungi and phosphate-solubilising bacteria (PSB) were assessed by culture-based methods at baseline and every two weeks for eight weeks. THB, coliform and fungal counts generally decreased with increasing concentration and exposure time, while untreated controls were comparatively more stable. At 0.70%, THB declined from 3.1 × 10⁷ to 3.7 × 10⁴ CFU/g with Lambdashi, from 2.35 × 10⁷ to 2.16 × 10⁴ CFU/g with Temifos, and from 5.9 × 10⁷ to 1.35 × 10⁴ CFU/g with Cyperforce. Coliforms under 0.70% Lambdashi decreased from 1.7 × 10⁵ to 1.81 × 10² CFU/g. Fungal counts at 0.70% fell from 1.21 × 10⁴ to 1.02 × 10² CFU/g with Lambdashi, from 2.5 × 10⁴ to 1.02 × 10³ CFU/g with Temifos, and from 6.9 × 10⁴ to 1.32 × 10³ CFU/g with Cyperforce. These reductions suggest marked suppression of culturable microbial groups and potential effects on decomposition, nutrient cycling and rhizosphere biological functioning. PSB counts in treated soils were reported only at baseline, so their response over time could not be determined; by contrast, PSB in untreated maize soil increased from 2.9 × 10³ to 4.3 × 10³ CFU/g by week 8. Given the marked reductions at 0.70% and subsequent non-recovery of PSB, this concentration should not be recommended for routine field use until replicated field trials establish an acceptable dose-response relationship. Pesticide use should remain within approved product-label rates, and post-treatment monitoring should specifically assess PSBs recovery alongside THB, coliform and fungal counts.