TOXICOLOGICAL EVALUATION OF GLYPHOSATE-BASED HERBICIDES IN SOIL AND MITIGATION BY TITHONIA DIVERSIFOLIA BIOCHAR (TITHOCHAR) TYPES.

Increasing hectarage of degraded farmland as indicated by loss of organic matter, poor nutrient holding capacity, disappearance of useful soil micro and macroorganisms such as earthworm and overall reduction in crop yield demands attention. Intensive and indiscriminate use of agrochemicals including herbicides have been implicated as major driver for soil degradation. Glyphosate is one of the commonly used herbicides in the world, indiscriminately used among farmers and non-farmers in Ogbomosho. This work therefore evaluated the toxicological effect of glyphosate-based herbicide on soil as well as on useful microorganisms like earthworm and its mitigation by Tithonia diversifolia (TD) Biochar types. The leaching column experiment was a factorial combination of two glyphosate types: GLY1 and GLY2, and two biochar types: Sole TD (STB) @ 350° C and TD+ bone meal (BM) @ 350°C (TDBMB) pyrolyzed for 20 minutes residence time in a muffle furnace and applied at application rate of 5t/ha (equivalent to 3 g/1.2 kg soil). Two checks which were herbicide alone (which received no biochar) and absolute control AC (which received neither biochar nor herbicide) were compared. Polyvinyl (PVC) pipes filled with 1.2 kg soil were treated with appropriate biochar and left to incubate at field capacity for two weeks. Each glyphosate was applied at manufacturer’s recommended rate of 6 L a.i./ha equivalent to 22.5 μL of glyphosate stock solution applied into two sets of 9 leaching column pipes containing GLY1 and GLY2 respectively (prepared by diluting 36 μL of each glyphosate into 225 μl water to serve 10 columns (1 allowance included). Afterwards, NPK 15:15:15 chemical fertilizer was applied at rate of 0.24 g/1.2 kg soil (60 kg N, 30 kg P and 30 kg K). One-time leaching simulation was done using 50 ml water. Two maize seeds were thereafter sown and nurtured for two weeks. For earthworm trial, 200 g same freshly sieved soil were placed into mini-cups and moistened to field capacity using 40 ml of appropriate leachates collected from the one-time leaching simulation. Other treatments such as: soil that received tap water (TW), freshly prepared 100% GLY 1 and 100% GLY 2 were included and compared. This was followed by placement of two earthworm (initial weights recorded) each into each treated mini cup. The set up was covered with foil paper for 28 days incubation and earthworms’ weight were monitored at 7,14, 21 and 28 days of incubation. Data were collected on plants (Dry biomass weight, Pb and Cd contents), soil (Pb, Cd, pH, EC, ORP, available P, organic carbon), leachate (pH, EC, Pb and Cd) and earthworms (Initial and weekly body weight, weight loss/gain and mortality/new birth and evidence of cast formation). All the data collected were subjected to two-way analysis (and one-way for earthworm trial) of variance using Genstat statistical package and significant means were separated using LSD (and DMRT for the one-way trial) at 5% probability level. Sole Glyphosate applications (especially GLY1) reduced soil organic carbon by 3%, leachate pH by 8.9% and increased cadmium (Cd) in water by 100%, leachate volume by 0.9% and leachate EC by 26.4%. They however enhanced soil available P by 87.2% (GLY1) compared to AC. Biochar pretreatment however increased soil organic carbon (SOC) by 56.1% (by STB in GLY1), available P by 185.9% (in TDBMB in GLY2), leachate pH by 8.8%(STB in GLY1), dry biomass weight of maize by 264.4% (by TDBMB in GLY2) and reduced leachate volume by 8.8% (by TDBMB in GLY1) and Cd in leachate by 600% (by STB in GLY1). Biochar pretreatment also increased leachate EC by 8.5% (TDBMB) indicating ready availability of nutrient ions in them upon reaction with soil moisture. Biochar pretreatment contributed significantly highest to earthworm weight gain (TDBMB in GLY2) and contributed significantly to the highest earthworm weight loss (STD in GLY1) within 28 days. Under pretreatment with STB, GLY1 proved to be more toxic to earthworms than GLY2. Generally across the treatments, Absolute control recorded the highest earthworm weight loss. Conclusively, this research work highlighted toxicity potential of glyphosate-based herbicides applied at manufacturer’s recommended rate. The effectiveness of tithonia biochar pretreatment in mitigating toxicity effects in glyphosate-treated soil was also established in this trial. Glyphosate type 1 posed to have more toxic effects on the environment over type 2 and should therefore be used with caution such as pretreatment of soil with appropriate biochar prior glyphosate application. Repetition of the trial under field condition is recommended to evaluate weed killing efficacy of the tested glyphosate under biochar pretreatment.