Increasing livestock grazing intensity reduces soil organic carbon and infiltration capacity in Miombo woodlands of Tanzania.

Description

The intensity of livestock grazing has been increasing in Miombo woodlands as a result of rising livestock numbers and limited grazing resources in alternative vegetation types. This study investigated the effect of light, medium, and high grazing intensities (GI) on soil organic carbon (SOC) and field-saturated soil hydraulic conductivity (Kfs) across nine Miombo woodlands spanning three districts in Tanzania with diverse climates and landforms. We hypothesize that high grazing intensities will negatively affect SOC and soil infiltration capacity. Grazing intensities were classified using two methods; stocking rate method based on official livestock population records and scoring method based on field observations. Data for this study was collected from nine Miombo woodlands found in villages performing livestock grazing in Miombo woodlands of Tanzania. We collected 188 soil samples for SOC and Bulk Density (BD) determination and 117 sample plots for soil infiltration measurements. Our results indicate that medium and high grazing intensities substantially reduced SOC content and Kfs compared to light grazing intensity. These findings suggest that Miombo woodland soil is degraded under medium and high grazing intensities. Efforts should be directed towards promoting management practices that reduce the number and frequency of livestock grazing in Miombo woodlands for biodiversity and water conservation.

Steps to reproduce

Sampling design: purposive sampling used to select three Tanzanian districts representing dry (Handeni), intermediate (Kilosa), and wet (Kilombero) Miombo woodlands. Villages within these districts with Miombo and grazing activities were listed. Only three villages were random chosen per grazing intensity category based on official livestock statistics. The village area was classified into forest and non-forest strata, where sampling plots were randomly allocated within forest stratum. Grazing intensity estimation: Grazing intensity was estimated using stocking rate method and scoring method. Stocking rate (SR) was calculated as total number of livestock (Total TLU)/Grazing area (Woodland). SR ≤ 0.2 TLU/ha/yr indicated Light GI, 0.3 < SR < 0.4 TLU/ha/yr indicated Medium GI, and SR ≥ 0.5 TLU/ha/yr indicated High GI. TLU was derived by multiplying number of cattle by 0.7 and shoots by 0.1 conversion factor. Grazing area was adjusted by removing 35% for unsuitable grazing land. Scoring method: Involve field assessment and scoring of grazing parameters ie; livestock presence, paths, and grazed vegetation from 0 to 3 based on coverage in plot. Grazing intensity was determined by total scores ie; Light GI (1-3), Medium GI (4-6), and High GI (7-9). Soil sampling: Rectangular plots (50m x 20m) with five sub-plots was used to collect topsoil samples. Composite samples were obtained from sub-plot centers. Bulk density (BD) samples were taken from the center sub-plot using a steel soil core ring (10cm height and 6.4 cm diameter). Soil organic carbon (SOC) was analyzed using the Walkley-Black method. BD was determined from oven-dried soil mass. Soil infiltration measurement: A single-ring infiltrometer with 30 cm diameter and 30 cm height was used. The ring was inserted 5 cm into the soil. Pre-wetting was done for 15 minutes, then water levels were recorded every 5 minutes for the first 30mins, every 10 minutes for the first hour, and every 20 minutes. Infiltration rates were calculated for each interval by subtracting the final water level from the initial one and dividing it by the time interval. Topsoil field-saturated hydraulic conductivity (Kfs) was estimated from the infiltration rates.

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