Potential of perennial forages on soil carbon sequestration across agroecological zones with varying management practices in Meru County, Kenya

The decline in soil organic carbon (SOC) due to continuous cultivation practices threatens soil productivity and compromises climate change mitigation efforts. This study investigates the effect of agro-ecological zones (AZ), management practices (MP), and specific forages on SOC and its related fractions: particulate organic carbon (POC) and mineral-associated organic carbon (MAOC), in Meru County, Eastern Kenya. Thirty-five predetermined perennial forage fields with Brachiaria (B), Panicum (Panicum maximum) (P), Napier grass (P.P. Schumach) (N), paired with maize (Zea mays) (M) growers, and with two MP [farmyard manure (FYM) and inorganic fertilizer application] were selected per agro-ecological zone. A factorial plot design was employed, with zones as the main plots, the MP as split plots, and the forages as split-split plots. Soil samples were collected from 1 m-by-1 m plots across 35 farms at depths of 0–50 cm, and analyzed for pH, bulk density, SOC content, POC, MAOC, and other micronutrients. Soil samples were statistically analyzed using two-way ANOVA. From the survey, 70% of the respondents had perennial forages of more than 3 years except for panicum which majority had planted for <3 years. Most of the farmers interviewed applies organic manure. Bulk density ranged from 1.07 to 1.19 g cm−3, with the highest values under inorganic fertilizers and the lowest under FYM, showing significant differences between paired plots (p = 0.0011). Soil pH ranged from 5.13 to 5.36, and MP significantly affected all micronutrients, with FYM combined with Panicum having the highest cation exchange capacity (19.45 cmol/kg) and manganese (96.30 ppm), and FYM combined with maize showing the highest available phosphorus (54.08 cmol/kg). Results showed no significant differences in mean values of POC and MAOC across zones, though the Lower Zone (LZ) had higher POC (6.63 g C kg−1) and MAOC (7.24 g C kg−1) compared to the Mid Zone (MZ) and Upper Zone (UZ). Different forages exhibited varying levels of MAOC, POC, and SOCs, with Brachiaria showing the highest SOC (15.69 g C kg−1) and Panicum the lowest (14.84 g C kg−1). Particulate organic carbon and MAOC content were significantly different across forages and maize (p < 0.05), with Brachiaria having higher MAOC (5.37 g C kg−1) and POC (5.97 g C kg−1). FYM combined with Brachiaria resulted in the highest POC (6.09 g C kg−1) and MAOC (5.44 g C kg−1). POC and MAOC concentrations varied significantly with soil depth, showing higher values in the top 10 cm. The Mid Zone recorded significantly higher SOC, POC, and MAOC than the UZ and LZ. This study concludes that AZ and MP substantially influence SOC sequestration, with FYM and Brachiaria being most effective in LZ.