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Grassland management practices that improve carbon uptake by increasing productivity or reducing carbon losses can lead to net accumulation of carbon in soils. A study was conducted to quantify the amounts of shoots and roots biomass of Brachiaria grass cultivars and their effects on soil carbon in two sites, Ithookwe and Katumani in semi-arid tropical Kenya. The grass cultivars were Brachiaria decumbens cv. Basilisk, B. brizantha cvs. Marandu, MG4, Piatã and Xaraes, B. humidicola cv. Llanero and B. hybrid cv. Mulato II. These were compared with two locally cultivated grasses (Chloris gayana cv. KAT R3 and Pennisetum purpureum cv. Kakamega 1). The grass treatments were evaluated with fertilizer application (40 kg P applied at sowing and 50 kg N ha-1 in each wet season) and with no fertilizer application. Shoots biomass of the Brachiaria cultivars ranged from 3.0 to 11.3 t ha-1 and 5.5 to 8.3 t ha-1 at Ithookwe and Katumani sites respectively in year 1. The highest shoots biomass recorded at Ithookwe was from cv. Piata while cv. MG4 gave the highest biomass at Katumani. Similar trends were recorded in year 2 of growth though the shoots biomass was lower at Katumani. However, the yields were significantly lower than those recorded from control, Napier grass in both years. The cv. Marandu, Xaraes, Basilisk and Piata had higher roots biomass than the controls (Rhodes grass and Napier grass) indicating greater potential for the Brachiaria grasses to sequester more carbon in the soil. The results of this study indicate that introduction of Brachiaria grasses in the semi-arid tropics of Kenya and in other similar environments can help increase soil carbon stocks that would mitigate the adverse effects of climate change and have greater economic returns.

Soil aggregation is a key short term indicators of soil quality attributed to changes in land management. A study was conducted to investigate changes in the size distribution and stability of soil aggregates in a structurally unstable sandy loam soil following cultivation of Brachiaria grass in semi-arid region of Kenya. Brachiaria grass cultivars included Brachiaria decumbens cv. Basilisk, B. brizantha cvs Marandu, MG4, Piata and Xaraes, B. humidicola cv. Llanero and B. hybrid cv. Mulato II which were compared with two locally cultivated forage grasses (Chloris gayana cv. KAT R3 and Pennisetum purpureum cv. Kakamega 1) and a bare plot (negative check). The grass treatments were evaluated with fertilisers application (40 kg P applied at sowing and 50 kg N ha−1 in each wet season) and with no fertiliser applications. Aggregate size fractions were isolated using the wet sieving method. Aggregation based on the proportion of small macro-aggregates (250–2000 μm) increased in soils cultivated with all grass types compared to the control and was greatest in soils under B. hybrid cv. Mulato II. Aggregate stability in terms of mean weight diameter (MWD) differed among the grasses and was highest in soils under cv. Mulato II and cv. Marandu with MWD of 4.49 and 4.31 mm, respectively. Changes in small macro-aggregates fraction was positively correlated with particulate organic matter (POM) (r=0.9104, P= 0.001), microbial biomass carbon (MBC) (r=0.5474, P= 0.01), soil organic carbon (SOC) (r=0.3654, P= 0.05) and root biomass (r=0.4977, P= 0.01). This indicated that the binding agents were important in the aggregation of soils cultivated with Brachiaria grasses.

A study was conducted to investigate the changes in microbial biomass carbon (C), nitrogen (N) and phosphorus (P) following cultivation of Brachiaria grasses in semi-arid region of Kenya. The Brachiaria grass cultivars included Brachiaria decumbens cv. Basilisk, B. brizantha cvs Marandu, MG4, Piatã and Xaraes, B. humidicola cv. Llanero and B. hybrid cv. Mulato II which were compared with two locally cultivated forage grasses (Chloris gayana cv. KAT R3 and Pennisetum purpureum cv. Kakamega 1) and a bare plot (negative check). The grass treatments were evaluated with fertilizers application (40 kg P applied at sowing and 50 kg N ha-1 in each wet season) and with no fertilizer applications. Microbial biomass C, N and P were determined on field moist rhizosphere soil (18-23% by weight) from a depth of 10 cm. Microbial biomass was influenced (p < 0.01) by grass cultivars and N and P fertilizers. Generally, microbial biomass N was higher in plots with grasses than in the bare plots. A significant enrichment of organic matter was noted in the microbial biomass when Brachiaria grasses are grown with N and P fertilizers. Among Brachiaria cultivars, the highest microbial biomass C was recorded in plots with cv. Mulato II and the lowest from the plots with cv. MG4. Brachiaria grasses with fertilizers application accumulated the highest microbial C and N compared to grasses without fertilizers, but no interaction was observed between fertilizer and grass cultivars. The cv. Marandu had the highest microbial biomass N (21.2 mg N kg-1) in fertilizer treatments whereas cv. Mulato II hybrid had the highest microbial N (14.6 mg N kg-1) in no fertilizer treatments.