This study evaluated the effects of organic and inorganic soil fertilization on forage grass (Brachiaria brizantha cv. xaraes) yields, soil N availability, and soil greenhouse gas (GHG) emissions in Central Kenya. A field experiment was conducted at the International Livestock Research Institute (ILRI) farm in Nairobi, Central Kenya. A completely randomized block design was set up with three replications in blocks (20 m × 15 m) approximately 50 m apart from each other, each containing six plots (4 m × 2 m) with Brachiaria brizantha cv. xaraes. Treatments included one inorganic and four organic soil fertilizers, namely NPK fertilizer, farm-yard cattle manure (FYM), FYM plus biochar (FYM-BC), FYM converted to bioslurry via anaerobic digestion, legume intercropping with Lablab (Lablab purpureus), and control (zero fertilizer amendment). Greenhouse gas emissions (N2O, CO2 and CH4) were measured using the static chamber approach for a period of eight months. In addition, soil samples were taken for determination of mineral N concentrations in the forms of nitrate (NO3 - ) and ammonium (NH4 + ). Plant biomass sampling for Brachiaria brizantha cv. xaraes grass yields was conducted every ten weeks and above-ground plant dry matter was determined. All fertilizer types were applied at a rate of 45 kg N ha-1 one week after each harvest, except for Lablab intercropping, which relied solely on biological nitrogen fixation via the legume (rate not determined in this study). The study was conducted between October 2018 and August 2019 comprising of four harvest seasons of 10 weeks each: short rains (SR, October 2018 to January 2019), hot dry season (HD, January 2019 to March 2019), long rains (LR, March 2019 to June 2019), and short rain 2; cold dry season (CD, June 2019 to August 2019). Treatment and season significantly influenced daily CH4 uptakes (p <0.01 and p = 0.009) but did not show significant interaction (p = 0.093). Methane uptake was similar across all the treatments following the order of Control > Lablab > FYM > FYM-BC > NPK, except for Bioslurry which exhibited significantly lower (-2.69±4.47) CH4 uptake (p< 0.01). Within the xiv seasons, significantly lower (-11.43±4.42) and higher (-21.23±1.11) CH4 uptakes were recorded during the HD and CD seasons, respectively while SR and LR seasons had similar CH4 uptake. Treatment and season had significant (p < 0.01 two-way ANOVA) effect on CO2 emissions. CO2 emissions in FYM-BC and FYM alone were on average lower by 61.6% compared to the control which had the highest (94.76±19.32). Seasonal CO2 emissions followed the order of CD>HD>LR>SR seasons, respectively. Treatment and season also interacted significantly (p<0.01 two-way ANOVA) to influence CO2 emissions. Lower (44.33±15.67) emissions occurred under FYM alone during the HD season while the highest (157.54±2.77) CO2 emissions was recorded under the control treatment during the SR season. FYM-BC and FYM alone had significantly (p < 0.01 two-way ANOVA) lower (6.70±14.48 and 8.20±15.67 respectively) N2O emissions compared to the control which had the highest (12.95±3.61). Significantly higher N2O emissions were recorded during the SR season while HD, LR and CD seasons had similar emission rates. Significant (p < 0.01 two-way ANOVA) interaction between treatment and season was also observed with NPK recording the lowest (4.21±0.83) emissions during the second season relative to control which had the highest (27.16±0.90) N2O emissions during the first season. Furthermore, fertilizer treatments significantly influenced NH4 + and NO3 - availability in the soil (p < 0.001). The highest NH4 + concentration was recorded in the NPK treatment 14 days after fertilization (21.20±27.01 µg g -1 DM), while the lowest NH4 + concentration was recorded in the Lablab treatment (6.62±8.02 µg g-1 DM). Similar to NH4 + , significantly higher NO3 - -N concentration was observed in the NPK plots 14 days after fertilization (61.41±38.81 µg g-1 DM), while the lowest NO3 - concentration was found in the Lablab plots 14 days after the last harvest (37.09±25.10 µg g-1 soil). Brachiaria brizantha cv. xaraes yields for the four harvests followed the order Control > FYM > NPK > FYM-BC > Bioslurry > Lablab, but these differences were not significant (p ₌ 0.957). There were, however, significant differences in yields of Brachiaria across the four seasons (p<0.01), with highest yields recorded in the long rains at 4.72±1.47 Mg DM ha-1 and lowest yields recorded in the cold dry season at 1.54±0.51 Mg DM ha-1. The total mean biomass for the entire study period (8 months) was 10.4t ha-1±1.3. Taken together, our findings do not show any significant effect of different soil fertilizers on Brachiaria brizantha cv. xaraes yields. This could partly be attributed to the short study period of eight months in a newly established area. Furthermore, the soil had been ploughed before grass planting, which could have mobilized N and other nutrients from soil organic matter mineralization and therefore might have masked a potential fertilizer effect. Whether fertilizer effects become more clearly distinguishable in the long term requires long-term measurements. Concerning soil GHG emissions, the findings have shown that at the applied fertilization rate, organic fertilizers did not increase soil N2O emissions in this tropical site, indicating a potential option for low-emission forage grass production in SSA.

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