Effects of Nitrogen Forms on Growth, Yield and Nutritional Quality of Amaranth (Amaranthus Species) in Kiambu and Kirinyaga Counties, Kenya

Amaranthus spp is amongst the most preferred African leafy vegetables in the tropics. It contains numerous vitamins, minerals and bioactive phytochemical compounds for nutritional and health benefits. Nitrogen is a vital mineral element for plant growth which affects not only the biomass accumulation but also the nutritional quality of higher plants. This study was carried out to evaluate the effect of different N forms on growth, mineral elements, anti-nutrient and phytochemicals accumulation in Amaranthus species. In order to select the most preferred African leafy vegetables (ALVs) by farmers, field survey was done in Kiambu and Kirinyaga counties. Semi-structured questionnaires were administered on 67 respondents by purposive sampling technique. The survey revealed that Amaranthus spp was the most preferred with 84% and 93% in Kiambu and Kirinyaga respectively. Thus, amaranth was chosen for the greenhouse and field experiments. The field experiment was laid in a split plot arrangement in a randomized complete block design, with three amaranth varieties (AB5, AB6 and AB7) being the main plots and three N forms; sole ammonium (NH4 + ) stabilized with Piadin ® as nitrification inhibitor, sole nitrate (NO3 ), ammonium/nitrate mixture (NH4NO3) and control (where no N form was applied) constituted the subplots. While greenhouse was a completely Random Design (CRD). The treatments were replicated three times. Field survey data was analyzed by use of SPSS software while growth, biochemical and phytochemical data were subjected to analysis of variance (ANOVA) using SAS software. Separation of means was done using least significance difference (LSD). About 59% and 66.7% respondents in Kiambu and Kirinyaga respectively grew ALVs for nutrition or health benefits. Field and greenhouse experiments revealed that growth of three amaranth varieties were significantly (P≤0.05) affected by different N forms. Compared to control, nitrate treatment increased plant height by 54.3%, ammonium nitrate by 46.4% and sole ammonium by 29.2%. Nitrate treatment enhanced shoot dry weight 8 folds and leaf area 3 folds while ammonium treatment was 3 folds in relation to the control. This trend was also observed with leaf area in relation to the control. Root dry weight increased by 72.3% under nitrate provision and 36% under ammonium treatment in the greenhouse experiment. Amaranths treated with sole nitrate notably increased plant tissue calcium (Ca) and zinc (Zn) compared to ammonium-treated plants while iron (Fe) content was not significantly different. Compared to the control, nitrate elevated oxalate accumulation unlike ammonium treatment which on the contrary inhibited oxalate buildup. Under ammonium treatment, total flavonoids contents increased by 17.3% while total phenolics content increased by about 37.3% in greenhouse experiment. Likewise, NH4 + N form had higher antioxidant DPPH scavenging activity. Similar trends were observed in the field experiments. Ammonium induced rhizosphere acidification while nitrate treatment resulted to rise in rhizosphere pH. This affected concentration of mineral elements, anti-nutrients (oxalates) and phytochemical constituents as well as anti-oxidant inhibiting capacity of amaranth plants. It is therefore recommended that ammonium nitrate be used on vegetable amaranth for optimal growth and nutritional benefits.