FATE OF NITROGEN FERTILIZER IN RAINFED MAIZE SYSTEMS OF TROPICAL HIGHLANDS OF KENYA

Abstract

Crop production in sub-Sahara Africa (SSA) is often constrained by declining soil fertility associated with negative nutrient balances as a result of continuous cropping with little or no replenishment of nutrients. To offset the negative nutrient balance, especially nitrogen (N), in SSA agroecosystems and meet the ever-increasing food demand, it is paramount to intensify N fertilizer use. However, this could prove deleterious to ecosystem functioning since increased N fertilizer is associated with elevated reactive nitrogen (Nr) loading in the environment. Therefore, a study comprising increasing N fertilizer rates, 0, 25, 50, 75, 100 and 125 kg N ha-1 was conducted in the highland tropics of the Rift Valley, Kenya. The objective was to evaluate fertilizer-induced soil mineral N (Nmin) changes within the soil profile and nitrous oxide (N2O) emissions in maize monoculture systems. On average, the N application of 125 kg N ha-1 exhibited the largest NO3 - (14.8 mg kg-1 ) and NH4 + (11.85

mg kg-1 ) concentration across depth and different N rates. The annual N2O fluxes ranged from 0.33 to 0.77 kg N2O-N ha-1

across the different N rates. Additionally, yield- scaled emissions ranged from 0.07 to 0.14 g N2O-N kg-1

grain yield across different N rates. Increasing N fertiliser rates increased grain yield by 7, 14, 24, 37 and 46 % while applying 25, 50, 75, 100 and 125 kg N ha-1

, respectively. Application of 25 kg N ha-1 gave indications of soil N mining, with a nitrogen use efficiency (NUEgrain) value of > 1, whereas the rest of the N rates had an NUEgrain value of < 1. The optimum physiological efficiency and partial-factor productivity were realised when N was supplied at 75 kg N ha-1

. Overall, the results suggest that increasing N application to 75 kg N ha-1 would not only improve grain yield but also increase soil N intensity, N2O emissions and optimise NUE. Therefore, matching crop demands through judicious N use will be critical to ensure efficient use of fertiliser N and reduce losses to the ecosystem.