Effects of N fertilization on soil chemistry dynamics in Ferralsols of the High Potential Maize Zone, Kenya

Nitrogen-based inorganic fertilizers have been crucial in crop production globally. For a long time, SSA agriculture has been characterised by low fertilizer use and negative nutrient balances. However, recently fertilizer use has increased drastically. Unfortunately, increased use of synthetic N fertilizers alters soil properties directly and indirectly, and N losses to the ecosystem contribute to environmental degradation and climate change. Limited studies have focused on the effect of increased N application rates on agricultural soils in the tropical highlands. It is crucial to investigate and understand N flows in tropical soils to predict potential ecological impacts of increased synthetic N-fertilizer use while meeting the food demand in SSA.

This study aimed to investigate the effects of increasing N rates on soil N dynamics, chemical properties and N use efficiency in maize-monocrop systems in the tropical highlands of the Rift Valley region, Kenya. A field experiment consisting of six N-fertilizer rates (0, 25, 50, 75, 100 and 125 kg N ha^-1) in triplicate was set up in Eldoret, Kenya. Soil samples were collected at depths of 0-20, 20-40 and 40-60 cm throughout the maize cropping season and analysed for mineral N (NH₄⁺-N and NO₃^--N), soil organic carbon and pH. Results indicate a significant change in the soil chemistry due to fertilisation. The response magnitude varied across the three soil depths. For instance, NO₃^- -N increased with increased N application rate, which peaked at 14 (55.81 mg kg^-1) and 42 (34.99 mg kg^-1) days after treatment application in the top 20 cm and 20-40 cm depths, respectively. Similar trends were also observed in the NH₄⁺-N concentration across different depths, with high N application rates tending to exhibit relatively high concentrations compared to treatments with lower N rates. We also observed a considerable decline in soil pH for plots treated with N fertilizer in the first 14 days, which then stabilized and rose gradually throughout the maize growing stages. However, the lower fertilizer plots tended to have higher pH in contrast to the other treatments. There was also a consistent increase in soil organic carbon (SOC), with slight fluctuations, throughout the cropping season.  

These results indicated low mineral N movement below the effective root zone depth during the active growth phase of the crop. Thus, a clear indicator of increased plant uptake and implies a reduced risk of loss through leaching in Ferralsols. We also expect that meteorological conditions coupled with crop phenological processes to play a significant role in the soil chemistry variability, as exhibited by the differences in response to the treatments. We will therefore consider crop phenological processes and how they influence soil nutrient cycles. The results of this study will help to inform sustainable N use in maize cropping systems and further improve understanding of N cycle in tropical soils.