Nitrogen balance and nitrogen use efficiency in an East African tropical montane catchment characterised by smallholder farming

The input of reactive nitrogen sourced from anthropogenic activities such as smallholder farming as well as changes in its transport and fate in the environment may alter the nitrogen balance of a catchment and the nitrogen use efficiency for crop production. Both have the potential of causing detrimental effects on agricultural productivity and the environment such as water bodies. Insufficient nitrogen addition for crop production could lead to soil nitrogen depletion, while too high input rates could lead to excess nitrogen polluting water bodies. In the Mau Forest Complex (MFC) in Kenya, fertilizers and livestock management have been assumed to be associated with the increase in annual riverine export of reactive nitrogen. Twice the annual export of nitrogen from a catchment dominated by smallholder agriculture was reported compared to that from the native forest. To assess the role of smallholder agriculture in nitrogen losses, this study aims at determining the nitrogen balance and nitrogen use efficiency of a 27 km² headwater catchment characterized by smallholder farming in the MFC. Anthropogenic inputs and outputs were estimated from a household survey (n=185), field measurements involving precipitation collectors in 10 different locations as well as literature review. The nitrogen flows in the native forest were obtained from the literature.Results show that at farm scale, about one third of investigated farms have negative nitrogen balances, while at the catchment scale the aggregate nitrogen balance is −10.9 kg N ha−1 yr−1. This is in contrast to the positive nitrogen balance of the native forest of 26.5 kg N ha−1 yr−1. With respect to the nitrogen use efficiency only 20% and 18% of the maize fields as well as 7% and 12% of the potato fields, recorded nutrient use efficiency between 50% and 90% in 2018 and 2019, respectively.The study shows that inorganic fertilisers, atmospheric deposition and biological fixation are the most important sources of reactive nitrogen, while crop harvest, denitrification and leaching were identified as major loss pathways. The wide range of nitrogen surplus and deficits among farms and the subsequent potential for eutrophication and soil mining highlight the need to better educate farmers on the optimal use and timing of fertiliser application to close the deficit gap and prevent pollution.