Nitrogen inputs from smallholder farming in a tropical montane catchment - current state and outlook

Abstract

<p>Rising human populations increase the demand for food and lead to the intensification of agriculture and nitrogen fertilization to sustain productivity. In the tropical montane Mau Forest Complex in Kenya, the annual export of nitrogen from a catchment dominated by smallholder agriculture were reported to almost double those from the native forest. Despite the assumption that this excess nitrogen originates from fertilizer application, there are no studies that provide empirical information on the amount and spatial distribution of nitrogen inputs from smallholder agriculture into these catchments. Given the fact that the Mau Forest complex lost 25% of its forest cover to agriculture and other encroachment activities, such information is essential to better quantify the effect of smallholder farming practices on the nitrogen cycle and its contribution to catchment nitrogen export.</p><p>This study aimed at quantifying spatial distribution of fertilizer inputs in a smallholder catchment in the Mau Forest Complex using a household survey (n=185).</p><p>Results show that almost all farmers (99.4%) use inorganic fertilizers with an average nitrogen (N) application rate of 41±7.8 kg N ha<sup>-1</sup> yr<sup>-1</sup> diammonium phosphate (DAP). Among the DAP users, 16% apply in addition 79±3.9 kg N ha<sup>-1</sup> yr<sup>-1</sup> as NPK fertilizer, and 11% add 29±5.3 kg N ha<sup>-1</sup> yr<sup>-1</sup> as calcium ammonium nitrate (CAN). Overall, the average nitrogen input from inorganic fertilizers is 64±13.7 kg N ha<sup>-1</sup> yr<sup>-1</sup>. Only 6% of the cropland is fertilized using manure and other farmland residues with 79% of farmers anticipating to increase their inorganic fertilizer application rates in the future.</p><p>In conclusion, a future increase in nitrogen application rates on farmland in combination with continued conversion of natural forest to agricultural land raises a concern on whether nitrogen export will increase further, posing a threat to drinking water quality and the health of aquatic ecosystems downstream. To balance the trade-off between food production and the catchment nitrogen balance, there is a need to train farmers on appropriate methods, timing and optimal amounts of fertilizer application to prevent unnecessary losses.</p>