Inputs of reactive nitrogen from wet deposition and fertilization in a tropical montane catchment characterised by smallholder farming.

Reactive nitrogen from anthropogenic inputs such as fertilizers and its changes in the transport and fate in the environment as a consequence of changes in land management may alter the nitrogen balance of a catchment and thus its concentrations in water bodies. This can be enhanced by increased wet and dry deposition of nitrogen from anthropogenic activities. In the Mau Forest Complex in Kenya, the annual export of nitrogen from a catchment dominated by smallholder agriculture was reported to be almost double those from the native forest. Fertilization and livestock management are assumed to have contributed to this shift, but no empirical evidence is available to support this. Furthermore, the contribution of the nitrogen wet deposition through rainfall and throughfall to the nitrogen balance has not been quantified yet. This study aims at determining the contribution of nitrogen wet deposition and anthropogenic inputs to the nitrogen balance of a 27 km² headwater catchment characterized by smallholder farming in the Mau Forest Complex. Rainfall and throughfall samples were collected from precipitation collectors in 10 different locations within 24 hours after 11 rainfall events in the span of 9 weeks. Anthropogenic inputs were estimated from a household survey (n=185). 

Median and Interquartile range (IQR) concentrations of total dissolved nitrogen (TDN) were slightly higher in rainfall i.e. 0.7 (0.4–0.9) mg N L⁻¹ than in throughfall i.e. 0.6 (0.4–0.8) mg N L⁻¹, resulting in median wet deposition of 0.04 (0.02–0.06) kg N ha⁻¹ from rainfall and 0.03 (0.01–0.06) kg N ha⁻¹ from throughfall per sampled rainfall event (4.7 mm; 2.5–8.1 mm). Extrapolated to the full year, this leads to an estimated nitrogen input from wet deposition of 11.7 (9.5-12.5) kg N ha⁻¹ yr⁻¹ from rainfall and 6.4 (5.7-9.6) kg N ha⁻¹ yr⁻¹ from throughfall, although this estimate does not consider seasonal changes in TDN concentrations in rain- and throughfall. Median inputs of nitrogen from inorganic fertilizer was 25 (15.2–40.2) kg N ha⁻¹ yr⁻¹, whereas annual inputs from livestock were 76 (49-125) kg N ha^-1 yr⁻¹.

Reactive nitrogen inputs from farming and livestock are higher than the estimated wet deposition and could therefore significantly impact the catchment nitrogen balance. It follows therefore that a continual and increase use of nitrogen inputs from manures and fertilization with inorganic fertilizers, as well as further forest cover loss for agricultural expansion may lead to future elevated levels of nitrogen in the environment, leading to risks of progressive enrichment of water bodies and further nitrogen imbalances. To keep these in check, appropriate manure management strategies, fertilizer application and control of forest conversion cannot be overemphasized.