Gross Nitrogen Turnover of Natural and Managed Tropical Ecosystems at Mt. Kilimanjaro, Tanzania

In our study at Mt. Kilimanjaro, East Africa, we quantified gross rates of ammonification, nitrification, nitrogen immobilization, and dissimilatory nitrate reduction to ammonium in soils across different land uses, climate zones (savanna, montane forest ecosystems, extensive agroforest homegarden, and intensively managed coffee plantation), and seasons (dry, wet, and transition from dry to wet season) to identify if and to what extent conversion of natural ecosystems to cultivated land has affected key soil microbial nitrogen turnover processes. Overall variation of gross soil nitrogen turnover rates across different ecosystems was more pronounced than seasonal variations, with the highest turnover rates occurring at the transition between dry and wet seasons. Nitrogen production and immobilization rates positively correlated with soil organic carbon and total nitrogen concentrations as well as substrate availability of dissolved organic carbon and nitrogen r > 0.67, P < 0.05), but did not correlate with soil ammonium and nitrate concentrations. Soil nitrogen turnover rates were highest in the montane Ocotea forest (ammonification 29.84, nitrification 12.67, NH₄ ⁺ immobilization 38.92, NO₃ ⁻ immobilization 10.74, and DNRA 1.54 µg N g⁻¹ SDW d⁻¹) and progressively decreased with decreasing annual rainfall and increasing land-use intensity. Using indicators of N retention and characteristics of soil nutrient status, we observed a grouping of faster, but tighter N cycling in the (semi-) natural savanna and Ocotea forest. This contrasted with a more open N cycle in managed systems (the homegarden and coffee plantation) where N was more prone to leaching or gaseous losses due to high nitrate production rates. The partly disturbed (selected logging) lower montane forest ranged between these two groups.