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 lgNg-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.