Carbon and nitrogen mineralization in cultivated and natural savanna soils of Northern Tanzania

In the present study, soil C and N mineralization and nutrient availability were compared: (1) in savanna woodland soils under natural acacia vegetation; (2) at termite sites; (3) in degraded woodland where acacias were selective logged for charcoal production; (4) in agricultural fields which were cultivated for 3 and 15 years, and (5) in traditional homestead fields which regularly received animal manure for about 10 years. Soil C and N mineralization dynamics were measured by incubation under controlled conditions for 120 days. Labile and stable soil C and N pools were determined by fitting double-exponential models to the measured cumulative mineralization. Selective removal of acacias from the woodland and short-term cultivation for 3 years did not affect available nutrient contents but significantly decreased total C and N contents and mineralization (P < 0.05). Mainly the labile soil N pool decreased during the first 3 years of continuous cropping, whereas after 15 years the stable N pool, total S, available Ca and Zn contents were also depleted. Even after 15 years, however, the decrease of nutrient availability (apart from N) was less severe than that of soil organic matter stability. Additionally, not only the labile but also the more stable soil C and N pools decreased and controlled total mineralization as determined by the incubation experiments. Homestead fields with manure additions were shown to have elevated soil nutrient and organic matter contents. However, the manure should be mixed into the soil to improve organic matter stabilization. Soil regeneration in degraded savannas and recently cultivated fields might rapidly be achieved, whereas the 15-years-cultivated fields may require longer fallows to restore soil fertility.