Carbon and nitrogen mineralization in cultivated and natural savanna soilsof Northern Tanzania

In the present study, soil C and N mineralization and nutrient availability were compared: (1) in savanna woodland soils under natural acacia vegetati on; (2) at termite sites; (3) in degraded woodland where acacias were selec tive logged for charcoal production; (4) in agricultural fields which were cultivated for 3 and 15 years, and (5) in traditional homestead fields whic h regularly received animal manure for about 10 years. Soil C and N mineral ization dynamics were measured by incubation under controlled conditions fo r 120 days. Labile and stable soil C and N pools were determined by fitting double-exponential models to the measured cumulative mineralization. Selec tive removal of acacias from the woodland and short-term cultivation for 3 years did not affect available nutrient contents but significantly decrease d total C and N contents and mineralization (P < 0.05). Mainly the labile s oil N pool decreased during the first 3 years of continuous cropping, where as 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 matte r stability. Additionally, not only the labile but also the more stable soi l 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 stabili zation. Soil regeneration in degraded savannas and recently cultivated fiel ds might rapidly be achieved, whereas the 15-years-cultivated fields may re quire longer fallows to restore soil fertility.