Microbial biomass, mineral nitrogen and carbon content in savanna soil aggregates under conventional and no-tillage

In the tropics, few studies have considered the size and quality of the active compartment of the soil organic matter (SOM) associated with soil aggregates and the concomitant changes induced by tillage management. The main objectives of this study were firstly to determine the distribution of nitrogen (N) and carbon (C) of the microbial biomass (MB) in the aggregates of savanna soils under conventional tillage (CT) and no-tillage (NT). Secondly, to examine the importance of MB in the maintenance of water stable aggregates in savanna derived soils during the two contrasting climatic seasons. Samples taken at a depth of 0-5 and 5-10 cm were collected from a cultivated ultisol under CT for 13 years and another under CT for 8 years followed by 5 years under NT. A native-savanna (NS) was used as control. Sampling was in April and September corresponding to the dry and rainy seasons, respectively. I A drastic decrease in water stable macroaggregates was observed in the cultivated soils compared to NS. The decrease was more accentuated in the case of CT vs NS during the rainy season (92.3 vs 38.8%), whereas it was of lesser extent in the case of NT (92.3 vs 64.5%). Savanna and NT soils had a higher proportion of macroaggregates than microaggregates, but a reverse situation occurred in the soil under CT. Microbial C and N contents were consistently higher in the macroaggregates than in the microaggregates. Soil aggregates under CT did not show changes in MB in both seasons. Within microaggregates, microbial and mineral N was not significantly affected by intensity of tillage, whereas in macroaggregates variation in MB was affected by both the tillage and the season in the NT soil. The use of NT in degraded savanna ecosystems under intensive cereal cropping and subjected to strong wet-dry changes caused by the two season climate seems to be promising since it improves the SOM and increases the structural stability which are crucial aspects of the agroecosystem's functioning. (C) 2002 Elsevier Science Ltd. All rights reserved.