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Clearing and cultivation in crusting soils from Mazowe (Zimbabwe) has lead to severe changes in most physico-chemical characteristics related to the concentration and distribution patterns of plant nutrients and to the total amount of soil organic matter. Nevertheless, the concentration of the different humus fractions showed lower intensity changes, as did the mineralization rates of the organic matter.The most significant effects of cultivation on the soil chemical characteristics coincided with those considered to favor clay dispersion and crusting phenomena, including generalized desaturation of the exchange complex and losses of divalent ions with a potential bridging effect between soil particles. Concerning the soil organic matter, the humic acid tended to concentrate in the cultivated soils as a probable consequence of selective biodegradation of the other humic fractions. The composition and activity of soil humus suggest low-performance organo-mineral 2112 GIAMPAOLO, PARDO, AND ALMENDROS interactions: in these soils the active turnover of the plant wastes is not regulated by intense physico-chemical interactions with the soil mineral fractions, or by physical encapsulation of organic particles. In consequence, the mineralization rates were relatively constant in the soils studied and unrelated to soil organic matter concentration. The results suggests that there is a possibility to revert the early degradation stages of these soils through a rational management of suitable amounts of crop wastes.

The potential of Nostoc 9v for improving the nitrogen (N) 2 -fixing capacity and nutrient status of semi-arid soils from Tanzania, Zimbabwe, and South Africa was studied in a laboratory experiment. Nostoc 9v was inoculated on nonsterilized and sterilized soils. Inoculum rates were 2.5 mg dry biomass g 21 soil and 5 mg dry biomass g 21 soil. The soils were incubated for 3 months at 27 uC under 22 W m 2 illumination with a photoperiod of 16 h light and 8 h dark. The moisture was maintained at 60% of field capacity. In all soils, Nostoc 9v proliferated and colonized the soil surfaces very quickly and was tolerant to acidity and low nutrient availability. Cyanobacteria promoted soil N 2 fixation and had a pronounced effect on total soil organic carbon (SOC), which increased by 30-100%. Total N also increased, but the enrichment was, in most soils, comparatively lower than for carbon (C). Nitrate and ammonium concentrations, in contrast, decreased in all the soils studied. Increases in the concentration of available macronutrients were produced in most soils and treatments, ranging from 3 to 20 mg phosphorus (P) kg 21 soil, from 5 to 58 mg potassium (K) kg 21 soil, from 4 to 285 mg calcium (Ca) kg 21 , and from 12 to 90 mg magnesium (Mg) kg 21 soil. Positive effects on the levels of available manganese (Mn) and zinc (Zn) were also observed.