CARBON TURNOVER (DELTA-C-13) AND NITROGEN MINERALIZATION POTENTIAL OFPARTICULATE LIGHT SOIL ORGANIC-MATTER AFTER RAIN-FOREST CLEARING

Soil samples from under a rainforest, a papaya plantation, a pure Brac hiaria humidicola pasture and a B. humidicola/Desmodium ovalifolium sw ard from the humid tropical Bahia, Brazil, were analyzed for fertility parameters, litter quality, particulate light fraction [PLF>100/53-10 0 mu m, < 1.8 g cm(-3) density (sodium polytungstate)] and delta(13)C signatures. Under the grass pasture total soil C was reduced to 80 and 95% of the initial forest value after 18 years in the surface 2 cm an d the 5-15 cm horizons, respectively, largely due to the sharp decline in rainforest-derived C after deforestation. Losses of rainforest-der ived C amounted to 73% (0-2 cm) and 40% (5-15 Cm) of the initial soil C content. The contribution of newly added B. humidicola-derived soil C was estimated to be 24 and 5 mg C g(-1) soil and accounted for 68 an d 37% of total C in the surface and lower soil layer, respectively, af ter 18 years. The C content of the particulate light fraction (PLF-C) was largest in the rainforest and smallest in the papaya plantation. T he PLF-C accounted for 12-36% in the surface soil and 10-16% in the 5- 15 cm layer. The size of the PLF was not only influenced by the quanti ty of C inputs, but also strongly reflected the quality of incoming li tter. According to its delta(13)C signature the large (> 100 mu m) PLF was of younger age than the finer PLF or whole soil samples. The age of the 53-100 mu m PLF was very close to that of stabilized organic ma tter as indicated by C-13 data, its low C-to-N ratio (15-17:1) and its turnover time was about 30 y. The PLF appeared not to consist of one uniform pool and thus may have to be subdivided to be useful for model ling purposes (e.g. active, passive pools and charcoal). The contribut ion of PLF and above ground litter to total soil N mineralization (ana erobic incubation) was highest in the undisturbed rainforest (12/16% f or the PLF/litter, respectively, in the 0-2 cm layer), but much less i n the other systems. However, mineralization of PLF from different sou rces was unpredictably altered by the density agent. Introducing D. ov alifolium into these pastures increased, significantly, the amounts of N in PLF, decreased the C-to-N ratio of PLF and tended to increase th e mineralization potential despite its high lignin and polyphenol cont ent. The combination of delta(13)C analysis and size/density separatio ns, of surface samples in particular, allowed sensitive detection of c hanges in soil organic matter dynamics and soil fertility. (C) 1997 El sevier Science Ltd.