Changing enzymatic activities and mycorrhizal infections in a chronosequence of secondary and mature forests of eastern Amazonia Cl á udio Jos é Reis de Carvalho 1, Eric A. Davidson 2, Tereza Primo dos Santos 3, F á bio Carneiro Dutra 4, Bruno de Oliveira Serrão 4 1 Embrapa Amazônia Oriental; 2 The Woods Hole Research Center, 3 Instituto de Pesquisa Ambiental da Amazônia, 4 Bolsista ITI/DTI, CNPQ-LBA Amazônia Oriental The Woods Hole Research Center

The availability of N and P may limit rates of regrowth of secondary forests in Amazonia.

Losses of a 7-year-fallow vegetation after burning (31 t/ha DM) Losses in kg/ha and in % of biomass Carbon (98%) Nitrogen205 (96%) Potassium 39 (48%) Phosphorus 4 (47%) Calcium 107 (35%) Magnesium 18 (40%) Sulfur 14 (76%) Sodium 6 (30%) Carbon (98%) Nitrogen205 (96%) Potassium 39 (48%) Phosphorus 4 (47%) Calcium 107 (35%) Magnesium 18 (40%) Sulfur 14 (76%) Sodium 6 (30%) Hölscher 1995 Losses of a 7-year-fallow vegetation after burning (31 t/ha DM) Losses in kg/ha and in % of biomass Carbon (98%) Nitrogen205 (96%) Potassium 39 (48%) Phosphorus 4 (47%) Calcium 107 (35%) Magnesium 18 (40%) Sulfur 14 (76%) Sodium 6 (30%) Carbon (98%) Nitrogen205 (96%) Potassium 39 (48%) Phosphorus 4 (47%) Calcium 107 (35%) Magnesium 18 (40%) Sulfur 14 (76%) Sodium 6 (30%) Hölscher 1995

The highly weathered soils are nutrient poor, and numerous cycles of slash and burn can further impoverish nutrient stocks and lead to site degradation.

The mineralization of N and P from soil and litter by enzymatic activity and the acquisition of nutrients by mycorrhizae could be important for the recuperation of nutrient cycles during secondary forest succession.

These processes were studied in a secondary forest chronosequence (5, 8, 12, 22, 42, and 72 years), a recently abandoned black pepper plantation, and a remnant mature forest in the municipality of São Francisco do Par á.

At the end of the rainy season (May), samples of soil and roots were collected from the litter-soil interface and at 0-5, 5-15, and cm depths in mineral soil. Available P, total N, extractable NH 4 + and NO 3 -, urease and acid phosphatase activity, and concentrations of total and readily extractable glomalin were measured. The number of fungal spores and infections of mycorrhizae (VAM) were counted in roots.

The number of spores and mycorrhizal infections decreased with increasing age of the forest, whereas the activity of acid phosphatase increased with forest age.

The concentration of total and readily extractable glomalin tended to increase with forest age.

The concentration of NH 4 + and NO 3 - tended to increase with forest age, and were significantly higher in the mature forest soil.

Litterfall and decomposition

The results indicate that some, but not all, of these indicators of nutrient mineralization and acquisition become similar to the mature forest within 8 years of secondary forest succession.