Abstract

In Chile, commercial forests plantations have increased during the last decades due in part to replacement of native forests and conversion of agricultural soils. Pinus radiata D. Don has been the main tree planted, due to its rapid growth and adaptability. In the present study we proposed that biological activity varies along the year due to changes of precipitation, temperature and soil water content and mainly because the conversion of native forest to exotic P. radiata plantations alters the soil chemistry, N and C mineralization and the potential enzymatic activity in these soils. This hypothesis was examined in a Nothofagus obliqua (Mirb) Oerst forest and P. radiata plantation in central-southern Chile (40°07' S, 72° W). In this study, the rate of nitrogen mineralization (N-min), carbon mineralization (C-min) and the potential enzymatic activity of soils (urease, protease and fluorecein diacetate hydrolysis) were determined between September 2003 and May 2005. Results showed that soil biological activity was significantly different between forest and plantations (Wilk's Lambda = 0.022; F 5, 80 = 733, P < 0,0001), and among the months (Wilk's Lambda = 2.01E-6, F 100,395 = 84, P < 0,0001), with values for N. obliqua forest significantly higher (Tukey < 0.01) than those for P. radiata plantation. N-min and C-min rates showed seasonal variability with the higher values in spring and summer months. For enzymatic activity there was not a clear seasonal variation. The higher values of C and N-min and enzymatic activity in the N. obliqua forest could be explained by differences in composition and chemistry of litter. Consequently, P. radiata plantation changed soil chemistry and C/N ratio resulting in lower decomposition rates of organic matter and in consequence, reduced mineralization of C and N and soil biological activity compared to the original soil of N. obliqua forest.