Nutrient and soil organic matter dynamics in forestry systems summary

Doctoral thesis abstract

Abstract

In the recent decades, afforestation with Eucalyptus sp. and Pinus taeda) on previous grassland/crop soils have determined changes in some soil chemical properties. This aspect is particularly relevant because soils affected by the afforestation have low soil fertility, associated with low clay and soil organic matter contents. The objectives of this research were: i) to quantify nutrients export by harvest and to quantify nutrients recycled through the decomposition of harvest residues; ii) to characterize changes in soil chemical properties associated with changes in afforestation soil use; and iii) to characterize changes in soil organic carbon content and it distribution, and evaluate the potentiality of afforested soils to sequester CO2 from the atmosphere. Results have shown the importance to maintain harvest residues on the site, which contain on average 70% of the nutrients absorbed by trees. The residues decomposition and recycling of nutrients to the soil usually develops slowly, and depending on the nutrient, the type of residue, and residues management practices. However, this allows counteract the decline in some of the soil nutrients (such as calcium and magnesium) during the plantation cycle, although its concentration in the soil profile increase in the upper layers as consequence harvest residues and litter decomposition. An increase in soil acidity -associated with calcium and magnesium decrease in the soil- was also observed. Soil organic carbon content tended to decrease in afforested soils compared to native pasture use during the first turn of planting (10-years-old). However, carbon isotopic analysis techniques indicated a substitute and compensatory effect of native soil organic carbon by the input of carbon derived from the forest vegetation. This trend shows to increase in older forest plantations.