Abstract
In Uruguay, Pinus taeda is usually planted a few months after harvest of the former turn, therefore; decomposing residues represents a nutrient source for the new plantation. The aim of this study was to determine the biomass and nutrient extraction off site, following the harvest of a P. taeda plantation. Residue decomposition patterns, and nutrient release were also examined. The site will be referred as S1, corresponding to the clear cut of a 22-year-old P. taeda plantation. Before the clear cut 10 trees were harvested, and logs, branches, twigs, and needles separately weighed. Additionally, forest litter at harvest time was quantified in three different areas. To assess decomposition, mesh bags with residues were allocated in three areas over the forest litter, and samples were taken periodically for 26 months. The remaining biomass, N, P K, Ca, and Mg contents were determined in the different fractions, calculating decompositon rates. Most of the harvested biomass was removed in logs, but the proportion of nutrients exported was considerably lower. Needles showed the highest biomass loss and only 39.1% remained after 26 months, while branches presented high rates in the first two months after cut, but slower thereafter, and at the end of the study more than two thirds of the woody residues remained. Potassium was rapidly released from the residues, while Ca, and Mg, were slowly released, and there was evidence of N and P immobilization in the early stages of decomposition. It was concluded that, although a lower proportion of nutrients were exported, compared to biomass, in the long term, nutrient export with logs could be significant for the sustainability of this production system. While K release from residues did not depend on biomass decay, the slow decomposition, and release of the other nutrients, indicates that this process could have been delayed by nutrient scarcity.
Highlights
Forest production in Uruguay has significantly increased in the last three decades, and the area covered by forests has been multiplied several times
We studied decomposition of P. taeda residues in the field and the patterns of nutrient release
The quantified forest litter accounted for 13 Mg·ha−1, that should be added to the biomass left on the site by the plantation
Summary
Forest production in Uruguay has significantly increased in the last three decades, and the area covered by forests has been multiplied several times. In consequence harvest residue decomposition accompanies the growth of the new plantation, and the decomposed material is expected to release nutrients to the growing trees (Tutua et al, 2008; Turner & Lambert, 2013; Mavimbela et al, 2018). The exportation of nutrients with logs, and the pace of residue decomposition on site, are likely to alter the soil carbon and nutrient balances (Ruiz-Peinado et al, 2013; Bravo-Oviedo et al, 2017). In their work increases of Ca, Mg and K content of the upper soil layer was observed, which they attributed to recycling of the cations from deep soil layers, through the forest litter. To characterize the nutrient cycling in the forest is a key step to assess the sustainability of this production system
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