Export of Forest Harvesting Residues to Energy. Nutrient Loss and Replacement Cost

The use of forest harvesting residues for energy raised the discussion whether intensification in land use processes can increase soil degradation due to the increased export of biomass and nutrients, among other things. The aim of this study was to quantify the additional removal of aboveground biomass of Pinus taeda and nitrogen (N), phosphorus (P) and potassium (K) content in it, as a result of the export of waste for bioenergy. The experiment was conducted in two sites P. taeda plantations of 20 and 26 years of age, in the NO of the province of Misiones, Argentina (S1: 26º 00 '08' 'S 54º 38' 54 '' O and S2 : 25 ° 58' 52 'S and 54º 22' 65''O). The climate of the region is humid subtropical while the soil is a Kandiudult. Whole-tree harvesting (WH) system was compared to traditional stem-only harvesting (SH). WH harvest included felling, removal of the entire tree to court collection, limbing and bucking the shaft, loading and transportation. Meanwhile, in SH after felling and limbing blunting the shaft at the site was performed, extracting only the commercial bole to court collection. At each site and crop type, forest residue was estimated at 15 plots of 100 m². In each plot, forest residue was performed in 10 sub-plots of 0.5 m². The material was divided into five categories: branches> 5 cm in diameter, branches between 1-5 cm in diameter, branches <1 cm in diameter, needles and cones. The N, P and K concentration was determined in a sample of each type of crop at each site. The data were extrapolated to the hectare. To analyze information comparing the two populations through their sample means was used. To test the hypothesis test "t" was used at the significance levels of 95 and 99%.

The study showed that age differences between sites and stand density did not affect the results. The amount of forest residue collected after harvest was higher in SH, at both sites. In S1 was 74 Mg ha^-1 compared to 49 Mg ha^-1 in WH. In S2 was 84 Mg ha^-1 compared to 55 Mg ha^-1 in WH. When evaluating residue categories, the largest component exports are the branches > 5 cm Ø with 78% of the stratum S1 and S2 47%. In other categories, WH additional harvest was 17% lower in S1 and S2 36%. The differences between categories could be due to the drag of felled trees, causing the breakdown of crown and smaller fractions (branches <1, needles and cones), being in greater proportion in the site. Nutrients had the same pattern biomass being both S1 and S2, higher in SH. The remaining contents were significantly different for the three nutrients in five compartments. In S1, SH, the content of N, P and K was 424, 28 and 119 Kg.ha^-1 respectively, while WH were 306, 20 and 85 kg ha^-1 of N, P and K respectively. In S2 the quantities of N, P and K were 486, 21 and 177 respectively for Kg.ha^-1 and SH 336, 15 and 122 Kg.ha^-1 to WH.

The harvest of WH represented a further extraction of 25 and 29 Mg.ha^-1 biomass in S1 and S2 respectively. The additional loss of nutrients with respect to SH was 118 and 150 kg ha^-1 of N; 6 and 8 kg ha^-1 of P and, 34 and 55 kg ha-1 of K. If urea ($ 350/50 kg), triple superphosphate calcium ($ 420/50 kg), and potassium chloride ($ 526 / 50 kg) were applied to replenish nutrients exported, the replacement cost would be in S1 1796, 311 and 684 $ .ha^-1 for N, P and K respectively. For S2 the cost is 3078.5, 268 $.ha^-1 and 1100 $ ha^-1. A equivalent 1U $ = 14.9 $ gives a total cost of 177 and 207 U$. ha^-1. These values are underestimated because it does not include the cost of transfer, application efficiency of 100% was considered and it is assumed that there are no losses.