Biogeochemistry of a small catchment after the clearcutting of a 7-year old Eucalyptus plantation

Abstract

This work studied the effect on nutrient cycling and quantities of mineral nutrients exported with the woody biomass after the clearcutting of a 7-year old plantation of Eucalyptus saligna Smith. The experiment was carried out on lands of Votorantim Celulose e Papel S/A, Fazenda Bela Vista III, located in the municipality of Santa Branca, State of São Paulo, with coordinates 25°25' South and 45°54' West from Greenwich, and 695m average altitude. The climate is classified as Cwa type, according to Köeppen, and is characterized by dry winter temperatures, and by an annual average precipitation of 1,562mm. The soils in the region are predominantly of the Hapludulf type. The experimental catchment has an area of7.0 ha,3.3% of which is covered with riparian vegetation, with average slope of 19.6%. The total experimental period comprised eight years, with comparison of the results being made between data from the 7 years before the clearcutting and data obtained during the first year after the clearcutting. Rain water and streamflow samples were obtained weekly during the experimental period, and were analyzed for N, K+, Ca++, and Mg++, with the purpose of quantifying the catchment geochemical cycling. The amount of nutrients exported with the harvesting of the woody biomass was obtained from the estimates of the arboreous biomass and the nutrients content (phytomass and mineralmass, respectively). The total, above soil biomass production of theE. saligna plantation for the seven years amounted to 178.1t.ha, distributed as follows: 145.2t as wood, 14.7t as bark, 13.62t as branches and4.6t as leaves. The nutrients contents in the total biomass were: N 200.8Kg, P 52.8Kg, K 308.3Kg, Ca 796.1Kg, Mg 133.33kg, and S3.06Kg. The nutrient concentrations for the different parts of the tree, respectively, wood, bark, branches, and leaves, were as follows: N0.07,0.21,0.19,0.92 (%); P0.02,0.09,0.04,0.11 (%); K0.11,0.38,0.34,1.01 (%); Ca0.10,3.34,0.90,0.84 (%); Mg0.03,0.39,0.15,0.26 (%); S0.01,0.05,0.02, and0.13 (%). For the entire eight-year period, total inputs in precipitation, in Kg.ha-1, were as follows: N=77.6, K=32.2, Ca=46.7 and Mg=11.9. For the same period, total outputs in streamflow were: N=7.1, K=24.1, Ca=31.7 and Mg=7.4. Results are discussed in terms of maintenance of site productivity considering the exchangeable amount contained in the soil catchment, as well as the harvesting method, that is whole-tree x wood only harvesting.