A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Organic nutrients in throughfall and soil solution of mixed tree cropping systemsand forests of central AmazôniaJohannes Lehmann 1 and Klaus Kaiser 21 Department of Crop and Soil Sciences, Cornell University, Ithaca, NY 14853, USA, Email:CL273@cornell.edu; 2 Institute of Soil Science, University of Bayreuth, 95440 Bayreuth,GermanyDissolved organic matter comprises a large portion of total nutrients in throughfall, stemflowand soil solution of forests. In most ecosystems, the organically bound nutrients are mobile andcontribute largely to the leaching from soil. Under the strong leaching conditions of the humictropics, soluble organic nutrient forms may be even more mobile and thus control nutrientlosses.We tested this hypothesis in soils under species of agroforestry systems (Bactris gasipaesKunth., Theobroma grandiflorum (Willd. Ex Spreng.) K. Schum., Pueraria phaseoloides), of asecondary (Vismia spp) and of a primary forest (Oenocarpus bacaba, Eschweilera spp) incentral Amazônia. We sampled throughfall, stemflow, and soil water at 10, 60, and 200 cmdepth in January 1999 and measured inorganic and total organic N, S and P and organic carbontherein. In addition, a fractionation procedure was used to separate labile hydrophilic andrefractory hydrophobic (humic) compounds.Under the species of the agroforestry systems, the concentrations of dissolved organic carbon(DOC) were larger in the soil solution at 10 cm depth than in stemflow and throughfall. Withincreasing soil depth the DOC concentrations decreased. Under Eschweilera and Vismia, theconcentrations of DOC in stemflow were larger than at 10 cm depth. The DOC concentrationsunder both primary forest species increased with increasing soil depth and decreased onlyslightly under Vismia. Dissolved organic carbon in throughfall and soil solutions of theagroforestry and secondary forest stands were mainly in the hydrophilic fraction. Theproportions of hydrophobic DOC in soil water decreased with depth under the agroforestryspecies. In contrast, the proportions of hydrophobic DOC under secondary and primary forestspecies were largest in the subsoil. This result is opposite to those of temperate forests wereDOC typically decreases with soil depth and DOC in the subsoil is mainly hydrophilic.Dissolved organic nitrogen (DON) in all solutions comprised up to 90% of total N. Theconcentrations of DON under all investigated species remained unchanged with increasing soildepth or even increased. This suggests that the organic N is more mobile compared with DOCwhich is in agreement with observations in temperate forests. But in contrast to these systems,DON is predominately in the hydrophobic fraction. The largest dissolved organic sulphur (DOS)concentrations occurred in the topsoil. Here, up to 80% of total S were organically bound. Theconcentrations of DOS increased strongly with soil depth. Thus, DOS was less mobile in thesesoils than DOC. In all compartments of the investigated forests, DOS was entirely in thehydrophilic fraction. Dissolved organic phosphorus (DOP) was not detected in any solution.Our results show that dissolved organic matter contained the major part of the nutrients inthroughfall, stemflow and soil solutions not only in natural forest ecosystems but also managedand fertilized agroforestry systems of central Amazônia. Organic nutrient forms contributeespecially to the leaching of N into the subsoil. Thus, organically bound nutrients are of muchhigher relevance in tropical soils than in temperate soil.