Programa e Resumos - I Congresso Ibérico de Ciência do Solo 2004

I Congresso Ibérico da Ciência do Solo – 15 a 18 de Junho de 2004, Bragança, Portugal
P adsorption and desorption in andisols from the Azores, Portugal.
E. Auxtero, M. Madeira & E. Sousa
Departamento de Ciências do Ambiente Instituto Superior de Agronomia, Tapada da Ajuda,
1013-911 Lisboa, email: edgardo@isa.utl.pt
Resumo
Comunicação: Painel
Andisols of the Azores contain large amounts of active Al and Fe, allophane, and
organic C, which greatly influence the capacity of the soil to adsorb and desorb P. In
areas of the Azores, where P is heavily applied, it is suspected that a potential losses of
P especially from agricultural soil to nearby bodies of water may cause undesirable
effects on water quality brought about by eutrophication. Thus, P chemistry and its
effects on the environment need to be studied, regarding the ability of the soil to adsorb
P complemented with information relative to its desorptive properties. Information for
sound P management of andisols of the Azores is not yet available. Having this in view,
a study was conducted on surface and subsurface horizons of eight pedons of andisols
from the Azores occurring in the Faial, Pico and São Miguel islands to 1) determine the
P adsorption capacity using Langmuir isotherm, 2) determine the P desorption capacity
using 8 successive extractions with 0.01M CaCl2, and 3) assess the relationships
between adsorption-desorption parameters and selected soil properties in order to reduce
P eutrophication risks in the Azores. Results showed that P adsorption isotherm patterns
of the studied soils differ widely (0.04-50 g P kg-1 in the surface horizons, and 0.12-17
g P kg-1 in the subsurface horizons). P sorption isotherms of studied soils fitted the
Langmuir equation, and values of their Langmuir P adsorption maxima (ads max) were
strongly correlated with Fe and Al contents, as sufficient sites for P sorption. Vitrandic
Haplustepts and Typic Vitrands having low contents of Fe and Al adsorbed P poorly
(0.05-12 and 0.15-0.57 g P kg-1, respectively) and showed very high percentage P
desorbability (65-100% and 70-100%, respectively). Addition of P fertilizer to such
soils is not necessary and soil management to minimize the loss of P is important to
prevent eutrophication of nearby lakes. Conversely, Acrudoxic Hydrudands and Typic
Placudands adsorbed P greatly (100 and 20 g P kg-1, respectively). They have very low
percentage P desorbability (18 and 12%, respectively), suggesting that P management
on these soils should be focus on maximum P release and crop uptake.
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