PARENT SESSION
Oral Session #68: Nutrient Cycling. Presiding: D. Johnson.
Thursday, August 9, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.

Biotic versus abiotic controls on P availability in soils of Hawaiian wet tropical forests.

OLANDER, LYDIA¹, VITOUSEK, PETER¹, ¹

ABSTRACT- Phosphorus is likely to be a limiting or co-limiting nutrient in wet tropical forests, especially in those with soils that are high in iron and aluminum oxides (oxisols and ultisols). These minerals sorb phosphorus (P) strongly. We evaluated partitioning of P into abiotic versus biotic pools using Hawaiian soils, which vary in their P sorption capacity, from relatively low, to significantly higher than that observed in most continental tropical soils. When 1.7 mg P/g soil dry weight (50 to 150 kg/ per hectare of orthophosphate-P) is added to these soils, 36 to 74% of it is sorbed. To evaluate partitioning of P we measured the disappearance of added P from the soil solution pool. We compared soils incubated at 22°C, where both sorption and microbial immobilization can occur, to soils incubated at 0°C where sorption still occurs, but microbial activity is negligible. From this we observed how much P must be added before sorption capacity is filled and microbial immobilization begins to dominate. In soils with the highest sorption capacities, microbial uptake of P becomes significant only with additions of over 200gP/g soil (approximately 10 kg P/ha). However, in soils with lower sorption capacities, microbial immobilization more strongly dominates, and begins to do so at additions of only 30gP/g soil (between 1 and 5 kg P/ha). These results suggest that abiotic controls dominate at lower levels of P addition.

KEY WORDS: phosphorus, soil , sorption, microbe