PHOSPHORUS AND POTASSIUM-DEPLETION BY ROOTS OF FIELD-GROWN SLASH PINE- AEROBIC AND HYPOXIC CONDITIONS

Slash pine (Pinus elliottii Engelm. var, elliottii) is planted extensi vely on flatwood and coastal savanna soils of the southeastern USA. Pi ne roots growing in these soils encounter shallow water tables. Althou gh the fine-root system of pine trees growing in the surface 20-26 cm of those soils will not be continuously submerged, they will encounter short-term reduced soil conditions, with a severely reduced O-2 suppl y which might affect their ability for ion uptake from the soil soluti on. The objectives were: (i) to compare P and K depletion by lateral r oot systems of slash pine roots subjected to a shortterm hypoxic treat ment, and (ii) to document K and P depletion patterns by lateral root systems of slash pine roots following the removal of the hypoxic treat ment. Our purpose was to evaluate the uptake ability of these roots of changing aeration. For the experiment, 17 intact lateral roots from t welve-year-old slash pine trees were inserted into nutrient-uptake roo t chambers. The chambers were filled with a nutrient solution containi ng 6.25 mu M P (phosphate), and 25.66 mu M K. P and K depletion rates were monitored in six consecutive treatments, each lasting five days. Treatments of aerobic and hypoxic conditions were sequentially applied in the following order: aerobic-hypoxic-aerobic. This sequence was re peated twice. Uptake of P and K by slash pine roots was affected by ox ygen availability, but the degree of response differed. Under hypoxic nutrient solution conditions, K depletion from solution by pine roots was totally inhibited, resulting in net efflux of K. In contrast, P de pletion was not inhibited under hypoxic nutrient solution conditions. Results suggest that pine roots grown in aerobic soil conditions of su rface horizons are capable of P depletion when reduced soil conditions are present. (C) 1998 Elsevier Science B.V.