Phosphorus and potassium uptake by woody roots of twelve-year-old slash pine trees

Under natural conditions, woody roots contribute over 80% of the fine-root system of slash pine (Pinus elliottii Engelm. var. elliotii) and have the potential to play a major role in nutrient uptake of field grown trees. However, although nutrient uptake by 'woody roots' of trees has been reported, these studies have used brown coloration to define woodiness of roots, despite color being a poor indicator of anatomical attributes that defines secondary growth. Nutrient uptake by brown roots has also generally been measured using excised roots or intact roots from tree seedlings grown under laboratory or greenhouse conditions and which may have little relevance to field conditions. This study documented P and K depletion of intact fine roots for 12-year old slash pines growing under field conditions. The objectives were: (i) to compare P and K depletion between whole root systems and woody root systems of slash pine (Pinus elliotii Engelm. var. elliotii); and (ii) to compare P and K depletion between the whole-root system and woody-root systems when N-2 gas was used as a metabolic inhibitor of ion absorption. Woody-root components were isolated from whole-root systems (woody and non-woody) by waxing or cutting. Nz gas was also used as a metabolic inhibitor of P and K depletion and compared to P and K depletion when air gas was used. We concluded that woody roots function in P and K uptake, and that an active uptake mechanism is implicated. Rates of P and K uptake for woody roots were similar per unit root length to rates of ion uptake of whole (woody and non-woody) roots. (C) 2000 Elsevier Science B.V. All rights reserved.