THE CRITICAL LOAD OF NITROGEN FOR SITKA SPRUCE FORESTS ON STAGNOPODSOLS IN WALES - ROLE OF NUTRIENT LIMITATIONS

A study of Sitka spruce plantations of varying age on stagnopodsols in Wales has shown that relatively little nitrate leaches from forests o f under 30 years, but it does so to a statistically significant and in creasing extent from older forests. It has been hypothesised that P an d K deficiencies develop in later stages of the forest rotation which effectively limit N demand by the trees resulting in nitrate leaching. To test this hypothesis, bioassays to determine tree nutrient demands relative to supply, based on rates of uptake of isotopically labelled ions (P-32, Rb-86 and N-15) by live roots were applied to four stands on stagnopodsol soils in each of five Welsh forests (Beddgelert, Dyfi , Dyfnant, Hafren and Tywi). Older forest sites where stream nitrate c oncentrations were generally over 0.16 mg N 1(-1) tended to have a hig her demand for P and K, but a low demand for N, indicating that nitrat e leaching may be associated with deficiencies in supply of P and K. M ultivariate analysis of root bioassay data showed that sites leaching nitrate could be distinguished at P < 0.01 from those not leaching nit rate. The critical load for pollutant N inputs to the Welsh forests wi ll therefore need to be calculated using the mass balance equation mod ified to take account of P and K deficiencies on certain soil types.