NUTRIENT AVAILABILITY AND PLANT-GROWTH LIMITATION IN BLANKET MIRES INIRELAND

Nutrient availability in two Irish fens and two blanket bogs was asses sed by analysing soil samples for total and extractable N and P and mi neralisation. A two-year (1989 and 1990) fertiliser application experi ment with N, P or N + P was carried out to determine the growth-limiti ng nutrient(s). (i) The total N content of the soil ranged between 12. 2g and 28.8g kg(-1) dry weight. The total P ranged from 0.14g to 0.75g kg(-1). Both showed significant differences between all sites. Soil N /P ratios ranged from 16 in a fen up to 125 at a bog site, indicating a shortage of P. (ii) Ex-tractable N-H2O was significantly higher in t he fens than in the bogs; however, N-KCl showed no differences. Extrac table P appeared to be higher in bogs than in fens. (iii) The minerali sed N (H2O and KCl) was lower in the Kylemore fen than in the Kylemore bog, but the opposite was found for the Crossmolina fen, where releas ed amounts were higher than in the Crossmolina bog. Released P (H2O) w as significantly higher in the fens than in the bogs; no differences w ere found for bioavailable P (P-lact). (iv) The response of biomass to fertilisation was greater in 1990 than in 1989. Combined N + P and P applications resulted in a significantly greater harvest in 1990 than the control. The amounts of nutrients per m(2) increased significantly after P and N + P application but not after N application. The specie s composition remained the same. (v) The high N/P, C/N (17-32) and C/P (424-3946) ratios of the soil and N/P (13-28) ratios of plant materia l, combined with the increase in biomass and nutrients in plant materi al after fertiliser application, suggest P and/or N + P limitation of Irish mires.