| Summary: | Different doses of nitrogen (N) and phosphorus (P) were applied in a factorial design experiment to seedlings planted in nutrient poor and nutrient rich Andisols in Iceland to determine the effect of initial fertilization on ectomycorrhizal colonization, survival, growth, and nutrient status of downy birch (Betula pubescens) seedlings. One year after planting the highest NP fertilizer dosage reduced ectomycorrhizal colonization at the nutrient poor site significantly, while at the nutritional rich site the N and P fertilization had no significant effect. Three years after planting the difference in ectomycorrhizal colonization between the unfertilized and fertilized seedlings had vanished on both sites. At that time the average colonization at the rich and the poor site was 60% and 79%, respectively. No clear effect on the composition of ectomycorrhizal morphotypes was found by the various NP combinations. The initial fertilization increased the dry weight of birch on both soil types significantly. One and three years after planting the increase was up to 3.6 and 14 times, respectively. On the rich site, the largest dosages of NP had no additional effect over that of smaller NP dosages, while at the nutrient poor site the larger the dosages, the more the seedlings grew. Fertilization increased the foliar concentrations of N and P 3.6 and 7.7 times, respectively. NP fertilization during establishment of downy birch on Andisols in Iceland is recommended, as site-adapted moderate dosages had a positive effect on the growth and nutrient status and no adverse effect on the ectomycorrhizal development three years after planting. Árið 1998 var gerð áburðartilraun á birki (Betula pubescens) á tveimur stöðum á Suðurlandi. Markmið tilraunarinnar var að athuga áhrif mismunandi skammta af nitri (N) og fosfór (P) á lifun, vöxt, næringarástand og svepprótarmyndun á birki, annarsvegar á frjósömu landi (Kollabæ) og hins vegar á rýru landi (Markarfljótsaurum). Borið var á plöntur einu sinni við gróðursetningu í júní 1998 og ...
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