VEGETATION EFFECTS ON PHOSPHORUS FRACTIONS IN SET-ASIDE SOILS

As increasing amounts of arable land are being set aside, it is of importance to study the effect of vegetation on soil fertility. The fractionation of soil P under grassland, beech and spruce vegetation was investigated in sites previously fertilized with P by extracting sequentially with Resin, NaHCO3, NaOH, HCI and finally NaOH after ultrasonic pretreatment. Under beech a large part of extractable P was found in inorganic fractions which are considered to be available for plants (Resin P(i) and Bicarbonate P(i)). Under grass, a large part of the extractable P was found in potentially labile organic forms (Bicarbonate P(o) and Fulvic acid P(o)). After 25 years of permanent grass vegetation, the extractability of soil P was comparable to that from an adjacent arable plot. On spruce covered soils most of the added fertilizer P was rendered unextractable 20-30 years after application. However the available data does not allow a clear interpretation of this phenomena, as effects of soil parent material as well as vegetation may be taken into consideration. No decrease in P-extractability was found between beech and grass covered soils which had been fertilized for more than 200 years, when compared to less rich soils from the same area. On the basis of the current data it may be concluded that the vegetation affects the distribution of soil phosphorus fractions, and thus soil fertility. In the soils under investigation, grassland and beech vegetation conserved the phosphate availability to a high extent.