VEGETATION EFFECTS ON PHOSPHORUS FRACTIONS IN SET-ASIDE SOILS

As increasing amounts of arable land are being set aside, it is of imp ortance to study the effect of vegetation on soil fertility. The fract ionation of soil P under grassland, beech and spruce vegetation was in vestigated in sites previously fertilized with P by extracting sequent ially with Resin, NaHCO3, NaOH, HCI and finally NaOH after ultrasonic pretreatment. Under beech a large part of extractable P was found in i norganic fractions which are considered to be available for plants (Re sin P(i) and Bicarbonate P(i)). Under grass, a large part of the extra ctable 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 w as rendered unextractable 20-30 years after application. However the a vailable 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 th e basis of the current data it may be concluded that the vegetation af fects the distribution of soil phosphorus fractions, and thus soil fer tility. In the soils under investigation, grassland and beech vegetati on conserved the phosphate availability to a high extent.