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Lower P-input levels in organic than conventional farming can decrease soil total and available P, which can potentially be resupplied from soil organic P. We studied the effect of 30 y of conventional and organic farming on soil P forms, focussing especially on organic P. Soil samples (0-20 cm) were taken in a field experiment with a nonfertilized control, two organic systems receiving P inputs as animal manure, and two conventional systems receiving only mineral P or mineral P and manure. Soils were analyzed for total, inorganic, organic, and microbial P, by sequential P fractionation and by enzyme additions to alkaline soil extracts. Samples taken prior to starting the experiment were also analyzed. Average annual P balances ranged from -20 to +5 kg ha -1 . For systems with a negative balance, labile and moderately labile inorganic P fractions decreased, while organic and stable inorganic P fractions were hardly affected. Similar quantities and proportions of organic P extracted with NaOH-EDTA were hydrolyzed in all soils after addition of an acid phosphatase, a nuclease, and a phytase, and enzyme-stable organic P was also similar among soils. Thus, neither sequential fractionation nor enzyme addition to alkaline soil extracts showed an effect of the type of applied P (manure vs. mineral) on organic P, suggesting that organic P from manure has largely been mineralized. Thus far, we have no indication that the greater microbial activity of the organic systems resulted in a use of stable P forms.

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Additions of enzymes involved in organic phosphorus (P) hydrolysis can be used to characterize the hydrolyzability of molybdate-unreactive P (MUP) in soil water extracts. Our aim was to test the feasibility of enzyme additions to soil water suspensions with respect to (1) suitable enzyme preparations and (2) recovery of molybdate-reactive P (MRP). To this end, we compared the substrate specificity of seven commercially available enzyme preparations (acid and alkaline phosphomonoesterase, phytase, and nuclease preparations) and optimized the assay conditions in microplates. We then measured MRP release after the addition of the enzymes to soil water suspensions and filtrates of two Swiss grassland soils (midland and alpine). In some cases, commercial preparations of the same enzyme differed in their specificity, presumably due to contamination with other enzymes, and also in their efficiency in soil suspensions. Addition of EDTA to the buffer was required to decrease sorption of released P in soil suspensions. Enzymatic release of P was consistently equal or higher in soil suspensions than in soil filtrates. However, also more dissolved MUP was present in soil suspensions than in filtrates, since the buffer interacted with the solid phase. Of the total dissolved MUP in soil suspensions, 94 and 61 % were hydrolyzable in midland and alpine soil, respectively. More specifically, 60 and 17 % of MUP were in nucleic acids, 6 and 39 % in simple monoesters, and 28 and 5 % in inositol hexakisphosphate in midland and alpine soil, respectively. Thus, we show that the characterization of hydrolyzable organic P in soil suspensions with hydrolytic enzyme preparations may be useful to better understand the availability of soil organic P to enzymatic hydrolysis, but that it requires soilspecific adaptation for optimum P recovery.

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