PROTEASE EXTRACTION FROM SOIL BY SODIUM PYROPHOSPHATE AND CHEMICAL CHARACTERIZATION OF THE EXTRACTS

Two arable soils and one pasture soil had previously been air-dried fo r 6 d and stored at room temperature. The enzyme activities remaining after this treatment were constant. The soils were then extracted with 140 mM sodium pyrophosphate at pH 7.1. Amino acid N and total organic C con tent of soils and soil extracts, together with humic and fulvic acids content of soil extracts were determined. Total organic C was d etermined in soil residues obtained after extraction. Chemical charact erization of the organic matter of soils, soil extracts and soil resid ues was carried out by pyrolysis-gas chromatography (Py-GC). Protease activity was determined in soil extracts and soil residues by using th ree different substrates: N-benzoyl-L-argininamide (BAA), specific for trypsin; N-benzyloxy-carbonyl-L-phenylalanyl L-leucine (ZPL), specifi c for carboxypeptidases, and casein, essentially nonspecific. Comparat ive studies between specific activities referred to organic in soils, soil extracts and soil residues and their corresponding pyrogram compo sition, and also between total extracted or residual activity and the humine or unhumified organic matter content of the corresponding soil, allowed us to establish hypotheses about the type of organic matter t he enzymes are associated with. From 12% to 21% of the soil organic C (33% to 39% of which were humic acids) and from 3% and 18% of amino ac id N were extracted from soil using pyrophosphate. Py-GC analyses show ed that pyrophosphate was effective in extracting condensed humic subs tances and glycoproteins and that the organic matter present in soil e xtracts was especially rich in intact or partially-decomposed fresh re sidues of carbohydrate origin and also in certain humus-associated pro teins. Extracted BAA-hydrolysing activity accounted for 11% to 36% of the soil activity, depending on soil type. Extracted ZPL- and casein-h ydrolysing activities were, with one exception, remarkably high, accou nting for about 100% or even more of the soil activity, depending on s oil type. According to the results BAA-hydrolysing proteases are proba bly mostly associated with highly condensed humus, ZPL-hydrolysing pro teases with less condensed humic substances and casein-hydrolysing pro teases with fresh organic matter. (C) 1998 Published by Elsevier Scien ce Ltd. All rights reserved.