Properties of Fine and Water-Soluble Fractions of Several Composts

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Abstract
Organic forms of nitrogen, neutral sugars, and muramic acid, a constituent of bacterial cell walls, were analyzed in particle size fractions (F1, sedimented for 16 h at 4°C; F2, sedimented by centrifugation at 10,000 × g; F3, sedimented by centrifugation at 100,000 × g; F4, supernatant of F3) separated from four composts. Amino acid-N and ammonium-N were the predominant constituents of hydrolyzable organic nitrogen in all the fractions and amino sugar-N accounted for only a small proportion. The percentage of amino acid-N in organic nitrogen was higher than that of ammonium-N for fractions F1-F3, the reverse being true for fraction F4. The amino sugar-N content was significantly correlated with the amount of mineralizable organic N in fraction F4. The neutral sugars released by acid hydrolysis accounted for 10-35% of the dry matter in fraction Fl, but only for 2-10% in fractions F2-F4. Hexoses were the predominant sugars in all the fractions and glucose generally represented the largest portion of the hexoses. The greater part of the neutral sugars contained in fraction F1 consisted of hexoses, and glucose accounted for more than 60% of the total neutral sugars. The percentage of glucose decreased and that of mannose, galactose, and methylpentoses increased with the decrease of the particle size in the fraction. The percentage of total C contributed by neutral sugar-C was higher than that of amino acid-C for fraction Fl, the reverse being true for fractions F2-F4. Amino acid-, amino sugar-, and neutral sugar-C, however, accounted for less than 37% of total C in the fractions. Muramic acid was concentrated in fraction F2 and a significant correlation between the muramic acid content and the amount of mineralizable organic N in fraction F2 was observed. The above results and previous findings suggested that bacterial cells and their debris were the main constituents of the fine fractions (F2 and F3), whereas microbial extracellular metabolic products coexisted with humic substances in the water-soluble fraction (F4). The materials derived from the microbial biomass and metabolic products may become a source of readily mineralizable organic nitrogen in the fine and water-soluble fractions.