INFLUENCE OF SIEVE MESH SIZE ON ESTIMATES OF MICROBIAL CARBON AND NITROGEN BY FUMIGATION EXTRACTION PROCEDURES IN SOILS UNDER PASTURE

Measurements were made with five soils from perennial, grazed pastures of the influence of sequential sieving through a 5.6 and 2 mm mesh on estimates of (a) extractable C and N, (b) flushes of C and N, determined by fumigation-extraction methods, and (c) mineral-N flush determined by a fumigation-incubation procedure. Wet winter samples, partially dried winter samples, and drier summer samples were used. Levels of extractable C and N were highest in smeared and compacted, < 2 mm winter samples, largely because of a sieving artefact. In the summer samples, levels were mainly unaffected by sieve mesh size. Flushes of extractable C and N were generally lower in < 2 mm soil than in < 5.6 mm soil; they were highest in root residues remaining on the 2 mm mesh. Estimates of mineral-N flush were, however, influenced little by sieve mesh size. Partial drying of the winter samples had no effect on flushes of extractable C, but slightly lowered some extractable-N flush values. In contrast, mineral-N flush values of three of the soils were higher after partial drying. Implications of these results for the assessment of k(EC)- and k(EN)-factors, to convert flushes of extractable C and N to microbial C and N, respectively, are discussed. Sieving effects did occur, but they appeared, overall, less marked than other variables influencing these factors.