Mean-weight diameter of aggregation as affected by initial screen size of two fine-textured soils

Soil aggregation is considered a key indicator of soil health to protect soil against erosion, enhance organic C storage, and offer habitat for soil organisms. Various methods to assess aggregation may change interpretations of management, and therefore should be cross-calibrated. We assessed the impact of initial sieve opening size (8 or 4.75 mm) prior to determination of dry-stable and water-stable mean-weight diameter (MWD) from two fine-textured soils—a Rhodic Hapludox from São Paulo, Brazil and a Rhodic Kanhapludult from North Carolina, United States. Both soils were subjected to management expected to alter surface soil conditions. As expected, initial sieving through 8 mm led to greater dry-stable MWD (3.37 ± 0.60 mm) than initial sieving through 4.75 mm (1.94 ± 0.28 mm). However, soil stability index (water-stable MWD/dry-stable MWD) was not affected by initial sieve size opening (0.56 ± 0.13 mm mm−1 under both initial sieve openings). Management interpretations were consistent with both approaches as well, and in particular to detect the strong depth effect on water-stable MWD (i.e., declining with depth). Water-stable macroaggregates had 32% ± 25% greater C concentration than microaggregates; similarly under both initial sieving conditions. Soil stability index when initially sieved through 4.75 mm was highly associated with aggregate stability of 1–2-mm sized dry aggregates, which is a more common procedure. We conclude that passing soil through a screen with 4.75-mm openings to conduct a diversity of soil analyses can be appropriate for obtaining reasonable estimates of and interpretations about surface soil aggregation.