The effects of soil structure on protozoa in a clay-loam soil

Physical disruption of a clay-loam soil by slaking, grinding and compaction was used to determine the specific effects of soil structure on soil protozoa. Individual air-dry aggregates (1–2 cm diameter) were wetted slowly, or had their structure disrupted by slaking or grinding. They were then moistened with nutrient solution and incubated, at a matric potential of ?10 kPa. The nutrient solution had to supply at least 400 μg C g^?1 dry soil before protozoan populations increased measurably. Numbers of ciliate protozoa were enhanced by grinding, regardless of the amount of nutrient added. The moisture content and, therefore, the final nutrient concentration of the disrupted aggregates, changed markedly and could account for the observed increase in protozoan biomass from slowly wet to slaked to ground aggregates. There were no differences in protozoan biomass when the applied nutrient concentration was adjusted so that all treatments contained the same amount of nutrient. Soil cores were prepared from sieved (2–4 mm diameter), ground and ground/compacted soil. Thin sections revealed that the pore space accessible to protozoa decreased in these three treatments from 24% to 13% to 9%, respectively. Protozoan biomass was unaffected by grinding but showed a greater than 30-fold decrease following compaction that could not be accounted for solely by the reduced pore space. Grinding and compacting could have favoured anaerobic conditions in the core which would have reduced protozoan activity. Soil structure had no measurable direct effect on protozoan populations, but it had a much greater indirect effect through its influence on moisture content and aeration status.