Denitrification in Soil Aggregates of Different Sizes

Abstract

Abstract The denitrification potential in different size aggregates of a silt loam soil was studied to explain spatial variability of denitrification. Seven aggregate size ranges (<0.25, 0.25–0.5, 0.5–1, 1–2, 2–5, 5–10, and 10–20‐mm diam.) were obtained by gentle dry sieving of the sampled soil. The denitrification rate was determined by the acetylene block method following imposition of several different treatments. It was observed that the denitrification rate was much larger with the smaller aggregates and generally decreased as aggregate size increased. Crushing of the aggregates to <0.25‐mm diam. resulted in some diminishment of this relationship. The supply of NO − 3 was not limiting in the different size aggregates but the addition of glucose suggested that C substrate supply in larger aggregates was limiting denitrifier activity. Carbon dioxide production during aerobic incubation was greatest in microaggregates (<0.25 mm) and decreased with increasing aggregate size, further substantiating the deficiency in C substrate in large macroaggregates. It was found also that biomass C was greatest in microaggregates and decreased with increasing aggregate size when determined for both aerobic and anaerobic incubation conditions. The results of this study may be interpreted to suggest that considerable variability of available substrate C occurs in association with different size aggregates and could partly explain the large spatial variability of denitrification that occurs over relatively short distances in field soil