Turnover of clay-associated organic nitrogen in the different aggregate-size classes of a cultivated silty loam

Soil aggregates are known to protect organic matter against microbial attack, and thereby stabilize it. The scale at which aggregation is effective and the degree of control it exerts on organic matter transformations are not well known. I investigated whether rapid and slow pools of organic nitrogen bound to the <2-mu m clay fraction could be related to different locations of the clay fraction in the soil structure. Dry aggregates were separated from soil to which N-15 had been added for 4 years in a field experiment. Soil samples were at maximal immobilization of a labelled inorganic N dressing, and after 1 and 3 years of the beginning of the remineralization phase. Clay-sized particle fractions (<2 mu m) were separated from the different aggregate-size classes, and their total C and N contents, and N-15 isotopic excess, were determined. It seems that recently immobilized N associated with the clay fraction was rapidly sequestered in microaggregates (<0.1 mm). The results also suggest a relation between the fast decay of part of the clay-associated N and its location on the macroaggregate scale (>0.1 mm) of soil structure.