Effects of long-term fertilization on aggregates size distribution and carbon stock in black soil

Soil samples (0-20 cm) were collected from an experimental field with 22 years fertilization to study the effects of different fertilizations (no fertilization, NP, NPK, and NPK plus organic manure) on the composition of aggregates, organic carbon contents, and carbon stock in black soil. The aggregates were separated into five size classes (2, 1-2, 0.25-1, 0.05-0.25, and 0.05 mm) by wet sieving and dry sieving. The results showed that the aggregates were dominated by the size class 0.25-1 mm. Through wet sieving, different size water-stable aggregates were redistributed in their amounts, i.e., the size class 0.25-1 mm still dominated, but the amount of macro-aggregates decreased while that of micro-aggregates increased. Under long-term application of chemical fertilizers and of NPK plus organic manure, especially of the latter, the amounts of macro-(2 mm) and micro-(0.05-0.25 mm) aggregates and the carbon contents of different size class aggregates increased significantly, soil carbon stock increased, soil structure improved, and soil C sequestration enhanced.