Aggregate-associated carbon in a cultivated soil under short-term planted cover crop

Cultivation and agronomic practices (cropping density) influences soil carbon (C) and changes the distribution and stability of soil aggregates. Aggregation and aggregate-associated C content were studied in three depths of a cultivated soil planted with Mucuna jaspedea at three plant densities (25, 50, and 75%) in southwestern Nigeria. Dry and water stable aggregates were separated into six classes ( 0.25, 0.25 – 0.5, 0.5 – 1, 1 – 2, 2 – 4, and 4 mm). The largest proportions of dry and water-stable aggregates (WSA) in the soil surface (0 – 15 cm) depth were in the order: 0.25 – 0.5 mm 0.5 – 1 mm microaggregate ( 0. 25 mm) size classes, but the smallest proportion (9%) of dry aggregate distribution was found in the 2 – 4 mm size class. Cultivation resulted in reduction of the proportion of large ( 4 mm) dry and water-stable macroaggregates. Consequently, there was a shift in size distribution of aggregates 4 mm to the 1 – 2 mm aggregate size. Among the plant densities, the largest concentrations of C were found in the 0.25 - 2 mm dry macroaggregates isolated from 0 – 15 and 15 – 30 cm depths, while the largest C concentration was within the 0.5 – 1 mm WSA. The results show cultivation enhanced the disruption of aggregates and redistribution of C which predisposes the loss of C. It was also indicated that macroaggregates were sensitive to cultivation especially at the soil surface, thus, could be an indicator in evaluating the impacts of soil management.