Soil organic matter fractions and microaggregation in a Ultisol under cultivation and secondary forest in south-eastern Nigeria

The study of the role of soil organic carbon (SOC) in restoration of soil fertility and stability of soil microaggregates is of importance in soils that degrade rapidly. We studied 4 profiles in a Ultisol under secondary forest and cultivation to identify the SOC microaggregate-associated fractions and their roles in microaggregate stability. The soils are coarse-textured, deep, and low in soil nutrients and SOC, probably due to high rates of mineralisation. Microaggregate-associated SOC was also low with most of the SOC protected by the < 63 mu m fractions. Clay content was negatively correlated with < 2, 63 -2, and < 63 mu m associated SOC (r= -0.45*, -0.42*, -0.40*, respectively). Clay flocculation index and clay dispersion ratio were significantly correlated with < 2 and 63 - 2 mu m associated SOC, whilewater-stable aggregates < 0.25 mm were negatively correlated with all SOC fractions determined. Principal component analysis revealed that SOC fractions associated with 2000 - 63 mu m aggregate sizes were the SOC fractions that best explained the variance in aggregated silt + clay, indicating their contribution to microaggregate stability. This was attributed perhaps to the production of polysaccharides and materials released by microbial activities from this recently deposited or incompletely decomposed SOC. The other soil properties that in addition to SOC contributed to either dispersion or microaggregate stability of these soils were exchangeable Na+, Mg2+, and CEC.