Soil organic matter fractions under different agricultural land-use in Ile-Ife, Nigeria

The study investigated the impact of long-term agricultural land-use on the distribution of density and particulate fractions of soil organic matter and examined the potential of soil aggregate size fractions to stabilize soil organic matter. Soil samples were collected at 0-15 and 15-30 cm soil depths from paddock, undisturbed secondary forest, continuously cropped land, teak, oil palm, and cacao plantations at the Teaching and Research Farm, Obafemi Awolowo University, Nigeria. The soil samples were air-dried, and segregated into three different aggregate-size classes (0.063-0.25, 0.25-1 and 1-2 mm). Bulk soil and soil aggregates were analyzed for particulate, and density organic matter fractions. Soil organic matter and total nitrogen contents of the density fractions were determined. The soil's content of particulate organic matter fraction was significantly highest under oil palm plantation (30.96 gkg^-1) and least under continuous cultivation (9.8 gkg^-1). Cacao, teak, and secondary forest land-use types had higher heavy organic matter fractions (HFOM) of 18.92, 13.93 and 10.49 gkg^-1 respectively, while soil light organic matter fraction (LFOM) contents were not significantly different under the land-use types except continuously cropped land that had the least content. Therefore, using HFOM as index, cultivation of tree crops and afforestation stores and protect carbon in the soil. This implies that, rather than leaving the land to native fallow, cultivation of the studied economic tree crops will positively impact the carbon sequestration potential of the land. The HFOM, LFOM, and C:N ratio was higher in 0.063-0.25 mm aggregate-size class. Hence, smaller sized soil aggregates (0.0630.25 mm) were better at storing and stabilizing soil organic matter compared to the larger size aggregates.