Aggregate Stability in Relation to Hydrolysable Organic Carbon in Humid Tropical Ultisol under Manure-Fertilizer Amendments

Improving soil aggregate stability (AS) and soil organic carbon (SOC) through manure and fertilizer management is a wellestablished agricultural practice.This study investigates the impact of poultry manure (PM) and inorganic fertilizers (IFs) applied at varying rates to hybrid maize on AS and soil organic carbon fractions (SOCFs) in Nsukka, Nigeria.Both PM and IF, whether separately or together, enhanced mean weight diameter (MWD) in 2013, notably in PM-treated plots in subsequent years, supported by positive potential structural enhancement index (PSEI).PM decreased clay dispersion index (CDI) and dispersion ratio (DR) while increasing aggregated silt plus (ASC), especially with higher PM rates.Aggregate density (AD) decreased with increasing PM rates, while non-hydrolysable soil organic carbon (NHC) and hydrolysable soil organic carbon (HOC) increased.Both NHC and HOC showed significant positive correlations with MWD and PSEI, with only HOC exhibiting a negative association with AD.The SOCFs correlated negatively with CDI and DR, and positively with ASC, except for the non-significant association of NHC with DR.Overall, the positive effects of manure-fertilizer applications on AS indices, and SOCFs, highlighted the importance of enhancing HOC and NHC through organic inputs for soil structural stability and long-term SOC and labile C stabilization.