Soil aggregate characteristics and organic carbon concentration after 45 annual applications of manure and inorganic fertilizer

Soil management practices that increase soil carbon sequestration can improve soil quality and reduce agricultural contribution to carbon dioxide (CO2) emissions. The long-term (45 years) effect of applications of manure (FYM) and inorganic (NPK) fertilizers on soil aggregate characteristics and soil organic carbon (SOC) concentration was studied in surface soil of a Typic Haplustalf sandy loam. Soils from four treatments: inorganic fertilizer in combination with farmyard manure (FYM + NPK), fertilizer only (NPK), farmyard manure only (FYM) and a control receiving neither NPK nor FYM were studied. Results indicate that long-term application of manure with or without NPK improved aggregate stability by increasing mean weight diameter (MWD) of dry aggregate and fraction of dry aggregate > 2.0 mm. Higher values of MWD were also recorded in the water-stable aggregate size distributions for soils amended with either FYM or NPK. Soils amended with FYM + NPK sequestered more SOC than all other treatments. There was a low, but significant positive correlation between MWD of dry aggregates and SOC. Highest values of SOC were recorded in the > 2.0 mm aggregate fractions. This tendency towards greater SOC concentration for > 2.0 mm dry aggregate is suggestive of the effectiveness of this aggregate size range to sequester SOC in savanna Alfisols. These results demonstrate that continuous application of soil amendments like FYM and NPK over a long-term promotes soil aggregate stability and long-term SOC sequestration. Hence, for sustainable production in savanna Alfisols, there is a need to design policies that will promote soil aggregate stability and long-term carbon sequestration under continuous cultivation. Such policies must give attention to soil management practices that maintain higher level of SOC. Here, FYM with or without NPK was essential to increase stability of dry aggregates and SOC sequestration. However, the NPK amended soils possess higher fraction of water-stable aggregates.