Tillage effects on the aggregate-associated organic carbon and bulk density in some South African soils with different texture

Tillage operations disrupt the soil structure resulting in aggregates of various sizes and altered bulk density. Moreover, tillage influences soil carbon pools and many other soil physical properties. The objectives of this study were to determine, in various South African soils under different tillage systems, the following. (1) Amount of aggregate-associated soil organic carbon (SOC), (2) soil compressibility, and (3) relationship between compressibility, texture and the aggregate-associated SOC. The soil samples used in this study were collected from six different sites in Eastern Cape Province, South Africa. Soil samples were taken from conventional tillage (CT) and no-till (NT) land. To keep the soil aggregates intact sampling was done using a spade and carefully carried to the laboratory in rigid containers. For SOC determination, treatments were the two tillage systems, CT and NT, and four aggregate sizes. The experimental design was completely randomized design with a factorial layout and was replicated three times. Aggregate-associated SOC was determined using Walkley-Black method. Proctor compaction test was used determine the dry bulk density with varying moisture content and consequently the maximum bulk density (MBD) and critical water content (CWC). The aggregate-associated SOC content differed with tillage system and was significant higher (p < 0.05) in CT than NT. The amount of aggregate-associated SOC was 1.67 times higher in CT than NT plots. The MBD ranged between 1.77 g/cm3 and 10.27 g/cm3 and the CWC ranged from 9.1 percent to 10.3 percent. The higher amounts of SOC in CT were attributed to the annual crop residue returns while the lower amounts of SOC in the NT fields were due to grazing. Therefore, tillage influenced the amount of aggregate-associated organic carbon irrespective of the resulting size of the aggregate. The positive relationship between tillage and aggregate-associated SOC challenges the conversion of land to no-till for carbon sequestration. The overall gradient for correlation between the MBD and CWC was negative with r2 = 0.23 and a p value of 0.0076. The compressibility curves indicated higher values under CT if the texture class was silt clay