Soil organic carbon, glomalin related soil protein and related physical properties after 15 years of different management practices in a subtropical region of South Africa.
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Soil aggregation is an important mechanism, which plays a significant role in soil fertility as it decreases soil erosion and mediates air permeability, water infiltration, and nutrient cycling. Aggregation depends on a variety of aggregate binding agents, including carbon and its fractions, interrelating concurrently at different spatial scales. However, biologically active fractions of organic matter, such as microbial biomass carbon (MBC) and water-extractable organic carbon (WOC) could better reflect the changes in soil quality. Recent studies have highlighted the existence of a thermostable, water-insoluble soil glycoprotein operationally referred to glomalin-related soil protein (GRSP) that is crucial for preserving SOC. However, the relationship between SOM fractions and GRSP, and effects of different land uses on these parameters and relationships in humid environments are not clearly understood. The study sought to determine the relationships between soil organic carbon fractions, GRSP and aggregate stability under different management practices. The study was conducted on a farm located in the south-east of Howick, in the uMgungundlovu District Municipality, KwaZulu Natal province of South Africa. Soil samples were collected at 0-5, 5-10, 10-20 and 20-30 depths from three management practices i.e. long-term no till (NT), conventional tillage (CT), and native Forest (F). The native forest soils served as the control. Glomalin was assayed. The soils were further analysed for Ca, Mg, K, and Na, microbial biomass carbon (MBC), water soluble organic carbon (WSOC), soil bulk density (BD). Interaction between land-use and soil depth had significant effects on SOC content. There was general decrease in SOC as depth increased for all management practices except for no till, where no significant differences were observed in SOC across the four soil depth layers. The interaction between land-use and soil depth had significant effects (p < 0.05) on both easily iii extractable (EE-GRSP) and total (T-GRSP) glomalin related soil proteins. The NT treatment had the lowest concentration of EE -GRSP than the other two treatments in the 0-5 cm depth whilst no differences among the management practices at deeper soil layers were observed. The concentration of T-GRSP for soils under forest and NT tended to decrease with depth, while in CT the 10-20 and 20-30 cm depths had higher concentrations than the 0-5 and 5-10 cm depths. Land use also had significant effects (p < 0.05) on soil aggregate stability. Soils under Forest were the most stable with an MWD of just over 3, whilst soils under Conventional Tillage had the lowest MWD value of 1.24. The observed aggregate stability was significantly influenced by GRSP as evidenced by a significant positive relationship between both EE-GRSP (R2 = 0.72) and T-GRSP (R2 = 0.82). Therefore, management practices that mimic natural forest favour the accumulation of SOC and T-GRSP and should be widely adopted.