Effects of long-term super absorbent polymer and organic manure on soil structure and organic carbon distribution in different soil layers

Super absorbent polymer (SAP) and organic manure (OM) may improve soil structure and change soil organic carbon (SOC) composition and agroecosystem functioning. However, the understanding of the effects of SAP and OM on SOC composition, specifically in deeper soil layers, is still not clear. The objectives of this study were to examine the effects of long-term application of SAP and OM (8 years) on changes in SOC and soil structure down to a soil depth of 100 cm. Therefore, in order to investigate the stability of soil structure and the distribution of organic carbon at different soil depths (0-10 cm, 10-20 cm,…, 90-100 cm) under long-term application of SAP, OM, and a control treatment, soil structure and soil organic carbon content were analyzed in mixed soil samples and undisturbed soil samples. The results indicated that with depth, the proportion of large aggregates (0.5-2.0 mm) decreased gradually, while the proportion of small aggregates (0.5 mm soil aggregates in the 0-30 cm and 40-60 cm soil layers (P < 0.05) and 0.25-0.5 mm in the 0-50 cm soil layer (P < 0.05), and while the OM treatment had the same effect in the 0-30 cm soil layer (P < 0.05). The total organic carbon content (TOC) and labile organic carbon content (LOC) of bulk soil increased (0-20 cm) initially with depth, and then decreased (20-70 cm) approximately. In the 0-50 cm soil layer, the TOC and LOC under SAP treatment were higher than those under OM treatment. The SAP treatment was more beneficial to the increase of the TOC and LOC of 0.5-2.0 mm and 0.25-0.5 mm aggregates in the 10-40 cm soil layer compared with the control and OM treatment (P < 0.05). The SAP treatment was also more beneficial in increasing the contribution rate of organic carbon (CROC) in> 0.5 mm aggregates in 0-40 cm soil depth, while the OM treatment was more beneficial in increasing the CROC of > 0.5 mm aggregates in 0-30 cm soil depth. The SAP treatment improved the stability of the soil structure in 0-30 cm and 40-60 cm depths, and the OM treatment had the same effect in the 0-30 cm depth. Compared with the control and OM treatments, SAP treatment has shown to be the most beneficial in improving soil structure and increasing organic carbon content.