Effect of polymer materials on soil structure and organic carbon under drip irrigation

Polymer materials have broad potential applications for saving water and increasing crop yields in irrigated agricultural systems. However, little is known about how polymers affect the distribution and transformation of soil organic carbon. The purpose of this study is to evaluate the effects of humic acid (H) (an alkali extract of cottonseed meal), modified polymer (P) (a mixture of anionic polyacrylamide, polyvinyl alcohol, and manganese sulfate), and composite polymer (HP) (a 1:1 mixture of H and P) applied with drip irrigation on soil aggregate stability and organic carbon stability. The results demonstrated that the H and P treatments significantly increased soil aggregate stability and organic carbon content, compared with the HP treatment. Compared with the controls for corpped soils (CK) and uncorpped soils (CK-NP), the DR0.25 (>0.25 mm soil mechanical-stable aggregate), W-MWD (mean weight diameter of water stable aggregate), and W-GMD (geometric mean diameter of water stable aggregate) of the H treatment increased by 3.9%, 33.1%, and 23.2%, respectively (p < 0.05); the WR0.25 (>0.25 mm soil water stable aggregate), W-MWD, and W-GMD of the H-NP treatment (H treatment for uncorpped soils) increased by 44.7%, 38.0%, and 37.6%, respectively (p < 0.05); the WR0.25 and W-GMD of the P treatment increased by 29.8% and 26.8%, respectively (p < 0.05); and the W-GMD of the P-NP treatment (P treatment for uncorpped soils) increased by 16.5% (p < 0.05). Although the HP treatment also increased the organic carbon content of soil aggregates, it was less effective for improving soil aggregate stability. During the first 20 days of incubation, the mineralisation rate of the CK was the highest. Then, the mineralisation rates for the H and HP treatments increased more rapidly than that of the CK (p < 0.05), with increases of 27.6% and 54.4%, respectively. Results from field experiments (soil microbial biomass carbon content) supported this result. Compared with the CK, the H and P treatments rapidly increased soil labile organic carbon (LOC). The LOC content for the H treatment was 28.9% and 21.6% higher than that of the CK after 30 and 90 days, respectively (p < 0.05), whereas the LOC content for the P treatment was 32.8% and 20.2% higher than that of the CK after 60 and 90 days, respectively (p < 0.05). Redundancy analyses revealed that cultivation vs fallow treatments affected how polymer materials transform soil organic carbon. Cultivation allowed humic acid to improve soil aggregate mechanical-stability, promoting microbial decomposition of carbon and increasing organic carbon content. Modified polymer improved soil aggregate water-stability and reduced aggregate destruction, increasing the proportion of soil organic carbon that can be easily oxidised.