Tuo Jin

Polyacrylamide (PAM), as a widely used effective soil conditioner, can decrease sandy soil infiltration, but its function may decline significantly in a short time. Previous study results showed that the annual degradation rate of PAM in soil is about 10%, and the migration ability of PAM in soil is fairly weak; thus we hypothesized that the functional group of PAM is prone to aging caused by physical and biological factors, which is different from degradation caused by the breaking of long main chains into short ones. The sandy loam soil was selected to conduct column infiltration experiments to (1) determine the effects of PAM application and drying and wetting intensity on infiltration and (2) identify the causes of the aging effect. Soil samples were treated with three doses of PAM (0, 1, and 2 g·kg⁻¹) and incubated in three soil water conditions (constant wetting, moderate and strong drying/wetting cycles). Under constant wetting condition, the stable infiltration rates of soils were decreased by PAM. However, after two strong drying and wetting cycles, the decrement of infiltration rates of PAM-treated soils was reversed. The results of FTIR suggested that drying and wetting cycles led to the hydrolysis of amide groups in PAM, resulting in the weakening of PAM’s function on soil infiltration characteristics. The leaching amounts of NH₄⁺-N generated by the amide group hydrolysis increased through the drying/wetting alternation and the application of PAM. Therefore, based on the findings of this column study using a specific sandy loam soil under controlled intense drying–wetting cycles, reapplication of polyacrylamide (PAM) after two cycles may facilitate the sustained lowering of infiltration. However, this recommendation should be confined to analogous experimental conditions and necessitates further validation under field scenarios or for alternative soil types.

Biodegradable mulches have become the focus of attention, as pollution caused by leftover plastic mulch material becomes increasingly severe. However, the impact of biodegradable mulches to the soil needs to be further investigated. An experiment was conducted to evaluate the impact of no-mulch, biodegradable film mulch (BM) and polyethylene film mulch (PM) on the soil’s physical, chemical and biological properties after six years (2013–2019) of mulching in garlic growing season in a garlic-maize rotation. Results showed that the soil bulk density of the 10–20 cm soil layer under BM decreased by 12.09–17.17% compared with that under PM. The soil total nitrogen content increased significantly by 14.75–28.37%, and the soil available phosphorus and potassium content increased by 64.20% and 108.82%, respectively. In addition, BM increased the soil’s microbial, soil urease, and soil catalase activities compared with those for PM. To sum up, BM can reduce soil bulk density, and long-term use of BM does not cause a decrease in soil nutrient content and microbial activity. On the contrary, it can improve soil quality. This study helps accumulate data for the environmental safety evaluation of BM and provides theoretical and technical support for the large-scale promotion of biodegradable mulches.