Continuous plastic-film mulching increases soil aggregation but decreases soil pH in semiarid areas of China

Plastic-film mulch is a globally applied agricultural practice. However, its effects on soil aggregation and pH have been hardly studied. We assessed the effects of plastic-film mulch on soil aggregate water-stability and pH in fields used for maize (Zea mays L.) production, at two sites in a cold semiarid environment, China. Four treatments were used: (i) no plastic-film mulch and no straw incorporation, (ii) plastic-film mulch only, (iii) straw incorporation only, and (iv) straw incorporation plus mulch. Seven years after continuous treatment application, the use of plastic-film mulch increased the proportion of water-stable macroaggregates (>0.25 mm) in the top 15-cm soil layer by 16-28%, across sampling times and sites. Straw incorporation similarly increased the proportion of water-stable macroaggregates. However, the effects of mulch, on increasing the mean weight diameter of aggregates were greater in non-straw-incorporated soils than in straw-incorporated soils, and vice versa. Soil bulk density in the top 15-cm soil layer marginally increased in the mulched treatments, relative to the non-mulched treatments, while it decreased in straw-incorporated treatments. In addition, there was a decrease in soil pH, by 0.19-0.54 units, in the mulched, relative to the non-mulched soils. Combining our previous results, we suggested that increases in maize root growth and microbial activity were linked to the increased soil aggregation while accumulation in soil nitrate resulting from the stimulated soil nitrogen mineralization was responsible for the marginally decreased soil pH in the plastic-film mulched relative to the non-mulched soils, due to increased soil hydrothermal conditions.