Shallow groundwater table fluctuations: A driving force for accelerating the migration and transformation of phosphorus in cropland soil

The accumulation of phosphorus (P) in soil profiles of intensive cropland and the losses caused by runoff and leaching have been widely concerned. However, the loss of soil P due to shallow groundwater table (SGT) fluctuations driven by seasonal changes is often neglected, and the migration and transformation mechanisms of soil P are still unclear. On the basis of the long-term monitoring of cropland soil P accumulation and SGT fluctuations around Erhai Lake, the characteristics of soil P loss driven by SGT fluctuations and the corresponding mechanisms were investigated through a 260-day microcosm experiment. The results revealed that the fluctuations in SGT significantly changed the content and form of P in the soil profile. The soil P loss mainly occurred in dissolved form, mainly involving inorganic P, accounting for 75 %. Compared with those under continuous saturated conditions, soil total P (TP) release during SGT fluctuations significantly increased by 9.5 %, and soil TP storage was reduced by 2 %. SGT fluctuations increased the complexity of microbial networks in the soil profile, stimulated the expression of functional genes for soil P cycling, and promoted soil organic P mineralization. The SGT fluctuations caused an increase in the soil TP loss from cropland to 88.5 kg/ha, which was 70 and 25 times greater than that via leaching and runoff, respectively. These results indicated that SGT fluctuations accelerated the P loss from soil profile of cropland. Therefore, some measures should be comprehensively applied to prevent its loss, such as reducing external P input, improving surface soil P storage capacity and soil P utilization efficiency, reducing surface P leaching into deep soil, and reducing P accumulation in deep soil profiles.