Salt redistribution and nutrient loss after soil salt washing in plastic shed vegetable field in East China

Soil salt washing is very common in plastic shed vegetable field in East China, and understanding salt movement and nutrient fate during soil salt washing is vital for agricultural production and environmental protection. However, little attention was paid to salt redistribution after salt washing, which would lead to nutrient loss, and then induce eutrophication and water pollution. An on-site experiment of undisturbed soil columns with different salt contents was implemented to investigate the salt redistribution and nutrient loss after soil salt washing in six intensive farms with plastic shed vegetable field in Shanghai suburbs. The contents of salt and nutrient in the different depths of soils after salt washing were determined, and the losses of nitrogen and phosphorus from salt washing were calculated. After the soil salt washing, the contents of total soluble salt (TSS) and nitrate nitrogen (nitrate N) in the 0–20 cm soil layers at all sites decreased more than 35% and 15%, respectively, while the total phosphorus (TP) only performed slight decreasing trends of less than 10%. The pollution loads of total nitrogen (TN) from soil salt washing greatly increased following the increase of TSS contents in the surface soil, and ranged from 24.80 to 215.64 kg ha−1, while the TP ranged from 0.37 to 1.43 kg ha−1. The pollution loads of TN, nitrate N, TP, and soluble P were significantly correlated with the TSS contents of the 0–20-cm soil layer (P < 0.05). The salt washing could effectively eliminate soil secondary salinization, and cause nutrient loss and water pollution in the meantime. In the plastic shed vegetable field, soil salt washing performed very positive effect on the reduction of soil salinity, and can remarkably decrease the nitrate contents in the surface soil layers, while there was no obvious impact on the downward vertical movement of phosphorus. Meanwhile, salt washing from soils decreased nitrogen use efficiency and increased the risk of water pollution in the plain network area with shallow groundwater table.