Migration of nutrient and heavy metals impacted by biochar and pectin under the irrigation with livestock wastewater

Objectives Livestock wastewater contains abundant nutrients, but also contains some heavy metals. Migration of nutrient and heavy metals impacted by biochar and pectin application in soil-plant system irrigated by livestock wastewater was investigated to improve the use efficiencies of nutrients and control heavy metals in the livestock wastewater.

Methods A rhizobox was used to carry out experiment in maize. The tested soil was collected from the suburb of Xinxiang City, and biochar and pectin were individually added in ratio of 1% into soil. Maize was irrigated with distilled water (DW) or fermented livestock waste water (FW). The nutrients and heavy metal contents in soil and plants were determined, and the immigration of them in the plant-soil system was discussed.

Results There was no difference of plant shoot biomass between the FW and DW treatments, pectin showed more effective in improving plant growth than biochar did. Under FW irrigation, the root and shoot biomass in pectin treatments were 25.38% and 31.21% higher than in no pectin addition. The rhizosphere and bulk soil pH were generally decreased by FW irrigation, there was no significant difference among the pectin, biochar and no addition treatments. The electric conductivity of bulk soil was enhanced by the FW irrigation, and biochar addition increased it more than pectin did. The soil total N, available P, readily available K and organic matter contents were increased by the FW irrigation. The rhizosphere soil total P, available N, available Fe and available Mn in the pectin treatments were higher than those in the biochar treatments, while the total and readily available K contents in both rhizosphere and bulk soil of the biochar treatments were higher than those of the pectin treatments. Compared to the DW irrigation, the N and Ca contents in the plant roots and stems were enhanced by FW irrigation. Though the contents of N in roots, stems and leaves, P in roots and stems, K in stems, Ca in roots, stems and leaves, and Mg in roots and stems of the biochar treatments were all higher than those of the pectin treatments, the nutrient translation coefficients were higher in the pectin treatments. With the FW irrigation, the available Cu and Zn contents, especially available Zn contents, in rhizosphere and bulk soils were all increased; compared with the no addition treatment, biochar decreased the Cu, Pb and Ni contents in rhizosphere soil but pectin increased their contents on the contrary. The Cu, Zn and Pb contents in roots and stems were increased because of the FW irrigation. The Cu, Zn, Pb, Ni and Cd contents of plant roots were high in the pectin treatments, but their translocation amounts to the shoots were low.

Conclusions In northern alkaline soils, livestock wastewater irrigation could improve the soil fertility and plant nutrient contents of soil. Addition of biochar could decrease the heavy metal contents in plants by decreasing the available heavy metals in soil. Pectin could increase the available heavy metals in soil, but decrease their translocation to the shoots, avoiding the accumulation of heavy metals in plants to some extent.