Phytoextraction Potential Of Monocropping And Intercropping Of Maize And Legumes In Weak Alkaline Soils

The pollution of agricultural soils by heavy metals occurs worldwide and continues to deteriorate, which will decrease the yield of crops and at the same time cause the pollution of produced food, further giving rise to the health risk to inhabitants. There is an urgent need to remediate these polluted agricultural soils. The weak alkaline soils of the agricultural district in Baiyin around the drainage ditch Dongdagou were polluted mainly by heavy metals Cu, Zn, Pb and Cd due to wastewater irrigation in history. The present study chose Liangzhuang Village in Baiyin agricultural district as the experimental site. Nine growth stages of monocropping maize including vegetative stage with three leaves (V3), vegetative stage with six leaves (V6), vegetative stage with eight leaves (V8), vegetative stage with twelve leaves (V12), tasseling of the vegetative stage (VT), silking of the reproductive stage (R1), blistering of the reproductive stage (R2), doughing of the reproductive stage (R4) and physiological maturity of the reproductive stage (R6) were chosen to study the dynamic process of heavy metals phytoextraction by monocropping maize(Zea mays) (MM). Besides, the phytoextraction performance of monocropping and/or intercropping of maize and legumes soybean (Glycine max), pea (Pisum sativum), alfalfa (Medicago sativa) was also compared. The results were as follows:1. Monocropping maize extracting and translocating heavy metals depended mainly on transpiration. Old leaves retained the highest concentration of heavy metals, while grains kept it the lowest, indicating that translocation of heavy metals along with organic nutrients rarely happened. Moreover, high concentration of heavy metals was detected in organs like shoots and leaves which with photosynthesis capacity and high stoma density, proving that transpiration was the key power for phytoextraction of heavy metals.2. R6 stage was the optimum time for harvesting to get the best phytoextraction performance. The extraction content of heavy metals by MM increased over growth degree-days (GDD) slowly in the beginning, exponentially in the middle and decelerated in the end. Nonlinear regression analysis results indicated that the sigmoidal model fit best for the dynamic process of MM extracting heavy metals. At R6 stage, maize possessed the maximum biomass production, low heavy metals accumulation level and high extraction efficiency for heavy metals. Therefore, R6 stage was chosen to compare the heavy metals phytoextraction efficiency for different intercropping groups.3. Maize-soybean intercropping (IS) enhanced the phyextraction performance and showed no reduction in the biomass. There was no significant difference in the amount of biomass produced in intercropping groups comparing with MM. Besides, IS enhanced the extraction content of Cu and Zn significantly comparing with MM. Accordingly, IS could enhance the phytoextraction efficiency of heavy metals and at the same time produce no less biomass than MM. Moreover, heavy metals concentration in the biomass produced in the present study was accord with the legal threshold for fodder, providing the convenience of using the biomass.4. The activation of heavy metals in rhizosphere soils maintained high phytoextraction performance rather than deteriorated the contamination of deep layer soils. Diethylenetriamine pentaacetic acid (DTPA) extractable heavy metals concentration in most rhizosphere soils was higher than that in non-rhizosphere soils, indicating the activation of soils heavy metals. Although the activation was favorable for guaranteeing stable phytoextraction efficiency, still there existed potential risk of activated heavy metals leaching into underground or aboveground water. Further study was done to assess the potential risk with soils in different depth under soybean rhizosphere. Results showed that the activation did not contribute much to the DTPA extractable heavy metals in deep soils, except that the DTPA extractable Zn and Cd in rhizosphere soils in depth of 40-60 cm were significantly higher than that in non-rhizosphere soils. Therefore, it would not increase available portion of heavy metals in deep layers or the risk of heavy metals leaching into underground or aboveground water due to MS, or to IS deducing from its much lower level of activated Zn and Cd.In all, harvesting maize at R6 stage could gain the best phytoextraction performance, biggest biomass production with low enough heavy metals concentration to meet the China’s national legal thresholds for fodder use. IS possessed high phytoextraction efficiency for the four heavy metals and caused no reduction in the total biomass. The activation of heavy metals in rhizosphere soils would help to guarantee stable phytoextraction efficiency rather than contributed to the leaching possibility of heavy metals in deep soils, underground and aboveground water.