| Agricultural soils cadmium contaminated pose threats to the safety of agricultural products and human health in southern China.Phytoextraction is considered to be a cost-effective and less destructive in-situ technology for contaminant removal from agricultural fields without secondary pollution.Using napier grass（Pennisetum purpureum Schumach）for phytoextraction is a promising technology.In order to assess the practical phytoextraction capacity of napier grass,we conducted a field experiment with three different soil types（loam,sandy loam,clay loam）and Cd concentrations(0,0.25,0.5,1,2,4,8 and 16 mg kg-1).Napier grass were harvested at two different periods（70 days and120 days）to identify the optimal harvest time for extraction efficiency.Meanwhile,plants was harvested by mowing method,which was performed twice in the 120-day growing period.The main research contents and conclusions were as follows.1.In the range of soil cadmium content≤16 mg·kg-1,napier grass can grow normally.When the soil Cd content≤0.5 mg·kg-1 can slightly promote the growth of napier grass,and when the soil Cd content≥8mg·kg-1,it significantly（P<0.05）inhibits the growth of napier grass.The biomass of each part of the napier grass is in order stems,leaves and roots.With the increase of soil Cd content,especially when the soil Cd content≥2 mg·kg-1,the Cd concentration and extraction amount of each part of the napier grass were greatly increased,and significantly higher than the control（P<0.05）.In all treatment,roots had the highest Cd concentration,followed by stems and leaves,Cd is mainly stored in the roots.The results showed that all treatments had TF<1（translocation factor）,and BCFDTPA-Cd>1（bioconcentration factor base on DTPA-Cd concentration calculation）at the 70 days growth period,BCFDTPA-Cd<1 at the 120 days growth period.These results show that roots have strong Cd absorption abilities,but the capacity to transport Cd from root to shoot is weak.2.Comparing the two different growth periods of 70 days and 120days in first mowing,the shoot biomass of the napier grass in the loam,sandy loam and clay loam was 1400.07-1857.50,1094.47-2051.38,and1138.73-2078.53 g·plant-1 at 120 days growth period.The shoot biomass of napier grass at 120 days is higher than that 70 days,which is3.45-8.06,2.82-6.87 and 3.06-7.51 times,respectively.It indicates that70-120 days is the period of rapid accumulation of the shoot biomass of napier grass.The shoot of Cd concentration in the loam,sandy loam and clay loam was 0.27-5.91,0.32-6.00 and 0.28-4.89 mg·kg-1 at 70 days growth period,which are higher than 2.31-4.59,2.05-3.97,and 2.36-5.55times at 120 days,respectively.And the TF and BCFDTPA-Cd values of the napier grass in the 70 days were higher than the 120 days growth period,indicating that the shoots of napier grass at 70 days growth period had a stronger ability to transport and absorb Cd than the 120-day growing period.In the loam,sandy loam and clay loam,the Cd extraction of napier grass shoots was 0.06-2.03,0.10-2.32,0.06-1.82 mg·plant-1（70days）and 0.17-3.73,0.19-3.19,0.19-2.40 mg·plant-1（120 days）.The shoot Cd extraction amount of napier grass in 120 days is higher than 70days,and harvesting at 120 days can improve extraction efficiency.3.Compare the effect of mowing frequency on the extraction efficiency of napier grass at 120 days growth period.In the loam,sandy loam and clay loam,the biomass of shoot of the second mowing were1.80-2.23,1.43-2.16,and 1.73-2.46 times that of the second mowing,respectively,while the shoot Cd concentration of the first mowing were1.00-2.04,0.86-1.82,and 0.87-2.46 times that of the second mowing,respectively.The TF and BCFDTPA-Cd values of the second mowing were lower than that of the first mowing,indicating that the Cd transport and absorption capacity of the shoot of the second mowing decreased compared with the first mowing.In the loam,sandy loam and clay loam,the Cd extraction of napier grass shoots was 0.31-4.66、0.33-4.32 and0.29-2.68 mg·plant-1 in second mowing,it was slightly higher than that of the first mowing.Therefore,mowing is an effective agronomic enhancement measure for Napier grass extraction to remedition Cd contaminated soil.4.Shoot and root Cd concentration were selected as the dependent variables and soil soil physicochemical properties（total Cd,DTPA-Cd,p H,soil organic matter,clay content,CEC,etc.）were selected as the independent variables,and use multivariate stepwise regression for correlation analysis.The results showed that the Cd concentrations in the shoots and roots of the napier grass were positively correlated with the DTPA-Cd and total Cd concentrations in the soil,but negatively correlated with the soil p H and clay content.Among all soil factors,the DTPA-Cd concentration is the main factor affecting the shoot and root Cd concentration.The Cd extracted from shoots and roots increased significantly with increases to the DTPA-Cd concentration to soil（P<0.05）.This indicates that DTPA-Cd is the main absorption speciation for napier grass.There was no significant difference in the biomass of grasses grown in the three soil types.When the soil Cd-added concentration and harvest time were the same,the Cd extraction from the shoots of the three types of soil showed that loam>sandy loam>clay loam.In summary,when using the napier grass to remedition Cd-contaminated soil,the content of soil DTPA-Cd is the key soil factor affecting extraction efficiency.Properly extending the growth time of the grass,choosing to harvest in the 120 days growing period can improve the extraction efficiency.Harvesting by mowing method can effectively improve extraction efficiency while saving workload. |
