Accumulation Of Heavy Metals In Particulate Organic Matter (POM) From Purple Paddy Soils

Soil organic matter is an important component of soils and is one of the most important properties affecting the migration,transformation and bioavailability of heavy metals in soils.It has been reported that the effect of organic matter on the migration and transformation of heavy metals in soils is not only related to its contents,but also to its compositions.Soil particulate organic matter（POM）,referring to the soil organic matter fractions with the particle size in 0.053²mm,is an active intermediate of animal and plant residues into the soil humus with a low humification and high active degree of organic carbon pool.Recent studies have shown that POM has a significant enrichment ability of heavy metals in soil.Purple paddy soil is an important agricultural soil in Southwest China,but the content of POM in purple paddy soil and its characteristics of heavy metal accumulation are still not clear.Therefore,this study can provide the some basis for the accumulation mechanisms of heavy metals in soil.Seventy-five purple paddy soil samples developed from 6 typical soil parent materials were collected in Sichuan and Chongqing.The soil properties such as pH,organic matter（OM）and cation exchange capacity（CEC）of were measured,POM was separated,the contents of C,N and heavy metals including Cu,Zn,Pb and Cd were determined.The relationship among the soil properties,the contents and amounts of POM-C and the content and amount of heavy metals in POM were analyzed.The conclusions were as followed:（1）The content of POM in purple paddy soil of the sampling area is significant lower than the fine soil fraction and sand fraction,ranged from 0.44%⁹.68%,with an average level of 3.39%.With the particle size decreasing,the content of POM elevated.0.053⁰.25 mm was the main components in POM,and the content and the average value was 42.14%⁸6.03% and 70.83%,respectively.Compared with the existing reports,the content of POM in purple paddy soil collected in this study was higher than those in farmland.（2）The content of POM-C/ POM-N in purple paddy soil of the sampling area showed an enrichment characteristic of soil organic carbon（SOC）/total nitrogen（TN）,with the contribution rate ranged from 11.77%⁸5.24%/4.71%⁴2.48%,and the average value was 30.18%/15.36%,which represented a large part of the soil organic carbon pool/ toal nitrogen.The content of POM-C decreased with the particle size decreasing,and the maximum content are ranged from 168.15²99.63 g·kg^-1 in POM（1²mm）,with an average of 231.72 g·kg^-1.The contribution to SOC had increased with the decreasing of particle size,and the POM（0.053⁰.25mm）had the highest contribution rate ranged from 33.28⁸2.49%,with the average value is 56.55%.0.25¹mm is the advantage particle size for POM-N,ranged from 6.06¹8.13 g·kg^-1,and the average value was 10.65 g·kg^-1.The POM-N contribution increased with the particle size decreasing,particle size of 0.053⁰.25 mm of POM was the largest contribution rate in the range of 42.14⁸6.03%,and the average value is 70.83%.（3）The POM content in purple paddy soils of Suining formation,Feixianguan formation and Ziliujing formation were observed by Scanning electron microscope showed the complete shape,while the POM in Shaximiao formation,jiaguan formation,Penglaizhen formation had a higher degree of degradation.The Fourier transform infrared showed that,with the particle size of POM decreasing,the number and the variety of functional groups in POM were significantly reduced,so did the aromaticity.The absorption peaks of Feixianguan formation and Suining formation were lower than those in each particle size of POM,and the strong absorption peaks showed in Penglaizhen formation at the particle size of 0.053⁰.25 mm.（4）POM in purple paddy soil of the sampling area was enriched in Cu,Zn,Pb and Cd,and the enrichment ability and contribution rate of Cu and Pb were much higher.Cu,Zn,Pb and Cr in POM of purple paddy soil mainly bounded with the speciation of the organic tightly bound and complex state.The proportion of organic tightly bound of those heavy metals in POM was decreased with particle size decreasing.The average percentage content of organic tightly bound of Cu,Pb in POM were showed much higher than Zn and Cd,however,the average percentage content of complex state of Zn,Cd were much higher than Cu and Pb.The content of exchangeable fraction of Cd in each particle was the highest.（5）The highest of the POM content in purple paddy soil of the sampling area was the soil developed in Jiaguan formation with the average of 5.86±1.95%,and the rest of the soil had no significant difference.SOC,TN,CEC,clay and silt content was significant or extremely significant correlated the POM content,in which SOC is a key factor.The content of POM in Shaximiao formation was correlated with p H.（6）The minimums of the POM-C content in purple paddy soil of the sampling area was the soil developed in Penglaizhen formation with the average of 129.59±38.36 g·kg^-1,and the maximums of it was showed in soils developed in Ziliujin formation and Feixianguan formation,with the average content of 214.64±49.98 g·kg^-1,191.08±53.37 g·kg^-1,respectively,and the remaining 3 kinds of soils did not showed significantly difference.pH,TN,SOC had extremely significant effect on the content of POM-C,while TN,silt,clay,SOC were significantly related to the amount of POM-C.Soil properties had rare correlation wih POM-C content,and the soil properties in Shaximiao formation,Ziliujing formation and Penglaizhen formation had stong correlation with the amount of POM-C in sampling site.（7）The lowest content of heavy metals in the POM of the sampling area was the soil developed in Penglaizhen formation of purple soil,while greatest enrichment coefficient of Cu in POM was the soil developed in Feixianguan formation and POM in Jiaguan formation had a strong ability to enrich the Pb,Zn and Cd.Most of tested soil properties had showed significant or extremely significant correlation with the content of POM-Pb,while the other heavy metals in POM had no or weak correlation with it.TN,SOC,silt,clay content,total POM-C,SOC were extremely significantly related to the amount of Zn,Pb and Cd in POM,in which SOC was the key factor.The heavy metals in POM were more significantly affected by soil properties,and 34.9%⁸9.3% of the change could be explained by the soil properties.Comparing with other heavy metals,the amount content of zinc in POM has the highest regression coefficience and the closest relationship with soil properties.