The Distribution Characteristics And Evaluation Of Heavy Metals Pb、Cr、Cd、As In Different Agricultural Cycle Models Of Straw Utilization

Every year, there are a large number of straw resources which cannot be effectively used in agricultural production in China. The circular agriculture can effectively achieve the straw resource. There are three mainly circular agriculture patterns including straw-farmland circulation, straw-edible mushrooms circulation and straw-cattle circulation in Henan province.It is not only the migration of nutrient elements in agricultural cycle, but also the shifting of heavy metals, which can cause crop yield losses and harm human’s health by the food chain.So the comprehensive utilization of crop straw was focused on in straw-farmland circulation, straw-edible mushrooms circulation and straw-cattle circulation in this experiment, specifically the comprehensive utilization of wheat straw and corn stalk from 2013 to 2014. Straw was reused for pulverization returning, culturing mushrooms and feeding cattle, then mushroom residue and cow dung were returning to field too. The content of heavy metals(including Pb, Cr, Cd and As) of these samples, such as soil, straw, grain of crops, fungous material, mushroom, mushroom residue, fodder, beef and cow dung, were measured with inductively coupled plasma emission spectrometry(ICP-MS). And the distribution regulation of heavy metals in three agricultural circulation patterns was determined, the pollution risk of heavy metals was evaluated by the single pollute index method and the Nemerow Synthetical Pollution Index method, and the enrichment coefficient and the transport rate of heavy metals in soil-crop system was studied detailedly to control the migration of heavy metals in food chain.The main research results were as follows:1. In straw-farmland circulation pattern with straw returned to field directly after pulverization, the content of four heavy metals in soil was not more than the level of Soil Environment Quality Standard(GB15618-1995). Moreover, the content of Cr and Cd in the roots of wheat and corn was higher obviously than in the soil, but the content of Pb and As was dropped from root to stem and leaf, even to grain. And the pollution risk of Pb and As in wheat grain was higher. In short, the content of Pb, Cd and As in soil had a slight decline after a whole cycle, with the content of Cr in soil having no change.2. In straw-edible mushrooms circulation pattern with straw reused to culture mushroom, the content of the four heavy metals in the fungous material was closed to that in the crops straw; heavy metals were absorbed a small amount by edible mushroom, and residued in quantity in the mushroom residue. After the mushroom residue returned to wheat field, the content of the four heavy metals in the soil was lower than the level of Soil Environment Quality Standard(GB15618-1995). The distribution regulation of the four heavy metals in crops was similar to the straw-farmland circulation pattern. And there was a mild pollution risk only to the content of Pb in corn grain.3. In straw-cattle circulation pattern with straw reused to feed cattle, the heavy metals content in the fodder slightly was higher slightly than that in the crops straw, and the content of heavy metals in cow dung was basically flat with fodder. Whatever feeded by straw from wheat or corn, the heavy metal content in beef was far below the level of Food Safety Standard(GB 2762-2005) at all. After the cow dung returned to wheat field, the content of these four heavy metals was lower than the soil and environment standard too. In addition, the heavy metals content in crops was as root > stem and leaves > grain, and the grain of crops have no risk in the assessment of heavy metal pollution.4. Using single pollute index method and Nemerow Synthetical Pollution Index method to evaluate heavy metals pollution risk about the three agricultural circulation patterns of straw utilization, the evaluation results showed that, when without exceedjing the limit of the soil standard, grain of wheat and corn was vulnerable pollutioned by Pb and As. Moreover, the transport rate of heavy metals from straw to edible mushroom in straw-edible mushrooms circulation pattern was above that from straw to beef n straw-cattle circulation pattern, so edible fungi was more susceptible to heavy metal pollution. Therefore, the biggest enrichment coefficient of heavy metals was the root of the crops in all members of the three agricultural circulation patterns, while the transport rate of heavy metals was appeared in the stem and leaves. So root, stem and leaves should be the key members in the agricultural circulation to control heavy metals far from the food chain.