玛河流域不同连作年限棉田土壤质量分析及综合评价

Soil quality assessment of continuous cropping cotton fields for different years in Manas River Basin

以玛河流域不同连作年限(1、5、10、15、20 a和25 a)棉田土壤为研究对象,测定了土壤碱解氮、有机质、速效钾、速效磷、pH值、盐分及镉(Cr)、铬(Cd)、铅(Pb)、镍(Ni)、铜(Cu)、砷(As)共计12个指标,对比分析土壤养分特征和重金属含量,分别采用因子分析法和土壤质量综合评价指数法对不同连作年限棉田土壤质量状况进行定量评价并划分等级,采用Muller地质累积指数法评价棉田土壤重金属污染情况。结果表明:(1)pH值在开垦种植1 a和25 a连作时最高,均值为7.90,5、10 a连作时最小,pH值均值为7.75;土壤盐分含量由连作5 a的3.68 g·kg^(-1)逐年增加至连作25 a的3.87 g·kg^(-1);有机质含量在种植1 a时仅11.02 g·kg^(-1),随后缓慢增加,至连作10 a达19.97 g·kg^(-1),之后降低,25 a时降至1 a水平;速效钾含量变化趋势与有机质相似;全氮和速效磷从种植1 a开始逐年递增,连作25 a时达最大,全氮含量0.903 g·kg^(-1),速效磷含量55.1 mg·kg^(-1)。(2)土壤重金属Cr、Ni、Cu、Pb、As和Cd含量均随连作年限增加而增加,其中Cr、Ni和Cd在连作15 a时分别达93.87、33.18 mg·kg^(-1)和0.23 mg·kg^(-1),有重金属富集趋势;As在连作20 a和25 a分别为12.03 mg·kg^(-1)和13.53 mg·kg^(-1),为轻度污染;污染指数(ISHM)显示Pb在连作1~25 a均无污染(ISHM<0),Cu含量超标(ISHM>0)。(3)因子分析法得出,SQAV评分最高的是连作10 a(0.437),其次是5 a(0.247),1 a和25 a为负值(分别为-0.712和-1.253),不利于土壤质量提升;土壤综合质量指数评价法得出5 a和10 a棉田土壤质量为“高”,土壤综合质量指数(SQI)分别为1.02和1.10,1 a和25 a为“低”,SQI分别为0.63和0.61,土壤质量状况最差,15 a和20 a为“中”。两种评价方法得出的结论相似,说明本研究选取的评价方法合理,可以定性定量对不同连作年限棉田土壤质量进行评价。

The long-term continuous cropping of cotton is a common planting pattern in the Manas River Basin,Xinjiang Province,and it may inevitably lead to unbalance in soil nutrients,diminishing soil fertility and degrading soil quality.In 2017 and 2018,0~20 cm depth soil samples were collected from six continuous cropping cotton fields,with different cultivation times(1,5,10,15,20 years and 25 years respectively).This study aimed to evaluate soil quality across continuous cropping cotton fields for different years by analyzing six physical and chemical properties(organic matter,pH,salt,total N,available P and available K)and six soil heavy metals(As,Cd,Cr,Ni,Cu,and Pb).The factor analysis and the comprehensive soil quality index(SQI)were applied to quantitatively evaluate soil quality grades.Muller geo-accumulation index(ISHM)was calculated to assess the pollution of heavy metals.The results:(1)1 year and 25 years had the highest pH,with an average of 7.90,while 5 years and 10 years cropping had the lowest pH value.The salt content of the soil increased year by year,with an average salinity of 3.65 g·kg^(-1) at 5 years and 3.87 g·kg^(-1) at 25 years.The organic matter was the highest at 10 years(19.97 g·kg^(-1)),and the lowest for 1 year and 25 years.The variation trend of available K was similar to that of organic matter.The contents of total N and available P increased from 1 year,and reached the maximum at 25 years,with average total N of 0.903 g·kg^(-1),available P 55.1 mg·kg^(-1).(2)The concentrations of heavy metals(Cr,Ni,Cu,Pb,As and Cd)increased along with increasing years of cultivation,with an average Cr,Ni and Cd of 93.87,33.18 and 0.23 mg·kg^(-1) at 15 years respectively,and these numbers were greater than the background numbers in Xinjiang,indicating the accumulation of those elements in the soils.The content of As was 12.03 mg·kg^(-1) at 20 years and 13.53 mg·kg^(-1) at 25 years,indicating micro-pollution.Heavy metal pollution index(ISHM)revealed that Pb had no pollution in continuous cropping 1~25 years(ISHM<0)and the content of Cu exceeded the standard(ISHM>0).(3)Soil quality assessment values(SQAV)based on factor analysis showed that 10 years had the highest SQAV(0.437),followed by 5 years(0.247),1 year and 25years had the lowest numbers.The integrated soil quality index evaluation method(SQI)showed that the soil quality of 5 years and 10 years were“high”(SQI were 1.02 and 1.10 respectively),1 year and 25 years were“low”(SQI were 0.63 and 0.61 respectively),and 15 years and 20 years were“medium”.In summary,the SQAV based factor analysis and SQI were all the most suitable methods for evaluating the soil quality of cotton fields with different continuous cropping years qualitatively and quantitatively.