华中地区某县农田土壤黑碳分布特征及来源解析

摘要

Background, aim, and scope Black carbon (BC) is a continuum of thermally altered materials produced by incomplete combustion of biomass, biofuels, and fossil fuels. It is found globally in the water, atmosphere, snow, ice, sediments, and soils. BC generally resides in the soil for a long time acting as a long-term C sink, with a potential negative feedback on climate warming. BC is composed of char and soot corresponding to combusted solid residues and clusters of carbon particles formed by gas-phase processes, respectively. Many studies have focused on BC in urban and forestland soils, while little attention is paid to agricultural soils in China. Furthermore, many previous studies did not differentiate between different BC fractions (char and soot) in soils, thereby much important information regarding the sources and environmental behavior of these two major components is missing. In this paper, the variation characteristics of BC, char and soot concentrations in agricultural soils were studied. The impact factor and potential sources of BC were also studied.Materials and methods A total of forty-six topsoil samples (0—20 cm) were collected with steel shovel in farmland from Yangxin County of Hubei Province, central China. The soil types can be classiifed into three categories: paddy soil, red soil, and lfuvo-aquic soil, and the number of soil samples is twenty-three, eighteen, and ifve, respectively. The concentrations of BC, char and soot in soils were analyzed by thermal optical relfectance method following the IMPROVE_A protocol. Total organic carbon (TOC) content was determined using potassium dichromate oxidation method.Results BC, char and soot concentrations in the agricultural soils varied from 0.17 g∙kg−1 to 3.18 g∙kg−1, 0.03 g∙kg−1 to 2.37 g∙kg−1 and 0.09 g∙kg−1 to 1.50 g∙kg−1, with average value of 0.17 g∙kg−1, 0.03 g∙kg−1, and 0.09 g∙kg−1, respectively. The average contents of BC, char and soot in three different types of soils ranked as follows: paddy soil > red soil > lfuvo-aquic soil. The variation coefficient of char was higher than 100%, while those coefficients of BC and soot were 73.97% and 68.59%, respectively. BC fraction in the agricultural soils contributed to 1.45%—26.43% of TOC, with a mean value of 6.76%. The highest proportional contribution of BC to TOC was found in lfuvo-aquic soil (8.67%), followed by red soil (7.68%), while the smallest was in paddy soil (5.62%). Char/soot ratios varied from 0.02 to 3.44, averaging 1.18. The average char/soot ratios in the three types of soils ranked as follows: red soil > lfuvo-aquic soil > paddy soil. Strong positive correlation was found between BC, char and soot conentrations. Close correlations between the concentrations of BC and TOC and BC/TOC.Discussion The measured BC concentrations in our study were lower than some background soils in the world and urban soils in China. One possible reason for the variability in BC distribution is due to the source contribution in different regions. Moreover, different BC analytical methods in various studies may lead to these differences. The average proportion of BC to TOC was lower than agricultural soils in other regions, which is likely attribute to the differences in land cultivation methods or application content of farmyard manure (plant ash). The identiifcation of char/soot ratios showed that fossil fuels combustion, such as industrial coal combustion and vehicle exhaust emissions, might be the main source of soil BC in the county. Conclusions The results show that BC, char and soot were inhomogeneously distributed in agricultural soils. The positive close correlation between BC and TOC suggested that BC plays an important role in the accumulation of TOC. Fossil fuel combustion is possibly the main source of soil BC. Road fugitive dust, farmland straw burning, and applications of farmyard manure (plant ash) may be other contributed source to BC.Recommendations and perspectives The data provide scientific basis for different BC fractions in soils and their signiifcant roles in global carbon cycle and climatic effect. BC in agricultural soils were signiifcantly inlfuenced by human activities, especially industrial dust and vehicle exhaust.%以华中地区阳新县农田土壤为研究对象，采用热光反射法研究了不同类型农田土壤（水稻土、红壤、潮土）中黑碳、焦炭、烟炱含量的变化特征，并探讨了影响黑碳、焦炭和烟炱分布的影响因素以及黑碳的可能来源。结果表明，农田土壤中黑碳、焦炭和烟炱含量变幅较大，分别为0.17—3.18 g∙kg−1，0.03—2.37 g∙kg−1和0.09—1.50 g∙kg−1，平均值分别为0.85 g∙kg−1，0.45 g∙kg−1和0.40 g∙kg−1。不同类型土壤中黑碳、焦炭、烟炱含量的大小顺序是：水稻土＞红壤＞潮土。黑碳占有机碳的比例变化范围在1.45%—26.43%，平均值为6.76%。相关分析结果表明土壤黑碳含量与有机碳、焦炭和烟炱含量之间都呈显著的正相关关系。根据焦炭/烟炱比值分析结果推测农田土壤中的黑碳主要来源于化石燃料燃烧，如工业燃煤及机动车尾气排放等。