Abstract:

Long-term planting and incorporation of green manuring Chinese milk vetches may significantly enhance the content of soil organic matter (SOM) and dissolved organic matter (DOM), thus influencing the adsorption behavior and speciation of cadmium (Cd) in soils. Understanding the characteristics of Cd in the soil layers at different depths is important for spreading the planting of Chinese milk vetches. This study was conducted at long-term experimental sites in Yujiang District (YJ), Nanchang County (NC), and Fengcheng City (FC) of Jiangxi Province, where chemical fertilizer and Chinese milk vetch were employed together (F+M), as well as chemical fertilizer alone. Dissolved organic carbon (DOC), SOM, 3D-EEMs, and Cd speciation in soils at different depths were investigated to determine the effects of Chinese milk vetch planting on DOM composition and Cd speciation in different soil layers. The results demonstrated that the SOM content in the plow layer soil (0‒20 cm) under the F+M treatment at the NC and FC experimental sites was significantly higher than that under the F treatment, with increases of 16.80% and 10.47%, respectively. The bioavailability of Cd in the soils of the F+M treatment was generally higher than that in the soils of the F treatment. The DOM of Chinese milk vetch is primarily transported in the plow layer and middle soil layers (20‒40 cm). Compared with those under the F treatment at the YJ, NC, and FC experimental sites, the DOC content in the plow layer soil under the F+M treatment increased by 17.02%, 58.14%, and 33.72%, respectively, and those in the middle layer soil increased by 42.95%, 36.67%, and 11.21%, respectively. The application of Chinese milk vetch can reduce the proportion of protein-like components in DOM, while increasing the proportion of humus-like components. In the plow, middle, and deep layers (40‒60 cm) of soils, the average contributions of protein-like components in DOM under the F treatments of the three experimental sites were 7.34%, 16.72%, and 23.93%, respectively, which were about 1.26, 1.92, and 1.54 times higher than those in the F+M treatment group. Compared with humus-like components, protein-like components showed a higher migration potential to the deep soil layer. Stepwise regression analysis revealed that the proportions of biologically active Cd speciation were negatively correlated with SOM content (r= ‒2.170) and tryptophan-like components (r = ‒0.760), but positively correlated with the contents of fulvic-like components (r=0.239). The findings of this study suggest that the activation risks of Cd in soils may be posed by changes in SOM and DOM, which could be attributed to long-term planting and the incorporation of green manure Chinese milk vetches in the field.

Key words: Astragalus sinicus L., long-term returning, dissolved organic matter, three-dimensional fluorescence spectrum, Cd form

摘要：

长期种植和翻压紫云英能够显著提高土壤中有机质（SOM）和水溶性有机物（DOM）的含量，影响土壤颗粒对重金属镉（Cd）的吸附行为和Cd在土壤中的形态，了解该条件下Cd在不同土层中的特征对紫云英的进一步推广种植具有重要意义。以余江区（YJ）、南昌县（NC）、丰城市（FC）3地长期种植和翻压紫云英处理（F+M）和化肥处理（F）的定位试验点为研究对象，测试了不同层次土壤中SOM、水溶性有机碳（DOC）含量、Cd总量和形态、DOM三维荧光，以期探明紫云英长期还田模式下不同土层中DOM成分的差异及其对Cd含量和形态影响。结果显示，NC和FC试验点F+M处理耕作层土壤（0－20cm）中SOM的含量显著高于F处理，分别提高16.8%和10.5%。同一土层中土壤Cd活性呈现F+M处理>F处理的趋势。紫云英DOM主要在耕作层和中层（20－40 cm）土壤中迁移，YJ、NC、FC试验点中，F+M处理耕作层土壤中DOC含量比F处理分别提高17.0%、58.1%和33.7%，中层土壤中DOC含量分别提高43.0%、36.7%和11.2%。种植和翻压紫云英降低了DOM中类蛋白质组分的占比，增加了类腐殖质组分的占比，耕作层、中层和深层土壤（40－60 cm）中，3个试验点F处理DOM中类蛋白质的平均占比分别为7.34%、16.7%、23.9%，占比分别是F+M处理的1.26、1.92和1.54倍。类蛋白质组分的深层迁移能力大于类腐殖质组分。逐步回归分析表明，有生物活性的Cd形态占比与土壤SOM、类色氨酸含量存在负相关关系，与类富里酸含量存在正相关关系，其系数分别为−2.170、−0.760和0.239。常年种植和翻压紫云英通过影响土壤SOM总量以及DOM组分对土壤Cd表现出一定的活化潜力。

关键词: 紫云英, 长期还田, 水溶性有机物, 三维荧光光谱, Cd形态