石灰和双氰胺对红壤酸化和硝化作用的影响及其机制

施用石灰是改良酸性土壤的重要措施,但其对土壤硝化作用的增强不仅加速土壤酸化,也增加硝态氮流失风险。传统的硝化抑制剂双氰胺(Dicyandiamide,DCD)能否在石灰改变pH的条件下始终有效抑制硝化是当前红壤区生产中亟需解决的问题。采用短期土壤培养试验,探讨了不同用量石灰与DCD配合施用对土壤酸化和硝化作用的影响及其机制。结果表明:施用一定量的石灰(≤4 g·kg^–1)显著提高土壤pH,通过促进氨氧化细菌的生长以促进硝化作用。在不同pH条件下,DCD对红壤硝化过程均有显著抑制效果。在较高pH(pH 7.0~7.8)条件下,DCD主要通过降低氨氧化细菌的丰度以抑制硝化,而在低pH(pH<6.0)条件下,DCD对氨氧化古菌和氨氧化细菌的丰度均有抑制作用。此外,DCD通过抑制土壤硝化,显著提高了土壤pH。上述结果表明,适宜量(2~4 g·kg^–1)的石灰和DCD结合施用不仅能够减缓红壤酸化,而且能够抑制硝化作用,降低硝态氮的潜在环境风险。

Effects of Liming and Dicyandiamide(DCD)Application on Soil pH and Nitrification of Acidic Red Soil

Objective] Soil acidification is one of the most important limitations on agricultural production in the red soil regions of South China, where the production of staple food crops is negatively affected by acid soil because its high contents of aluminum, manganese and acid hinder efficient use of soil nutrients such as nitrogen. Therefore, there is an urgent need for dual improvement of soil pH and N use efficiency in the red soil regions. As a conventional method for ameliorating acidic soils, liming increases soil pH, but inevitably enhances soil nitrification. The enhanced nitrification not only accelerates soil acidification, but also increases the risk of nitrate loss through leaching and runoff in the red soil regions with abundant rainfall. To prevent nitrification, dicyandiamide (DCD) is a commonly used nitrification inhibitor, which will reduce nitrate loss and soil acidification. The use of DCD coupled with lime may be an effective approach to improving red soil productivity. Previous studies only focused on the effect of liming or DCD separately on soil nitrification, and the joint effects of liming and DCD on nitrification of acid red soil are rarely reported. Thus, the objectives of this study are to investigate effects of combined application of lime and DCD on soil pH and nitrification in acid red soil and their underlying mechanisms.Method] Soils collected from the Yingtan Red Soil Ecological Experimental Station in Jiangxi Province were cultivated for 35 days, and 200 mg N·kg^-1 urea was applied as substrate for nitrification. Two factors were involved in the study, i.e. liming rate (0, 2, 4 and, 6 g·kg^-1 soil) and DCD application rate (0 and 20 mg·kg^-1). The soil in each treatment was sampled every 7 days for analyzing soil pH, ammonium and nitrate contents to evaluate their variations. At the end of the culture, soil DNA was extracted and amoA gene copies of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) were determined.Result] It was found that liming at <4 g·kg^-1increased soil pH and enhanced nitrification, whereas liming at >4 g·kg^-1 increased soil pH a bit and did almost nothing on nitrification compared with 4 g·kg^-1. A parabolic relationship curve was observed between soil pH and apparent nitrification rate. Apparent nitrification rate peaked on D7, D14, and D21 in soil 6.9 or so in pH. Soil nitrification was mainly affected by AOA and AOB, either independently or jointly. Path analysis shows that abundance of AOA had certain direct, but negative effect on apparent soil nitrification rate, and also some indirect effect reducing the rate by negatively affecting the abundance of AOB. On the contrary, abundance of AOB had certain positive effect on apparent nitrification rate, indicating that the promoted nitrification was attributed mainly to AOB as a result of liming. Application of DCD displayed a significant inhibitory effect on nitrification in red soil, regardless of soil pH. In soils high in pH (pH 7.0~7.8), DCD inhibited nitrification by reducing the abundance of AOB, while in soils lower in pH (pH<6.0), DCD inhibited the growth of both AOA and AOB, thus reducing nitrification. Furthermore, soil pH was increased significantly due to the reduced nitrification when DCD was applied.Conclusion] Combined application of an appropriate amount of lime (2~4 g·kg^-1) and DCD can not only raise pH of the acidic red soils and alleviate acidification, but also reduce nitrification, thus mitigating the potential threat of nitrate loss to the environment. All the findings in this study may serve as theoretical support to and new ideas for amelioration of acid soils. However, as the present experiment was carried out under well-controlled condition, the effects of combined application of lime and DCD on soil pH and nitrification in fields in the red soil regions need to be further studied for validation.