Comparisons of the effects of different drying methods on soil nitrogen fractions: Insights into emissions of reactive nitrogen gases (HONO and NO)

ABSTRACT

Reactive nitrogen (N_r) emission from soils, e.g., nitrous acid (HONO) and nitric oxide (NO), is a key process of the global nitrogen (N) cycle and has significant implications for atmospheric chemistry. To understand the underlying mechanisms of soil N_r emissions, air-dried or oven-dried soils are commonly used in the laboratory. To date, few studies have compared the effects of different drying methods on soil N_r gas fluxes and N fractions. Here, the authors studied soil water content, pH, (in)organic N content, and N_r gas fluxes of air-dried, freeze-dried, oven-dried, and fresh soils from different land-use types. The results showed that the soil pH of air-dried and oven-dried samples was significantly lower compared with fresh soil from farmland and grassland, but higher compared with forest soil. The difference in soil pH between freeze-dried and fresh soil (mean ± standard deviation: 0.52 ± 0.31) was the lowest. In general, all drying methods increased the soil NH₄⁺-N, NO₃⁻-N, and dissolved organic N contents compared with fresh soil (P < 0.05). The maximum HONO and NO flux and total emissions during a full wetting–drying cycle of fresh soil were also increased by air-drying and oven-drying (P < 0.001), but comparable with freeze-dried soil (P > 0.2). In conclusion, all drying methods should be considered for use in studies on the land–atmosphere interface and biogeochemical N cycling, whereas the freeze-drying method might be better for studies involving the measurement of soil N_r gas fluxes.

Graphical Abstract

摘要

土壤活性氮气体 (HONO和NO) 排放是全球氮循环的关键过程, 对大气化学具有重要意义。本研究采集了农田, 草地和森林三种不同土地利用类型的土壤样品, 分别对样品进行了风干, 冻干和烘干三种处理, 并与未处理的新鲜土壤进行比较研究。结果表明, 三种类型土壤的冻干土与新鲜土样品的pH值差异最小。总的来说, 三种干燥处理都能提高新鲜土壤NH₄⁺-N, NO₃⁻-N和溶解态有机N的含量。与新鲜土壤相比, 冻干土壤HONO和NO的最大排放通量及完整干湿交替循环的总排放量没有显著增加, 而风干和烘干土壤显著增加。因此, 冷冻干燥方法可能更适合涉及土壤活性氮气体排放通量的研究。这一发现的重要启示是, 我们需要慎重评价以往基于不同干燥方法得到的有关陆地–大气界面活性氮交换过程和生物地球化学氮循环机制的认识。

KEYWORDS:

• Freeze-dried
• nitrogen cycling
• NO_x
• soil pH
• gas flux

关键词:

• 冷冻干燥
• 氮循环
• 氮氧化物
• 土壤pH
• 气体通量

Disclosure Statement

No potential conflict of interest was reported by the authors.

Supplementary Material

The supplemental data for this article can be accessed here.

Additional information

Funding

This work was sponsored by the National Natural Science Foundation of China [grant numbers 41807449, 41761144062, and 41730646], the Shanghai Pujiang Program [grant number 18PJ1403500], and the Fundamental Research Funds for the Central Universities.