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Carbon-nitrogen isotope coupling of soil organic matter in a karst region under land use change, Southwest China
Agriculture, Ecosystems & Environment ( IF 6.6 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.agee.2020.107027
Guilin Han , Yang Tang , Man Liu , Lukas Van Zwieten , Xiaomin Yang , Changxun Yu , Hailong Wang , Zhaoliang Song

Abstract The soil stable carbon (C) and nitrogen (N) isotopes are widely used to indicate C3/C4 vegetation history, N sources and transformation processes, respectively. However, land use change, particularly converting forest into farm land, alters soil organic matter (SOM) sources and processes in soils, resulting in a hard understanding of soil C and N fate. In the present study, soil organic carbon (SOC) and soil organic nitrogen (SON) contents, and their stable isotope compositions (δ13C and δ15N) were determined in the five soil profiles under land use change (i.e., conversion of native forest land into shrub land, grass land, maize field, and paddy land) in Lobo county, Guizhou province, Southwest China. A coupling of 13C and 15N isotope in SOM under land use change was verified whether it could provide more accurate indications of sources and transformation processes. The SOC and SON contents of native forest land at the 0∼20 cm depth were significantly larger than these under other transformed lands. The SOC and SON contents decreased exponentially with increasing soil depth under all land use types, and showed opposite trends with soil pH. The C/N ratios of SOM in the soils under undisturbed native forest decreased from 10 to 7 with increasing soil depth, while an irregular fluctuation along soil profile was shown in other transformed lands. Similarly to the most study in the soils under C3 forest, the δ13C and δ15N values of SOM in the soils under native forest at the 0∼50 cm depth increased with increasing soil depth, with the range of −27.7‰∼−25.7‰ and 6.5‰∼10.0‰, respectively. While decreasing trends of them in the soils below 50 cm depth were attributed to the mixing of 13C and 15N-depleted organic matters from bedrocks. However, the δ13C and δ15N values of SOM along the soil profiles under other transformed lands were intensively irregularly fluctuated between −29.1‰ and −19.0‰, 1.2‰ and 7.9‰, respectively. The single δ13C and δ15N signals in the soil profiles of transformed lands indeed revealed the alterations of historical C3/C4 composition and N transformation processes after land use change, but these indications were not specific. The result of the coupling of 13C and 15N isotope under native forest land reveals a positive relationship between them, which associated with full plant-absorption against 15N-depleted inorganic nitrogen derived from SOM mineralization. This study suggests that the coupling of C N isotope fractionation more likely occurs in the C3 forest ecosystem with high N utilization efficiency. However, the replacement of native forest by farm land or grass land will reduce soil N utilization efficiency.

中文翻译:

西南土地利用变化下喀斯特地区土壤有机质碳氮同位素耦合

摘要 土壤稳定碳(C)和氮(N)同位素被广泛用于指示C3/C4植被历史、氮源和转化过程。然而,土地利用的变化,特别是将森林转化为农田,改变了土壤中土壤有机质 (SOM) 的来源和过程,导致对土壤碳和氮的归宿难以理解。在本研究中,土壤有机碳 (SOC) 和土壤有机氮 (SON) 含量及其稳定同位素组成(δ13C 和 δ15N)在土地利用变化(即原始林地转化为贵州省罗波县灌木地、草地、玉米地和水田)。验证了土地利用变化下 SOM 中 13C 和 15N 同位素的耦合是否可以提供更准确的来源和转化过程指示。0~20 cm深度原生林地SOC和SON含量显着高于其他转化地。在所有土地利用类型下,SOC和SON含量随着土壤深度的增加呈指数下降,并与土壤pH值呈相反趋势。随着土壤深度的增加,未受干扰的原生林下土壤中 SOM 的 C/N 比从 10 下降到 7,而其他转化土地则显示出沿土壤剖面的不规则波动。与C3林下土壤中的大多数研究类似,0~50 cm深度原生林下土壤中SOM的δ13C和δ15N值随着土壤深度的增加而增加,范围为-27。分别为 7‰∼−25.7‰ 和 6.5‰∼10.0‰。而在低于 50 cm 深度的土壤中它们的减少趋势归因于基岩中 13C 和 15N 耗尽的有机物质的混合。而其他转化地下土壤剖面SOM的δ13C和δ15N值分别在-29.1‰和-19.0‰、1.2‰和7.9‰之间剧烈不规则波动。转化土地土壤剖面中的单一δ13C和δ15N信号确实揭示了土地利用变化后历史C3/C4组成和N转化过程的变化,但这些迹象并不具体。天然林地下 13C 和 15N 同位素的耦合结果揭示了它们之间的正相关关系,这与植物完全吸收 SOM 矿化产生的 15N 耗尽无机氮有关。本研究表明,在氮利用效率高的 C3 森林生态系统中,CN 同位素分馏耦合更可能发生。然而,以农田或草地替代原生林会降低土壤氮的利用效率。
更新日期:2020-10-01
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