Earth Science Frontiers ›› 2025, Vol. 32 ›› Issue (5): 524-533.DOI: 10.13745/j.esf.sf.2024.11.76

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The Impact and Mechanism of land reclamation and retirement on the coupling relationship between soil carbon and nitrogen in Karst areas

WU Zeyan1,2,3(), LI Qiang1,2,3, ZHANG Cheng1,2,3,*(), JIANG Zhongcheng1,2,3, LUO Weiqun1,2,3, HU Zhaoxin1,2,3, TU Chun1,2,3   

  1. 1. Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China
    2. Pingguo Guangxi, Karst Ecosystem, National Observation and Research Station/Pingguo Baise, Karst Ecosystem, Guangxi Observation and Research Station, Pingguo 531406, China
    3. Key Laboratory of Karst Dynamics Ministry of Natural Resources, Guangxi/International Research Center on Karst under the Auspices of United Nations Educational, Scientific and Cultural Organization, Guilin 541004, China
  • Received:2024-07-02 Revised:2024-12-02 Online:2025-09-25 Published:2025-10-14
  • Contact: ZHANG Cheng

Abstract:

During the process of ecological restoration in karst areas, most of the retired croplands were reclaimed in the past and subjected to deep tillage. But this has received relatively little attention, and there is a lack of understanding of whether it causes changes in the soil carbon-nitrogen coupling relationship. In this paper, typical reclaimed croplands were selected within the demonstration area for ecological restoration of rocky desertification, with traditional croplands as a comparison. A total of 39 soil samples were collected from the surface to the bedrock, and the soil organic carbon content (SOC), total nitrogen content (TN), the natural abundance of soil stable isotope 13C (δ13CSOC), and the carbon-to-nitrogen ratio (C/N) were analyzed. Correlation analysis and principal component analysis were used to study the soil carbon-nitrogen coupling relationship and its mechanism. The study shows that the soil carbon and nitrogen of traditional croplands exhibit coupled changes, mainly regulated by the input of vegetation-derived carbon and the degree of microbial decomposition of carbon. As soil depth increases, the input of carbon and nitrogen from vegetation sources gradually decreases, and microbial carbon decomposition activities are limited by the insufficient supply of carbon and nitrogen (known as “carbon-nitrogen limitation”). This is consistent with general patterns. In contrast, the mean contents of SOC and TN in the reclaimed slope croplands were 2.23%±0.33% and 0.25%±0.03%, respectively, which were 74.2% and 66.7% higher than those in the traditional croplands. There was no correlation between SOC and TN contents, and a positive correlation between δ13CSOC and SOC, showing a clear “decoupling” characteristic of soil carbon and nitrogen. The main reason is that land reclamation led to the replacement of surface and deep soils. Deep, originally nitrogen-poor soils receive additional atmospheric nitrogen deposition and nitrogen from rocks in the surface layer, while the originally nitrogen-rich surface soils replenish the nitrogen in the deeper soils. This lifted the “nitrogen limitation”. And the decomposition rate of the originally carbon-rich surface soil decreases after being buried at a depth of 45 to 65cm, lifting the “carbon limitation”. These findings of this study enrich the theory of soil carbon-nitrogen coupling in karst areas and provide an important reference for the evaluation of carbon and nitrogen distribution and storage under ecological restoration in karst rocky mountainous areas.

Key words: Karst, land reclamation and conversion, soil organic carbon, total nitrogen, 13C stable isotope, C/N

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