Discrepant responses of soil organic carbon dynamics to nitrogen addition in different layers: a case study in an agroecosystem

Highlights

● ¹³C isotope analysis was used to estimate the contribution of new and old carbon to SOC.

● The maize plot with high N rate improved SOC fixation than the maize plot with low N rate.

● The maize plot with high N rate transferred organic matter to a deeper soil layer.

● There are remarkable differences in turnover time of SOC under different N rates.

Abstract

Empirical research indicates that heightened soil nitrogen availability can potentially diminish microbial decomposition of soil organic carbon (SOC). Nevertheless, the relationship between SOC turnover response to N addition and soil depth remains unclear. In this study, soils under varying N fertilizer application rates were sampled up to 100 cm deep to examine the contribution of both new and old carbon to SOC across different soil depths, using a coupled carbon and nitrogen isotopic approach. The SOC turnover time for the plot receiving low N addition (250 kg·ha⁻¹·yr⁻¹ N) was about 20−40 years. Conversely, the plot receiving high N (450 kg·ha⁻¹·yr⁻¹ N) had a longer SOC turnover time than the low N plot, reaching about 100 years in the upper 10−20 cm layer. The rise in SOC over the entire profile with low N addition primarily resulted from an increase in the upper soil (0−40 cm) whereas with high N addition, the increase was mainly from greater SOC in the deeper soil (40−100 cm). Throughout the entire soil layer, the proportion of new organic carbon derived from maize C₄ plant sources was higher in plots treated with a low N rate than those treated with a high N rate. This implies that, in contrast to low N addition agricultural practices, high N addition predominantly enhances the soil potential for fixing SOC by transporting organic matter from surface soils to deeper layers characterized by more stable properties. This research offers a unique insight into the dynamics of deep carbon under increased N deposition, thereby aiding in the formulation of policies for soil carbon management.

Graphical abstract

Keywords

¹³C natural abundance / nitrogen addition / soil depth / organic carbon turnover

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Supplementary materials

The online version of this article at https://doi.org/10.15302/J-FASE-2024565 contains supplementary materials (Table S1).

Acknowledgements

We thank Fengyue Qiu (Peking University) and Yufu Jia (China Agricultural University) for help during the fieldwork. This work was financially supported by the National Key Research and Development Program of China (2017YFA0605003).

Compliance with ethics guidelines

Hualing Hu, Liang Zhao, Wenbing Tan, Guoan Wang, and Beidou Xi declare that they have no conflicts of interest or financial conflicts to disclose. This article does not contain any studies with human or animal subjects performed by any of the authors.

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The Author(s) 2024. Published by Higher Education Press. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0)