Effects of recovery models on organic carbon pathways: A method using ¹³C natural abundance

Highlights

• The pathways of organic carbon turnover had been clearly determined by ¹³C natural abundance.

• Natural grasslands have the highest soil organic carbon content under the same years of restoration.

• δ¹³C was more enriched in micro-aggregates than in large and small macro-aggregates.

• Artificial restoration altered the carbon flow pathway of transformation and reduced the sequestration of organic carbon.

Abstract

The rehabilitation of disaster-prone areas can enhance the stability of soil structure and is a common way to increase organic carbon storage. The response of soil carbon sequestration pathways to different recovery modes is not clear, especially in mountain soils. After 11 years of recovery, we evaluated soil organic carbon (SOC) sequestration pathways in plantations (dominated by Olea europaea ‘Leccino’), croplands [Zea mays (L.)] natural shrublands (Lycium chinense Mill), and natural grasslands [Setaria viridis (L.) Beauv]. The physical and chemical properties of the soil and the ¹³C natural abundance of each aggregate and its density components were studied. The results showed that, during the restoration process, the soil organic carbon content of natural grassland increased the most, while the δ¹³C value of soil of natural shrubs was the highest. The natural abundance of ¹³C was used to reveal the pathway of C flow in soil organic matter (SOM), as follows: free light fractions (ρ < 1.6 g cm⁻³)→mineral fractions (ρ > 2.0 g cm⁻³)→dense occluded fractions (ρ from 1.6 to 2.0 g cm⁻³) (in plantation, natural shrubland, and grassland). However, in cropland soil, C flowed as follows: mineral fractions→free light fractions→dense occluded fractions. Specifically, the SOC content decreased with aggregate particle size, and after entering the soil, plant litter was first stored in large aggregates and then decomposed into the free light fraction. The study revealed the mechanism of organic carbon sequestration in the restoration area, emphasizing that artificial restoration treatment can change the carbon conversion pathway, and reduced the sequestration of organic carbon.

Introduction

Soil is the largest pool of organic carbon in terrestrial ecosystems, which can store 2500 Pg of carbon, and slight fluctuations in soil organic carbon can significantly affect the atmospheric CO₂ concentration (Lal, 2003, Lehmann and Kleber, 2015, Liu et al., 2020). Plants generally fix organic carbon in the soil and reduce the concentration of CO₂ released into the atmosphere (Mazzoncini et al., 2011, Cotrufo et al., 2015, Ji et al., 2020). The effect of plants on soil carbon stability varies with vegetation type and litter input (Mazzoncini et al., 2011, Okolo et al., 2020). Vegetation restoration is widely regarded as a useful tool to improve soil structure and SOC. It can provide biophysical stimulation to increase soil density and slow down the mineralization of organic carbon. (Shen et al., 2017, Wang et al., 2019b).

As the fundamental unit of soil structure, soil aggregates provide good physical protection for soil organic carbon (Chaplot and Cooper, 2015, Jiang et al., 2018). Approximately 90% of the organic carbon in the topsoil of terrestrial ecosystems is fixed in soil aggregates (Six and Paustian, 2014, Li et al., 2020). In general, there are many factors that affect the stability of soil aggregates, including intrinsic factors and external factors (Amézketa, 1999). Intrinsic factors are related to primary soil properties such as soil minerals, organic matter content, and soil clay content, whereas external factors include climatic variables, vegetation type and cover, and agricultural practices such as tillage, irrigation, and fertilizer applications (Roscoe, 2003, Annabi et al., 2011, Zhao et al., 2017, Zeng et al., 2018). Under different vegetation types, organic carbon storage in the surface soil (0–20 cm) varies widely (Song et al., 2014, Jiang et al., 2019). Different sizes of soil particles vary in their ability to protect soil organic carbon (Lagomarsino et al., 2012, Gunina and Kuzyakov, 2014, Ferreira et al., 2020). Studies have shown that the SOC concentration increases with increasing aggregate size, and the SOC in microaggregates (< 0.25 mm) is older and more stable than that in macroaggregates (> 0.25 mm) (John et al., 2005, Jiang et al., 2019). Proposed by Liu et al. (2018), organic carbon sequestration of fresh organic matter generally begins in large macroaggregates, and, after decomposition and microbial consumption, decomposed organic matter is sequestered in microaggregates. However, the mechanism of this shift in carbon sequestration is still unclear.

In recent decades, stable carbon isotopes have been widely used to quantitatively assess the generation, stability, and turnover of SOM (Sharma et al., 2005, Poeplau et al., 2018). Because plant litter is the primary source of SOM, soil δ¹³C can reflect the comprehensive effect of litter fractionation (Deng et al., 2016). According to the material balance of carbon isotope content, the relative contribution of new organic carbon and old organic carbon can be estimated, thereby evaluating SOC turnover (Zhang et al., 2015; Dou et al., 2017; Jiang et al., 2019). Stable carbon isotopes can also link aggregate dynamics with organic matter, revealing long-term changes in SOC composition (Werth and Kuzyakov, 2010, Wang et al., 2020). Based on δ¹³C analysis in a 45-year-old forest and cultivated soil, Gunina and Kuzyakov (2014) proposed the following order of SOM fraction formation: free particulate organic matter (POM)→light occluded POM→heavy occluded POM→mineral fraction. Free particulate organic matter has a density of < 1.6 g cm⁻³ (free POM), light occluded particulate organic matter < 1.6 g cm⁻³ (occluded POM_<1.6), dense occluded particulate organic matter 1.6–2.0 g cm⁻³ (occluded POM_1.6–2.0), and mineral fraction soil organic matter > 2 g cm⁻³ (mineral fraction) (John et al., 2005, Gunina and Kuzyakov, 2014). Additionally, the stability of the SOM fractions was analyzed using C:N ratios and NMR spectroscopy. This approach has suggested the following order of SOM fraction formation: free light fraction (f-LF)→ low-density material fraction (m-LF) → high-density fraction (HF) (Wagai et al., 2008). Atere et al. (2020) found that carbon flows from minerals to free light fractions in rice fields treated with different fertilization treatments, contrary to what has been found in dryland soils. Werth and Kuzyakov (2010) considered that compared with soils in temperate/northern climates, in arid/semiarid climates, ¹³C fractionation pathways differ in soils where different carbon stabilization mechanisms are dominant.

Differences in environmental and soil conditions may drive different pathways of organic carbon sequestration (Jiang et al., 2020, Mary et al., 2020). Numerous studies have evaluated moist soil in the south and arid soil in the north (Gunina and Kuzyakov, 2014, Atere et al., 2020), but there are few studies on ecologically fragile areas in semiarid regions. Thus, exploring the response of mountain soil organic carbon in semiarid areas to recovery patterns will contribute substantially to understanding pathways of soil C flow. In this study, we used natural ¹³C abundance in soil to evaluate the impact of recovery on soil organic carbon flow channels and sequestration in different treatments and habitats. We hypothesized that 1) the distribution of SOM fractions would be related to the physical structure of soil and 2) that soil organic carbon flow channels would be affected by different recovery patterns.