Can deep tillage enhance carbon sequestration in soils? A meta-analysis towards GHG mitigation and sustainable agricultural management

https://doi.org/10.1016/j.rser.2020.110293Get rights and content

Highlights

  • Soil carbon sequestration is an opportunity to offset global CO2 emissions.

  • Meta-analysis was conducted to evaluate the effects of deep tillage on SOC.

  • Deep tillage has the potential to increase SOC compared to control treatment.

  • The effects of deep tillage change with environmental and agronomic conditions.

  • Strategies for effective deep tillage management were discussed.

Abstract

Sequestration of soil organic carbon (SOC) is regarded as a promising approach to offset global CO2 emissions. Deep tillage (DT) can alleviate high soil strength, influencing both the surface and subsoil carbon pools either directly or indirectly. However, field studies on the benefits of DT to SOC remain inconclusive, and comprehensive quantitative assessment has been lacking. This study used meta-analysis to assess the response of SOC storage to DT based on global data of 430 comparisons from 43 studies. In general, DT was found to significantly enhanced SOC by 7.79%. Specifically, subsoiling significantly increased SOC, augmenting it by 8.87%. Deep ploughing did not facilitate SOC sequestration to a significant extent for the whole soil profile, although it did significantly increase SOC in 20–50 cm layer. The individual response of SOC to DT was found to be highly site-specific. DT was found to bring greater benefits in soil under arid zones, which typically featured fine or medium textured soil, and relatively high background SOC (> 6 g kg−1) and BD content (> 1.3 g cm−3). Furthermore, agronomic practices played an essential role in constraining SOC responses to DT, where better SOC responses were observed under rotational cropping, DT/NT (no-tillage) rotational tillage, advisable DT depth difference, and moderate nitrogen application rate (200–300 kg ha−1 y−1) with prolonged experiment duration. In this regard, it is important to include site-specific environmental and agronomic conditions when identifying appropriate DT practices for enhancing SOC sequestration. The investigation into SOC storage capacity and DT technologies can provide scientific policy guidance for long-term global carbon management.

Introduction

The atmospheric concentration of CO2 is closely coupled with global warming and is increasing at an alarming rate of 4 Pg C per year [1,2]. The overall global soil carbon stock is approximately 2500 Pg, which is about four times the biotic carbon (560 Pg) and about three times the atmospheric one (760 Pg) [3]. Therefore, managing soil carbon proposes an opportunity to offset global CO2 emission [4].

The soil organic carbon (SOC) is estimated to be 615 Pg in the surface layer (measuring to a depth of 0.2 m) and 2344 Pg to a depth of 3 m. Subsoil stores more than 50% of the world's SOC and most of it presents low SOC content [5,6]. SOC turnover in subsoil is limited on account of unfavorable conditions for decomposers, restricted input of fresh organic matter, and low accessibility of decomposers to available SOC [7]. The incorporation of substances rich in SOC into subsoil with a high proportion of undersaturated mineral surface area may help sequester SOC by facilitating the stabilization of buried SOC and limiting SOC decomposition. Nevertheless, subsoil is still poorly understood and neglected constituent of terrestrial carbon pool. For instance, among the 360 studies surveyed focusing on the responses of soil organic matter (SOM) to land management, 90% of them investigated soil depth ≤ 30 cm [8]. The insufficient understanding of deep soil processes may, in turn, lead to misinterpretation of ecological evolution and the underlying processes [9].

SOC is governed by a collection of environmental and management factors and the regulating processes are highly complex. For instance, climatic factors, especially precipitation and temperature, are the most important determinants of SOC contents, due to their impacts on the quantity and quality of residue inputs and the rates of SOM mineralization and litter decomposition [10]. In the context of climate change, for instance, both field measurements and models with respect to climate change predict an incremental trend of drought intensity and frequency [11,12]. Plant development is being confronted with new challenges such as forthcoming supply shortages of water and nutrients. Therefore, it is necessary to provide new access to nutrient and water resources for crops and other plants. The subsoil below the topsoil, which is usually tilled, stores a large amount of nutrients and retains water even under water-restricted circumstance. In fact, it is capable of storing nearly 50% of total nitrogen stocks and 25–70% of total phosphorus stocks [13]. Nevertheless, various factors affect subsoil nutrient availability to plants.

Soil compaction restricts plant root development, impeding the acquisition of resources available in the subsoil, and thereby posing threats to plant productivity and ecological function of the soil [14]. Soil compaction may be caused by (1) large agricultural vehicles used for agricultural production, (2) usage of tillage equipment for many years at the same depth, (3) naturally occurring layers due to soil properties that tend to bind soil particles and eliminate porosity [15], and (4) long-term no tillage (NT) practices [16]. NT farming is widely used in many countries and is one of the most frequently studied agronomic practices for reinforcing carbon sequestration in croplands [[17], [18], [19]]. However, such options have proven to be limited in terms of their ability to mitigate climate change [20,21]. Accumulated SOC as a result of NT has been substantiated to be constrained to easily decomposable component and primarily to the topsoil, which is largely outweighed by subsoil carbon loss [22] and is easily lost by conventional cultivation. Furthermore, long-term NT farming introduces some challenges over years of use, such as herbicide resistance of weeds, proliferation of pests, excessive accumulation of nutrients in the surface soil layer, and soil compaction due to lack of soil mobilization [16]. In this respect, occasional tillage may assist addressing these challenges.

Tillage management, such as cropping intensity, or frequency, also affects SOC storage by disturbing soil and altering the amount of time that the soil is sustaining a crop [23]. DT is a tillage operation performed below the normal tillage depth to break up the plough pan, ameliorate the physical or chemical properties of a soil, alleviate high soil strength, promote deep root development, and facilitate the availability of subsoil resources (Fig. 1). Integrating subsoil in management decisions may provide a means by which for plants to mitigate the adverse effects of soil compaction and climate change, thus sequestrating more carbon. DT has been developed into various types, such as deep ploughing, subsoiling, and deep ripping [24]. The purpose of subsoiling is to loosen soil structure, thereby decreasing subsoil strength without disturbing soil horizons. Subsoiling may also refer to deep ripping or deep chiseling [24]. Deep ploughing, in contrast, disturbs soil horizons, inverses soil profile, and displaces the topsoil, burying it in the deep soil profile [25]. DT has been applied in countries such as Australia [26], China [27,28], Canada [29], Netherlands [30], United States [31], Italy [32], India [33], and Germany [25].

In recent years, several researchers have reported that long-term DT acts as a terrestrial carbon sink in the context of rising concerns over atmospheric CO2 emission. For instance, Schiedung et al. [34] reported that SOC stocks in the depth of 0–150 cm increased by 69% over 20 years after deep flipping in the pasture in New Zealand. Alcantara et al. [35] demonstrated that deep-ploughed soil comprised mean 42% more SOC than the reference soil after 45 years. However, some studies have revealed that tillage may accelerate the fragmentation of soil aggregates which is a major protection for SOC, thus increasing the possibility of organic matter mineralization and acting as a carbon source for atmospheric carbon pool [36]. Meanwhile, the extent to which DT as a regular tillage practice contributes to SOC sequestration under different environmental and management conditions is not well understood and merits further exploration. In this context, the objectives of this project are to (1) obtain a quantitative review of published DT impacts on SOC stock; and (2) identify the role of environmental variables and management practices in constraining SOC stock response to DT.

Section snippets

Data sources

A detailed review of the literature published in peer-reviewed journals for all years up to December 2019 was conducted. Publications were searched from Web of Science using the search terms shown in Table 1.

The following criteria were fulfilled: (1) article compared the SOC content or stocks of ordinary-tilled plot (control treatment, CT) with adjacent deep tillage plot (DT) based on field study; (2) DT and CT received similar agronomic treatments apart from tillage depth, and only studies

General dataset information

After the screening process, a total of 43 publications (from 1987 to 2019) covering 43 field experimental sites and representing 430 observations were selected (Table S1). The distribution of field experimental sites is shown in Fig. 2. Overall, the effect size of SOC under DT was found to exhibit a normal distribution (Fig. 3). The fail-safe number for publication bias analysis is 45,508, indicating that most of the results considered in this study were robust (Table S2).

The database was

The effects of DT on SOC

Soil carbon are mainly from: (1) accumulation of SOC due to the humification of plant residues and (2) root-borne substances and root exudates released into the rhizosphere during plant growth and root sloughing [52]. DT can be an effective tool for treating soil with physical barriers in order to loosen soil structure, alleviate high soil strength, and promote deep root development. In soils, the transformation of organic residues into recalcitrant substances is crucial to carbon

Conclusions

The effects of DT practice on SOC were assessed using meta-analysis in this study. In general, DT significantly enhanced SOC compared to CT. Specifically, deep ploughing was found to benefit subsoil carbon sequestration (20–50 cm), although it may depress surface SOC content. The SOC change rate at the subsoil (30–50 cm) was significantly negative relative to the SOC of topsoil (0–10 cm) under deep ploughing. It was found that, subsoiling, in contrast, did not carry this risk. The enhancement

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the Natural Sciences and Engineering Research Council of Canada. The authors are thankful for the comments and suggestions from Jonathan Tomalty, the editor and the anonymous reviewers.

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