Crops for increasing soil organic carbon stocks – A global meta analysis
Introduction
In 2015, 192 countries ratified the Paris Agreement at COP21 in Paris to limit the rise of global air temperature to 2°C above pre-industrial levels by the end of this century (Minasny et al., 2017). At that occasion, the 4 per mille Initiative: Soils for food security and climate (4p1000) was launched by the French government with the aspiration to transfer atmospheric carbon (C) to soils for a net increase in soil organic carbon (SOC) stocks (also termed soil carbon sequestration, e.g. Chenu et al., 2019) by 0.4% per year (Minasny et al., 2017). The Initiative envisaged that atmospheric C sequestration into soils would yield spillover benefits such as a reduction in the global temperature, improved soil fertility; favorable soil structure, high soil biodiversity, and reduced risk of soil erosion will accrue (Lal, 2016) to sustain higher crop productivity.
However, some practices proposed by Paustian et al. (2016) to meet the 4p1000 objectives, such as reduced tillage and land use conversion, are subjects of debate. For example, it has been argued that reduced tillage only cause redistribution of SOC within the soil profile with no net gain (Dimassi et al., 2014, Haddaway et al., 2017), while land use conversion from cropland to grassland or forest can have negative implications for SOC stocks and food security. Furthermore, there is a high tendency to rely on best agronomic practices for increasing SOC stocks, while neglecting the effects of plants despite abundant evidence that genetic variation, especially in root traits, influences C allocation in terrestrial ecosystems (Warembourg et al., 2003).
Carbon sequestration into soils is influenced by equilibrium C carrying capacity of the soil, the vegetation it supports and the attendant climatic factors (Gupta and Rao, 1994, Lal, 2004). Plants play a pivotal role in the global C cycle because more than 10% of the carbon dioxide (CO2) in the atmosphere is recycled through photosynthesis (Raich and Potter, 1995). After photosynthesis, plants deposit organic C into the soil via exudation and biomass C incorporation into the soil and a net increase in the soil C stocks is realized when the amount of C deposits are higher than C losses through microbial decomposition and root respiration (Fearnside and Barbosa, 1998). Plant C is either incorporated as structural C, released as exudates or respired as CO2 (Ostle et al., 2003). These processes vary with crops’ genetic characteristics, leading to potential differences in C fluxes to soils (Kuzyakov and Domanski 2000). Yet there is little empirical analysis on how crops differ in their ability to enhance SOC stocks (Wegener et al., 2015). The first step for assessing the ability of crops to sequester C in terrestrial ecosystems lies in the evaluation of the C fluxes to the different pools: shoots, roots and soil.
There have been concerted efforts to understand C fluxes in the plant/soil system (e.g. Remus and Augustin, 2016, Studer et al., 2014). Carbon fluxes are commonly estimated by measuring changes in SOC stocks over time (e.g. Hemminga et al., 1996). However, this method lacks precision when little changes in SOC stocks occur and is unable to distinguish between the preexistent and “new” SOC (e.g. Remus and Augustin, 2016). Isotopic C labeling, where C 12 atoms are replaced with either C 13 or 14 atoms, facilitate the quantification of C fluxes from atmosphere to soil through plants (Studer et al., 2014). Based on C labelling, it has been reported that plants transfer from 30 to 50% of their photosynthetic C below ground (Buyanovsky and Wagner, 1997). Furthermore, C labelling has proven that the translocation of C to roots may occur immediately after C assimilation in leaves with up to 10% of the photosynthetic C being detected in roots within 2 h (Kaštovská and Šantrůčková, 2007).
Carbon assimilated by plants is allocated to above ground (shoots and reproductive organs) and below ground (roots) biomass depending on plant genetic constitution (de Neergaard and Gorissen 2004), plant growth pattern, environmental conditions and interactions among these factors (Rangel-Castro et al., 2005). However, it is the root C fluxes, which are particularly important for increasing SOC stocks because root tissues and/or root exudates are in immediate contact with the decomposer community and the mineral phase, thus leading to lower mineralization rates as compared to shoot-derived C (Rasse et al., 2005). Labile C inputs such as those derived from above ground biomass are potentially easily decomposed, leading to losses of the total plant C stock of 15–45% in case of perennial plants and 27–60% in annuals (e.g. Warembourg and Paul, 1977).
Several compilations of individual studies have been performed to identify the trends in SOC dynamics (e.g., Guo and Gifford, 2002). From Guo and Gifford (2002), we learn that the conversion of natural vegetation to cropland is detrimental to SOC stocks with a worldwide average loss in the 0–0.3 m soil layer of 42% from native forest and of 59% from grassland. From this meta-study we also learn that afforestation of cropland can increase SOC stocks by as much as 50%, with Laganière et al. (2010) indicating greatest increase by using broadleaf tree species as compared to coniferous species. Meta-studies on land management impact on SOC stocks have also been performed. Poeplau and Don (2015), using 139 plots worldwide, showed the potential of cover crops to increase SOC stocks in the first 0.2 m of the soil to be 0.32 ± 0.08 Mg C ha−1 yr−1 or about 0.5% or 5p1000. The study by Ugarte et al. (2014), using data from 55 peer-reviewed studies, showed a 9% average increase of SOC stocks in the 0–0.3 m layer using diversified crop rotations and conservation systems as compared to the conventional controls, a result confirmed by Alison et al. (2017). The increase was 32% by using a combination of chemical and organic fertilization (Han et al., 2016) as compared to no fertilization. Shi et al. (2018) pointed to an 18% increase in SOC stocks using agroforestry under subtropical climate. All these studies considered the soil to a depth of 0.3 m.
While several individual studies worldwide have been published on the fluxes of atmospheric C to soils on one or several crops, these data have not been synthesized yet and the global trends are still unknown. This study aimed at compiling multiple trials across the world that quantified C fluxes from the atmosphere to the soil during the growing cycle using C labelling. The hypothesis behind this study is that some crops might be more efficient than others to build SOC and might thus be promoted.
Section snippets
Meta analysis
The study is a meta-analysis based on data collated from isotopic C enrichment experiments conducted across the world. The data was obtained from peer-reviewed journal articles published between 1985 and 2016. The journal articles were obtained from academic databases (Google Scholar, Refseek, Science Direct, SciFinder, Scopus, Springer Link and Web of Science) using key words and phrases, singly and/or in combination, such as “isotopic pulse labelling”, “C allocation”, “plant carbon
Variability in C allocation to plant parts
Table 4 presents the statistics of biomass and C allocation to the different plant parts from the field and greenhouse trials. As shown by the statistical t test (Table 4), there was no significant differences between field and greenhouse trials for all study variables, except for root biomass (Rb), which was about two times higher under field conditions than in greenhouses (36 vs 16 Mg C ha−1).
The average C allocation to shoots ranged from 0.20 Mg C ha−1 yr−1 recorded by for barley in the
Soil texture
Fig. 5 shows the variations in C allocation in response to soil clay content. For clayey soils, the mean C allocation to shoots was 2.36 ± 0.23 Mg C ha−1 yr−1 (Fig. 5a), which was 13% lower than that of loam textured soils (2.70 ± 0.25 Mg C ha−1 yr−1) with the widest variability (values between 0.5 and 4.6 Mg C ha−1 yr−1). There was a decrease in root C allocation from clayey and loamy (1.30 ± 0.13; 1.36 ± 0.16 Mg C ha−1 yr−1) to sandy soils (0.91 Mg C ha−1 yr−1) (Fig. 5b). In contrast, soil
The impact of crop species on C assimilation from atmosphere to shoots and roots
The wide variations in atmospheric C allocation to shoots from 0.4 to 5.6 Mg C ha−1 yr−1 was expected and is likely to result from variable environmental conditions, plant growth patterns and internal metabolism which are under genetic control (Warembourg et al., 2003). Maize exhibited a significantly higher assimilation of atmospheric C into shoots than ryegrass, which confirmed previous reports by Pausch and Kuzyakov, 2017, Atkinson et al., 2014. The differences in shoot C fluxes among annual
Conclusions
Three main conclusions can be drawn from this study of 227 research trials worldwide investigating atmospheric C allocation to crop shoots, roots and to the soil:
- (i)
Only a small fraction (about 7%) of the plant assimilated C is found in the soil after harvest with wheat showing the highest proportion (23%), followed by rice (20%), and maize and ryegrass with 19%;
- (ii)
The transfer of atmospheric C to the soil was highest for maize (1.00 Mg C ha−1 yr−1) followed by ryegrass (0.95 Mg C ha−1 yr−1), rice
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.
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