Carbon stocks in riparian buffer systems at sites differing in soil texture, vegetation type and age compared to adjacent agricultural fields in southern Ontario, Canada

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Highlights

Abstract

In any given watershed, riparian buffer systems (RBSs) can influence terrestrial carbon (C) sequestration and potentially enhance terrestrial C sinks. This study looked at the effects of soil texture, vegetation type and age on C stock sequestration in eight RBSs located within the Grand River Watershed (GRW), southern Ontario, Canada and compared the C stock with adjacent agricultural fields. Soil organic carbon (SOC) stocks on an equivalent mass basis were significantly higher (p<0.05) in mature buffers (192.7 Mg C ha-1) compared to adjacent agricultural fields (corn [Zea mays] –soybean [Glycine max] –wheat [Triticum aestivum] rotation), at 0-30 cm soil depth (88.2 Mg C ha-1), irrespective of soil texture and vegetation type of RBS. Sixty-year old deciduous tree buffers on clayey soils had the highest SOC concentration (5.94 %, p<0.05) at 0-30 cm soil depth while the young six-year old coniferous tree buffers on loamy soil had the lowest SOC concentration (2.87%). In mature deciduous [major species; sugar maple (Acer saccharum) and ash (Fraxinus excelsior)] buffers, biomass C stocks were 247 Mg C ha-1, which is up to two and half times greater than mature coniferous [major species; eastern white cedar (Thuja occidentalis) and white pine (Pinus strobus)] buffers (100 Mg C ha-1). Overall results support that increasing RBS within Ontario watershed areas can enhance C stocks and may contribute to Canada’s climate change mitigation efforts. In the context of riparian areas, deciduous species should be promoted over coniferous species to obtain higher terrestrial C sequestration within Ontario watersheds.

Introduction

There is worldwide consent related to increasing greenhouse gases (GHGs) in the atmosphere and their influence on increasing temperature (EPA, 2017). Global warming is partially caused by increasing atmospheric concentrations of GHGs. The key GHGs are carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), ozone (O3) and water vapor. The concentration of CO2 in the atmosphere is increasing by anthropogenic activities such as burning of fossil fuel and deforestation (EPA, 2017). Historically, conversion of land-use from forest and grasslands to intensive agricultural cropping systems has also contributed to the increase in atmospheric CO2 (Lal et al., 1999; Griscom et al., 2017). Atmospheric CO2 concentration has reached 413 ppm in January, 2020 [https://www.esrl.noaa.gov/gmd/ccgg/trends/ - accessed on February 13, 2020].

Carbon (C) sequestration is considered as one of the main cost effective tools (one of Natural Climate Solutions) to mitigate climate change via reducing GHG concentrations in the atmosphere (Amundson and Biardeau, 2018). In this context, riparian buffer system (RBS) (Thevathasan et al., 2004), where strips of perennial plants, shrubs and / or trees are planted along waterways mainly to control non-point source of pollutions reaching the water source (Palone and Todd, 1997), is a type of agroforestry land-use that can be adopted to enhance terrestrial C sinks. Agroforestry is recognized as an integrated land-use system promoting both productivity and environmental integrity (Nair, 2007). This system has been widely recognized by global organizations, such as the Intergovernmental Panel on Climate Change (IPCC) and United Nation’s Food and Agriculture Organization (FAO), as a land-use contributing to climate change mitigation and resilience (Jose and Bardhan, 2012; Thevathasan et al., 2018). Further, agroforestry systems are also now considered in the 2019 Refinement to the 2006 Guidelines for National Greenhouse Gas Inventories (https://www.ipcc-nggip.iges.or.jp/public/2019rf/index.html, accessed on April 17, 2020). RBS is also considered a best management practice to enhance water quality (Jose 2009; Vogt et al. 2015). However, RBSs also have the potential to sequester atmospheric CO2 in the above ground biomass, and belowground in roots and in soil, referred in this paper as total C stock.

In relation to C stock accumulation, RBSs and their contributions to enhance water quality have been studied in detail (Jose 2009; Zehetner et al., 2009; Vogt et al. 2015). However, the effect of trees, their age class and type and soil texture on C stock accumulation (above ground and below ground) by RBS is not well understood. There is therefore a research gap related to the quantification of C stock gain potentials in RBS by assessing both above and below ground biomass C as well as soil organic C (SOC) stock as influenced by soil texture, vegetation type and vegetation age. This study therefore addressed this research gap by quantitatively assessing C stock gain potential of RBSs established in different soil textures, and consisting of different vegetation types and ages. Eight RBS sites were selected within the Grand River Watershed (GRW) in southern Ontario, Canada to quantify C stocks. The Grand River Conservation Authority (GRCA) has digitized 11,000 km of linear length of degraded agricultural streams within the 7,000 km2 of their watershed. In relation to the above, this study seeks to accomplish the following objectives: (1) quantify C stock in RBS differing in soil texture (clay, loam), vegetation type (coniferous, deciduous) and age class [young (<15 years), mature (≥15 years)], and (2) compare land-use influence on SOC stock within RBS and in adjacent agricultural (AAg) fields. It is therefore hypothesized that RBS will increase terrestrial C stocks beyond that of adjacent agricultural field.

Section snippets

Study area

The study was conducted on selected RBS within the GRW, southern Ontario, Canada. A network of riparian buffer treatments comprised of a factorial array of 2 tree types (coniferous vs. deciduous) x 2 tree age classes [<15 years (young) vs. ≥ 15 years (mature)] x 2 soil texture classes (clay vs. loam) were identified. In 2017, eight RBS sites were chosen for this study and site descriptions for each site are given in Table 1.

Soil samples and vegetation data were collected from 5th of July to 8th

SOC stock comparison between riparian buffer systems and adjacent agricultural field

Comparison of SOC stocks (Mg C ha-1) in different land-uses (RBS vs. AAg fields) are given in Fig. 2.

Significant differences (p < 0.05) were observed in the mean SOC stocks at 0-30 cm soil depth between mature RBS and respective AAg fields. MDC (249.6) MDL, (209.6), MCC (164.0) and MCL (147.5) had significantly higher mean SOC (Mg C ha-1) compared to the associated AAg fields; 109.4, 85.4, 59.5 and 98.4 Mg C ha-1, respectively. With respect to four young RBSs and adjacent AAg fields, there were

SOC stock comparison between riparian buffer systems and adjacent agricultural field

SOC stock differences on an equivalent soil mass basis between all mature RBS and respective adjacent agricultural fields in the upper layer (0-30 cm) were significantly higher due to the presence of perennial vegetation in the RBSs (Fig. 2). However, a significant difference in SOC stocks was not observed in the young riparian systems, except YCC, and their associated agricultural fields showing that SOC gains are a long-term process. The amount of plant C input varies in different land-use

Conclusion

The soils in all mature RBS sites had accumulated significantly higher SOC stocks in 0-30 cm soil depth compared to AAg fields. Young RBS soils except YCC did not show significant differences in SOC stocks at the same depth when compared with AAg fields. However, all RBSs accumulated higher SOC than the respective AAg fields. This demonstrates the positive impact of RBS on SOC accumulation compared to conventional agriculture land-use systems. In addition, annual SOC accumulation in RBS also

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgement

This work was funded by Agriculture Agri-Food Canada (AAFC), under the Agricultural Greenhouse Gases Program Two (AGGP-2). The authors are also thankful to the Grand River Conservation Authority (GRCA) personnel, especially to Mr. Ron Wu-Winter, Ms. Anne Loeffler and Ms. Louise Heyming.

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