Land cover and carbon sequestration in agricultural landscapes

Over the past 200 years, large tracts of diverse biomes such as North American grasslands, the northern Kazakh steppe, the Brazilian cerrado, or Ontario’s Carolinian forest zone have been converted to commodity-based agricultural production (Comer et al. 2018; Petrick, Wandel,and Karsten 2013; Oliveira and Hecht 2016; Bowley 2016). The process of clearing more “natural” forms of land cover for commercial agricultural use has significantly compromised the carbon sequestration capacity of terrestrial biomes at the ecosphere scale (Zomer et al. 2017).

In most cases, the carbon sequestration potential of agricultural land cover categories cannot match the potential of well-managed, relatively natural forms of land cover (Deng, Liu, and Shangguan 2014; Yang et al. 2019; Purakayastha, Huggins, and Smith 2008; Castaño-Sánchez et al. 2021). For example, field-based research conducted in a south-central Ontario agricultural landscape demonstrates that areas of remnant forest contained 25% of farm-wide soil carbon even though remnant forest comprised only 15% of land cover at research sites (Mazzorato, Esch, and MacDougall 2022). These findings suggests that field boundary areas under perennial land cover such as trees and grasses provide more carbon sequestration capacity than areas under annual field crops.

Agricultural landscapes may serve as atmospheric carbon sinks if managed using practices that have been scientifically verified to sequester more carbon than they emit. It is important to stress that climatic and other environmental conditions may override the effects of changes to land use and land cover or shifts made in land management practices (Spengler 2011; Zomer et al. 2017). Recent research also points out that the agricultural sector continues to be a net source of GHG emissions and any practices that may sequester carbon over the long term may not provide additional capacity to sequester carbon from other economic sectors (Schlesinger 2022).

Carbon sequestration occurs in agricultural landscapes via two pathways: above-ground biomass growth and soil-based carbon sequestration. Above-ground biomass growth ties up carbon for relatively short timeframes (even woody biomass) because carbon stored in biomass is released back into the atmospheric carbon pool as plant matter decomposes. Therefore carbon stored as above-ground biomass is not seen as a viable means of long-term carbon sequestration (Zeng et al. 2013).

The second pathway is via soil-based carbon sequestration. Soil-based carbon sequestration occurs when organic forms of carbon are stored in soils for relatively long timeframes. The primary mechanism for sequestering carbon in the soil at longer timescales is via exudatessecreted by plant roots into the soil rhizosphere. Exudates are short-chain starch molecules that serve a variety of purposes in the rhizosphere. In relation to carbon sequestration, exudates can be converted to stable forms of soil carbon by soil microbiota. A secondary mechanism of soil-based carbon sequestration is the decomposition of plant biomass at the soil surface and in the soil profile (Kell 2012; Jones, Nguyen, and Finlay 2009).

The ability of agricultural soils to sequester carbon is determined by two biophysical factors: biotic processes and physical soil structure. Biotic processes of living plants, animals, fungi, and microorganisms drive carbon inputs and outputs in soils. The physical structure of soils is comprised of inorganic and organic particles in the soil profile. Biotic and physical soil properties vary by depth. Upper soil layers closer to the surface are more influenced by the environment and land use and land cover. Most soil carbon is found in soil horizons closer to the surface (O and A soil horizons) (World Bank 2021, 11).

In conclusion, soil-based carbon sequestration is the primary pathway of carbon sequestration in agricultural landscapes. Even with field crop land management practices that promote soil-based carbon sequestration, this may not be enough to offset GHG emissions from within the agriculture sector. More “natural” forms of land cover provide better carbon sequestration capacity than areas of annual crops. To maximize carbon sequestration in agricultural landscapes, more attention should be paid to field boundary areas, pastures, and the conversion of marginal annual cropping areas back to perennial forms of land cover.

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