finding 12.3 : key-message-12-3

Soil carbon stocks are sensitive to agricultural and forestry practices and loss of carbon-rich soils such as wetlands. Soils in North America have lost, on average, 20% to 75% of their original top soil carbon (0 to 30 cm) with historical conversion to agriculture, with a mean estimate for Canada of 24% ± 6%. Current agricultural management practices can increase soil organic matter in many systems through reduced summer fallow, cover cropping, effective fertilization to increase plant production, and reduced tillage. Forest soil carbon loss with harvest is small under standard management practices and mostly reversible at the century scale. Afforestation of land in agriculture, industry, or wild grasslands in the United States and Canadian border provinces could increase SOC by 21% ± 9% (high confidence).

This finding is from chapter 12 of Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report.

Description of evidence base: Converting native forests or pastures to cropland can reduce soil carbon by 42% to 59%, respectively (Guo and Gifford 2002). A meta-analysis for Canadian soils reported that, when native soil was converted to agricultural land, there was an average 24% loss of soil carbon (VandenBygaart et al., 2003). Estimates for Mexico also suggest that loss of soil carbon due to management remains significant (Huber-Sannwald et al., 2006).

Agricultural effects on soil carbon stocks, including effects of conservation measures, are reviewed and quantified in Angers and Eriksen-Hamel (2008), Hutchinson et al. (2007), Luo et al. (2010), Palm et al. (2014), Paustian et al. (2016), Powlson et al. (2014), and many others. Specific conservation measures for improved soil carbon retention have been shown to be effective in both Canada and the United States. In Canada, conservation measures, including reduced summer fallow and reduced tillage, have been widely adopted over many regions and have resulted in soil carbon increases and reduced erosion (Soil Conservation Council of Canada 2016). Agriculture and Agri-Food Canada (2016; AAFC) has 30 years of data showing that, in the Canadian Prairies, reduced tillage combined with reduced summer fallow have led to significant SOC increases. Improved residue management, including adding forage in crop rotations or adopting agroforestry, and practices that increase plant growth such as effective fertilization are effective in increasing soil carbon (Hutchinson et al., 2007; Palm et al., 2014). A meta-analysis by Angers and Eriksen-Hamel (2008) suggested that, although significant increases in surface soil carbon with reduced tillage are commonly observed, the slight decreases in soil below the plow layer also are common, thus making overall increases in total soil carbon profiles averaged across studies small but significant. In a more recent meta-analysis by Luo et al. (2010), increased soil carbon with reduced tillage was seen only for double-cropping systems, a finding which agrees with the AAFC result that reduced summer fallow and reduced tillage together caused significant increases in soil carbon.

Palm et al. (2014) point out serious methodological flaws with many tillage comparisons that include sampling by depth not equivalent soil mass, flaws which cause significant overestimates of soil carbon in no-till soils with higher bulk densities. In their 2014 meta-analysis, about half the paired comparisons showed small increases in soil carbon from reduced till but half did not, suggesting that increased residue retention is more significant than reducing tillage. A similar meta-analysis by Kopittke et al. (2017) that also corrected for changes in bulk density found an overall small positive (+9%) effect of conversion to no-till practices from conventional till. Powlson et al. (2014) point out that the gains in surface soil carbon with adoption of no-till methods can improve crop production and reduce erosion in many cases, but the reverse can be true in cool, wet climates or the wet tropics.

Several meta-analyses of afforestation effects on former croplands have been conducted, and there is general consensus that soil carbon gains may take more than 30 years to be seen (Barcena et al., 2014; Li et al., 2012; Nave et al., 2013) and can increase carbon stocks by 19% to 53% (Guo and Gifford 2002; Nave et al., 2013).

Data on forest harvest effects are from a comprehensive meta-analysis by Nave et al. (2010), who report variable and low changes in mineral soil carbon stocks with forest harvest but significant decreases in forest floor carbon. Several chronosequences support this meta-analysis. Dean et al. (2017) argue from a modeling standpoint that there are significant long-term losses of soil carbon with forest harvest of primary forests; however, much of this argument is based on assumptions about the relationship between plant inputs and soil carbon sequestration that are not necessarily supported by empirical studies. Wetland estimates are based on information in this report’s (SOCCR2) two wetland chapters. All chapters showed findings of strong evidence that loss of wetlands is a significant factor for total soil carbon loss, given the very high carbon density of wetland soils.

Wear and Coulston (2015), using data from the National Greenhouse Gas Inventory (NGHGI), report annual forest carbon accumulation, including both sequestration and land-use transfers in the United States as 223 teragrams of carbon (Tg C) per year, roughly 0.5% of the stored forest carbon. This likely translates into increased soil carbon storage, although this distinction was not made in the analysis. Similar estimates have not been made for Canada or Mexico.

New information and remaining uncertainties: The certainty for forest harvest effects on soil carbon appears to be very robust and based on many studies across North America, although a recent modeling study suggests that these other studies, carried out over decades, miss a multicentury-scale slow loss of soil carbon with forest harvest. However, there are no data to support that model result. Uncertainty arises because there are few empirical studies that compare soil carbon stocks in true primary forests to forests that have undergone centuries-long harvest cycles.

Uncertainties for agricultural effects have to do with site-specific variation in management implementation and lack of knowledge of deep soil carbon dynamics. However, convergence of the different meta-analyses on similar figures and research in this field is quite extensive (Li et al., 2012).

The wetland estimate also is quite robust given the high sampling density of the National Wetland Condition Assessment (NWCA) of the National Aquatic Resource Surveys. The NGHGI estimate of forest cover increase is quite robust given the quality of input data.

Assessment of confidence based on evidence: The meta-analyses of Nave et al. (2010, 2013) suggest very good agreement over forestry effects on soil carbon, although Dean et al. (2017) suggest that, over centuries, logging has had more significant effects on soil carbon. Given that the Dean et al. (2017) study is based on modeling with assumptions that are not supported in this analysis, such as that SOC is strongly related to biomass inputs, SOCCR2 is placing greater confidence in the Nave analyses (Nave et al., 2010, 2013).

The analysis by Paustian et al. (2016) suggests that there is some disagreement over agricultural management effects on SOC and that these effects are specific to local site and climatic conditions. The Li et al. (2012) meta-analysis suggests that afforestation of former croplands globally results in net SOC increases but that local results are so variable that local projection is difficult and results depend on soil type, management, and the type of tree species.

The wetland estimate is quite robust given the high sampling density of the NWCA.

This finding was derived from figure P.2: P.2. Likelihood and Confidence Evaluation

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