Executive Summary

Reducing (and eventually reversing) the increase in greenhouse gases (GHGs) in the atmosphere due to human activities, and thus reducing the extent and severity of anthropogenic climate change, is one of the great challenges facing humanity. While most of the man-caused increase in GHGs has been due to fossil fuel use, land use (including agriculture) currently accounts for about 25% of total GHG emissions and thus there is a need to include emission reductions from the land use sector as part of an effective climate change mitigation strategy. In addition, analyses included in the recent IPCC 5th Climate Change Assessment report suggests that it may not be possible to achieve large enough emissions reductions in the energy, transport and industrial sectors alone to stabilize GHG concentrations at a level commensurate with a less than 2oC global average temperature increase, without the help of a substantial CO2 sink (i.e., atmospheric CO2 removal) from the land use sector. One of the potential carbon sinks that could contribute to this goal is increasing C storage in soil organic matter on managed lands.

There are numerous land management practices that can be adopted to increase soil carbon storage in agricultural soils (e.g., changes in crop rotations, tillage, fertilizer management, organic amendments, etc.) which have been extensively reviewed and assessed in the scientific literature. One of the most effective means for increasing soil C sequestration is through changing land cover, such as converting annual cropland to forest or perennial grasses, which generally contribute much more plant residue to soils. However, if widely applied, such land use conversions would have negative consequences for food and fiber production from the crops that are displaced. An option that has not yet been widely explored is to modify, through targeted breeding and plant selection, crop plants to produce more roots, deeper in the soil profile where decomposition rates are slower compared to surface horizons, as an analogous strategy to increase soil C storage.

This report details a preliminary scoping analysis, to assess the potential agricultural area in the US –where appropriate soil, climate and land use conditions exist – to determine the land area on which ‘improved root phenotype’ crops could be deployed and to evaluate the potential long-term soil C storage, given a set of ‘bounding scenarios’ of increased crop root input and/or rooting depth for major crop species (e.g., row crops (corn, sorghum, soybeans), small grains (wheat, barley, oats), and hay and pasture perennial forages). The enhanced root phenotype scenarios assumed 25, 50 and 100% increase in total root C inputs, in combination with five levels of modifying crop root distributions (i.e., no change and four scenarios with increasing downward shift in root distributions). We also analyzed impacts of greater root production on the soil-crop nitrogen balance, from the standpoint of increased need for additional N inputs and consequences for increased N2O flux, as well as potential impacts if more and deeper roots contributed to reduced N leaching. In the enhanced root phenotype scenarios, the implicit assumption was that increases in overall plant production could be achieved (e.g., through increased CO2 assimilation, greater growth efficiency) without reducing the harvested yield – that is, we did not include potential leakage and land substitution effects from potential decreased crop yield in the analysis.

We found that around 87% of total US cropland (major annual crops plus hay/pasture land) had soils of sufficient depth and lacking major root-restricting soil layers to allow for crops with enhanced phenotypes. In general, the areas showing the largest potential soil C increases were in the northern tier of the Great Plains and Corn Belt agricultural area and in irrigated croplands in the western US, with smaller increases in C storage in the south-eastern US. Long-term soil C stock changes (i.e., change to a new equilibrium state) ranged from a 4% increase in stocks (0-2 m) with no increased root C inputs but a small downward shift in root distributions, to a 70% increase with no additional C inputs but with the deepest rooting scenario, to a 3.4 fold increase in soil C stocks with a doubling of current root C inputs and the deepest root distribution scenario (with annual crop roots having root distributions similar to that observed in some deep rooting perennial grasses). Changes to a new equilibrium soil C state would take place over a several hundred year period (due to the long turnover time of some of the more recalcitrant soil organic fractions). However, a significant portion of the change would occur over the first few decades after new crop plants were introduced and we estimated that about 30% of the total change to a new equilibrium would be achieved in the initial 30 year period. Based on this calculation, average annual (averaged over the initial 30 yr period) soil C accrual rates (assuming 100% adoption of improved phenotypes) ranged up to 280 Tg C per year (1026 Tg CO2eq) for the most optimistic scenario of a doubling of root C inputs and an extreme downward shift in root distributions. This is equivalent to an average rate of increase of almost 1.8 Mg C per hectare per year, similar to rates of soil C increase that have been observed with conversion of annual cropland to high productivity perennial grasses.

Including impacts on the soil N balance, reduced somewhat the total GHG mitigation potential of some of the improved root phenotype scenarios, due to increased demand for N inputs and hence increased GHG emissions from N2O and from embodied GHG emissions associated with fertilizer manufacture and distribution. Many cropland soils currently have a surplus N balance and thus for modest changes in root inputs and depth distributions, there is sufficient surplus N to meet the increased N demand and thus little impact on overall net GHG benefits due to altered N budgets. However, with increasing soil C storage as a consequence of greater root C inputs and deeper root distributions, the role of N in the overall GHG balance increases. For the most optimistic scenario of doubled root C inputs and an extreme downward shift in root distributions, total net GHG benefits were reduced by up to 28% (from 1026 to 746 Tg CO2eq) due to the increased N2O emissions associated with increased N inputs. However, a more detailed analysis using dynamic process-based models that couple plant and soil C and N fluxes, including N2O, other gaseous N losses and N leaching is needed to better evaluate net GHG consequences.

In addition to helping meet GHG mitigation goals, changes in crop root production and root distributions that increase soil organic matter stocks, can provide a wide range of other benefits to soil health and sustainability, including improved soil physical characteristics, nutrient storage and cation retention, improved water retention and water quality and enhanced soil biodiversity.

Published with ARPA-E's Rhizosphere Observations Optimizing Terrestrial Sequestration (ROOTS) Funding Opportunity Announcement.