Dynamic, Intermediate Soil Carbon Pools May Drive Future Responsiveness to Environmental Change

Abstract

Accurately capturing dynamic soil response to disturbance effects in agroecosystem models remains elusive, thereby limiting projections of climate change mitigation potential. Perennial grasses cultivated in zero-tillage management systems hold promise as sustainable agroecosystems. High-yielding tropical C grasses often have extensive rooting systems, and the belowground processes of root turnover, aggregate formation, and mineral stabilization drove rapid C accumulation after cultivation in a recent study. We sought (i) to understand and constrain the size and responsiveness of dynamic, intermediate-cycling C pools contributing to the observed C accrual rates, and (ii) to simulate C stocks over time under the disturbance of elevated temperature using soil incubation at multiple temperatures and physical fractionation via density and sonication. Three-pool transfer modeling of soil incubations revealed small pools of readily available (i.e., days to months) microbial substrate that were responsive to temperature, time since cultivation, and inputs. Larger, kinetically slow-cycling pools were more indicative of long-term (i.e., years to decades) changes in C stock and strongly connected to measured changes in physical fractions. Combining the sensitivity of readily available microbial substrate with three-pool transfer modeling of the physical fractions over time since cultivation revealed that dynamic transfers of inputs occurred between the free organic and aggregate-protected fractions, and from these fractions to the mineral-associated dense fraction. Under 5°C temperature elevation, increased transfer rates outweighed elevated decomposition losses to sustain soil C accrual into the future. To effectively plan managed landscapes and monitor sustainable agroecosystems for climate change mitigation, tools must incorporate the complexity of soil response to change.

Measurements and Models to Identify Agroecosystem Practices That Enhance Soil Organic Carbon under Changing Climate

“…The emphasis on improving crop and climate inputs to soil C cycling model components should not imply that our fundamental understanding of the soil processes regulating the transformation of plant residues and models depicting those processes are not also in need of improvement. As shown by both Crow and Sierra (2018) and Sherrod et al (2018), there is a need for better understanding and model representation of the responses of different pools of soil C to climate and management, as well as transfers of C among those pools.…”

“…Fourteen contributions to this special section applied dynamic process-based models to assess the effects of climate and/or management on SOC dynamics. As described below, seven papers used the CQESTR model, whereas other models included were DeNitrification-DeComposition (DNDC), DayCent, CENTURY, RothC, the Model for Nitrogen and Carbon in Agro-Ecosystems (MONICA), the Environmental Productivity Integrated Climate (EPIC) model, and a detailed mathematical model of soil C dynamics devised by Crow and Sierra (2018). Cavigelli et al (2018) used SOC data from the long-term Farming Systems Project and the CQESTR model to examine the impact of projected climate change on SOC to 50-cm soil depth for grain cropping systems in the Mid-Atlantic United States.…”

“…This study provided a platform to facilitate the prediction of SOC and uncertainties in the climate data that drive SOC dynamics. Crow and Sierra (2018) sought to understand the size and responsiveness of dynamic, intermediate C pools using a combination of field and laboratory experiments and detailed process modeling. They measured soil C dynamics under elevated temperature over time in a soil incubation study and measured physical fractionation of different soil C pools via density and sonication.…”

Integrating Complex Soil Dynamics Using the Non-equilibrium Effective Temperature

“…This mismatch has led to the conclusion that C-cycling in soils may not be thermodynamically limited, but rather that it is kinetically limited. Furthermore, kinetic limitations of C-turnover are linked to carbon sorption on particles [e.g., Crow and Sierra (2018)] and processes of aggregation (Kalinina et al, 2015;Crow and Sierra, 2018). However, because the effective temperature and the aggregate turnover are controlled by BSM, and that in the longtime limit Cmobility is essentially diffusive, this implies that carbon turnover rate is proportional to the effective temperature (Equation 13) through the temperature dependency of diffusion itself (Loi et al, 2011a).…”

Carbon flow through energycane agroecosystems established post‐intensive agriculture

“…For example, aggregate formation and stability were affected by soil type in giant miscanthus in the midwestern United States, thereby reducing the protection of recent C inputs from losses in some cases (Tiemann & Grandy, 2015). In contrast, the dynamic transfer of fresh inputs from roots through aggregates and into organo‐mineral stabilized pools drove the rapid soil C accumulation measured in tropical perennial C 4 grasses (Crow, Deem, Sierra, & Wells, 2018; Crow & Sierra, 2018). In another study, Plaza, Courtier‐Murias, Fernández, Polo, and Simpson (2013) similarly found that organo‐mineral interactions increased soil C by 16% under no‐till compared to conventional management.…”