Hoyoung Kwon

Soil carbon sequestration and land use change associated with biofuel production: Empirical evidence

Soil organic carbon (SOC) change can be a major impact of land use change (LUC) associated with biofuel feedstock production. By collecting and analyzing data from worldwide field observations of major LUCs from cropland, grassland, and forest to lands producing biofuel crops (i.e. corn, switchgrass, Miscanthus, poplar, and willow), we were able to estimate SOC response ratios and sequestration rates and evaluate the effects of soil depth and time scale on SOC change. Both the amount and rate of SOC change were highly dependent on the specific land transition. Irrespective of soil depth or time horizon, cropland conversions resulted in an overall SOC gain of 6–14% relative to initial SOC level, while conversion from grassland or forest to corn (without residue removal) or poplar caused significant carbon loss (9–35%). No significant SOC changes were observed in land converted from grasslands or forests to switchgrass, Miscanthus, or willow. The SOC response ratios were similar in both 0–30 and 0–100 cm soil depths in most cases, suggesting SOC changes in deep soil and that use of top soil only for SOC accounting in biofuel life cycle analysis (LCA) might underestimate total SOC changes. Soil carbon sequestration rates varied greatly among studies and land transition types. Generally, the rates of SOC change tended to be the greatest during the 10 years following land conversion and had declined to approach 0 within about 20 years for most LUCs. Observed trends in SOC change were generally consistent with previous reports. Soil depth and duration of study significantly influence SOC change rates and so should be considered in carbon emission accounting in biofuel LCA. High uncertainty remains for many perennial systems and forest transitions, additional field trials, and modeling efforts are needed to draw conclusions about the site‐ and system‐specific rates and direction of change.

A meta-analysis of soil organic matter response to soil management practices: An approach to evaluate conservation indicators

Increased understanding of the influences of management practices on soil properties and associated ecosystem function is needed to improve tools used to administer conservation programs in the United States. This study used meta-analysis to assess the influence of cropping systems (conventional, conservation with minimum tillage, conservation with no-till, and organic systems) and management practices (nitrogen [N] fertility and rotation length) on soil organic carbon (SOC). These factors are considered by tools that evaluate conservation performance and provision of ecosystem services. We also reviewed the literature to determine whether this approach could be applied to other proxy variables (erosion rates, soil erodibility factor [K values], available phosphorus [P], and nitrous oxide [N2O]). Data mining was used to populate a database with variables representing practices used by the Natural Resource Conservation Services Conservation Measurement Tool (CMT) to determine eligibility for the Conservation Stewardship Program. Data collected from 55 peer-reviewed studies was categorized based on sampling depth (0 to 10, 0 to 15, 0 to 20, and 0 to 30 cm [0 to 3.9, 0 to 5.9, 0 to 7.8, and 0 to 11.8 in]). The magnitude of the effect estimated by meta-analysis was then compared to scores assigned to practices in the soil quality module of the CMT. Meta-analysis of data from the 0 to 20 cm (0 to 7.8 in) depth suggested that rates of SOC accrual were similar in organic systems using diversified crop rotations and conservation systems using inorganic fertility sources, increasing SOC by 9% compared to the conventional control. In comparisons at the 0 to 30 cm (0 to 11.8 in) depth, results from conservation systems using no-till and organic systems diverged, with conservation systems relying on no-till producing no gains while organic systems produced a 29% increase in SOC. While the use of organic amendments generally increased SOC, the magnitude of the effect was more modest than suggested by current CMT weighting. In addition, our results suggested that quality of manure, which is not differentiated in the CMT, influences the magnitude of the effect and that addition of wet manure may decrease SOC. A comparison of rotation length showed cropping systems with rotations of 3 years or longer were better able to increase SOC than shorter rotations. These findings suggested that the CMT generally ranks practices appropriately and shows how meta-analysis could be used to adjust credits awarded for use of reduced or no-till practices or different fertility sources.

Ontology-Based Simulation Applied to Soil, Water, and Nutrient Management

Ontology-based simulation is an approach to modeling in which an ontology is used to represent all elements of a model. In this approach, modeling is viewed as a knowledge representation problem rather than a software engineering problem. Ontology-based techniques can be applied to describe system structure, represent equations and symbols, establish connections to external databases, manage model bases, and integrate models with additional information resources. Ontology reasoners have the potential to automatically compare, organize, search for, and discover models and model elements. We present an environment for building simulations based on the Lyra ontology management system, which includes Web-based visual design tools used for constructing models. An example application based on a model of soil, water, and nutrient management in citrus that uses the approach is also presented.

Use of inverse modeling to evaluate CENTURY-predictions for soil carbon sequestration in US rain-fed corn production systems

We evaluated the accuracy and precision of the CENTURY soil organic matter model for predicting soil organic carbon (SOC) sequestration under rainfed corn-based cropping systems in the US. This was achieved by inversely modeling long-term SOC data obtained from 10 experimental sites where corn, soybean, or wheat were grown with a range of tillage, fertilization, and organic matter additions. Inverse modeling was accomplished using a surrogate model for CENTURY’s SOC dynamics sub-model wherein mass balance and decomposition kinetics equations from CENTURY are coded and solved by using a nonlinear regression routine of a standard statistical software package. With this approach we generated statistics of CENTURY parameters that are associated with the effects of N fertilization and organic amendment on SOC decay, which are not as well quantified as those of tillage, and initial status of SOC. The results showed that the fit between simulated and observed SOC prior to inverse modeling (R2 = 0.41) can be improved to R2 = 0.84 mainly by increasing the rate of SOC decay up to 1.5 fold for the year in which N fertilizer application rates are over 200 kg N ha-1. We also observed positive relationships between C inputs and the rate of SOC decay, indicating that the structure of CENTURY, and therefore model accuracy, could be improved by representing SOC decay as Michaelis-Menten kinetics rather than first-order kinetics. Finally, calibration of initial status of SOC against observed levels allowed us to account for site history, confirming that values should be adjusted to account for soil condition during model initialization. Future research should apply this inverse modeling approach to explore how C input rates and N abundance interact to alter SOC decay rates using C inputs made in various forms over a wider range of rates.

Concepts and Methods of Global Assessment of the Economics of Land Degradation and Improvement

The Economics of Land Degradation (ELD) initiative seeks to develop a science basis for policy actions to address land degradation. The purpose of this chapter is to provide with a conceptual framework and sound and feasible methodological standards for ELD assessments at global and national levels. Only if some basic standards are identified and adhered to, comparative assessments can be conducted between countries and useful aggregation of findings, based on these case studies, can be achieved. Therefore, using the Total Economic Value (TEV) framework, the chapter identifies minimum core standards that need to be adhered to in all country case studies to generate comparable material for international assessment and ELD policy guidance.

Inverse Modeling of CO2 Evolved During Laboratory Soil Incubation to Link Modeled Pools in CENTURY With Measured Soil Properties

Abstract Challenges associated with initialization of CENTURY soil organic matter (SOM) model might be addressed through a useful constraint between its conceptually defined pools and measurable soil properties. To develop such constraint, we linked soil properties directly measured in Florida sandy soils with initial CENTURY SOM pool sizes inversely modeled from CO2 evolved during 87-day laboratory incubation in the same soils. We conducted this inverse modeling by using a surrogate model for CENTURY’s soil organic C (SOC) dynamics submodel. The model entails the mass balance and decomposition kinetics equations for residue and SOM pools identical to those of CENTURY that are coded in the programming language of the SAS statistical software to use various statistical functions embedded within a nonlinear regression procedure of SAS. We used the surrogate CENTURY to objectively estimate site-specific masses of SOC in three (active, slow, and passive) pools from the soils. From 28 of the total 100 soils modeled, we were able to estimate initial masses of SOC in active and slow pools, which were statistically significant (P < 0.05), by fitting the surrogate CENTURY to CO2 evolution data. In the 28 soils, we found that active and slow pool sizes were correlated with hot water–soluble C and clay percentage (R2 = 0.74 and 0.77, respectively). Applying the constraints to initialize CENTURY modeling of an independent validation set of an additional 39 soils showed a significant correlation between modeled and observed CO2 (R2 = 0.49), suggesting the potential for other practical applications to properly initialize CENTURY simulations. Findings further suggest that inverse modeling has promise to inform CENTURY initialization and other process-based models that adapt CENTURY’s SOM structure.

Conservation management and ecosystem services in midwestern United States agricultural systems

The recognition that agricultural systems provide services beyond food production has motivated federal legislation and associated programs that promote sustainable management and conservation of productive working lands. This study evaluates the ability of the USDA Natural Resources Conservation Services (NRCS) Conservation Measurement Tool (CMT), which has been used to determine eligibility for enrollment in the Conservation Stewardship Program, to reflect changes in ecosystem services resulting from the use of conservation management practices on croplands. This work evaluates four macroconcerns or outcomes (soil erosion, soil quality, water quality, and air quality) that are quantified within the tool by comparing CMT scores tabulated according to practices reported by farmers involved in grain producing operations in the US Midwest with soil health measures (dynamic soil properties [DSPs]) determined on 72 Illinois farm fields. Three management practice categories—Conventional, Conservation, and Organic—were evaluated. The CMT identified significant differences among management categories, and scores ranked consistently higher for Organic than Conventional or Conservation systems. Direct analysis of DSPs suggested that there were relatively small differences among soil health indicators measured on farms under varied management; this implies that the CMT might be overemphasizing the influence of selected management practices. The evaluation of multivariate relationships between the practices scored in the CMT and DSPs revealed that soil properties like soil pH and particulate organic matter-carbon (POM-C) can be used to improve the accuracy of the tool. They also show how relationships between questions (practices) and outcomes suggested by DSPs vary among farming categories and suggest this type of practice-based tool should be adapted by embedding factors to scale farming systems or, preferably, by adding questions that better describe important factors (e.g., plant productivity and fertilizer timing, rate, and form of application) and their interactions with other factors known to influence specific outcomes. This work shows how field data describing practices, properties, and outcomes can be used to statistically evaluate and refine conservation scoring tools.

Land management change greatly impacts biofuels’ greenhouse gas emissions

Harvesting corn stover for biofuel production may decrease soil organic carbon (SOC) and increase greenhouse gas (GHG) emissions. Adding additional organic matter into soil or reducing tillage intensity, however, could potentially offset this SOC loss. Here, using SOC and life cycle analysis (LCA) models, we evaluated the impacts of land management change (LMC), that is, stover removal, organic matter addition, and tillage on spatially explicit SOC level and biofuels’ overall life cycle GHG emissions in US corn–soybean production systems. Results indicate that under conventional tillage (CT), 30% stover removal (dry weight) may reduce baseline SOC by 0.04 t C ha−1 yr−1 over a 30‐year simulation period. Growing a cover crop during the fallow season or applying manure, on the other hand, could add to SOC and further reduce biofuels’ life cycle GHG emissions. With 30% stover removal in a CT system, cover crop and manure application can increase SOC at the national level by about 0.06 and 0.02 t C ha−1 yr−1, respectively, compared to baseline cases without such measures. With contributions from this SOC increase, the life cycle GHG emissions for stover ethanol are more than 80% lower than those of gasoline, exceeding the US Renewable Fuel Standard mandate of 60% emissions reduction in cellulosic biofuels. Reducing tillage intensity while removing stover could also limit SOC loss or lead to SOC gain, which would lower stover ethanol life cycle GHG emissions to near or under the mandated 60% reduction. Without these organic matter inputs or reduced tillage intensity, however, the emissions will not meet this mandate. More efforts are still required to further identify key practical LMCs, improve SOC modeling, and accounting for LMCs in biofuel LCAs that incorporate stover removal.

Global Estimates of the Impacts of Grassland Degradation on Livestock Productivity from 2001 to 2011

In response to the needs for estimating the cost of grassland degradation to determine the cost of inaction and for identifying cost-effective strategies to address the consequent loss of livestock productivity, we developed a modeling framework where global statistics databases and remote sensing data/analyses coupled with empirical/statistical modeling are designed to quantify the global cost of grassland degradation. By using this framework, we identified grassland degradation hotspots over the period of 2001 to 2011 and estimated changes in livestock productivity associated with changes in grassland productivity within the hotspots. Ignoring environmental benefits and losses in live weight of livestock not slaughtered or sold, the cost of livestock productivity was estimated about 2007 US

Ontology-based simulation in agricultural systems modeling

6.8 billion. Although on-farm cost is small in Sub-Saharan Africa due to the low livestock productivity, the impact on human welfare would be much more severe in the region where majority of the population is below the poverty line. This implies that addressing grassland degradation is even more urgent in the region, given the increasing demand for livestock products and the potential contribution to poverty reduction. Taking action toward grassland degradation could simultaneously reduce poverty and promote carbon sequestration while conserving socio-economic, cultural, and ecological benefits that livestock provide.