Ronald F. Follett

The potential of U.S. cropland to sequester carbon and mitigate the greenhouse effect

Objectives Basic Processes The Greenhouse Process Global Trends in Greenhouse Gas Emissions The Role of Agriculture in U.S. Emissions of Three GHGs The SOC Pool in U.S. Soils and SOC Loss from Cultivation Processes in Governing Emissions from the Pedosphere Plant Action Soil Processes Soil Quality Strategies for Mitigating Emissions from Cropland U.S. Cropland Sustainable Management Studies Soil Erosion Management Land Conversion and Restoration Conversion of Marginal Land Restoration of Degraded Soils Biofuels for Offsetting Fossil Fuel Intensification of Prime Agricultural Land Conservation Tillage and Residue Management Irrigation Water Management Improved Cropping Systems The Carbon Sequestration Potential of Arable Land U.S. Croplands Overall Potential to Mitigate the Greenhouse Effect Techniques for Sequestration Rates of SOC Sequestration Possible Implementation Obstacles Required Action Conclusions - The Win-Win Strategy Agricultural Profits from Environmental Improvements SOCs Monetary Value SOCs Environmental Value Global Potential Appendix 1: Definitions Appendix 2: Researchable Topics

Nitrogen in the Environment: Sources, Problems, and Management

Nitrogen (N) is applied worldwide to produce food. It is in the atmosphere, soil, and water and is essential to all life. N for agriculture includes fertilizer, biologically fixed, manure, recycled crop residue, and soil-mineralized N. Presently, fertilizer N is a major source of N, and animal manure N is inefficiently used. Potential environmental impacts of N excreted by humans are increasing rapidly with increasing world populations. Where needed, N must be efficiently used because N can be transported immense distances and transformed into soluble and/or gaseous forms that pollute water resources and cause greenhouse effects. Unfortunately, increased amounts of gaseous N enter the environment as N2O to cause greenhouse warming and as NH3 to shift ecological balances of natural ecosystems. Large amounts of N are displaced with eroding sediments in surface waters. Soluble N in runoff or leachate water enters streams, rivers, and groundwater. High-nitrate drinking water can cause methemoglobinemia, while nitrosamines are associated with various human cancers. We describe the benefits, but also how N in the wrong form or place results in harmful effects on humans and animals, as well as to ecological and environmental systems.

Crop Residues: The Rest of the Story

Sinking agricultural botanical and soil residues to the deep seafloor may not be a viable option for long-term carbon sequestration.

Soil carbon dynamics during a long-term incubation study involving 13C and 14C measurements

Soil organic matter is the earths largest terrestrial reservoir of carbon (C). Thus, it serves as a major control on atmospheric carbon dioxide (CO2) levels. To better understand these controls, decreases in soil organic C (SOC), soil microbial biomass (SMB) C, and the role of SMB as a source of mineralizable C were measured during a long-term incubation (853 days) without added substrate. The 2 soils used were a Weld loam (fine montmorillonitic, mesic, Aridic Paleustoll) from near Akron, Colorado, and a Duroc loam (fine silty, mixed mesic Pachic Haplustoll) from near Sidney, Nebraska. The Akron soil was uniformly cropped to small grain crop-fallow rotations until 1989 when wheat (Triticum aestivum L.) in conventional (stubble mulch) till-fallow, reduced till-fallow, and no-till fallow treatments were adopted. On additional rotation plots, continuous corn (Zea mays L.) or no-till corn, fallow, wheat, and no-till corn in a 4-year rotation were grown. The Sidney soil was broken from native sod in 1970 and planted to wheat-fallow with no-till, plow-tillage, and sod-plot treatments. Moist soil samples were collected and refrigerated until plant material removal by sieving and picking. The SOC and SMB-C decreased during incubation and rates of loss measured. The results from this study allow insights into contributions of SMB and changes in soil isotope C ratios not previously available. Soil microbial biomass C contributed an average of 31% of the evolved CO2-C across all treatments between day 10 and day 79 of incubation and an average of about 20% during the more extended times between later measurements thereafter. Until day 160, evolution of 13CO2 during incubation indicated that evolved C came from plant residues and was soil derived thereafter, including from the native grassland SOC. Where corn was grown, evolution of evolved C is hypothesized to have had a less negative 13CO2 isotope signature from days 630 to 720 of the incubation because of the delayed microbial breakdown of the cob materials. After 853 days of incubation and across all plots, the SOC remaining averaged 67% and was similar to the amount of observed hydrolysis residue C. Acid hydrolysis and 14C dating were also used to characterize the resistant SOC fraction and showed increased 14C age with hydrolysis but not with long-term incubation.

Advances in Nitrogen Management for Water Quality

Nitrogen (N) is an essential nutrient for crops and other plants and is needed for many plant physiological functions. Nitrogen is critically important for global sustainability of food and has been a key to the success of the green revolution. Due to its importance as a crop nutrient, N fertilizer is of great importance for maximizing crop production. Crop uptake of N follows a sigmoid function with N uptake necessarily preceding above-ground dry matter while early season N requirements of the root system are met. For most agricultural cropping systems, production cannot be maximized without additional N inputs. However, N inputs can also have environmental impacts because of increased losses of N to the environment (Cowling et al. 2002; Galloway et al. 2003). Management of this dynamic nutrient is a key to lessening its potential impact on the environment. Though N management is complex in many aspects, there are basic principles that can be used to reduce N losses via leaching and/or atmospheric losses (Meisinger and Delgado 2002; Mosier et al. 2002). The risk of environmental impacts is increased when recommended management practices for reducing N losses are not implemented (Meisinger and Delgado 2002; Mosier et al. 2002). Scientists have…

Chapter 2 – Transformation and Transport Processes of Nitrogen in Agricultural Systems

Publisher Summary This chapter reviews the fate and transport of nitrogen from the various sources used to supply the nitrogen-requirements of crops in the context of the nitrogen cycle. Nitrogen is ubiquitous in the environment. It is also one of the most important nutrients and is central to the growth of all crops and other plants. However, nitrogen also forms some of the most mobile compounds in the soil-plant-atmosphere system; and there is mounting concern about agricultures role in nitrogen delivery into the environment. Nitrogen represents the mineral fertilizer most applied to agricultural land. This is because available soil-nitrogen supplies are often inadequate for optimum crop production. Use of nitrogen budgets or a mass-balance approach is needed to understand the options for improving management of nitrogen in farming and livestock systems and for mitigating the environmental impacts of nitrogen. Fertilizing crops for crop nitrogen-uptake that will be near the point of maximum yield generally is an economically and environmentally acceptable practice.

CQESTR Simulations of Soil Organic Carbon Dynamics

A process-based carbon (C) model, CQESTR (sequester), was used to predict soil organic carbon (SOC) dynamics and examine the effect of agricultural management practices on SOC accretion in three diverse regions of the U.S. The three regions chosen had long-term experiments (LTEs) ranging from 23 to 134 years in duration. The range of management practices included crop residue harvest, burning or residue retention, tillage type, fertilizer or manure addition, fallow and crop intensification, monoculture and complex crop rotation. The CQESTR model captured temporal and spatial changes in SOC. Simulation results indicated cultivation and crop residue removal decreased SOC; however, with appropriate management such as the use of conservation tillage, organic amendments, and/or cropping intensification, SOC losses could be reversed. Using fertilizer alone is insufficient to overcome residue removal effects on SOC. Model validation and future predictions concerning SOC management can only be conducted using a long-term (>25-year) SOC database because stable SOC changes occur very slowly and require several decades to reach equilibrium.

Carbon Dynamics and Sequestration in Urban Turfgrass Ecosystems

Urbanization is a global trend. Turfgrass covers 1.9% of land in the continental US, occupying about 16 million ha. In this article, we review existing literature associated with carbon (C) pools, sequestration, and nitrous oxide emission of urban turfgrass ecosystems. Turfgrasses exhibit significant carbon sequestration (0.34–1.4 Mg ha−1 year−1) during the first 25–30 years after turf establishment. Several studies have reported that residential turfgrass soil can store up to twofold higher soil organic carbon (SOC) content than agricultural soils. Published research suggests that the dynamics of nitrogen (N) is controlled by C transformation. Turfgrass areas have high levels of SOC and microbial biomass creating a carbon-based “sink” for inorganic N. Therefore, lower than “expected” nitrate leaching and N2O emissions have been measured in the majority of the experiments carried out for turfgrass ecosystems. Increased SOC in turfgrass soil can result from: (1) returning and recycling clippings, (2) appropriate and efficient-fertilizer use, and (3) irrigation based on turfgrass needs. Some turfgrass management practices (such as fertilization, mowing, and irrigation) carry a carbon “cost”. Therefore turfgrass’s contribution to a sink for carbon in soils must be discounted by fuel and energy expenses and fertilizer uses in maintaining turf, and the flux of N2O. More work is needed to evaluate the carbon sequestration, total carbon budget, and fluxes of the other greenhouse gases in turfgrass systems.

Using pyrolysis molecular beam mass spectrometry to characterize soil organic carbon in native prairie soils

The goal of this study was to test if analytical pyrolysis coupled with molecular beam mass spectrometry and multivariate statistical analyses could provide a rapid and accurate methodology to identify and quantify soil carbon fractions and understand the fundamental chemistry that distinguishes these fractions. We analyzed soil organic carbon (SOC) contained in well-characterized agricultural soils with pyrolysis molecular beam mass spectrometry (py-MBMS) and then determined correlations between the mass spectra and associated soil characterization data. Both soil carbon chemistry and the organic forms in which SOC is sequestered (soil microbial biomass (SMBC), particulate organic matter carbon (POM C), and mineral-associated carbon (Cmin C)) were assessed by multivariate statistical analyses to discover existing correlations and if they could be developed into estimative models. The sample set consisted of well-characterized soils collected from native prairie sites in the western U.S. Corn Belt and Great Plains: 11 sites located within 8 midwestern states (CO, NE, IA, ND, MT, TX, MO, and MN). Sample characterization parameters included site, depth, %SOC, POM C, Cmin C, SMBC, and SOC calendar age (determined from 14C age). Correlations were found for samples collected across this large geographic region (at or greater than 0.90) for SOC, POM C, Cmin C, and SMBC. Soil organic carbon calendar age derived from radiocarbon-14 dating could be estimated for ustollic soils from MT, NE, and CO. These soils also contained deeper and younger eolian layers, whose ages were correctly estimated with this technique. The Py-MBMS analysis additionally showed that soils developed from water-sorted sediments on a tilled-floor lake plain (lacustrine soils) were significantly different from the other samples.

Soil N Dynamics Related to Soil C and Microbial Changes During Long-term Incubation

Knowledge of the pools and fluxes of C and N soil components is required to interpret ecosystem functioning and improve biogeochemical models. Two former grassland soils, where wheat or corn are currently growing, were studied by kinetic analysis of microbial biomass C and N changes, C and N mineralization rates, acid hydrolysis, and pyrolysis. Nearly twice as much C as N was mineralized during incubation. Modeling of changes during incubation demonstrated that two-pool first-order kinetics effectively described losses of microbial biomass C and N and concurrent N mineralization. Loss of microbial biomass N during incubation accounted for a significant portion of the N mineralized. Microbial biomass N content and soil N mineralization rates were strongly affected by soil type and soil management. Nitrification, but not N mineralization, was inhibited during the latter stages of incubation in one of the soils. We believe nitrifier populations had dropped below effective levels. Nonacid hydrolysable C was increased in both amount and mean residence time by cultivation and incubation. Hydrolysis removed a larger amount of N than incubation. Data after pyrolysis of soils, in argon at 550°C, closely reflected results for both C and N found after cultivation and incubation. This technique should be further investigated to identify the recalcitrant forms of C and N in soils. The dynamics of soil C and soil N, although related, are not identical; thus, management can be targeted to soil C or N cycling in ecosystem functioning or to soil organic matter dynamics in global change.