S. J. Del Grosso

Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO2

Atmospheric CO2 enrichment may stimulate plant growth directly through (1) enhanced photosynthesis or indirectly, through (2) reduced plant water consumption and hence slower soil moisture depletion, or the combination of both. Herein we describe gas exchange, plant biomass and species responses of five native or semi-native temperate and Mediterranean grasslands and three semi-arid systems to CO2 enrichment, with an emphasis on water relations. Increasing CO2 led to decreased leaf conductance for water vapor, improved plant water status, altered seasonal evapotranspiration dynamics, and in most cases, periodic increases in soil water content. The extent, timing and duration of these responses varied among ecosystems, species and years. Across the grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO2 enrichment were relatively greater in dry years. In contrast, CO2-induced aboveground biomass increases in the Texas C3/C4 grassland and the New Zealand pasture seemed little or only marginally influenced by yearly variation in soil water, while plant growth in the Mojave Desert was stimulated by CO2 in a relatively wet year. Mediterranean grasslands sometimes failed to respond to CO2-related increased late-season water, whereas semiarid Negev grassland assemblages profited. Vegetative and reproductive responses to CO2 were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to functional groups. Results suggest that the indirect effects of CO2 on plant and soil water relations may contribute substantially to experimentally induced CO2-effects, and also reflect local humidity conditions. For landscape scale predictions, this analysis calls for a clear distinction between biomass responses due to direct CO2 effects on photosynthesis and those indirect CO2 effects via soil moisture as documented here.

General model for N2O and N2 gas emissions from soils due to dentrification

Observations of N gas loss from incubations of intact and disturbed soil cores were used to model N2O and N2 emissions from soil as a result of denitrification. The model assumes that denitrification rates are controlled by the availability in soil of NO3 (e− acceptor), labile C compounds (e− donor), and O2 (competing e− acceptor). Heterotrophic soil respiration is used as a proxy for labile C availability while O2 availability is a function of soil physical properties that influence gas diffusivity, soil WFPS, and O2 demand. The potential for O2 demand, as indicated by respiration rates, to contribute to soil anoxia varies inversely with a soil gas diffusivity coefficient which is regulated by soil porosity and pore size distribution. Model inputs include soil heterotrophic respiration rate, texture, NO3 concentration, and WFPS. The model selects the minimum of the NO3 and CO2 functions to establish a maximum potential denitrification rate for particular levels of e− acceptor and C substrate and accounts for limitation of O2 availability to estimate daily N2+N2O flux rates. The ratio of soil NO3 concentration to CO2 emission was found to reliably (r2=0.5) model the ratio of N2 to N2O gases emitted from the intact cores after accounting for differences in gas diffusivity among the soils. The output of the ratio function is combined with the estimate of total N gas flux rate to infer N2O emission. The model performed well when comparing observed and simulated values of N2O flux rates with the data used for model building (r2=0.50) and when comparing observed and simulated N2O+N2 gas emission rates from irrigated field soils used for model testing (r2=0.47).

General CH4 oxidation model and comparisons of CH4 Oxidation in natural and managed systems

Fluxes of methane from field observations of native and cropped grassland soils in Colorado and Nebraska were used to model CH 4 oxidation as a function of soil water content, temperature, porosity, and field capacity (FC). A beta function is used to characterize the effect of soil water on the physical limitation of gas diffusivity when water is high and biological limitation when water is low. Optimum soil volumetric water content (W opt ) increases with PC. The site specific maximum CH 4 oxidation rate (CH 4max ) varies directly with soil gas diffusivity (D opt ) as a function of soil bulk density and FC. Although soil water content and physical properties are the primary controls on CH 4 uptake, the potential for soil temperature to affect CH 4 uptake rates increases as soils become less limited by gas diffusivity, Daily CH 4 oxidation rate is calculated as the product of CH 4max , the normalized (0-100%) beta function to account for water effects, a temperature multiplier, and an adjustment factor to account for the effects of agriculture on methane flux. The model developed with grassland soils also worked well in coniferous and tropical forest soils. However, soil gas diffusivity as a function of field capacity, and bulk density did not reliably predict maximum CH 4 oxidation rates in deciduous forest soils, so a submodel for these systems was developed assuming that CH 4max is a function of mineral soil bulk density. The overall model performed well with the data used for model development (r 2 = 0.76) and with independent data from grasslands, cultivated lands, and coniferous, deciduous, and tropical forests (r 2 = 0.73, mean error < 6%).

Simulated effects of dryland cropping intensification on soil organic matter and greenhouse gas exchanges using the DAYCENT ecosystem model

We present evidence to show that DAYCENT can reliably simulate soil C levels, crop yields, and annual trace gas fluxes for various soils. DAYCENT was applied to compare the net greenhouse gas fluxes for soils under different land uses. To calculate net greenhouse gas flux we accounted for changes in soil organic C, the C equivalents of N2O emissions and CH4 uptake, and the CO2 costs of N fertilizer production. Model results and data show that dryland soils that are depleted of C due to conventional till winter wheat fallow cropping can store C upon conversion to no till, by reducing the fallow period, or by reversion to native vegetation. However, model results suggest that dryland agricultural soils will still be net sources of greenhouse gases although the magnitude of the source can be significantly reduced and yields can be increased upon conversion to no till annual cropping.

Testing DAYCENT model simulations of corn yields and nitrous oxide emissions in irrigated tillage systems in Colorado.

Agricultural soils are responsible for the majority of nitrous oxide (N(2)O) emissions in the USA. Irrigated cropping, particularly in the western USA, is an important source of N(2)O emissions. However, the impacts of tillage intensity and N fertilizer amount and type have not been extensively studied for irrigated systems. The DAYCENT biogeochemical model was tested using N(2)O, crop yield, soil N and C, and other data collected from irrigated cropping systems in northeastern Colorado during 2002 to 2006. DAYCENT uses daily weather, soil texture, and land management information to simulate C and N fluxes between the atmosphere, soil, and vegetation. The model properly represented the impacts of tillage intensity and N fertilizer amount on crop yields, soil organic C (SOC), and soil water content. DAYCENT N(2)O emissions matched the measured data in that simulated emissions increased as N fertilization rates increased and emissions from no-till (NT) tended to be lower on average than conventional-till (CT). However, the model overestimated N(2)O emissions. Lowering the amount of N(2)O emitted per unit of N nitrified from 2 to 1% helped improve model fit but the treatments receiving no N fertilizer were still overestimated by more than a factor of 2. Both the model and measurements showed that soil NO(3)(-) levels increase with N fertilizer addition and with tillage intensity, but DAYCENT underestimated NO(3)(-) levels, particularly for the treatments receiving no N fertilizer. We suggest that DAYCENT could be improved by reducing the background nitrification rate and by accounting for the impact of changes in microbial community structure on denitrification rates.

Elevated atmospheric CO2 effects and soil water feedbacks on soil respiration components in a Colorado grassland

facilitated on all treatments by a 13 C disequilibrium between currently growing plants and soil organic matter. Decomposition rates were more than doubled by elevated CO2, whereas rhizosphere respiration rates were not changed. In general, decomposition rates were most significantly correlated with soil temperature, and rhizosphere respiration rates were best predicted by soil moisture content. Model simulations suggested that soil moisture feedbacks, rather than differences in substrate availability, were primarily responsible for higher total respiration rates under elevated CO2. By contrast, modeling demonstrated that substrate availability was at least as important as soil moisture in driving CO2 treatment differences in soil organic matter decomposition rates. INDEX TERMS: 1610 Global Change: Atmosphere (0315, 0325); 1615 Global Change: Biogeochemical processes (4805); 1851 Hydrology: Plant ecology; 1866 Hydrology: Soil moisture; KEYWORDS: decomposition, rhizosphere respiration, stable isotopes, 13 C/ 12 C, soil C cycling, shortgrass steppe

Chapter 16 – Simulated effects of land use, soil texture, and precipitation on N gas emissions using DAYCENT

This chapter discusses simulated effects of land use; soil texture; and precipitation on N gas emissions using DAYCENT. The chapter describes the N gas flux submodel used in the DAYCENT ecosystem model and demonstrates the ability of DAYCENT to simulate the low N gas emissions observed from native soils, the intermediate emissions associated with dryland agriculture, and the high emissions observed for irrigated agricultural soils. DAYCENT has been used to compare N gas emissions from soils for native range grass, winter wheat conventional till and no till, winter wheat/corn/fallow no till, irrigated corn and irrigated silage cropping. NO x made up the majority of N gas fluxes in all cases followed by N 2 O and N 2 . Soil water inputs, tillage, timing of crop/fallow periods, and fertilizer application interact to control N gas emissions so generalizations regarding land use are difficult to make. Switching to no till without changing the winter wheat cropping schedule resulted in higher N 2 O emissions because the increased soil water content induced by no till supported higher denitrification rates. Finally, the soil water savings associated with no till also allows a reduction in the fallow period and the 3-year winter wheat rotations had lower N 2 O and NO x emissions than the 2-year winter wheat/fallow systems considered.

Introducing the GRACEnet/REAP Data Contribution, Discovery, and Retrieval System.

Difficulties in accessing high-quality data on trace gas fluxes and performance of bioenergy/bioproduct feedstocks limit the ability of researchers and others to address environmental impacts of agriculture and the potential to produce feedstocks. To address those needs, the GRACEnet (Greenhouse gas Reduction through Agricultural Carbon Enhancement network) and REAP (Renewable Energy Assessment Project) research programs were initiated by the USDA Agricultural Research Service (ARS). A major product of these programs is the creation of a database with greenhouse gas fluxes, soil carbon stocks, biomass yield, nutrient, and energy characteristics, and input data for modeling cropped and grazed systems. The data include site descriptors (e.g., weather, soil class, spatial attributes), experimental design (e.g., factors manipulated, measurements performed, plot layouts), management information (e.g., planting and harvesting schedules, fertilizer types and amounts, biomass harvested, grazing intensity), and measurements (e.g., soil C and N stocks, plant biomass amount and chemical composition). To promote standardization of data and ensure that experiments were fully described, sampling protocols and a spreadsheet-based data-entry template were developed. Data were first uploaded to a temporary database for checking and then were uploaded to the central database. A Web-accessible application allows for registered users to query and download data including measurement protocols. Separate portals have been provided for each project (GRACEnet and REAP) at nrrc.ars.usda.gov/slgracenet/#/Home and nrrc.ars.usda.gov/slreap/#/Home. The database architecture and data entry template have proven flexible and robust for describing a wide range of field experiments and thus appear suitable for other natural resource research projects.

Adding ecosystem function to agent-based land use models

The objective of this paper is to examine issues in the inclusion of simulations of ecosystem functions in agent-based models of land use decision-making. The reasons for incorporating these simulations include local interests in land fertility and global interests in carbon sequestration. Biogeochemical models are needed in order to calculate such fluxes. The Century model is described with particular attention to the land use choices that it can encompass. When Century is applied to a land use problem the combinatorial choices lead to a potentially unmanageable number of simulation runs. Century is also parameter-intensive. Three ways of including Century output in agent-based models, ranging from separately calculated look-up tables to agents running Century within the simulation, are presented. The latter may be most efficient, but it moves the computing costs to where they are most problematic. Concern for computing costs should not be a roadblock.

Chapter 18. DAYCENT Simulated Effects of Land Use and Climate on County Level N Loss Vectors in the USA

Publisher Summary This chapter provides an overview of the DAYCENT ecosystem model and describes the nitrogen gas sub-model of DAYCENT. The model was used to explore how land use, precipitation, and soil texture impact total nitrogen losses and nitrogen gas emissions at the national scale using county level resolution simulations of cropped lands, grazed land, and native vegetation. Total nitrogen losses and the proportion of total losses due to NO3 leaching both tended to increase with nitrogen inputs. At the national scale, NO3 leaching was the major loss vector for both native and cropped/grazed systems because both nitrogen inputs and leaching are positively correlated with water inputs. However, leaching was responsible for less than half of total nitrogen losses for ∼50% of the counties under native vegetation and ∼15% of the counties for cropped/grazed systems.