Agricultural intensification and ecosystem properties.

Expansion and intensification of cultivation are among the predominant global changes of this century. Intensification of agriculture by use of high-yielding crop varieties, fertilization,irrigation, and pesticides has contributed substantially to the tremendous increases in food production over the past 50 years. Land conversion and intensification,however, also alter the biotic interactions and patterns of resource availability in ecosystems and can have serious local, regional, and global environmental consequences.The use of ecologically based management strategies can increase the sustainability of agricultural production while reducing off-site consequences.

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.

DAYCENT national-scale simulations of nitrous oxide emissions from cropped soils in the United States.

Until recently, Intergovernmental Panel on Climate Change (IPCC) emission factor methodology, based on simple empirical relationships, has been used to estimate carbon (C) and nitrogen (N) fluxes for regional and national inventories. However, the 2005 USEPA greenhouse gas inventory includes estimates of N2O emissions from cultivated soils derived from simulations using DAYCENT, a process-based biogeochemical model. DAYCENT simulated major U.S. crops at county-level resolution and IPCC emission factor methodology was used to estimate emissions for the approximately 14% of cropped land not simulated by DAYCENT. The methodology used to combine DAYCENT simulations and IPCC methodology to estimate direct and indirect N2O emissions is described in detail. Nitrous oxide emissions from simulations of presettlement native vegetation were subtracted from cropped soil N2O to isolate anthropogenic emissions. Meteorological data required to drive DAYCENT were acquired from DAYMET, an algorithm that uses weather station data and accounts for topography to predict daily temperature and precipitation at 1-km2 resolution. Soils data were acquired from the State Soil Geographic Database (STATSGO). Weather data and dominant soil texture class that lie closest to the geographical center of the largest cluster of cropped land in each county were used to drive DAYCENT. Land management information was implemented at the agricultural-economic region level, as defined by the Agricultural Sector Model. Maps of model-simulated county-level crop yields were compared with yields estimated by the USDA for quality control. Combining results from DAYCENT simulations of major crops and IPCC methodology for remaining cropland yielded estimates of approximately 109 and approximately 70 Tg CO2 equivalents for direct and indirect, respectively, mean annual anthropogenic N2O emissions for 1990-2003.

Impact of management practices on the tallgrass prairie.

The ELM ecosystem-level grassland model simulates the flow of water, heat, nitrogen, and phosphorus through the ecosystem and the biomass dynamics of plants, consumers, and the decomposers. This model was adapted to a tallgrass prairie site in northeastern Oklahoma, USA, the Osage Site of the U.S. International Biological Program Grassland Biome. Several range management manipulations were simulated by the model and the results compared to field data and literature information: (1) altering the grazing intensity, grazing system, and grazing time period; (2) adding nitrogen and phosphorus to the grassland; (3) adding water during the growing season; and (4) spring burning of the prairie.The model showed that cattle weight gain per head, above-ground and belowground plant production, transpiration water loss, standing dead biomass, and the net nitrogen balance decrease with increasing grazing intensity, while soil water content and bare soil water loss increase. A moderately stocked year-round cow-calf grazing system is more beneficial to the grassland than a more highly stocked seasonal steer grazing system because the former increases the aboveground and belowground primary production and the plant nutrient uptake rates. Range manipulations, such as fire, which stimulate uniform grazing of a pasture, increase primary production, cattle weight gains, and nutrient uptake of plants and animals. Model results indicated that adding fertilizer was the best strategy for increasing cattle weight gains per head, while adding water would produce the greatest increase in primary production. Simulation of yearly and triennial spring burns suggests that these treatments increase primary production, plant nutrient uptake, and cattle weight gain per head. Burning increases the nitrogen losses from the systems; however, these losses are greater with annual burns. The model results also suggest the spatial grazing pattern of cattle must be considered to correctly represent the impact of grazing on the prairie.The model is used to describe the behavior of the tallgrass prairie ecosystem, evaluate alternative management strategies, and identify future scientific research and management studies.

A simulation model of Bouteloua gracilis biomass dynamics on the North American shortgrass prairie.

A grassland primary producer model for simulating intraseasonal biomass dynamics as a function of temperature, moisture, light, and nitrogen was developed for Bouteloua gracilis (H.B.K.) Lag., the dominant C4 grass of the North American shortgrass prairie. Plant state variables included young and mature leaves, crowns, and roots from three depth categories while simulated processes included spring regrowth, photosynthesis, respiration, photosynthate allocation, death, and litterfall. Sensitivity analyses revealed the model was most sensitive to changes in photosynthesis and photosynthate allocation and least sensitive to changes in initial values of state variables, leaf dark respiration rates, and rate of spring regrowth.An abiotic submodel driven by observed weather data was used in conjunction with the primary producer model to simulate plant biomass dynamics under a variety of conditions including untreated controls (C), nitrogen fertilization (F), irrigation (I), and irrigation plus fertilization (IF). Model predictions of life shoot biomass (B s) and annual aboveground net primary production (NPP A) followed the same trends as field measurements with B sand NPP Aof IF>I>F>C. Failure of the model to accurately predict measured declines in peak B sand NPP Aafter several years of irrigation may have been caused by failure to account for growth lags following water stress, inadequate simulation of interspecific competition, or failure to simulate response to some mineral nutrients which had become limiting after several years of this treatment. A simulated annual carbon budget for plants in the four treatments suggests that from 61% (IF) to 80% (C) of the net carbon fixed above ground is ultimately translocated and utilized below ground.

An empirical model for estimating CO2 exchange of Bouteloua gracilis (H.B.K.) Lag. in the shortgrass prairie.

An empirical model for predicting net photosynthesis (P N ) and dark respiration (R D ) in the field was developed and tested for Bouteloua gracilis (H.B.K.) Lag., the dominant C4 grass of the North American shortgrass prairie. P N is predicted as a function of soil water potential, canopy air temperature, irradiance, and plant age, while R D is expressed as a function of soil water potential and temperature. The model accounted for 85% of the variability in the data base used to estimate parameter values. Results of a validation test showed good agreement between observed and predicted P N rates, suggesting this approach would be useful as a submodel of a grassland ecosystem model.

Water status of soil and vegetation in a shortgrass steppe.

In an attempt to describe some major relationships between soil and plant compartments in a shortgrass steppe, the process of water loss from the system and plant water relations throughout a drying cycle were studied. The water supply was manipulated and some soil and plant variables monitored throughout a drying cycle. Leaf conductance and leaf water potential of blue grama (Bouteloua gracilis) were measured periodically at predawn and noon. Soil water content and water potential of different layers were also monitored.Three different periods were distinguished in the water loss process throughout a drying cycle. These distinctions were made taking into account the relative contribution of different soil layers. Leaf conductance and water potential at noon slowly declined throughout the first 50 days of plant growth. After that, they rapidly decreased, reaching values of 0.29 mm s-1 and-5.0 MPa, respectively. The predawn leaf water potential remained unchanged around-0.5 MPa during the first 45 days, then rapidly decreased. This occurred when soil water of the wettest soil layer was near depletion.Predawn leaf water potentials were highly correlated with water potentials of the wettest layer. Leaf conductance and water potential at noon were correlated with effective soil water potential (soil water potential weighted by the root distribution in the profile). We concluded that root surface area limited the water flow through an important part of the day in this semiarid ecosystem. Axial root resistance did not appear important in determining the equilibrium status between leaves and the wettest soil layer.

Effects of available P and N:P ratios on non-symbiotic dinitrogen fixation in tallgrass prairie soils.

Prescribed burning is a major control over element cycles in Tallgrass prairie (Eastern Kansas, USA). In this paper we report potential effects of fire on nonsymbiotic nitrogen fixation. Fire resulted in additions of available P in ash, which may stimulate nitrogen fixation by terrestrial cyanobacteria. Cyanobacterial nitrogenase activity and biomass responded positively to additions of ash or P in laboratory assays using soil. Further assays in soil showed that cyanobacteria responded to changes in available N:available P ratio (aN:P) across a range of concentrations. Nitrogen fixation rate could be related empirically to aN:P via a log-linear relationship. Extrapolation of laboratory results to the field yielded a maximal estimate of 21 kg N ha-1 y-1. Results support arguments from the marine and terrestrial literature that P availability is central to regulation of ecosystem N budgets.

Long-term effects of clipping and nitrogen management in turfgrass on soil organic carbon and nitrogen dynamics: the CENTURY model simulation.

Experiments to document the long-term effects of clipping management on N requirements, soil organic carbon (SOC), and soil organic nitrogen (SON) are difficult and costly and therefore few. The CENTURY ecosystem model offers an opportunity to study long-term effects of turfgrass clipping management on biomass production, N requirements, SOC and SON, and N leaching through computer simulation. In this study, the model was verified by comparing CENTURY-predicted Kentucky bluegrass (Poa pratensis L.) clipping yields with field-measured clipping yields. Long-term simulations were run for Kentucky bluegrass grown under home lawn conditions on a clay loam soil in Colorado. The model predicted that compared with clipping-removed management, returning clippings for 10 to 50 yr would increase soil C sequestration by 11 to 25% and nitrogen sequestration by 12 to 28% under a high (150 kg N ha(-1) yr(-1) nitrogen (N) fertilization regime, and increase soil carbon sequestration by 11 to 59% and N sequestration by 14 to 78% under a low (75 kg N ha(-1) yr(-1)) N fertilization regime. The CENTURY model was further used as a management supporting system to generate optimal N fertilization rates as a function of turfgrass age. Returning grass clippings to the turf-soil ecosystem can reduce N requirements by 25% from 1 to 10 yr after turf establishment, by 33% 11 to 25 yr after establishment, by 50% 25 to 50 yr after establishment, and by 60% thereafter. The CENTURY model shows potential for use as a decision-supporting tool for maintaining turf quality and minimizing negative environmental impacts.

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.

Equilibration of the terrestrial water, nitrogen, and carbon cycles.

Recent advances in biologically based ecosystem models of the coupled terrestrial, hydrological, carbon, and nutrient cycles have provided new perspectives on the terrestrial biosphere's behavior globally, over a range of time scales. We used the terrestrial ecosystem model Century to examine relationships between carbon, nitrogen, and water dynamics. The model, run to a quasi-steady-state, shows strong correlations between carbon, water, and nitrogen fluxes that lead to equilibration of water/energy and nitrogen limitation of net primary productivity. This occurs because as the water flux increases, the potentials for carbon uptake (photosynthesis), and inputs and losses of nitrogen, all increase. As the flux of carbon increases, the amount of nitrogen that can be captured into organic matter and then recycled also increases. Because most plant-available nitrogen is derived from internal recycling, this latter process is critical to sustaining high productivity in environments where water and energy are plentiful. At steady-state, water/energy and nitrogen limitation "equilibrate," but because the water, carbon, and nitrogen cycles have different response times, inclusion of nitrogen cycling into ecosystem models adds behavior at longer time scales than in purely biophysical models. The tight correlations among nitrogen fluxes with evapotranspiration implies that either climate change or changes to nitrogen inputs (from fertilization or air pollution) will have large and long-lived effects on both productivity and nitrogen losses through hydrological and trace gas pathways. Comprehensive analyses of the role of ecosystems in the carbon cycle must consider mechanisms that arise from the interaction of the hydrological, carbon, and nutrient cycles in ecosystems.

Physiological responses of plant populations to herbivory and their consequences for ecosystem nutrient flow.

We explored how responses of two populations variable in grazing tolerance provide feedbacks to nutrient supply by controlling carbon supply to soil heterotrophs. The study focused on differences in production and carbon and nitrogen allocation patterns between the two populations. The grazing-tolerant population, or on-colony population, is found on intensively grazed prairie dog colonies, and a grazing-intolerant population, the off-colony population, is found in uncolonized grasslands. Equations describing the production and allocation responses to defoliation for the two ecotypes described were incorporated into CENTURY, a nutrientcycling simulation model. Simulations showed an increase in plant production that paralleled increases in net nitrogen mineralization. Production was greater with grazing and was maintained at higher grazing intensities for the on-colony than the off-colony population. Differences between the populations provided important controls over nitrogen losses. Feedbacks between plant responses to grazing and nitrogen cycling accounted for increased nitrogen availability with grazing. These feedbacks were more important determinants of ecosystem function than were fertilization effects of urine and feces deposition. The simulation results suggest that ecosystem function may be sensitive to physiological differences in population responses to periodic disturbances like herbivory.

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.