No‐tillage (NT) management has been promoted as a practice capable of offsetting greenhouse gas (GHG) emissions because of its ability to sequester carbon in soils. However, true mitigation is only ...possible if the overall impact of NT adoption reduces the net global warming potential (GWP) determined by fluxes of the three major biogenic GHGs (i.e. CO2, N2O, and CH4). We compiled all available data of soil‐derived GHG emission comparisons between conventional tilled (CT) and NT systems for humid and dry temperate climates. Newly converted NT systems increase GWP relative to CT practices, in both humid and dry climate regimes, and longer‐term adoption (>10 years) only significantly reduces GWP in humid climates. Mean cumulative GWP over a 20‐year period is also reduced under continuous NT in dry areas, but with a high degree of uncertainty. Emissions of N2O drive much of the trend in net GWP, suggesting improved nitrogen management is essential to realize the full benefit from carbon storage in the soil for purposes of global warming mitigation. Our results indicate a strong time dependency in the GHG mitigation potential of NT agriculture, demonstrating that GHG mitigation by adoption of NT is much more variable and complex than previously considered, and policy plans to reduce global warming through this land management practice need further scrutiny to ensure success.
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.
A 2-year study was conducted to investigate the potential of no-till cropping systems to reduce N2O and NO emissions under different N application rates in an irrigated corn field in northeastern ...Colorado. Flux measurements were begun in the spring of 2003, using vented (N2O) and dynamic (NO) chambers, one to three times per week, year round, within plots that were cropped continuously to corn (Zea mays L.) under conventional-till (CT) and no-till (NT). Plots were fertilized at planting in late April with rates of 0, 134 and 224 kg N ha-1 and corn was harvested in late October or early November each year. N2O and NO fluxes increased linearly with N application rate in both years. Compared with CT, NT did not significantly affect the emission of N2O but resulted in much lower emission of NO. In 2003 and 2004 corn growing seasons, the increase in N2O-N emitted per kg ha-1 of fertilizer N added was 14.5 and 4.1 g ha-1 for CT, and 11.2 and 5.5 g ha-1 for NT, respectively. However, the increase in NO-N emitted per kg ha-1 of fertilizer N added was only 3.6 and 7.4 g ha-1 for CT and 1.6 and 2.0 g ha-1 for NT in 2003 and 2004, respectively. In the fallow season (November 2003 to April 2004), much greater N2O (2.0-3.1 times) and NO (13.1-16.8 times) were emitted from CT than from NT although previous N application did not show obvious carry-over effect on both gas emissions. Results from this study reveal that NT has potential to reduce NO emission without an obvious change in N2O emission under continuous irrigated corn cropping compared to CT.
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 N(2)O emissions is described in detail. Nitrous oxide emissions from simulations of presettlement native vegetation were subtracted from cropped soil N(2)O 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 C(O)2 equivalents for direct and indirect, respectively, mean annual anthropogenic N(2)O emissions for 1990-2003.
Land-use change is an important driver of soil-atmosphere gas exchange, but current greenhouse-gas budgets lack data from urban lands. Field comparisons of urban and non-urban ecosystems are required ...to predict the consequences of global urban-land expansion for greenhouse-gas budgets. In a rapidly urbanizing region of the U.S. Great Plains, we measured soil-atmosphere exchange of methane (CH4) and nitrous oxide (N2O) for one year in replicated (n = 3) urban lawn, native shortgrass steppe, dryland wheat-fallow, and flood-irrigated corn ecosystems. All soils were net sinks for atmospheric CH4, but uptake by urban, corn, and wheat-fallow soils was half that of native grasslands ($-0.30 \pm 0.04 g C\cdot m^{-2}\cdot yr^{-1}$mean ± 1 SE). Urban ($0.24 \pm 0.03 g N\cdot m^{-2}\cdot yr^{-1}$) and corn ($0.20 \pm 0.02 g N\cdot m^{-2}\cdot yr^{-1}$) soils emitted 10 times more N2O to the atmosphere than native grassland and wheat-fallow soils. Using remotely sensed land-cover data we calculated an upper bound for the contribution of lawns to regional soil-atmosphere gas fluxes. Urban lawns occupied 6.4% of a$1578-km^2$study region, but contribute up to 5% and 30% of the regional soil CH4consumption and N2O emission, respectively, from land-use types that we sampled. Lawns that cover small portions of the landscape may contribute significantly to regional soil-atmosphere gas exchange.
When appraising the impact of food and fiber production systems on the composition of the Earth's atmosphere and the ‘greenhouse’ effect, the entire suite of biogenic greenhouse gases – carbon ...dioxide (CO2), methane (CH4), and nitrous oxide (N2O) – needs to be considered. Storage of atmospheric CO2 into stable organic carbon pools in the soil can sequester CO2 while common crop production practices can produce CO2, generate N2O, and decrease the soil sink for atmospheric CH4. The overall balance between the net exchange of these gases constitutes the net global warming potential (GWP) of a crop production system. Trace gas flux and soil organic carbon (SOC) storage data from long-term studies, a rainfed site in Michigan that contrasts conventional tillage (CT) and no-till (NT) cropping, a rainfed site in northeastern Colorado that compares cropping systems in NT, and an irrigated site in Colorado that compares tillage and crop rotations, are used to estimate net GWP from crop production systems. Nitrous oxide emissions comprised 40–44% of the GWP from both rain-fed sites and contributed 16–33% of GWP in the irrigated system. The energy used for irrigation was the dominant GWP source in the irrigated system. Whether a system is a sink or source of CO2, i.e. net GWP, was controlled by the rate of SOC storage in all sites. SOC accumulation in the surface 7.5 cm of both rainfed continuous cropping systems was approximately 1100 kg CO2 equivalents ha−1 y−1. Carbon accrual rates were about three times higher in the irrigated system. The rainfed systems had been in NT for >10 years while the irrigated system had been converted to NT 3 years before the start of this study. It remains to be seen if the C accrual rates decline with time in the irrigated system or if N2O emission rates decline or increase with time after conversion to NT.
Effects of large-scale weed invasion on the nature and magnitude of moisture-pulse-driven soil processes in semiarid ecosystems are not clearly understood. The objective of this study was to monitor ...carbon dioxide (CO2) and nitrous oxide (N2O) emissions and changes in soil carbon (C) and nitrogen (N) following the application of a water pulse in Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) communities dominated by the exotic annual grass cheatgrass (Bromus tectorum) and by the native perennial grass western wheatgrass (Pascopyrum smithii). Sampling locations were established in shrub interspaces dominated by B. tectorum and P. smithi and beneath shrub canopies adjacent to interspaces dominated by B. tectorum and P. smithi, where no grass was present. Soils were classified as fine-loamy, mixed, Borollic Haplargids. Soil samples (0–10cm) and air samples were collected at 0, 4, 8, 24, 49, 72, and 216h following additions of 25.4mm of water. Soil samples were analyzed for dissolved organic carbon (DOC), microbial biomass carbon (MBC), extractable ammonia (NH4+), extractable nitrate (NO3−), and dissolved organic nitrogen (DON). Grass species induced differences in soil nitrification, N2O and CO2 emissions, and the quantity and timing of labile C available to microbial populations responding to increased moisture availability. In the first 8-h phase after wetting P. smithii soils had the greatest CO2 emissions compared to other soils but B. tectorum soils had the greatest N2O emissions and the greatest increases in CO2 emissions relative to before wetting. Microbial biomass C in B. tectorum interspace soils increased rapidly but the response was short-lived despite sufficient water availability. After the first 8h of soil response to wetting, the observed MBC declines in B. tectorum interspace coincided with disproportional DOC and DON concentration increases. Similar DOC and DON increases were also observed in B. tectorum soils beneath shrub canopy. In contrast, DOC and DON concentrations in P. smithii soils remained unaffected by soil wetting and small MBC increases observed during the first 8-h phase did not decline as rapidly as in B. tectorum interspace soils. In conclusion, summer drying-wetting cycles that occur frequently in areas invaded by B. tectorum can accelerate rates of nitrification and C mineralization, and contribute significantly to trace gas emissions from sagebrush-steppe grasslands. With frequent summer rainfall events, the negative consequences B. tectorum presence in the ecosystem can be significant.
Contributors to the Intergovernmental Panel on Climate Change (IPCC) generally agree that increases in the atmospheric concentration of greenhouse trace gases (i.e., CO2, CH4, N2O, O3) since ...preindustrial times, about the year 1750, have led to changes in the earth's climate. During the past 250 years the atmospheric concentrations of CO2, CH4, and N2O have increased by 30, 145, and 15%, respectively. A doubling of preindustrial CO2 concentrations by the end of the twenty-first century is expected to raise global mean surface temperature by about 2 degrees C and increase the frequency of severe weather events. These increases are attributed mainly to fossil fuel use, land-use change, and agriculture. Soils and climate changes are related by bidirectional interactions. Soil processes directly affect climatic changes through the production and consumption of CO2, CH4, and N2O and, indirectly, through the production and consumption of NH3, NO(x), and CO. Although CO2 is primarily produced through fossil fuel combustion, land-use changes, conversion of forest and grasslands to agriculture, have contributed significantly to atmospheric increase of CO2. Changes in land use and management can also result in the net uptake, sequestration, of atmospheric CO2, CH4 and N2O are produced (30% and 70%, respectively) in the soil, and soil processes will likely regulate future changes in the atmospheric concentration of these gases. The soil-atmosphere exchange of CO2, CH4, and N2O are interrelated, and changes in one cycle can impart changes in the N cycle and resulting soil-atmosphere exchange of N2O. Conversely, N addition increases C sequestration. On the other hand, soil processes are influenced by climatic change through imposed changes in soil temperature, soil water, and nutrient competition. Increasing concentrations of atmospheric CO2 alters plant response to environmental parameters and frequently results in increased efficiency in use of N and water. In annual crops increased CO2 generally leads to increased crop productivity. In natural systems, the long-term impact of increased CO2 on ecosystem sustainability is not known. These changes may also result in altered CO2, CH4, and N2O exchange with the soil. Because of large temporal and spatial variability in the soil-atmosphere exchange of trace gases, the measurement of the absolute amount and prediction of the changes of these fluxes, as they are impacted by global change on regional and global scales, is still difficult. In recent years, however, much progress has been made in decreasing the uncertainty of field scale flux measurements, and efforts are being directed to large scale field and modeling programs. This paper briefly relates soil process and issues akin to the soil- atmosphere exchange of CO2, CH4, and N2O. The impact of climate change, particularly increasing atmospheric CO2 concentrations, on soil processes is also briefly discussed.
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.
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