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Nitrification inhibitors show promise in decreasing nitrous oxide (N2O) emission from agricultural systems worldwide, but they may be much less effective than previously thought when both direct and indirect emissions are taken into account. Whilst nitrification inhibitors are effective at decreasing direct N2O emission and nitrate ( NO3-) leaching, limited studies suggest that they may increase ammonia ( NH3) volatilization and, subsequently, indirect N2O emission. These dual effects are typically not considered when evaluating the inhibitors as a climate change mitigation tool. Here, we collate results from the literature that simultaneously examined the effects of nitrification inhibitors on N2O and NH3 emissions. We found that nitrification inhibitors decreased direct N2O emission by 0.2-4.5 kg N2O-N ha−1 (8-57%), but generally increased NH3 emission by 0.2-18.7 kg NH3-N ha−1 (3-65%). Taking into account the estimated indirect N2O emission from deposited NH3, the overall impact of nitrification inhibitors ranged from −4.5 (reduction) to +0.5 (increase) kg N2O-N ha−1. Our results suggest that the beneficial effect of nitrification inhibitors in decreasing direct N2O emission can be undermined or even outweighed by an increase in NH3 volatilization. [ABSTRACT FROM AUTHOR]

Understanding nitrogen (N) removal and replenishment is crucial to crop sustainability under rising atmospheric carbon dioxide concentration ([ CO2]). While a significant portion of N is removed in grains, the soil N taken from agroecosystems can be replenished by fertilizer application and N2 fixation by legumes. The effects of elevated [ CO2] on N dynamics in grain crop and legume pasture systems were evaluated using meta-analytic techniques (366 observations from 127 studies). The information analysed for non-legume crops included grain N removal, residue C : N ratio, fertilizer N recovery and nitrous oxide (N2O) emission. In addition to these parameters, nodule number and mass, nitrogenase activity, the percentage and amount of N fixed from the atmosphere were also assessed in legumes. Elevated [ CO2] increased grain N removal of C3 non-legumes (11%), legumes (36%) and C4 crops (14%). The C : N ratio of residues from C3 non-legumes and legumes increased under elevated [ CO2] by 16% and 8%, respectively, but the increase for C4 crops (9%) was not statistically significant. Under elevated [ CO2], there was a 38% increase in the amount of N fixed from the atmosphere by legumes, which was accompanied by greater whole plant nodule number (33%), nodule mass (39%), nitrogenase activity (37%) and %N derived from the atmosphere (10%; non-significant). Elevated [ CO2] increased the plant uptake of fertilizer N by 17%, and N2O emission by 27%. These results suggest that N demand and removal in grain cropping systems will increase under future CO2-enriched environments, and that current N management practices (fertilizer application and legume incorporation) will need to be revised. [ABSTRACT FROM AUTHOR]

The article studies the effects of global warming potential and greenhouse gas intensity in irrigated cropping systems in northeastern Colorado. A no-till (NT) cropping systems study initiated in 1999 to evaluate soil organic carbon (SOC) sequestration potential in irrigated agriculture was used in this study to make trace of gas flux measurements for 3 years to facilitate a complete greenhouse gas accounting of global warming potential (GWP) and greenhouse gas intensity (GHGI). These multi-year data suggest that there is year-to-year variability in trace gas exchange, as demonstrated by Dobbie et al. (1999) in other systems and inter-annual variability in SOC exchange as well.

The article presents the research that measures the fluxes of nitrous oxide and methane in non-urban and urban ecosystems. The topics discussed include the vital role of change of land-use on the soil-atmosphere gas exchange, the net sinks in soils, and the contribution of urban lawns in methane consumption and nitrous oxide emission in regional soils.

© New Phytologist (2004) doi: 10.1111/j.1469-8137.2004.01054.x [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The article presents a study which aims to explore the CO2 response of the shortgrass steppe used for livestock grazing in the North American Great Plains. The study uses three with ambient air and three with air CO2 chambers as well as three unchambered controls to evaluate responses to rising atmospheric CO2. It also suggests the enhancement of the rising atmospheric CO2 production of lower quality forage along with a species composition shift.

Nitrogen recommendations for Upland cotton (Gossypium hirsuturn L.) in the western USA are based on spring soil NO[sup -, sub 3]-N tests. In-season monitoring of plant N status is another approach. Our primary objective was to test spectral reflectance and chlorophyll meter measurements as in-season N decision aids for irrigated cotton, and to compare these with soil test-based N management. The secondary objective was to determine the fate of [sup 15]N as affected by N management and irrigation modes. Urea ammonium nitrate was applied with low energy precision (LEPA) center-pivot, surface drip, and subsurface drip irrigation. Microplots received 3 atom% [sup 15]N. Soil test N application was based on 0- to 60-cm soil NO[sup -, sub 3]-N and 1400 kg lint ha[sup -1] expected yield. Thirty-four kilograms of N per hectare was applied when green vegetative index (GVI) or chlorophyll meter readings relative to well-fertilized plots were <0.95. Lint yield responded to N at Lubbock in 2000 and 2001, but not at Ropesville. Nitrogen applied with in-season monitoring in 2000 at both sites was 34 to 101 kg N ha[sup -1] less than soil test N application of 134 kg ha[sup -1], with similar yields. In Lubbock, 2001 lint yields were near the expected yield, and in three of four cases, N applications with in-season monitoring equaled soil test N applications of 101 kg ha[sup -1]. Nitrogen-15 recovery in plants ranged from 19 to 38%, and was affected by N management in two of three site-years, but not by irrigation. This study indicates that basing N applications on in-season monitoring can reduce N applications in low yielding seasons and match the yield potential in high-yielding seasons. [ABSTRACT FROM AUTHOR]

The article presents a study which examines biotic and abiotic controls over soil nitric oxide (NO) and nitrous oxide (N2O) emissions across a gradient of aboveground Prosopis biomass growing on two soil types. The correlation of NO emissions and nitrification potential with temperature is stated. It is inferred that mesquite encroachment in grasslands increases NO emissions in a spatially explicit manner influenced by soil type and aboveground biomass.

Summary Six open-top chambers were installed on the shortgrass steppe in north-eastern Colorado, USA from late March until mid-October in 1997 and 1998 to evaluate how this grassland will be affected by rising atmospheric CO2. Three chambers were maintained at current CO2 concentration (ambient treatment), three at twice ambient CO2, or approximately 720 μmol mol-1 (elevated treatment), and three nonchambered plots served as controls. Above-ground phytomass was measured in summer and autumn during each growing season, soil water was monitored weekly, and leaf photosynthesis, conductance and water potential were measured periodically on important C3 and C4 grasses. Mid-season and seasonal above-ground productivity were enhanced from 26 to 47% at elevated CO2, with no differences in the relative responses of C3/C4 grasses or forbs. Annual above-ground phytomass accrual was greater on plots which were defoliated once in mid-summer compared to plots which were not defoliated during the growing season, but there was no interactive effect of defoliation and CO2 on growth. Leaf photosynthesis was often greater in Pascopyrum smithii (C3) and Bouteloua gracilis (C4) plants in the elevated chambers, due in large part to higher soil water contents and leaf water potentials. Persistent downward photosynthetic acclimation in P. smithii leaves prevented large photosynthetic enhancement for elevated CO2-grown plants. Shoot N concentrations tended to be lower in grasses under elevated CO2, but only Stipa comata (C3) plants exhibited significant reductions in N under elevated compared to ambient CO2 chambers. Despite chamber warming of 2.6 °C and apparent drier chamber conditions compared to unchambered controls, above-ground production in all chambers was always greater than in unchambered plots. Collectively, these results suggest increased productivity of the shortgrass steppe in future warmer, CO2 enriched environments. [ABSTRACT FROM AUTHOR]

Inappropriate management of irrigation water and fertilizer N in irrigated corn (Zea mays L.) has resulted in excess N leaching from the rooting zone and NO{sub}3{end}{sup}-{end} contamination of groundwater. A better understanding of microbially mediated conversion of plantavailable N to gaseous N forms is needed for more efficient N use, to evaluate the impact of radiatively important gas production such asnitrous oxide (N{sub}2{end}O) on air quality, and to determine the potential of microbial denitrification for remediating high-NO{sub}3{end}{sup}-{end} irrigation water. Soil denitrification and N{sub}2{end}O losses from pivot irrigated corn were measured during the 1991 and1992 growing seasons using surface gas chambers at the Management System Evaluation Area (MSEA) site for water quality research in the Central Platte Valley near Shelton, NE. Denitrification and N{sub}2{end}O losses remained low throughout most of the growing season, generally ranging from 10 to 50 g N ha{sup}-1{end} d{sup}-1{end}, when soil water contents were generally below 60% water-filled pore space (WFPS). Nitrous oxide was the major gaseous N product under these conditions. When soils were wetted by irrigation or rainfall to >70% WFPS, denitrification losses ranged from 0.2 to 1.4 kg N ha{sup}-1{end} d{sup}-1{end} with N{sub}2{end} comprising 80 to 98% of the denitrification gases measured. Monthly average denitrification losses across yearswere highly correlated (r = 0.693) with time of growing season, suggesting a possible link between plant growth and denitrification. Seasonal gaseous N losses via denitrification and N{sub}2{end}O emission from irrigated corn represented 1 to 5% of the N applied as fertilizer or in irrigation water during a relatively dry 1991 season and the 1992 season with near normal precipitation. Results of our 2-yr studyindicate that under good irrigation and N management practices gaseous N losses from denitrification and N{sub}2{end}O production pose little [ABSTRACT FROM AUTHOR]