B24A-01
Effect of Tillage and Non-tillage Agricultural Practice on Nitrogen Losses as NO and N2O in Tropical Corn Fields at Guarico State, Venezuela.
We evaluated the effect of agricultural practices on NO and N2O emissions from corn fields at Northern Guárico, one of Venezuelan largest cereal production regions. Historically, the most common agricultural practice in these regions has been mono cropping. Tillage (T) and non-tillage (NT) of soils represent approximately 30 and 70% of the planted area, respectively. Comparative studies of the nitrogen losses associated with these agricultural practices are not available for these regions. This study was conducted at the farm "Tierra Nueva", Guárico State (9° 23' 33'' N, 66° 38' 30'' W) in two corn fields under tillage and non-tillage agricultural practice during the growing season (June-August 2006). A dry tropical forest, the primary ecosystem of the region, was evaluated for the same period of time. The corn and the forest fields were adjacent; therefore, they were exposed to the same meteorological conditions. The mean annual precipitation of the area is 622±97.3 mm (last 5 years). The soils are Vertisols (Typic Haplusterts). Nutrient soil concentrations (as nitrate and ammonium), water soil content and pH soil were measured in the fields for the same period of time. Soils were fertilized and planted simultaneously by a planting machine provided with a furrow opener where the fertilizer and seeds are incorporated between 0-10 cm depths. Tillage soils were fertilized on June 1st 2006 with 65 kgN/ha of NPK (13:18:16/3MgO, 3S; N as NH4Cl), whereas non-tillage soils were fertilized the next day with 56 kgN/ha of NPK (12:25:12/3MgO, 3S; N as NH4Cl). Second fertilization of both fields was done thirty-seven days later by broadcast adding 58 kgN/ha approximately, using nitrophosphate as fertilizer (NP 33-3: 33% N total; 16.7% N- NO3- and 16.6% N- NH4+). In general, NO and N2O soil emissions from both corn fields increased after fertilization events, and depend on water soil content and nutrient soil concentration. N2O soil emissions were 11 and 9 times larger in comparison to the forest values for the 60-day evaluation period for NT and T, respectively. On the other hand, NO soil emissions were 1.5 and 5 times larger in NT and T fields, respectively, in comparison to forest values. The fertilizer-induced emissions factors (FEI) for NO and N2O show that the nitrogen gaseous losses are mostly in the form of N2O for NT (NO-FEI = 0.2% and N2O-FEI=3.6%) and T (NO-FEI=1.7% and N2O- FEI=2.8%) practices. However, NO losses are higher in T than NT soils, probably due to the higher porosity in the former that promotes NO production under aerobic conditions. These results imply that tillage agricultural practice leads to a higher stimulation of these nitrogen gaseous emissions than non-tillage agricultural practice. Our N2O-FEI values for direct emissions are higher than the proposed by the IPPC national guidelines 2006 (FEI = 1%, Volume 4, Chapter 11). This implies that tropical agroecosystems are more susceptible to increase emissions after fertilization than temperate regions. Our results will be used to produce better estimates of direct N2O emissions from tropical agriculture and improve the current Venezuelan national greenhouse gas inventory.
B24A-02
Nitrogen transport and deposition during the Rocky Mountain Airborne Nitrogen and Sulfur (RoMANS) study
A number of deleterious effects have been noted due to increasing deposition of nitrogen compounds in Rocky Mountain National Park (RMNP). The Rocky Mountain Airborne Nitrogen and Sulfur (RoMANS) study was conducted to improve our understanding of the sources and transport of airborne nitrogen and sulfur species within RMNP as well as their deposition pathways. Two field campaigns were conducted, in spring and summer 2006, to characterize pollutant transport and deposition during seasons with historically high nitrogen inputs. Several measurements sites were operated within the park, at locations west and east of the park boundaries, and at locations near the NE, NW, and SE boundaries of the state of Colorado. Measurements at several sites included 24-hour integrated gas concentrations (ammonia, nitric acid, sulfur dioxide), PM2.5 composition, and wet deposition. A core measurement site in the park included more detailed and higher time resolution chemical, optical, and particle size distribution measurements. An overview of study findings will be presented including the composition of collected PM2.5, concentrations of key trace gas species, and observations of wet and dry deposition composition and fluxes. Concentrations of N species in RMNP varied significantly with local and regional transport patterns. High concentrations of nitrate/nitric acid and ammonia/ammonium observed routinely on the eastern plains of Colorado reflect a mixture of urban and agricultural emissions. The highest concentrations of N species in RMNP were generally associated with upslope transport from the east. Nitrogen deposition in RMNP during the spring campaign was dominated by a single, upslope snowstorm. A combination of high pollutant concentrations and heavy precipitation during this upslope event acted to produce N deposition fluxes that far outweighed other spring precipitation events. During the summer study, by contrast, numerous events contributed more equally to total N wet deposition fluxes. Organic nitrogen, ammonium, and nitrate were all important contributors to N wet deposition. Wet deposition of N substantially exceeded dry deposition inputs.
B24A-03 INVITED
Distinguishing NOx Source Contributions to Wet and Dry Nitrate Deposition in the United States using Stable Isotopes
Nitrate isotopes in wet deposition are useful indicators of NOx source contributions to nitrate formation and NOx oxidation pathways at local and regional scales. Here, we examine whether nitrogen and oxygen isotopes (d15N and d18O, respectively) provide similarly useful information in: 1) wet deposition at the continental scale; and 2) dry deposition at the regional scale. For wet deposition analyses, weekly archived samples (2000) from 156 NADP sites across the United States were pooled into bimonthly, volume-weighted composites and analyzed for d15N and d18O of nitrate. For dry deposition analyses, weekly nitric acid and particulate nitrate samples from eight CASTNET sites in New York, Pennsylvania, and Ohio were pooled into monthly composites from April 2004- April 2005 and analyzed for d15N and d18O. We present spatial and temporal variations in both N and O isotopes, and investigate the critical question of whether these variations are a function of atmospheric processes and/or NOx source contributions. Building on our results from the Northeastern US (Elliott et al., in press), we examine relationships between d15N values in wet and dry deposition and surrounding NOx emissions. At the national scale, we determined that d15N in wet deposition is strongly correlated with the distribution of major NOx sources, including stationary, mobile, and biogenic emissions. Correlations with biogenic sources are strongest during the warmer months and areas of intensive agriculture, particularly the mid-west. Although d15N at high elevation site is correlated with major NOx sources, the correlations are distinct from other regions. d18O values are strongly correlated with temperature and solar radiation, furthering the notion that seasonal variations in photochemistry influence d18O values. In nitric acid and particulate nitrate dry deposition, d15N and d18O exhibit seasonal trends similar to those of wet nitrate deposition, although d15N values are significantly higher in the dry fractions. Similar to the case with wet deposition, d15N values in dry deposition are strongly correlated with surrounding NOx emissions from stationary sources. Finally, we discuss the use of isotopes in wet and dry deposition as a potential tool for monitoring tool long-term reductions in source contributions.
B24A-04 INVITED
Microbial and Nutrient Responses to Enhanced Availability of Nitrogen, Phosphorus, and Carbon Dioxide in a California Grassland
We here report results from a full factorial manipulation of CO2, N, and P availability, using mesocosms planted with six species common to California grasslands. The experiment was conducted in the field as a companion study to the Jasper Ridge Global Change Experiment and was designed to evaluate the role of P limitation as a constraint on ecosystem responses to rising CO2 concentrations and N deposition. This presentation will focus on belowground responses, although the larger study included many aboveground components. Response variables considered here include nutrient fluxes through soil (measured using resin strips), bacterial production and potential nitrification. Preliminary analyses revealed that enhanced N deposition led to increased soil N trapped on resin strips, but only under ambient CO2 and P. Bacterial production and potential activity of nitrite oxidizers both increased in response to P additions, and bacterial production responded positively to N but only under ambient P. These data suggest an increase in belowground allocation by plants in response to enhanced CO2. Increased C availability belowground would be expected to increase production of heterotrophic but not autotrophic microbes (i.e., nitrifiers). Increased nutrient demand under elevated CO2 suggested by resin strip data could result either from direct plant uptake or from enhanced microbial activity. The positive response of both heterotrophic and autotrophic bacteria to P additions suggests that microbial immobilization, in addition to plant uptake, explains the decline in resin N in response to P enrichment.
B24A-05
Nitrogen deposition and photosynthetic performance relations at high-N-loading forests in Switzerland and at a highly variable-N-loading Rocky Mtn. subalpine forest: Fluorometry and the estimation of photosynthetic-efficiency-based critical N loads
Chlorophyll fluorescence (fluorometry) for quantitative plant photosynthetic performance determination is widely accepted in the biological sciences but less so for ecosystem analysis studies. Portable fluorometry has provided plant photosynthetic performance and plant stress status data in a small number of eco-physiological field studies that have investigated several specific atmospheric pollutants' impacts at forested ecosystems. We used fluorometry in N-amendment studies at forest canopies (where amended N is best applied to consider atmospheric N deposition impacts at forested ecosystems). Fluorometry measurements in the context of canopy N-amendment studies have been undertaken at the Niwot (LTER) Colorado subalpine forest (Sievering etal.: Part I, Tellus B 59(3), 483-492; Tomaszewski and Sievering: Part II, Tellus B 59 (3), 493-505) and, more recently, in Europe at several Swiss ILTER forest ecosystems. Our fluorometry measurements have shown (among other results to be presented): ▪ N amendments increased photosynthetic efficiency at the Niwot LTER subalpine forest. Modeling analysis indicates amended N enhanced needle Rubisco content and increased photosynthetic efficiency at this N-limited and 7 kgN ha-1 yr-1 (wet+dry) N loading Spruce-Fir subalpine forest ecosystem; ▪ Beech, Oak and Spruce trees responded differently to N amendments of about twice (wet+fog) growing- season N deposition at Swiss forest ecosystems receiving 20-35 kgN ha-1 annual N loadings.
B24A-06
Estimating Critical Nitrogen Loads for a California Grassland
Rigorously established critical nitrogen loads to protect biodiversity can be effective policy tools for addressing the insidious impacts of atmospheric N-deposition on ecosystems. This presentation describes methods for determining critical N-loads to a California grassland ecosystem by careful examination of the continuum from emissions, transport, atmospheric chemistry, deposition, ecosystem response, and impacts on biodiversity. Nutrient-poor soils derived from serpentinite bedrock support diverse native grasslands with dazzling wildflower displays and numerous threatened and endangered species, including the Bay checkerspot butterfly. Under moderate atmospheric N-deposition, these sites are rapidly invaded by introduced nitrophilous annual grasses in the absence of appropriate grazing or other management. Critical loads to this ecosystem have been approached by measurements of atmospheric concentrations of reactive N gases using Ogawa passive samplers and seasonally averaged deposition velocities. A regional-scale pollution gradient was complemented by a very local-scale pollution gradient extending a few hundred meters downwind of a heavily traveled road in a relatively unpolluted area. The local gradient suggests a critical load of 5 kg-N ha-1 a-1 or less. The passive monitor calculations largely agree with deposition calculated with the CMAQ model at 4 km scale. Emissions of NH3 from catalytic converters are the dominant N-source at the roadway site, and are a function of traffic volume and speed. Plant tissue N-content and 15N gradients support the existence of N-deposition gradients. The complexities of more detailed calculations and measurements specific to this ecosystem include seasonal changes in LAI, temporal coincidence of traffic emissions and stomatal conductance, surface moisture, changes in oxidized versus reduced N sources, and annual weather variation. The concept of a "critical cumulative load" may be appropriate over decadal time scales in this ecosystem and other semi-arid systems where N-export is minimal. http://www.creeksidescience.com
B24A-07
Nitrogen Flux in Watersheds: The Role of Atmospheric Deposition, Waste Water Treatment Plants and Climate Oscillations in Nitrogen Exported to the Coastal Ecosystems
Quantifying the flux of nitrate from different sources in watersheds is important to understand the increased flux of nitrogen to coastal ecosystems. Recent technological advances in chemical sensor networks has demonstrated that chemical variability in aquatic environments are chronically under-sampled, and that many chemical monitoring programs with monthly or daily sampling rates are inadequate to characterize the dominate seasonal, daily or semi-diurnal process and episodic storm event fluxes. The RiverNet program has measured the nitrate flux in the Neuse River Basin, NC on a 15 minute interval over the past six years. Significant diurnal variation has been observed in nitrate concentrations during high and low flow periods associated with waste water treatment plants in the basin. Other species of riverine nitrogen do not show this type of concentration variation. Comparison of 15 minute versus 24 hour nitrate flux calculations show that daily monitoring programs underestimate N flux by 10-40%. Two RiverNet stations were used to estimate nitrate gains in the river from biosolid application fields at one waste water treatment plant. Over a 4 year period non-point source nitrate entering the river from the fields was 50% of the nitrogen released in plant effluent. Non-point source flux from biosolid application fields is event driven and can not be determined from daily or weekly sampling. These results suggest that the importance of waste water treatment plant N flux has been under-estimated in current models. The δ15N and δ 18O composition of nitrate has been used to assess importance of atmospheric sources to watershed N flux, but because of transformations contaminant source tracing with these isotopes has been complicated. We have used multiple isotope tracers of nitrate δ 15N, Δ 17O, δ 18O to distinguish between different N contamination sources, areas of extensive denitrification, and areas of atmospheric N. Areas of extensive denitrification are associated with hydric soils and can be delineated with GIS distribution of hydric soils on watershed scales. Most discrete surface water samples have low concentrations of nitrate Δ 17O, suggesting the importance of atmospheric N in riverine N flux has been overestimated in some studies. Δ 17O of nitrate in groundwater is high in forested areas and low in agricultural areas. Nitrate Δ 17O have distinct peaks during storm events in forested and urban areas during falling discharge. When the atmospheric N flux is integrated over discharge events, atmospheric N can approach 30% of the total N riverine flux in urban areas. Discharge and N flux in the basin has significant inter- annual variations associated with El Nino oscillations modified by the North Atlantic oscillation. Positive JMA and NAO indexes are associated with increased groundwater levels and estuary fish kills. Future changes in these climate oscillations have important implications for water resources policy. http://rivernet.ncsu.edu
B24A-08
The Role of the Nitrogen Cycle in the Climate System
The Fourth Assessment Report of Intergovernmental Panel on Climate Change was released earlier this year and has generated world-wide attention This was the first Working Group 1 report to take an explicit look at the global nitrogen cycle and how changes in the N cycle have impacted the climate system. The Working Group 1 report states the following: "Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. It is very likely that the increase in the combined radiative forcing from carbon dioxide, methane and nitrous oxide has been at least six times faster between1960 to 1999 than over any 40 year period during the two 50 millennia prior to the year 1800. " Changes to the cycling of reactive nitrogen, not the stable atmospheric N2, play an important role in the climate system. The most obvious is the rise in the atmospheric abundance of nitrous oxide since 1750. Nitrous oxide is an important atmospheric tracer that allows us to track global changes to the nitrogen cycle. Nitrogen plays a role in many other aspects of the climate system that are not immediately obvious. Biologically available nitrogen is required for carbon uptake which helps fuel both oceanic and terrestrial carbon uptake. Without the nitrogen fueled carbon uptake, the air-borne fraction of the carbon dioxide released from fossil fuel combustion will increase according to the first coupled climate, carbon and nitrogen simulations done with the NCAR Community Climate System Model (CCSM). NOx (NO+NO2) is one of the necessary precursors for ozone formation that has increased more than thirty eight percent since the pre-industrial era. Understanding the role of sources other than fossil fuel emissions, including soil NOx emissions and lightning formation of NOx are important to understanding ozone formation. Recent European Space Agency satellite observations of NO2 from the GOME and SCHIAMACHY satellites and CCSM simulations underscore the importance of soil emissions of NOx. Nitrogen availability can play an important role in soil methane oxidation and consumption which helps regulate atmospheric methane concentrations. Nitrogen, as ammonia, ammonium and nitrate are important in the formation of aerosols that reflect incoming solar radiation and provide a net cooling to the Earth's surface. Reactive nitrogen, e.g. N2O, NOx, NOy and NHx plays a key role in the regulating the abundance of four of the top five greenhouses gases.