|Groffman, Peter - INST. ECOSYSTEM STUDIES|
|Verchot, Louis - INTL. CTR AGRO-FORESTRY|
|Magill, Alison - UNIV. NEW HAMPSHIRE|
|Aber, John - UNIV. NEW HAMPSHIRE|
|Steudler, Paul - THE ECOSYSTEM CENTER|
Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 4, 2002
Publication Date: March 1, 2003
Citation: VENTEREA, R.T., GROFFMAN, P.M., VERCHOT, L.V., MAGILL, A.H., ABER, J.D., STEUDLER, P.A. NITROGEN OXIDE GAS EMISSIONS FROM TEMPERATE FOREST SOILS RECEIVING LONG-TERM NITROGEN INPUTS. GLOBAL CHANGE BIOLOGY. 2003. 9:346-357. Interpretive Summary: Air pollution deriving from fossil fuel combustion and agricultural fertilizer use results in increased amounts of nitrogen (N)-containing chemicals in the atmosphere, which can subsequently be deposited on land. Additions of atmospheric N to forest ecosystems may have several negative effects, including loss of nutrients, acidification of soils and streams, and leaching of nitrate into groundwater. Very little is known regarding exactly how these added N compounds react within forest soils. It is suspected that some of the N may be released back into the atmosphere as gases after being processed by soil micro-organisms. The objective of this research was to measure rates of N oxide gas emissions from the soil surface in long term experimental plots which have been treated with excess N since 1988. One particular form of gas, nitric oxide (NO), had never been measured in the plots prior to this study. Once NO is released into the atmosphere it can react to form ozone. Ozone in the troposhere is a major air pollutant which can damage forest vegetation and agricultural crops. Ozone is also a greenhouse gas and may contribute to global warming. Over a two-year period, we found that NO emissions were equivalent to 15 - 30% of nitrate leaching rates in some plots, and over 100% of nitrate leaching rates in other plots. We also found that soil acidity generated from the excess N was contributing to NO emissions due to chemical reactions favored at lower pH. This suggests that liming of soils to reduce acidity could reduce NO emissions. Our results also suggest that the high organic matter content of these forest soils may also be promoting NO production. Another important finding was that plots dominated by red pine trees exhibited high NO emission rates when treated with 50 kg of N per hectare per year, but plots containing mainly oak and other deciduous trees showed very little NO emissions with this level of N addition. Only at the higher N treatment level (150 kg N per hectare per year) did the deciduous plots exhibit elevated NO emissions, indicating that the deciduous forest has a greater capacity to absorb N inputs. These findings contribute to our understanding of how fundamental soil processes respond to N additions, and will also aid in the evaluation of regulations governing the emissions of N oxides from fossil fuel burning plants and other sources which may impact terrestrial ecosystems via N deposition.
Technical Abstract: From spring 2000 through fall 2001, we made monthly measurements of nitric oxide and nitrous oxide fluxes in two temperate forest sites in Massachusetts that have been treated since 1988 with different levels of nitrogen (N) to simulate elevated rates of atmospheric N deposition. Plots within a red pine stand which were treated with either 50 or 150 kg N per hectare per year displayed consistently elevated NO fluxes (100 - 200 ug N per square meter per hour) compared to control plots, while only the higher N treatment plot within a mixed hardwood stand displayed similarly elevated NO fluxes. Nitrous oxide fluxes in the N-treated plots were generally < 10 % of NO fluxes. Net nitrification rates and NO production rates measured in mineral and organic soils in the laboratory displayed patterns that were consistent with field NO fluxes. Total N oxide gas flux was positively correlated with contemporaneous measurements of soil nitrification rates and nitrate concentrations. Treatment of soils with acetylene resulted in inhibition of both nitrification and NO production, indicating that autotrophic nitrification was responsible for the elevated NO production. Soil pH was negatively correlated with annual rates of N deposition. Low levels (3 - 11 ug N per kg soil) of nitrite were detected in mineral soils from both sites. Kinetic models describing NO production as a function of the protonated form of nitrite (nitrous acid) adequately described the mineral soil data. The results indicate that atmospheric deposition may generate losses of gaseous NO from forest soils by promoting rapid nitrification, and that the response may vary significantly between forest types. The lowering of soil pH resulting from nitrification and/or directly from atmospheric deposition may also play a role by promoting abiotic reactions involving nitrous acid.