|Groffman, Peter - INST. ECOSYSTEM STUDIES|
|Verchot, Louis - INTL. CTR AGRO-FOREST|
|Magill, Alison - UNIV. NEW HAMPSHIRE|
|Aber, John - UNIV. NEW HAMPSHIRE|
Submitted to: Forest Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 22, 2004
Publication Date: July 12, 2004
Citation: Venterea, R.T., Groffman, P.M., Verchot, L.V., Magill, A.H., Aber, J.D. 2004. Gross nitrogen process rates in temperate forest soils exhibiting symptoms of nitrogen saturation. Forest Ecology and Management. 196:335-349. Interpretive Summary: Long-term experiments at the Harvard Forest in Massachusetts have been investigating the responses of a red pine and a mixed hardwood forest to experimental inputs of excess nitrogen (N). The experimental additions are designed to simulate inputs of N which many forests located downwind of intensive industrial or agricultural areas receive via deposition from the atmosphere. Studies to date have shown that both forests are exhibiting symptoms of "N saturation". That is, a significant amount of the added N is being leached below the root zone as nitrate or emitted as N oxide gases back into the atmosphere. Both of these effects have negative environmental consequences. The objective of this study was to examine the fundamental processes within the soils which are controlling these N loss pathways. We measured several pathways simultaneously, including N mineralization, nitrification, immobilization, and N oxide gas production consumption. Nitrification rates were elevated in soils from both forests that have been receiving N inputs, while N mineralization rates did not differ with the level of N addition. Immobilization of N by the hardwood soils tended to decrease with the level of N addition, suggesting that the capacity of the soils to retain N has been depleted after 12 years of N inputs. In contrast, immobilization rates tended to increase with N addition in the pine forest.Differences in nitric oxide gas production rates were consistent with patterns of nitrification, and accounted for up to 19 % of nitrification rates. Nitric oxide was found to be highly reactive once produced in the soil, with half-lives estimated on the order of 2 - 20 sec. The nitric oxide gas production decreased and the nitrous oxide: nitric oxide ratio increased with increasing soil water content. These results help to improve our fundamental understanding of the processes controlling nitrate leaching and N oxide gas emission in temperate forests which are subject to persistent atmospheric N deposition, and also provide some quantitative basis for estimating rates of the underlying processes. The findings should be useful for both scientists and policy-makers concerned with regulating the levels of N which are deposited onto our forests.
Technical Abstract: We made concurrent measurements of N mineralization, nitrification, nitrate and ammonium immobilization, nitrous and nitric oxide production, and nitric oxide consumption rates in soils from long-term N addition plots at Harvard Forest. Gross nitric oxide production rates were elevated above controls in soils from the Pine forest receiving 50 and 150 kg N per ha per year and in soils from the Hardwood forest receiving 150 per ha per year, consistent with previously observed patterns in net process rates. Production of nitric oxide accounted for up to 19 % of gross nitrification rates (NRs) in N-amended soils. Rates of nitrous oxide gas production accounted for < 3 % of NRs, and differences with respect to level of N addition were generally not significant. Values of gross nitric oxide production decreased with increasing soil water content in soils from the high-N plots, and increases in the nitrous oxide: nitric oxide ratio with increasing soil water content were well-described using simple exponential growth equations (r2 = 0.99). Nitric oxide consumption rates were equivalent to half-lives on the order of 2 - 20 sec, indicating that nitric oxide is rapidly consumed in these soils. Nitric oxide consumption rates generally did not differ with level of N addition. Measurements of field subsurface nitric oxide production in the Hardwood forest further demonstrated that > 95 % of the nitric oxide produced in the upper 0.15 m was consumed prior to being emitted at the soil surface. Gross nitrification rates varied in a manner that was consistent with patterns of nitric oxide production. Gross NRs represented < 3 % to > 90 % of gross N mineralization rates (MRs). Gross MRs did not differ between treatments. Absolute rates of ammonium immobilization and rates of nitrate immobilization per unit of substrate in Hardwood organic soils decreased with level of N addition, suggesting that the N retention capacity of the Hardwood forest soil has been partly depleted by persistent N inputs. In contrast, immobilization rates increased or did not vary with N addition in soils from the Pine forest.