An Unexpected Nitrate Decline in New Hampshire Streams

Goodale, Christine L.; Aber, John D.; Vitousek, Peter M.

doi:10.1007/s10021-002-0219-0

Cited by 131 publications

(108 citation statements)

References 59 publications

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“…We analyzed archived samples of stream water and bulk deposition since 1990 and fresh samples from 2008 (Methods and SI Results and Discussion). The δ 15 N-NO 3 − signature in archived bulk precipitation [0.47 ± 0.78‰ (average ± SE), n = 8] did not differ from the precipitation in 2008 samples (−1.07 ± 0.44‰, n = 6; Wilcoxon rank sum test, z = −1.48, P > 0.05) and showed no trend with nitrate concentration (Fig. 4C).…”

Section: Resultsmentioning

confidence: 95%

“…Stream nitrate and δ 18 O-NO 3 − values were both low during base flow, but they increased substantially during large snowmelt events (Fig. 4 A and B).…”

Section: Resultsmentioning

confidence: 97%

“…This mechanism might seem unlikely on theoretical grounds, because denitrification generally is high in nitrate-rich (i.e., early HBEF record) but low in nitrate-poor conditions (late HBEF record) (19,20). We used a stable isotope approach that takes advantage of the substantial isotopic discrimination of δ 15 N-NO 3 − that occurs during microbial denitrification (21). We expected a negative correlation between δ 15 N-NO 3 − and nitrate concentration if denitrification was the primary determinant of the stream water nitrate trend.…”

Section: Resultsmentioning

confidence: 99%

“…Because climate is an overriding and powerful driver of biological process, we pay particular attention to whether the observed changes in ecosystem N dynamics were linked to climate change over the past five decades. Previous examinations of this trend have discounted effects of changing N deposition and forest maturation, and they have suggested that there may exist a previously unrecognized ecosystem sink for N (3,4), possibly associated with the soil (5,6). We evaluate these hypotheses and other competing hypotheses using an analysis that integrates several unique long-term records of biological, physical, and biogeochemical factors within the HBEF.…”

mentioning

confidence: 97%

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Complex response of the forest nitrogen cycle to climate change

Bernal

Hedin

Likens

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

136

111

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Climate exerts a powerful influence on biological processes, but the effects of climate change on ecosystem nutrient flux and cycling are poorly resolved. Although rare, long-term records offer a unique opportunity to disentangle effects of climate from other anthropogenic influences. Here, we examine the longest and most complete record of watershed nutrient and climate dynamics available worldwide, which was collected at the Hubbard Brook Experimental Forest in the northeastern United States. We used empirical analyses and model calculations to distinguish between effects of climate change and past perturbations on the forest nitrogen (N) cycle. We find that climate alone cannot explain the occurrence of a dramatic >90% drop in watershed nitrate export over the past 46 y, despite longer growing seasons and higher soil temperatures. The strongest climate influence was an increase in soil temperature accompanied by a shift in paths of soil water flow within the watershed, but this effect explained, at best, only ∼40% of the nitrate decline. In contrast, at least 50-60% of the observed change in the N export could be explained by the long-lasting effect of forest cutting in the early 1900s on the N cycle of the soil and vegetation pools. Our analysis shows that historic events can obscure the influence of modern day stresses on the N cycle, even when analyses have the advantage of being informed by 0.5-century-long datasets. These findings raise fundamental questions about interpretations of long-term trends as a baseline for understanding how climate change influences complex ecosystems.forest ecosystems | long-term monitoring | streamwater chemistry | precipitation chemistry | nutrient cycles O ur understanding of how climate change impacts complex ecological systems depends on our conception of a baseline against which change can be judged and knowledge of how this baseline has been shaped by historical conditions. At the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, for example, we know that current concentrations of nitrate in watershed streams are the lowest in 46 y of measurement and that ecosystem nitrate losses have decreased by >90% over this time (Fig. 1A). If we were to take the early high nitrate period (1969)(1970)(1971)(1972)(1973)(1974)(1975)(1976) as the historical reference, we would estimate that nitrate export has dropped by a total of ∼125 kg nitrogen (N) ha −1 during the 30 y of the decline (Fig. 1A). Such a large drop in N export is ecologically relevant and constitutes a dramatic shift in the ecosystem N cycle: from a leaky cycle that retained only ∼30% of external inputs in the high stream water nitrate period to a highly retentive cycle that currently captures ∼90% of atmospheric inputs (Methods).We adopt a watershed mass balance approach (1, 2) to examine the factor(s) responsible for this dramatic change in the forest N cycle (Fig. 2). Because climate is an overriding and powerful driver of biological process, we pay particular attention to whether the observed changes i...

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Section: Resultsmentioning

confidence: 95%

“…Stream nitrate and δ 18 O-NO 3 − values were both low during base flow, but they increased substantially during large snowmelt events (Fig. 4 A and B).…”

Section: Resultsmentioning

confidence: 97%

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 97%

See 2 more Smart Citations

Complex response of the forest nitrogen cycle to climate change

Bernal

Hedin

Likens

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

136

111

View full text Add to dashboard Cite

show abstract

“…Alternatively, elevated levels of atmospheric CO 2 could be reducing N availability by stimulating microbial immobilization of N (9,10). To date, the only estimates of historical changes in terrestrial N availability are derived from changes in streamwater nitrate concentrations in forested catchments that suggest that forest N availability has declined in many parts of the northeastern United States since the 1970s (11,12). Unfortunately, streamwater nitrate concentrations ambiguously reflect terrestrial N availability because they can be influenced by in-stream chemical processing (13,14).…”

mentioning

confidence: 99%

Changes in nitrogen cycling during the past century in a northern hardwood forest

McLauchlan

Craine

Oswald

et al. 2007

Proc. Natl. Acad. Sci. U.S.A.

113

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Nitrogen (N) availability, defined here as the supply of N to terrestrial plants and soil microorganisms relative to their N demands, limits the productivity of many temperate zone forests and in part determines ecosystem carbon (C) content. Despite multidecadal monitoring of N in streams, the long-term record of N availability in forests of the northeastern United States is largely unknown. Therefore, although these forests have been receiving anthropogenic N deposition for the past few decades, it is still uncertain whether terrestrial N availability has changed during this time and, subsequently, whether forest ecosystems have responded to increased N deposition. Here, we used stable N isotopes in tree rings and lake sediments to demonstrate that N availability in a northeastern forest has declined over the past 75 years, likely because of ecosystem recovery from Euro-American land use. Forest N availability has only recently returned to levels forecast from presettlement trajectories, rendering the trajectory of future forest N cycling uncertain. Our results suggest that chronic disturbances caused by humans, especially logging and agriculture, are major drivers of terrestrial N cycling in forest ecosystems today, even a century after cessation.15 N ͉ land-use history ͉ Mirror Lake ͉ nitrogen availability ͉ paleoecology

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Climate change decreases nitrogen pools and mineralization rates in northern hardwood forests

et al. 2016

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Abstract. Nitrogen (N) supply often limits the productivity of temperate forests and is regulated by a complex mix of biological and climatic drivers. In excess, N is linked to a variety of soil, water, and air pollution issues. Here, we use results from an elevation gradient study and historical data from the longterm Hubbard Brook Ecosystem Study (New Hampshire, USA) to examine relationships between changes in climate, especially during winter, and N supply to northern hardwood forest ecosystems. Low elevation plots with less snow, more soil freezing, and more freeze/thaw cycles supported lower rates of N mineralization than high elevation plots, despite having higher soil temperatures and no consistent differences in soil moisture during the growing season. These results are consistent with historical analyses showing decreases in rates of soil N mineralization and inorganic N concentrations since 1973 that are correlated with long-term increases in mean annual temperature, decreases in annual snow accumulation, and a increases in the number of winter thawing degree days. This evidence suggests that changing climate may be driving decreases in the availability of a key nutrient in northern hardwood forests, which could decrease ecosystem production but have positive effects on environmental consequences of excess N.

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An Unexpected Nitrate Decline in New Hampshire Streams

Abstract: Theories of forest nitrogen (N) cycling suggest that stream N losses should increase in response to chronic elevated N deposition and as forest nutrient requirements decline with age.

Cited by 131 publications

References 59 publications

Complex response of the forest nitrogen cycle to climate change

Complex response of the forest nitrogen cycle to climate change

Changes in nitrogen cycling during the past century in a northern hardwood forest

Climate change decreases nitrogen pools and mineralization rates in northern hardwood forests

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