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The goal of this project is to test the overarching hypothesis that positive feedback mechanisms involving changes in seasonal cycles that diminish N availability to plants such that plant N demand is not met by soil N availability in northern forests. Specifically, we hypothesize that increasing N demand by plants (induced by increasing temperatures, longer growing seasons, and other environmental changes) leads to greater N resorption by trees in autumn, increased C:N in litter, and greater net immobilization of N by soil microbes in the following spring. However, the timing of snowmelt and soil freezing in spring may further affect net mineralization and N availability for plants. These hypotheses are being tested with a combination of observational, experimental, and modeling approaches at Hubbard Brook Experimental Forest in New Hampshire: 1) measurements at 14 previously established sites along an elevation/aspect climate gradient; 2) litter and snow manipulation experiments at six sites along the climate gradient to create variation in soil climate conditions and microbial N immobilization during spring. We leveraged 14 sites previously established along an elevation and aspect-driven climate gradient at Hubbard Brook as a “natural climate experiment" to test our hypothesis that a positive feedback between N cycling during fall senescence and spring contributes to declining N availability in northern forests. This elevation gradient encompasses variation in mean annual air temperature of ~2.5 °C that is similar to the change projected to occur with climate change over the next 50–100 years in the northeastern U.S. There is relatively little variation in soils along the gradient. We are utilizing three sites at higher elevation (~550-660 m, north facing) and three sites at lower elevation (~375-500 m, south facing) for the litter and snow manipulation experiments to maximize the differences in temperature among the 14 sites. Litterbox manipulation: The objective of the litterfall manipulation experiment is to determine whether increases in autumn litter C:N ratios contribute to greater N immobilization by microbes and reductions in net mineralization and plant N uptake in spring, and ultimately, N oligotrophication in northern forest ecosystems. We applied early (low C:N litter that is lost from from hardwood foliage in the first two weeks of autumn) and late (high C:N litter that falls in the last two weeks of autumn) season litter in October 2022 that was collected in fall 2021 at rates equal to standing mass of litter (300 g m2). We also applied native litter that was collected from the forest floor of each intensive site to represent background levels of C:N in litter samples. This litter was applied to one litterbox at each of the six intensive sites. Following application of litter, we installed deer netting around and on top of each of the litterboxes to eliminate litter loss from wind. Soil samples were collected from these plots in November 2021, April 2022, May 2022, June 2022, November 2022, April 2023, May 2023, June 2023 and anlalysed for microbial biomass and activity, as described in the methods section. Snow manipulation: The objective of the snow manipulation experiment is to determine whether the timing of spring snowmelt, length of the spring, and soil freezing in spring affect microbial N immobilization, hydrologic losses, net mineralization, and plant N uptake. The snow manipulation treatment was conducted in the spring of 2022 and 2023. We manually halved (Removal treatment) or doubled (Addition treatment) snow water equivalent (SWE) in experimental plots in March of 2022 and 2023 to accelerate or delay by an average of one week, respectively, the onset of spring snowmelt. Soil Samples were collected from these plots in November 2021, April 2022, May 2022, June 2022, November 2022, April 2023, May 2023, June 2023 and anlalysed for microbial biomass and activity, as described in the methods section. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station.
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Hubbard Brook Nitrogen Oligotrophication (HBNO): In-situ Nitrogen Mineralization and Nitrification, 2021-2023
The goal of this project is to test the overarching hypothesis that positive feedback mechanisms involving changes in seasonal cycles that diminish N availability to plants such that plant N demand is not met by soil N availability in northern forests. Specifically, we hypothesize that increasing N demand by plants (induced by increasing temperatures, longer growing seasons, and other environmental changes) leads to greater N resorption by trees in autumn, increased C:N in litter, and greater net immobilization of N by soil microbes in the following spring. However, the timing of snowmelt and soil freezing in spring may further affect net mineralization and N availability for plants. These hypotheses are being tested with a combination of observational, experimental, and modeling approaches at Hubbard Brook Experimental Forest in New Hampshire: 1) measurements at 14 previously established sites along an elevation/aspect climate gradient; 2) litter and snow manipulation experiments at six sites along the climate gradient to create variation in soil climate conditions and microbial N immobilization during spring. We leveraged 14 sites previously established along an elevation and aspect-driven climate gradient at Hubbard Brook as a “natural climate experiment" to test our hypothesis that a positive feedback between N cycling during fall senescence and spring contributes to declining N availability in northern forests. This elevation gradient encompasses variation in mean annual air temperature of ~2.5 °C that is similar to the change projected to occur with climate change over the next 50–100 years in the northeastern U.S. There is relatively little variation in soils along the gradient. We are utilizing three sites at higher elevation (~550-660 m, north facing) and three sites at lower elevation (~375-500 m, south facing) for the litter and snow manipulation experiments to maximize the differences in temperature among the 14 sites. Litterbox manipulation: The objective of the litterfall manipulation experiment is to determine whether increases in autumn litter C:N ratios contribute to greater N immobilization by microbes and reductions in net mineralization and plant N uptake in spring, and ultimately, N oligotrophication in northern forest ecosystems. We applied early (low C:N litter that is lost from from hardwood foliage in the first two weeks of autumn) and late (high C:N litter that falls in the last two weeks of autumn) season litter in October 2022 that was collected in fall 2021 at rates equal to standing mass of litter (300 g m2). We also applied native litter that was collected from the forest floor of each intensive site to represent background levels of C:N in litter samples. This litter was applied to one litterbox at each of the six intensive sites. Following application of litter, we installed deer netting around and on top of each of the litterboxes to eliminate litter loss from wind. Soil samples were collected from these plots in November 2021, April 2022, May 2022, June 2022, November 2022, April 2023, May 2023, June 2023 and anlalysed for Nitrogen mineralization and nitrification, as described in the methods section. Snow manipulation: The objective of the snow manipulation experiment is to determine whether the timing of spring snowmelt, length of the spring, and soil freezing in spring affect microbial N immobilization, hydrologic losses, net mineralization, and plant N uptake. The snow manipulation treatment was conducted in the spring of 2022 and 2023. We manually halved (Removal treatment) or doubled (Addition treatment) snow water equivalent (SWE) in experimental plots in March of 2022 and 2023 to accelerate or delay by an average of one week, respectively, the onset of spring snowmelt. Soil Samples were collected from these plots in November 2021, April 2022, May 2022, June 2022, November 2022, April 2023, May 2023, June 2023 and analysed for Nitrogen mineralization and nitrification, as described in the methods section. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station.
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- PAR ID:
- 10573265
- Publisher / Repository:
- Environmental Data Initiative
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Declining nitrogen (N) availability relative to plant demand, known as N oligotrophication, is a widespread phenomenon that has been particularly well documented in northern hardwood forests of the northeast U.S. It is hypothesized that later fall senescence contributes to this trend by increasing tree resorption of N, resulting in higher carbon:nitrogen ratios (C:N) in litterfall and reduced N availability in soil. To examine the effects of litterfall C:N on soil N cycling, we conducted a litter quality manipulation experiment comparing low C:N and high C:N litter with native litter along an elevation and aspect gradient at Hubbard Brook Experimental Forest, NH, USA. We found that potential net ammonification and mineralization rates were positively correlated with litter N and negatively correlated with litter C:N under high C:N litter, but these relationships were not present under native or low C:N litter. Differences in nitrate pools and net mineralization rates between high- and low-quality litter treatments were greater at colder sites where native litterfall tends to have lower C:N than at low elevation sites. Together, these results demonstrate that higher C:N litter and a warming climate likely contribute to N oligotrophication through effects on microbially driven N cycling rates in organic soils. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station.more » « less
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Declining nitrogen (N) availability relative to plant demand, known as N oligotrophication, is a widespread phenomenon that has been particularly well documented in northern hardwood forests of the northeast U.S. It is hypothesized that later fall senescence contributes to this trend by increasing tree resorption of N, resulting in higher carbon:nitrogen ratios (C:N) in litterfall and reduced N availability in soil. To examine the effects of litterfall C:N on soil N cycling, we conducted a litter quality manipulation experiment comparing low C:N and high C:N litter with native litter along an elevation and aspect gradient at Hubbard Brook Experimental Forest, NH, USA. We found that potential net ammonification and mineralization rates were positively correlated with litter N and negatively correlated with litter C:N under high C:N litter, but these relationships were not present under native or low C:N litter. Differences in nitrate pools and net mineralization rates between high- and low-quality litter treatments were greater at colder sites, where native litterfall tends to have lower C:N than at low-elevation sites. Together, these results demonstrate that higher C:N litter and a warming climate may contribute to N oligotrophication through effects on microbially driven N cycling rates in organic soils.more » « less
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