ecology - Nitrogen and terrestrial ecosystems

docx

School

Lehigh University *

*We aren’t endorsed by this school

Course

152

Subject

Biology

Date

Jan 9, 2024

Type

docx

Pages

Uploaded by AmbassadorTurkeyPerson359

Allie P, Taryn J, Hannah H, Mairin L Nitrogen and terrestrial ecosystems CASE STUDY A Introduction The Hubbard Brook clear-cut experiment is probably the most quoted ecosystem-level study. Herb Bormann and Gene Likens chose this region of the White Mountains in New Hampshire because the Hubbard Brook Experimental Forest could be partitioned into reasonable sized, discrete watersheds drained by obvious streams. Impermeable bedrock beneath the site allowed the researchers to conduct input/output measurements by sampling precipitation and stream water at the base of the watersheds. For this study they monitored plant nutrient (calcium, potassium, nitrate) loss from the 6 watersheds by placing a V-notched weir across the stream exiting a watershed; the weir created a small dam and stream gauges continuously measured flow and water height. Water samples collected at the notch gave an integrated measure of nutrient loss from the watershed. In the Watershed 2 study, all vegetation was cut during fall and winter of 1965; herbicide (Bromacil) applied for 3 years inhibited regrowth. They measured concentration of major ions in stream water before and after deforestation, including the years when herbicide was used. See the figure legend for more details about their methods. Working in groups you should interpret the results in the attached figure, and answer the following questions. 1. Why did the researchers use herbicides to inhibit plant growth for 2 years following deforestation? a. The researchers used herbicides to inhibit plant growth so that they were better able to understand the loss of nutrients evident in the stream ecosystem. The researchers used 2 watershed studies and in that sector applied the use of herbicides. They were then able to better interpret the results 2. Streamflow increased in streams exiting the deforested watershed. Why? Where did the water come from? a. Due to deforestation the movement of water through the watershed had a higher quantity because there was less water being absorbed by trees and other forms of vegetation. This would cause an increase in streams exiting the deforested watershed. 3. Draw the nitrogen cycle in a forest ecosystem like the Hubbard Brook watershed, including all inputs, outputs, and transformations. Indicate on your drawing which processes increased and which processes decreased after vegetation was removed.

4. The Hubbard Brook researchers selected a site with impermeable bedrock. Why was this critical to their experiments? a. Impermeable bedrock doesn’t allow any nutrients or water to escape the system and seep into the earth. This allows researchers to more accurately monitor plant nutrient loss in a watershed. 5. What effects might clear cutting of forests have on aquatic systems? a. The researchers show that when the forest was clear cut streamflow increased and particulate matter increased. Calcium and potassium also increased in aquatic systems. This alters aquatic system environments. 6. What are some of the implications of the Hubbard Brook research for conservation and pollution research and policy? a. The Hubbard Brook research shows the impact of herbicides and deforestation on plant growth. It shows that deforestation can alter forests and the surrounding ecosystem. In order to prevent this from happening conservation efforts would be needed. Figure 1. Precipitation, stream flow, net export of nutrients and particulate matter, and aboveground production in 2 watersheds in the Hubbard Brook Experimental Forest. Watershed 2 (open circle) was clearcut in 1966 and herbicides were applied monthly from fall of 1966 to winter 1969. Watershed 6 (closed circles) remained unaltered. Nutrients and particulates were measured in a stream draining the watershed. CASE STUDY B Introduction

Nitrogen saturation of forests occurs when the availability of inorganic nitrogen exceeds demand by plants and microbes (Aber et al. 1989). It is a response to human-caused increases in nitrogen deposition - specifically oxides of nitrogen (nitrate and NOx) mainly from fossil fuel emissions and also ammonium from production and use of fertilizers. With increasing recognition of this problem ecologists have focused more effort on the capacity of forests to retain high inputs of nitrogen and also effects of excess nitrogen on vegetation. In addition, some of these studies target effects of high nitrogen loading in receiving waters. Two aspects of the Aber et al. paper are especially important to emphasize: 1) the non- linear response of key system phenomena (e.g. N cycling processes, N loss to ground and surface waters) to N inputs that is due to a "saturation" point beyond which fundamental changes take place and 2 ) a shift from positive fertilization effects of N to negative effects once this point is crossed. High rates of nitrogen deposition to forests lead to a cascade of effects (Vitousek et al. 1997). Increasing concentration of ammonium in soil stimulates nitrifiers with an accompanying rise in hydrogen ions. Nitrate is mobile in soils and its leaching also results in loss of cations including calcium, magnesium, and potassium. Loss of these nutrients can lead to stunted tree growth and tree mortality as a result of nutrient imbalances in tree roots and leaves. Finally, with calcium depletion and soil acidification, aluminum ions become mobile and are a potential threat to tree roots or aquatic organisms. Since the 1980's, numerous nitrogen saturation studies have documented: 1) increase in nitrate concentrations in streams and rivers, 2) increased loss of nutrient cations from soils, 3) nutrient imbalances, higher rates of insect and pathogen damage, and reduced frost hardiness, 4) declines in tree growth, especially of evergreens, and 5) higher rates of nitrous oxide emissions (Peterjohn et al. 1999). The impacts of nitrogen saturation were first seen in Europe in the early 1980's with observations of increased nitrate in some streams and rivers and also yellowing plus needle loss in spruce and other conifers. The problem is most severe in parts of northern Europe compared to North America because nitrogen deposition rates are higher. Japanese watersheds are also experiencing N saturation in urbanized areas (Ohrui and Mitchell 1997). Effects in the U.S. are more limited to high elevations with high inputs and shallow, poorly buffered soils (Vitousek et al. 1997). The Broader Context As Jefferies and Maron (1997) point out, a century ago scientists debated whether atmospheric inputs of N were sufficient for plant growth. Today we recognize that more N is fixed each year by humanity than by natural processes in terrestrial ecosystems. This shifts the ecological focus to understanding the fate of this anthropogenic N and its effects on systems.

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

Galloway et al (2003) take the phenomenon of nitrogen saturation further with their "nitrogen cascade" idea. They divide N compounds into reactive and nonreactive, and they propose that in prehuman times, reactive N did not accumulate due to a balance of nitrogen fixation and denitrification. In contrast today reactive N is accumulating on all spatial scales. Results from numerous studies done in the 1990's now allow more synthetic analysis of nitrogen saturation in terrestrial ecosystems. For example, in a return to the question "Is the N status of northeastern forests being altered by N deposition?" Aber et al. (2003) say: "... our analysis suggests that the answer to this question is yes, although the degree of response varied greatly across the three different categories of indicators we examined. The surface water data suggest a strong relationship between NO 3- concentration and flux across the N deposition gradient. The soil data show strong relationships between N deposition, soil C:N ratio, and nitrification in several cases, but the strength and significance of these trends differed among forest types and soil horizons. Finally, in the foliar data set, significant relationships with N deposition did not emerge beyond the covarying effects of climate and elevation". Support a Statement (groups of 3-4) Read the explanation below about the phenomenon called nitrogen saturation and a examine the figures from a study by Peterjohn et al. (1999) on Virginia forests. Then, with this information plus evidence in and your interpretation of Figs. 2A, 2B, and 2C write a few sentences colectively in which you support the following statement from the Peterjohn study: “Not all forests in West Virginia are negatively affected by high concentrations of nitrogen in precipitation. Differences in symptoms of nitrogen saturation are related to aspect (e.g. south or east-facing slopes).” Develop your argument in support of this statement logically — in a logical order. This means that you should 1) introduce the focus and intent of your essay, 2) give the general ecology reader some background information about nitrogen saturation — what it is and why it is important to study, 3) describe Peterjohn et al.'s study — the question(s) they were asking and their overall approach, 4) describe their findings, and 5) explain the significance of these findings to the overall issue of nitrogen saturation in forests. ___________________________ 1) The focus of this study was to determine the impact of high concentrations of nitrogen from precipitation on the health of a forest system. The relative differences in nitrogen are related to the aspect of the slopes, which was shown in the data collected. 2) Nitrogen saturation of forests occurs when the availability of inorganic nitrogen exceeds the demand by plants and microbes. This occurs in response to human caused increase in nitrogen deposition, from fossil fuel emissions and fertilizers. Nitrogen saturation is important to the study because it looks at how the effects

of high nitrogen concentration affect the vegetation, receiving waters, and forest’s ability to retain nitrogen. 3) Peterjohn et al.’s studied how root growth was affected by the addition of nitrogen and phosphate (2A). He then looked at the concentration of nitrate that was leached from the soils (2B) and how that affects the population of tree species in certain regions of the watershed (2C). 4) The first study, Figure 2A demonstrated the impact of nitrogen and phosphorus on plant growth on both the east and south facing slopes. On the east slope both phosphorus and nitrogen had a similar impact on plant growth, however on the south slope nitrogen had a significantly larger effect on plant growth. The second figure (2B) used in this study was showing the average quantity of nitrate in water leaching down the mountainside. The easterly side had high concentrations of nitrate leaching down the mountain, but on the contrary the southerly slope had hardly any. The final figure used was to reveal the amount of woody vegetation on the slopes (figure 2C). This data was compiled in a pie chart and revealed that on the easterly aspect the primary species identified was acer saccharum and on the southern aspect the primary species was more evenly distributed. 5) These findings relate to the overall issue of nitrogen saturation in forests because it displays levels of high nitrogen saturation, where leaching occurs. When nitrogen leaching occurs other nutrients are also leached and loss of these nutrients can stunt tree growth and result in nutrient imbalances in trees and roots. The study also displays areas of normal nitrogen concentration and compares the effects to when there is too much nitrogen saturation. References Aber, J. D., J. M. Melillo, K. J. Nadelhoffer, J. Pastor, and R. D. Boone. 1991. Factors controlling nitrogen cycling and nitrogen saturation in northern temperate forest ecosystems. 1991. Ecological Applications 1: 303-315. Aber, J. D., et al. 2003. Is nitrogen deposition altering the nitrogen status of northeastern forests? BioScience 53: 3750389. Galloway, J. N., et al. 2003. The nitrogen cascade. BioScience 53: 341-356. Jefferies, R. L., and J. L. Maron. 1999. The embarrassment of riches: atmospheric deposition of nitrogen and community and ecosystem processes. Trends in Ecology and Evolution 12: 74-78. Ohrui, K., and M. J. Mitchell. 1997. Nitrogen saturation in Japanese forested watersheds. Ecological Applications 7: 391-401.

Peterjohn, W. T., C. J. Foster, M. J. Christ, and M. B. Adams. 1999. Patterns of nitrogen availability within a forested watershed exhibiting symptoms of nitrogen saturation. Forest Ecology and Management 119: 247-257. Vitoursek, P. M., J. D. Aber, R. W. Howarth, G. E. Likens, P. A. Matson, D. W. Schindler, W. H. Schlesinger, and D. G. Tilman, 1997. Human alteration of the global nitrogen cycle: causes and consequences. Figure 2A. Stimulation of root growth from nitrogen and phosphate addition on roots growing into soil cores on east and south-facing slopes in a West Virginia forest. Bars are 1 standard error. Dotted line indicates no stimulation by fertilization. W. T. Peterjohn, C. J. Foster, M. J. Christ, and M. B. Adams. 1999. Patterns of nitrogen availability within a forested watershed exhibiting symptoms of nitrogen saturation. Forest Ecology and Management 119: 247-257. Figure 2B. Average concentrations of nitrate in water leaching down into the soil on east and south-facing slopes in a West Virginia forest. Bars represent 1 standard error of the mean. W. T. Peterjohn, C. J. Foster, M. J. Christ, and M. B. Adams. 1999. Patterns of nitrogen availability within a forested watershed exhibiting symptoms of nitrogen saturation. Forest Ecology and Management 119: 247-257. Fig. 2C. The importance percentages of woody species growing on slopes with different aspects in watershed 4 (WS 4). Only species with the greatest aspect-related differences in their importance percentages are identified. W. T. Peterjohn, C. J. Foster, M. J. Christ, and M. B. Adams. 1999. Patterns of nitrogen availability within a forested watershed exhibiting symptoms of nitrogen saturation. Forest Ecology and Management 119: 247-257.

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

ecology - Nitrogen and terrestrial ecosystems

Related Documents