Bernard T. Bormann

Nitrogen excess in North American ecosystems: predisposing factors, ecosystem responses, and management strategies

Most forests in North America remain nitrogen limited, although recent studies have identified forested areas that exhibit symptoms of N excess, analogous to overfertilization of arable land. Nitrogen excess in watersheds is detrimental because of disruptions in plant/soil nutrient relations, increased soil acidification and aluminum mobility, increased emissions of nitrogenous greenhouse gases from soil, reduced methane consumption in soil, decreased water quality, toxic effects on freshwater biota, and eutrophication of coastal marine waters. Elevated nitrate (NO3−) loss to groundwater or surface waters is the primary symptom of N excess. Additional symptoms include increasing N concentrations and higher N:nutrient ratios in foliage (i.e., N:Mg, N:P), foliar accumulation of amino acids or NO3−, and low soil C:N ratios. Recent nitrogen-fertilization studies in New England and Europe provide preliminary evidence that some forests receiving chronic N inputs may decline in productivity and experience greate...

Rapid soil development after windthrow disturbance in pristine forests

Summary I We examined how rapidly soils can change during secondary succession by observing soil development on 350-year chronosequences in three pristine forest ecosystems in south-east Alaska. 2 Soil surfaces, created by different windthrow events of known or estimated age, were examined within each of three forest stands (0.5-2.0 ha plots; i.e. a within-stand chronosequence method). Soil surfaces are more likely to have developed under common climate and vegetation conditions within stands than in the spatially separated ecosystems used in traditional chronosequence studies. 3 We observed rates of change that were higher than those previously reported for secondary succession, and were similar to those described for primary succession. Well-developed spodic and albic (podzol) horizons with characteristic C, Fe, and Al signatures were found in soil surfaces less than 150 years old. Carbon accumulated linearly at 21 g m -2 year -1 ; mineral P and N became increasingly immobilized in the spodic horizon as time passed. We found no trend toward an equilibrium in C or N accumulation over the 350-year chronosequences in any of the three stands examined. 4 These rapid changes in soil and a shift in rooting from mineral to organic horizons appeared likely to reduce productive capacity of the soil during a single generation of trees. Windthrow or disturbances that mimic windthrow may be required at intervals of about 200-400 years to maintain soil productive capacity in these ecosystems.

Rapid, plant-induced weathering in an aggrading experimental ecosystem

To evaluate whether rates of weathering of primary minerals are underestimated in watershed mass-balance studies that fail to include products of weathering accumulating in plants and in developing soil, changes in the calcium and magnesium content of vegetation and soil fractions were measured in large, monitored lysimeters (sandbox ecosystems) at Hubbard Brook Experimental Forest, New Hampshire. Weathering was evaluated over 4–8 yr in sandboxes planted with red pine (Pinus resinosa Ait.) and kept mostly free of vegetation (nonvegetated). Three mass-balance equations were used that cumulatively include (a) Ca and Mg in precipitation inputs and drainage outputs, (b) accumulation of Ca and Mg in vegetation, and (c) changes in products of weathering in soils. Soil products were evaluated with an extraction process designed to avoid removing ions from primary minerals. Relative to the input-output equation, the estimated rate of weathering increased 2.4 (Ca) and 1.8 (Mg) times when accumulation of Ca and Mg in pine biomass was accounted for, and 8 (Ca) and 23 (Mg) times when changes in soil products were also included. Weathering estimates that included accumulation in vegetation and soil products were 261 (Ca) and 92 (Mg) kg ha-1 yr-1 in the pine sandbox. These rates were 10 (Ca) and 18 (Mg) times higher than the rates in the nonvegetated sandbox, which were not significantly greater than zero. This study raises the possibility that weathering can play a significant role in the release of nutrients available to plants over short periods. Faster rates like this become extremely important where managers are trying to balance nutrients available to plants from precipitation and weathering release with outputs including harvest removals.

Rapid N^2 Fixation in Pines, Alder, and Locust: Evidence From the Sandbox Ecosystems Study

Not all nitrogen (N) inputs have been accounted for in forested ecosystems. We sought to account for N2 fixation and dry deposition using a lysimeter mass—balance approach. Large sand—filled, field lysimeters were used to construct 5—yr nitrogen budgets for two N2—fixing trees, two pines, and a nonvegetated control soil. This approach is a promising and straightforward technique for quantifying otherwise difficult—to—measure fluxes. Accurate assessment of changes in N storage combined with direct measurement of N inputs in precipitation and losses from leaching allowed as to estimate fluxes. Gains of N in pine systems were greatest in vegetation and litter, overshadowing combined losses from mineral soil and leaching by about threefold. Rapid acetylene reduction in pine rhizospheres and in cultures from washed roots suggests that unexplained gains are due to associative N2 fixation. These results provide strong evidence for N2 fixation in pine systems of °50 kg°ha—1°yr—1 N. The symbiotic N2—fixing trees black locust and black alder fixed 2 and 5 times more N2, respectively, than did pines. In all systems, input in precipitation and dry deposition were relatively unimportant to the N budget. Unexplained losses of N from the nonvegetated control suggest that denitrification is an important flux. Mineral soil organic matter declined sharply and significantly in pines (20%) and even more so in the nonvegetated control (40%). Symbiotic N2—fixing trees caused a small, nonsignificant increase in mineral soil organic matter and large, significant increases in litter layer organic matter and large, significant increases in litter layer organic matter. Bulk density (0—20 cm) declined by 5% under symbiotic N2—fixing trees and increased by 5% in one pine sandbox. Correction for soil expansion or collapse did not greatly alter estimates of unexplained N or N2 fixation. Pines with rhizospheres that fix N2 at the rates we observed might be used to restore degraded land and to create silvicultural systems that are N self sufficient. We first need to better understand the microbiology, tree genetics, and soil conditions that lead to rapid N2 fixation in pine ecosystems.

Tripartite associations in an alder: effects of Frankia and Alpova diplophloeus on the growth, nitrogen fixation and mineral acquisition of Alnus tenuifolia

The role of tripartite associations among Frankia, Alpova diplophloeus (an ectomycorrhizal fungus) and Alnus tenuifolia in growth, nitrogen fixation, ectomycorrhizal formation, and mineral acquisition of A. tenuifolia was investigated. Seedlings of A. tenuifolia were planted in pots containing a mixture of ground basalt–perlite, or perlite alone, which served as the control. The seedlings were inoculated with Frankia isolated from root nodules of alder, followed by spores of A. diplophloeus and grown for 5 months in a greenhouse. The seedlings grown in the pots with a mixture of ground basalt–perlite after dual inoculation with Frankia and A. diplophloeus had the heaviest shoots and root nodules in dry weight, and showed the greatest nitrogen-fixing ability measured by acetylene reduction. Ectomycorrhizae formed with A. diplophloeus increased when this fungus was inoculated together with Frankia. The mineral composition (P, K, Ca, Fe, Mg, Mn, Na, Si and Al) in the seedlings was also determined. The results of these experiments showed that the tripartite associations could improve the growth, nitrogen fixation and mineral acquisition (rock solubilization) of A. tenuifolia.

Lessons from the Sandbox: Is Unexplained Nitrogen Real?

AbstractIn their review of 24 studies of forest nitrogen (N) budgets, Binkley and others (2000) found that only one of them supported the conclusion that an N accumulation of more than 25 kg N ha−1 y−1 is possible without known symbiotic N2–fixing plants. They contended that, given how well the N cycle is known, new N accumulation pathways are unlikely. They also concluded that the Hubbard Brook sandbox study (Bormann and others 1993) was insufficiently replicated and had low precision in vegetation and soil estimates. Here we reevaluate and extend the sandbox analysis and place the findings in a broader context. Using multiple methods of estimating vegetation N accumulation in pine sandboxes, we arrived at results that differed from the reported rates but still strongly supported large biomass N accumulation. The original studys conclusions about soil N changes were strengthened when new evidence showed that N accumulated in lower horizons and that the sandboxes were successfully homogenized at the beginning of the experiment. Unexplained ecosystem N accumulation ranged from about 40 to 150 kg ha−1 y−1, with 95% confidence intervals that did not include zero. No evidence was found that could balance the sandbox ecosystem N budgets without adding unexplained N. Unreplicated experiments, such as the sandboxes, can explore the possibility that N can accumulate in ways not explainable by mass balance analysis, but they cannot quantify the frequency and extent of the phenomenon. New studies should combine substantive microbiological, mass balance, and process research using multiple direct measures of N2 fixation.

Can intensively managed forest ecosystems be self-sufficient in nitrogen?

Bormann, B.T. and Gordon, J.C., 1989. Can intensively managed forest ecosystems be self-sufficient in nitrogen? For. Ecol. Manage., 29: 95-103. A simple input-output model was constructed to evaluate the long-term net nitrogen (N) gain or loss or N self-sufficiency of several silvicultural systems. In general, short-rotation, whole-treeharvest systems are not self-sufficient in N. If they include an active biological nitrogen fixation (N2 fixation) component, however, they are highly N self-sufficient even when they sustain large losses of N in harvested biomass. Forestry systems, with or without substantial N2 fixation, appear to be greatly superior in this respect to most annual agricultural crops with or without N2 fixation. All N inputs and outputs must be measured in field experiments, however, before N self-sufficiency can be accurately evaluated. Better estimates of loss through harvest removal, and gain through N2 fixation and atmospheric inputs, over more than one rotation are particularly needed.

Forest Structure Affects Soil Mercury Losses in the Presence and Absence of Wildfire

Soil is an important, dynamic component of regional and global mercury (Hg) cycles. This study evaluated how changes in forest soil Hg masses caused by atmospheric deposition and wildfire are affected by forest structure. Pre and postfire soil Hg measurements were made over two decades on replicate experimental units of three prefire forest structures (mature unthinned, mature thinned, clear-cut) in Douglas-fir dominated forest of southwestern Oregon. In the absence of wildfire, O-horizon Hg decreased by 60% during the 14 years after clearcutting, possibly the result of decreased atmospheric deposition due to the smaller-stature vegetative canopy; in contrast, no change was observed in mature unthinned and thinned forest. Wildfire decreased O-horizon Hg by >88% across all forest structures and decreased mineral-soil (0 to 66 mm depth) Hg by 50% in thinned forest and clear-cut. The wildfire-associated soil Hg loss was positively related to the amount of surface fine wood that burned during the fire, the proportion of area that burned at >700 °C, fire severity as indicated by tree mortality, and soil C loss. Loss of soil Hg due to the 200,000 ha wildfire was more than four times the annual atmospheric Hg emissions from human activities in Oregon.

Long-Term Forest Productivity

Planning for forest sustainability has been a hallmark of US national resource management, beginning with the work of several visionaries of the previous century, including Gifford Pinchot and US presidents Grover Cleveland and Theodore Roosevelt. Their efforts created the US national forests in 1905 to address concerns about sustainable, long-term supplies of both water and timber. Congress subsequently passed the Multiple-Use Sustained Yield Act of 1960 to fulfill needs beyond water and timber resources. The National Forest Management Act of 1976 better assured sustainably by defining it as “the achievement and maintenance in perpetuity of a high-level annual or regular periodic output of the various renewable resources of the national forests without impairment of the productivity of the land.”