William T. Peterjohn

Nutrient dynamics in an agricultural watershed: observations on the role of a riparian forest

Nutrient (C, N, and P) concentration changes were measured in surface runoff and shallow groundwater as they moved through a small agricultural (cropland) watershed located in Maryland. During the study period (March 1981 to March 1982), dramatic changes in water-borne nutrient loads occurred in the riparian forest of the watershed. From surface runoff waters that had transited : 50 m of riparian forest, an estimated 4.1 Mg of particulates, 1I kg of particulate organic-N, 0.83 kg of ammonium-N, 2.7 kg of nitrate-N and 3.0 kg of total particulate-P per ha of riparian forest were removed during the study year. In addition, an estimated removal of 45 kg ha- yr-t of nitrate- N occurred in subsurface flow as it moved through the riparian zone. Nutrient uptake rates for the cropland and riparian forest were estimated. These systems were then compared with respect to their pathways of nutrient flow and ability to retain nutrients. The cropland appeared to retain fewer nutrients than the riparian forest and is thought to incur the majority of its nutrient losses in harvested crop. The dominant pathway of total-N loss from the riparian forest seemed to be subsurface flux. Total phosphorus loss from the riparian forest appeared almost evenly divided between surface and subsurface losses. Nutrient removals in the riparian forest are thought to be of ecological significance to receiving waters and indicate that coupling natural systems and managed habitats within a watershed may reduce diffuse-source pollution.

Symptoms of nitrogen saturation in two central Appalachian hardwood forest ecosystems

By synthesizing more than twenty years of research at the Fernow Experimental Forest, we have documented 7 symptoms of nitrogen saturation in two adjacent watersheds. The symptoms include: 1) high relative rates of net nitrification, 2) long-term increases in stream-water concentrations of nitrate and base cations, 3) relatively high nitrate concentrations in solution losses, 4) little seasonal variability in stream-water nitrate concentrations, 5) a high discharge of nitrate from a young aggrading forest, 6) a rapid increase in nitrate loss following fertilization of a young aggrading forest, and 7) low retention of inorganic nitrogen when compared with other forested sites. These data support current conceptual models of nitrogen saturation and provide a strong, and perhaps the best, example of nitrogen saturation in the United States.

NUTRIENT LIMITATION IN SOILS EXHIBITING DIFFERING NITROGEN AVAILABILITIES: WHAT LIES BEYOND NITROGEN SATURATION?

The nature of nutrient limitation in large areas of temperate forest may be changing due to human activities. As N availability in these forests increases, other nutrients could increasingly constrain productivity and other ecosystem processes. To determine the nature of nutrient limitation (N, P, and Ca) in forest soils exhibiting differing N availability, we conducted three field studies in the Fernow Experimental Forest, West Virginia, USA. The first used a ubiquitous herbaceous species, Viola rotundifolia, to compare indices of N availability to the activity of root-associated phosphomonoesterase (PME) activity at two spatial scales. The second study used fertilized, root in-growth cores to assess the extent of N, P, and Ca limitation. Finally, we measured the root-associated PME activity of V. rotundifolia growing in experimental plots that have received various combinations of nutrient additions and harvest treatments. For entire watersheds, stream water nitrate concentrations were positively related to PME activities (R2 = 0.986). For small plots, PME activities were positively associated with soil nitrate availability (R2 = 0.425), and to a lesser extent with the leaf N concentrations (R2 = 0.291). Root growth into microsites fertilized with P was greater than growth into microsites fertilized with either N or Ca, especially in watersheds with high N availability. Experimental additions of N increased the root-associated PME activity of V. rotundifolia, supporting the causality of the relationship between N availability and PME activity. Collectively, our results indicate that, as N availability increases, P becomes increasingly limiting at the sites examined. Understanding how nutrient limitations change during N saturation should improve ecosystem models and better inform our attempts to mitigate any undesired effects.

Aspects of spatial and temporal aggregation in estimating regional carbon dioxide fluxes from temperate forest soils

We examine the influence of aggregation errors on developing estimates of regional soil-CO2 flux from temperate forests. We find daily soil-CO2 fluxes to be more sensitive to changes in soil temperatures (Q10 = 3.08) than air temperatures (Q10 = 1.99). The direct use of mean monthly air temperatures with a daily flux model underestimates regional fluxes by approximately 4%. Temporal aggregation error varies with spatial resolution. Overall, our calibrated modeling approach reduces spatial aggregation error by 9.3% and temporal aggregation error by 15.5%. After minimizing spatial and temporal aggregation errors, mature temperate forest soils are estimated to contribute 12.9 Pg C yr−1 to the atmosphere as carbon dioxide. Georeferenced model estimates agree well with annual soil-CO2 fluxes measured during chamber studies in mature temperate forest stands around the globe.

Nitrification potentials and landscape, soil and vegetation characteristics in two Central Appalachian watersheds differing in NO3- export

Two watersheds within 1 km of each other in the Central Appalachian mountains of West Virginia have similar management histories and receive 13 kg of N in atmospheric deposition, but NO3 � export from one watershed (W4) has increased over the last 30 years, and is now approximately five times greater than NO3 � export from the other (W10). We measured net nitrification potentials (NNP) and other landscape, soil, and plant community variables (1) to determine whether differences in leaching could be attributed to differences in NNP, (2) to identify other significant differences between the watersheds, (3) to identify variables that could account for both between- and within-watershed variability in NNP, and (4) to identify readily measured variables that distinguish plots with relatively high or low NNP. NNPs in W4 were 0.84 kg N ha � 1 per day, approximately three times higher than those on W10. Watershed 4 lay at a slightly higher elevation, had gentler slopes, a thinner forest floor, lower C:N in the 0‐10 cm soil layer, lower tree density, greater basal area in Acer saccharum, less basal area in Quercus prinus and Amelanchier arborea, more frequent occurrences of A. saccharum seedlings, Laportea canadensis, Polystichum acrostichoides, Trillium sp., Uvularia sessilifolia and Viola spp., and fewer occurrences of Gaultheria procumbens, and Viburnum acerifolum. NNPs were correlated with many soil characteristics related to base cation supply, C:N and water holding capacity (WHC). Several two- and three-variable regression models, which were mostly based on soil characteristics, accounted for a large proportion of the variability in NNP (adjusted R 2 > 0:60), as well as for the difference between watershed means (t-test of residuals indicate no significant difference). A regression model based on basal area of A. saccharum and A. rubrum and the presence or absence of Trillium accounted for 50% of the variability in NNP. At this highdeposition site, plots with soils that had higher pHs, greater base cation supply and WHC, and lower C:N were more susceptible to NO3 � leaching and N saturation. # 2002 Elsevier Science B.V. All rights reserved.

Patterns of nitrogen availability within a forested watershed exhibiting symptoms of nitrogen saturation

Watershed 4 (WS 4) at the Fernow experimental forest in West Virginia shows several symptoms of N saturation. Surprisingly, however, past measurements of N2O production suggest that a portion of WS 4 (the south-facing slopes) may still be strongly limited by the lack of available N. To examine this possibility more fully, we measured four indices of N availability in two sections of WS 4-slopes with easterly aspects that are located on one side of the main stream channel and slopes with southerly aspects that are located on the opposite side of the stream. Compared to the portion of WS 4 with more east-facing slopes (78% of the total area), we found that south-facing slopes had lower extractable NO3 ˇ pools (14 versus 554 mg N m ˇ2 ), lower rates of net nitrification (2 versus 35 mg N m ˇ2 day ˇ1 ), a greater response of root growth to N-rich microsites (4.5 versus 1.6 > controls), and lower concentrations of NO3 ˇ in water leaching below the A, B, and C soil horizons (ca. 0.066 versus 2.3 mg N l ˇ1 ). The two sections of WS 4 also differed in the composition of their woody vegetation. Slopes with more easterly aspects had a greater relative importance of Acer saccharum and Prunus serotina. The south-facing slopes were characterized by a greater importance of Nyssa sylvatica and Fagus grandifolia. From these results we hypothesize that aspect-related differences in species composition can strongly influence the susceptibility of a forested stand to the early onset of N saturation. If this hypothesis is proven, then community composition may account for a significant proportion of the variable response of forested watersheds to similar levels of elevated N deposition. It would also imply that management practices which favor certain species might delay or accelerate the onset of N saturation and the potentially negative changes associated with this process. # 1999 Elsevier Science B.V. All rights reserved.

The influence of elevated ultraviolet-B radiation (UV-B) on tissue quality and decomposition of loblolly pine (Pinus taeda L.) needles

Stratospheric ozone depletion is expected to elevate the influx of ultraviolet-B radiation (UV-B) to the biosphere. Increased levels of UV-B may, in turn, alter important ecosystem processes such as decomposition. Previous studies have shown that growth under elevated UV-B can alter leaf quality in angiosperm species and thereby indirectly change subsequent rates of leaf decay. In this experiment, we determined if elevated UV-B would alter the chemical composition and decay of needle tissue from two seed sources of the gymnosperm Pinus taeda L. Maryland and Virginia seed sources of P. taeda were grown in the field for 3 years beneath lampbanks supplying either ambient, low elevated or high elevated UV-B. These levels of UV-B corresponded to 0, 16 and 25% stratospheric ozone depletion at the experimental site in Beltsville, MD (39 degrees N). Needles were collected from six randomly chosen plants for each combination of seed source and UV-B level. The needle samples were analyzed for total C and N, UV-B absorbing compounds, and carbon fractions. Decay rates were also determined by measuring rates of CO(2) evolution from needle material decomposed under laboratory conditions. UV-B did not significantly alter the chemical composition of needles from the Virginia seed source. In contrast, needles from the Maryland seed source tended to have elevated lignin/N ratios and a lower holocellulose content when grown under the highest level of UV-B. Furthermore, while needles from the Virginia pines did not have UV-B altered decay rates, Maryland needles grown under low elevated UV-B conditions decomposed 36% more rapidly than needles from other treatments. Results from this experiment illustrate at least three characteristics about the indirect effect of UV-B on decomposition, (1) UV-B can modify decomposition of tissue from gymnosperms as well as angiosperms; (2) UV-B effects on tissue chemistry and decay may not only be species-specific but also seed-source specific; and (3) UV-B effects on decomposition may not increase with increasing UV-B dose.

N FERTILIZATION EFFECTS ON DENITRIFICATION AND N CYCLING IN AN AGGRADING FOREST

We investigated N cycling and denitrification rates following five years of N and dolomite amendments to whole-tree harvested forest plots at the long-term soil productivity experiment in the Fernow Experimental Forest in West Virginia, USA. We hypothesized that changes in soil chemistry and nutrient cycling induced by N fertilization would increase denitrification rates and the N2O:N2 ratio. Soils from the fertilized plots had a lower pH (2.96) than control plots (3.22) and plots that received fertilizer and dolomite (3.41). There were no significant differences in soil %C or %N between treatments. Chloroform-labile microbial biomass carbon was lower in fertilized plots compared to control plots, though this trend was not significant. Extractable soil NO3- was elevated in fertilized plots on each sample date. Soil-extractable NH4+, NO3-, pH, microbial biomass carbon, and %C varied significantly by sample date suggesting important seasonal patterns in soil chemistry and N cycling. In particular, the steep decline in extractable NH4+ during the growing season is consistent with the high N demands of a regenerating forest. Net N mineralization and nitrification also varied by date but were not affected by the fertilization and dolomite treatments. In a laboratory experiment, denitrification was stimulated by NO3- additions in soils collected from all field plots, but this effect was stronger in soils from the unfertilized control plots, suggesting that chronic N fertilization has partially alleviated a NO3- limitation on denitrification rates. Dextrose stimulated denitrification only in the whole-tree-harvest soils. Denitrification enzyme activity varied by sample date and was elevated in fertilized plots for soil collected in July 2000 and June 2001. There were no detectable treatment effects on N2O or N2 flux from soils under anaerobic conditions, though there was strong temporal variation. These results suggest that whole-tree harvesting has altered the N status of these soils so they are less prone to N saturation than more mature forests. It is likely that N losses associated with the initial harvest and high N demand by aggrading vegetation is minimizing, at least temporarily, the amount of inorganic N available for nitrification and denitrification, even in the fertilized plots in this experiment.

Oak contribution to litter nutrient dynamics in an Appalachian forest receiving elevated nitrogen and dolomite

Ecosystem nitrogen (N), phosphorus (P), and calcium (Ca) fluxes are affected by inputs of atmospheric N. Oak litter may additionally affect these fluxes because of its high-lignin content. We analyzed nutrient dynamics in ambient mixed-species litter in an aggrading hardwood stand at the Fernow Experimental Forest in West Virginia. We separated oak from the mix for analysis (oak) and compared it with total litter (all species) to understand how oak affects nutrient fluxes in the litter layer. The study was conducted under ambient atmospheric deposition, under elevated atmospheric dep- osition, and under elevated deposition plus mitigation with dolomite. N flux between litterfall and 12 months later indi- cated a net loss in all-species litter of up to 7.3 kg Nha -1 and a retention of up to 0.6 kg Nha -1 in oak. P flux included losses in all species in ambient and in dolomite treatments of up to 0.19 kg Pha -1 and gains of up to 0.12 kg Pha -1 in oak in all treatments. Oak mineralized Ca at an average across treatments of 4.6 kg Caha -1 compared with 16 kg Caha -1 in all species, with half of that when trees were dormant. Percent immobilization and release over initial litter were greater in oak than in all species, but nutrient fluxes were lower in oak than in all species because of low oak litter mass. Elevated deposition lowered N and increased P immobilization. Dolomite appeared to affect early N dynamics only. With an in- crease in litterfall mass when forests mature, these effects are also likely to increase.

SOIL CHEMICAL RESPONSE TO EXPERIMENTAL ACIDIFICATION TREATMENTS

One of the conclusions reached during the Congressionally mandated National Acid Precipitation Program (NAPAP) was that, compared to ozone and other stress factors, the direct effects of acidic deposition on forest health and productivity were likely to be relatively minor. However, the report also concluded “the possibility of long-term (several decades) adverse effects on some soils appears realistic” (Barnard et al. 1990). Possible mechanisms for these long-term effects include: (1) accelerated leaching of base cations from soils and foliage, (2) increased mobilization of aluminum (Al) and other metals such as manganese (Mn), (3) inhibition of soil biological processes, including organic matter decomposition, and (4) increased bioavailability of nitrogen (N). Sulfate-induced acidification occurs as the sulfate (SO