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DIVISION S-7—FOREST & RANGE SOILS

Sampling-Induced Increases in Net Nitrification in the Brush Brook (Vermont) Watershed

Department of Plant and Soil Science, University of Vermont Burlington, VT 05405

* Corresponding author (donald.ross{at}uvm.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Accurate measurement of nitrification rates in disturbance-sensitive soils can be problematic. A series of experiments illustrated the extent of net nitrification increases and explored possible causes. In highly disturbed soils, weekly leachates of columns containing homogenized Oi/Oe material had NO^-₃ concentrations over 5 mmol L^-1 after 20 wk and over 1 mmol L^-1 after 2 yr. Small-scale impact disturbance in the field (footprints) resulted in a tripling of NO^-₃ concentrations after a 2-wk incubation. After disturbance by sampling, bulked Oa horizons showed rapid increases in NO^-₃ concentration, with change detectable within 1 h. Initial net nitrification rates were as high as 74 µmol kg^-1 h^-1 (300 mg N m^-2 d^-1). Ammonium concentrations increased over the first 10 h but decreased afterwards. Incubation with acetylene stopped nitrate accumulation but NH⁺₄ continued to increase. Intact cores also showed increases in NO^-₃ after 1 to 3 d but at a much lower rate than bulked samples. Gross rates of nitrification in intact cores were much lower than net rates in bulk samples. The addition of NH⁺₄ to intact cores caused an increase in net nitrate after 20 h. These results suggest that sampling stimulates nitrification rapidly through an increase in NH⁺₄ availability. The NH⁺₄ increase may be because of a stimulation of ammonification caused by mixing, a decrease in spatial heterogeneity, or a disturbance in other types of NH⁺₄ consumption. In soils that exhibit these disturbance effects, nitrification rate measurements will not be accurate but still may represent the potential for nitrification with increased NH⁺₄ supply.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
FOREST SOIL DISTURBANCE has been found to increase net nitrification creating artifact in field plot studies (Miller et al., 1992), soil column studies (Johnson et al., 1995; Nelson and Robarge, 1996), root dynamics studies (Joslin and Wolfe, 1999), and soil sampling (Ross and Bartlett, 1990; van Miegroet, 1995). Disturbance-induced increases in stream NO^-₃ export have been demonstrated in watershed studies (e.g., Likens et al., 1969; Vitousek et al., 1979) and after episodes of severe freezing (Mitchell et al., 1996). Observation of this phenomenon is not new. In a treatise on the humus layer of northeastern U.S. forest soils, Romell (1935)(p. 6) observed: "Even very heavy and pronounced northeast-American forms of mor (belonging to or approaching the greasy type) react with an ease surprising to a European observer. They seem to be activated by the slightest disturbances. The reason for their greater reactivity has not been explained." This "activation" refers to the onset of net nitrification and eventual conversion of a "bothersome" mor to a more mull-like condition. A complete explanation for the cause of the disturbance effect is still lacking.

Nitrification in forest soils is of special interest in ecosystems that may be impacted by increased N deposition. Changes in nitrification rates could have a profound effect on NO^-₃ leaching. Thus, the potential magnitude and rapidity of NO^-₃ release after experimental or sampling disturbance present problems for monitoring and research efforts aimed at measuring ecosystem N fluxes. For example, Johnson et al. (1995) found extremely high soil solution NO^-₃ concentrations (peaking at 5 mmol L^-1) in a controlled-environment pot study using the Ap horizon of an Aiken clay loam (Xeric Haplohumult). Miller et al. (1992) found O horizon solution concentrations as high as 1.8 mmol L^-1 in a trenched plot experiment on a Typic Cryohumod. These elevated concentrations would almost certainly mask the effect of any reasonable treatment.

Dramatic changes in soil NO^-₃ concentrations take place shortly after sampling, even when soils are stored near freezing. For example, we found that the NO^-₃ concentrations in soil solution from two Oa-horizon samples nearly doubled after 24 h of storage at 3°C (Ross and Bartlett, 1990). Van Miegroet (1995) studied changes after sampling the top 10 cm of mineral soil taken in the Great Smoky Mountains National Park. She found as much as a seven-fold increase in NO^-₃ concentrations between extracting in the field immediately after sampling and extracting in the laboratory after <24 h of storage on ice. This rapid increase obviously dictates field extraction if in situ soil NO^-₃ concentrations are required.

The mechanism for the rapid increase in NO^-₃ after sampling is not clear. An increase in net nitrification could result from either a decrease in NO^-₃ consumption (e.g., microbial assimilation, plant uptake, abiotic fixation, denitrification), an increase in the gross rate of nitrification, or a combination of both. The gross rate of nitrification may be controlled by the availability of NH⁺₄ or by the presence of inhibitory factors. Davidson and Hackler (1994) and Korsaeth et al. (2001) hypothesized that in situ spatial heterogeneity of available NH⁺₄ limits the ammonia oxidizers even when bulk NH⁺₄ concentrations are relatively high. The chemolitho-autotrophic ammonia oxidizers use inorganic C, while the heterotrophic decomposers use organic C. These communities may be spatially separated. Tight cycling among the heterotrophs and higher plants would limit the NH⁺₄ available to the autotrophs. Disturbance may stimulate nitrification by decreasing this spatial heterogeneity. It may also stimulate NH⁺₄ production (and resulting nitrification) simply through mixing and providing the heterotrophs with more accessible substrate. Numerous responses to disturbance are possible (Vitousek et al., 1982) and it is likely that no single mechanism explains the rapid increase in net nitrification.

It is important to understand both the extent and the magnitude of disturbance-induced net nitrification. Are field studies inherently limited on sites with soils displaying this phenomenon? This study examined a range in disturbance, from the homogenization that occurs when samples are bulked to simple heavy footprints. Different common approaches for measuring nitrification rates were compared—for example, incubation of intact cores vs. bulk samples. Intact core sampling should create less disturbance, although both methods obviously cause cessation of plant uptake. Experiments were also designed to investigate the effect of spatial heterogeneity by adding NH⁺₄ to cores already containing substantial concentrations of extractable NH⁺₄. Gross rates of both NO^-₃ and NH⁺₄ production were measured to compare with net rates. Additionally, autotrophic ammonia oxidation was inhibited to reveal the pattern of net NH⁺₄ production in the absence of nitrification. The experiments document the rapidity of disturbance-induced net nitrification and attempt to elucidate a mechanism.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
All experiments were performed in Camels Hump State Forest in Huntington and Duxbury, VT (44°19'N lat., 72° 53'W long.). Previous work in this area showed rapid increase in soil NO^-₃ after sampling (Ross and Bartlett, 1990). Except for the column study, all soils were mapped as Houghtonville sandy loams (Coarse-loamy, isotic, frigid Typic Haplorthods) or variants with deeper lithic contact. Vegetation was mixed northern hardwoods (primarily Betula alleghaniensis and Acer saccharum) with a contribution of Picea rubens and Abies balsamea, and pockets of Tsuga canandensis. More details on the research watershed can be found in Ross et al. (1994). For convenience, experiments are numbered and site conditions summarized in Table 1.

Small-Scale Impact (Footstep) Study (Exp. 2)
This study was designed to determine if a relatively minor disturbance in the field would have an effect on soil inorganic N status. The experiment was initiated by establishing a series of ten alternating footprints approximately 70 cm apart ("giant steps"). Each print was made by two impacts of sufficient force to leave a 0.5- to 1.0-cm heel impression. A flag was inserted just off the front inside corner of the heel. After 2 wk, an identical set of footprints 25 cm to the left or right of the original prints were made so that 10 pairs of prints were established over a distance of about 6.5 m. A 4 by 4 cm square, 2 to 4 cm thick, sample of the A horizon from under the heel area was taken from each new print and the associated old print within 2 min. of the second treatment. After mixing, duplicate 5-mL subsamples were added to 50-mL centrifuges tube containing 20 mL of 1 M KCl. An additional 5-mL subsample was saved to obtain composite samples of each treatment. Samples were returned to the lab and centrifuged. Concentrations of NO^-₃ and NH⁺₄ in this and all subsequent experiments were determined using the Cd-reduction method and the salicylate-nitroprusside method, respectively, on an automated flow-injection analyzer (Lachat QuikChem AE, Milwaukee, WI). The dry weights of the soil samples were determined by rinsing the extracted samples with distilled water to remove excess KCl and drying at 55°C. In this and all subsequent experiments we measured pH in H₂O (2:1 v/v) and %C and %N by total elemental analyzer.

Short-Term Net Nitrification (Exp. 3)
These experiments were designed to determine the rapidity of the disturbance response and observe changes in the hours following sampling. Oa horizon bulk samples were taken from the side of small pits and homogenized by hand. Volumetric subsamples were put into a 50-mL centrifuge tube with 25 mL of KCl solution. Experiment 3a used duplicate 5-mL subsamples and 1 M KCl, and Exp. 3b and 3c used triplicate 10-mL subsamples and 2 M KCl. In Exp. 3a, soil solution was also extracted with the syringe-pressure method (Ross and Bartlett, 1990) and NO^-₃ determined by ion chromatography. After the initial sampling, soils and centrifuge tubes were placed in a cooler until the return to the lab (2–3 h later). Samples from Exp. 3a were stored at 4°C and those from Exp. 3b and 3c were stored at 12°C. Additional subsamples were taken and extracted over a 10-h period, and after 1 and 2 d. For the acetylene treatment in Exp. 3c, 10-mL subsamples were incubated in 100-mL gas-tight glass bottles fitted with septa. The treatment was prepared in the field within about 30 min of sampling by adding 1 mL of acetylene with a syringe. At each extraction time, 25 mL of KCl was added directly to each of three bottles and the samples shaken. Dry weights and moisture content were measured on separate volumetric subsamples.

Intact Core vs. Bulk Sample Study (Exp. 4)
This experiment examined the difference in the disturbance effect between two sizes of intact cores and bulk samples. Cores were inserted flush with the forest floor by carefully cutting around the outside edge with a sharp knife as downward pressure was applied. Cores were either open-ended metal cans (11.5 cm high with an 8.5-cm diam.) or 125-mL polyethylene bottles with the bottoms removed (6 cm high with a 4.8-cm diam.). Six blocks containing two each of the different cores were established. Within each block, the distance between core centers averaged 11 cm. Treatments, intact or disturbed, were assigned randomly for each pair of similar cores. The intact cores were lifted from the soil, put in a polyethylene bag, and returned to their original position. The disturbed cores were taken apart and the Oa horizon was separated and thoroughly mixed by hand. Duplicate subsamples (10 mL for the larger and 5 mL for the smaller cores) were extracted with 25 mL of 1 M KCl for NO^-₃ and NH⁺₄ analysis as described above. The remaining Oa sample was put in a polyethylene bag, replaced in the original sampling hole and covered with the Oi/Oe layer from that core. After 3 d, all remaining intact cores were taken apart and all treatments were extracted in the field as described above. Dry weights of 5 or 10 mL of soil and soil moisture content were determined on separate subsamples.

Ammonium Addition Studies (Exp. 5)
In these studies, NH⁺₄ was added to intact cores to determine if net nitrification could be increased. The hypothesis was that the native NH⁺₄ was not as available to the ammonia oxidizers, probably because of heterogeneity. Intact cores were sampled in open-ended metal cans (11 cm high with an 8.5-cm diam.) as described above. Each of five blocks contained four cans with treatments randomly assigned. Two cans were treated with 10 mL of (NH₄)₂SO₄ solution (2.0 mmol L^-1 in Exp. 5a and 2.5 mmol L^-1 in the final two) and two with 10 mL of distilled H₂O. The amount of added NH⁺₄ was intended to increase the soil NH⁺₄ concentration by about 0.3 mmol kg^-1. The solutions were added using 10-cm needles on a 5-mL syringe. A series of 8 to 10 repeated injections were made between 4- and 8-cm depth in a pattern around the volume of the core. Within 20 min of treatment, one core of each type of solution was taken apart and the Oa horizon separated and mixed by hand. Triplicate 10-mL samples were extracted in the field with 1 M KCl for analysis of NH⁺₄ and NO^-₃ as described above. All remaining Oa samples and remaining intact cores were put into polyethylene bags, placed in coolers with ice-packs and transported to a cold laboratory room (12–13°C). The intact cores were taken apart after about 20 h and all treatments were extracted as described above.

Gross Rate Measurements (Exp. 6)
Gross rates of ammonification and nitrification were measured using the isotope dilution method (Hart et al., 1994) to determine if there was high internal cycling. Blocks of four intact cores (11 by 8.5 cm) were carefully cut into the forest floor and two each were treated with 5 µmol of ¹⁵N as either (NH₄)₂SO₄ or KNO₃. Four blocks were used in Exp. 6a and 6b and five blocks used in the others. The solution was added evenly between a depth of 4 and 8 cm. One set of cores was dismantled and extracted in the field and the second set extracted after about 24 h of incubation at 12°C in the laboratory. The Oa horizon from the depth-treated was homogenized and 120 mL extracted with 240 mL of 2 M KCl. The extracted inorganic N was prepared for isotopic analysis using the Mason-jar diffusion method of Khan et al. (1998). Each sample was spiked with 100 µg of N as (NH₄)₂SO₄ to provide sufficient N for analysis. Concentrations of inorganic N were measured as described above and gross rates were derived using the calculations and assumptions described in Hart et al. (1994). Net rates in disturbed samples were measured in composite bulk samples of the Oa or A horizon taken from the immediate area of the intact cores. Repeated extractions for NO^-₃ and NH⁺₄ were performed as described above for Exp. 3b.

Statistical Analysis
Statistical analysis was performed using the general linear models procedure of the SAS Institute (1994). A complete random block design was used to test for treatment differences using the average of the replicated measurements. All data was log transformed to create normal distributions. Because the footstep treatment in Exp. 2 was not randomized, a separate ANOVA was performed to test the effect of order (left-right vs. right-left), and no effect was found. Experiments 4 and 5 had fours cores per block but only two 0-time measurements (the disturbed treatment). The ANOVA was performed only on the second set of complete measurements. Experiments 5a through 5c were combined into one ANOVA by treating each as a replicate. Pairwise contrasts were used to examine the effect of added N in each soil treatment (intact cores or disturbed).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Reconstructed Column Study (Exp. 1)
The disturbed-column leaching study showed an enormous conversion of organic N to NH⁺₄ and NO^-₃ (Fig. 1) . Weekly leaching of Oi/Oe horizon samples held at 10°C resulted in peak NO^-₃ concentrations of 6 mmol L^-1 and NH⁺₄ of 4.5 mmol L^-1 within the first 20 wk. Nitrate concentrations were still >2 mmol L^-1 after 75 wk of leaching. Nitrogen loss was calculated both by summing the export of inorganic N (16.4% loss) and by measuring differences in total N before and after treatment (22.7% loss). Overall, the Oi/Oe columns lost 11.8% of their original weight. Treatment effects (simulated acid vs. nonacid rain) were masked by both the high ionic strength and the acidification from nitrification (pH declined by 0.7 units). Other researchers have found similar problems with reconstructed soils (Johnson et al., 1995; Neslon and Robarge, 1996). In these cases, disturbance is high and the effects are dramatic. No conclusions can be drawn from such experiments except that the columns do not mimic field conditions. This study demonstrated an extreme in disturbance probably now avoided by most researchers.

View larger version (19K): [in this window] [in a new window]   Fig. 1. Inorganic N in weekly leachates from a laboratory study of highly disturbed soils (Exp. 1). Soil columns contained a homogenized mixture of Oi and Oe horizon material.

View larger version (22K): [in this window] [in a new window]   Fig. 2. Repeated measurements of inorganic N in a bulked Oa sample (Exp. 3a), beginning shortly after sampling. Separate subsamples were extracted over time by two different methods.

View larger version (22K): [in this window] [in a new window]   Fig. 4. Repeated measurements of inorganic N in a bulked Oa sample (Exp. 3c). Separate subsamples were extracted over time with 2 M KCl. An additional set of subsamples was incubated in a 1% acetylene atmosphere before extraction.

View larger version (18K): [in this window] [in a new window]   Fig. 3. Repeated measurements of inorganic N in a bulked Oa sample (Exp. 3b). Separate subsamples were extracted over time with 2 M KCl.

View larger version (33K): [in this window] [in a new window]   Fig. 5. Nitrate in the Oa horizon from two sizes of cores (Exp. 4). The disturbed treatment consisted of bulked Oa samples from cores dismantled just after sampling. Error bars represent the standard error of each treatment across blocks. The difference between 3 d disturbed and intact cores was significant (P < 0.05).

View larger version (37K): [in this window] [in a new window]   Fig. 6. Nitrate in the Oa horizon of intact cores incubated with and without added NH+4 (Exp. 5a–c). Each graph represents a different replication of the experiment (see Table 1). Error bars represent the standard error of each treatment across blocks. With the three experiments combined, there was a significant (P < 0.05) increase in nitrate in the intact cores in response to added NH+4.

Thus, it appears that disturbance increases NH⁺₄ supply and decreases spatial limitation. The high rates of intact-core gross ammonification indicate that a sufficient rate of NH⁺₄ supply may exist before disturbance. Alternatively, the initial increase in NH⁺₄ could be a simple response to the creation of a readily available C source caused by mixing. This concept is not new. Schloesing (1897) demonstrated that mixing increased decomposition independently of aeration. A portion of the readily available C may be from bacterial and fungal cells lysed by the disturbance (Piatek, 1999). Nitrification has often been found to respond to the NH⁺₄ supply (Vitousek et al., 1982; Robertson, 1982). If this hypothesis is valid, higher in situ NO^-₃ concentrations should be found after disturbances (e.g., freeze/thaw) and when competition for NH⁺₄ is reduced (off-season). Watershed-scale disturbances have been found to increase NO^-₃ concentrations in streams, as have episodes of severe freezing (Mitchell et al., 1996). Seasonal fluctuations in stream NO^-₃ export are well documented with concentrations increasing through the fall and winter. This could be explained by having significant nitrification only during periods of low plant uptake and greater NH⁺₄ supply (rather than having greater NO^-₃ consumption during the growing season). Significant net nitrification can occur at low temperatures (Ross and Bartlett, 1990; van Miegroet, 1995). It is likely that the ammonia oxidizing community is adapted to the cycle of available NH⁺₄.

Alternate hypotheses exist to explain the disturbance phenomenon and need to be explored. Different scenarios for the inhibition of nitrification in the field have been offered over the years (e.g., Rice and Pancholy, 1972; Bremner and McCarty, 1988; Paavolainen et al., 1998). Gaseous or volatile inhibitors would be dissipated by disturbance, allowing nitrification to proceed. Another intriguing possibility is the recently confirmed occurrence of anaerobic ammonia oxidation (to N₂ gas), found in sewage treatment plants (Jetten et al., 1999). Introduction of oxygen into this system immediately stops the process and NH⁺₄ accumulates. An interaction with denitrification is also possible, that is, the introduction of oxygen stops the process and NO^-₃ accumulates. These alternative hypotheses cannot easily explain all our findings but they do need further investigation.

Relationship to Other Studies
Differences in the extent of disturbance-induced nitrification among soils are still not well understood. Vitousek et al. (1982) found a wide range in net nitrification in disturbed samples from 14 sites across the USA, with many sites showing no net NO^-₃ accumulation. Recent research has linked nitrification rates to various parameters, including landscape position (Ohrui et al., 1999), land-use history (Goodale and Aber, 2001), elevation (Bohlen et al., 2001), moisture and temperature variation (Gilliam et al., 2001), and tree species (Lovett et al., 2000). The factors that contribute to differences in nitrification rates are complex and are interrelated. For example, landscape position can affect species composition (Peterjohn et al., 1999), elevation may have a moisture interaction (Bohlen et al., 2001) and land-use history may determine soil C/N ratios (Goodale and Aber, 2001). Differences in rates may also relate to methodology, as measurements in "reactive" soils may be sensitive to disturbance. Most of the studies cited above measured initial nitrate concentrations 1 to 2 d after sampling. If the soils behaved in a similar manner to those in the Brush Brook watershed, this initial reading could be much higher than that found in the field and interpretation of differences between studies may be difficult. Differences in sampling and incubation procedures could also result in variations in the results from sensitive soils, again making comparisons between studies difficult.

It should be reiterated that not all sites show a similar response. Christ et al. (1995) found little nitrification of added NH⁺₄ in a beech (Fagus grandifolia)-dominated stand in the Hubbard Brook Experimental Forest, about 150 km from the study site. Some European sites have not exhibited significant nitrification even with added NH⁺₄ (e.g., Emmett et al., 1995). In a study of forested sites in Sweden and Denmark, Persson et al. (2000) found little difference in net N mineralization between intact cores and sieved soils. Similar results were found by Laverman et al. (2001) in samples from a forested site in the Netherlands. Reasons for the differences between sites may be related to the C/N ratio. Using sites in a European north-south transect, Persson et al. (2000) found a good relationship between Oe/Oa C/N ratio and relative net nitrification below a C/N ratio of about 25. Above this, no net nitrification occurred. Working in New Hampshire's White Mountains, Goodale and Aber (2001) showed a similar relationship between total soil profile C/N ratio and net nitrification rates with little or no net nitrification above a ratio of 20. Gundersen et al. (1998) and Dise et al. (1998) working in Europe, and Goodale and Aber (2001) in New Hampshire, all found a relationship between nitrification and stream or soil water NO^-₃ export, again with a critical ratio C/N between 20 and 25. A common theme with most of these results is high variability, especially below the critical C/N value. The average C/N ratio of the soils that we studied was 16.8, well below the critical value. Thus, these soils would be expected to exhibit net nitrification.

More work is needed to clarify the mechanism of the response and the cause of differences between sites. The task is difficult because experimentation invariably causes disturbance. Regardless of the cause of the phenomenon, the implications are serious for research on these soils. How does one study them without producing artifacts? Is a soil that produces large concentrations of NO^-₃ upon disturbance a soil that is a good nitrifier or one in which nitrification in the field is limited by factors that we do not understand? The correlation found between stream export and C/N ratio (Gundersen et al., 1998; Dise et al., 1998; Goodale and Aber, 2001) suggests that soils that are sensitive to sampling disturbance will also produce NO^-₃ without disturbance. Although absolute rate measurements may not be possible, perhaps the potential rate measurements are useful for predicting ecosystem response to perturbations.

	ACKNOWLEDGMENTS

 
We thank Gary Badger of UVM's Medical Biostatistics Department for assistance with the statistical analysis, and Patrick Keane and Guinivere Fredriksen for field and lab support. We also thank Sandy Wilmot and the Vermont Dep. of Forest, Parks, and Recreation for permission to perform the research within Camels Hump State Park. Experiment 1 was carried out under the direction of Dr. F.R. Magdoff and Dr. R.J. Bartlett of the University of Vermont. This study was partially funded by award 99-35101-8235 from the USDA NRICGP.

Received for publication March 5, 2002.

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