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Soil Science Society of America Journal 67:878-888 (2003)
© 2003 Soil Science Society of America

DIVISION S-7—FOREST & RANGE SOILS

Mineral and Dissolved Organic Nitrogen Dynamics along a Soil Acidity-Fertility Gradient

Zengshou Yu, Tamara E. C. Kraus, Randy A. Dahlgren*, William R. Horwath and Robert J. Zasoski

Department of Land, Air and Water Resources, One Shields Avenue, University of California, Davis, CA 95616

* Corresponding author (radahlgren{at}ucdavis.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Mineral (NH+4 + NO-3) and dissolved organic nitrogen (DON) dynamics were investigated along a soil chronosequence in northern California that ranges in age from about 100 000 to 500 000 BP. Younger soils are slightly acidic and fertile supporting highly productive grasslands and mixed-conifer forests. Older soils are highly acidic and infertile supporting forests of dwarf (<3 m) conifers and Ericaceous species. This edaphic gradient provides an ideal opportunity to examine changes in N dynamics on soils that are progressively older and less fertile. We examined in situ net mineralization rates using closed-top tubes and we examined mineralization and nitrification rates using 15NH+4 and 15NO-3 pool dilution techniques. Net N mineralization rates per unit of organic C decreased as soil age increased. Net mineralization rates (per unit C) were more strongly related to differences in rates of immobilization than gross mineralization. However, gross mineralization results normalized to soil N levels (N activity basis) were similar across soil ages. A similar rate of N turnover across this edaphic gradient indicates that the size of the total N pool is an important factor regulating N mineralization. It further suggests that litter quality does not appreciably hinder N mineralization. Pool dilution of added 15NO-3 indicated that nitrification is active across all sites and that microbial assimilation consumed the majority of the NO-3 produced. Dissolved organic N makes a larger relative contribution to dissolved N in older soils indicating a shift in the dominant N cycling pathway from mineral to organic forms in older less fertile soils.

Abbreviations: DON, dissolved organic nitrogen • GNM, gross N mineralization • NNM, net N mineralization


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
INTERACTIONS AMONG FACTORS controlling mineral N and DON availability and turnover are rarely considered in assessing ecosystem productivity. In the absence of soil rejuvenating processes (e.g., erosion, deposition), soils generally become progressively more acidic and less fertile with time. Evidence from the Ecological Staircase soil chronosequence (100 000 to 500 000 BP) in northern California suggests that the dominant N cycling pathway shifts from mineral to organic forms as soils become older (Northup et al., 1995b; Yu et al., 1999). Increased polyphenol concentrations in vegetation associated with older soils may be a feedback affecting edaphic conditions (Muller et al., 1987; Northup et al., 1995a). Polyphenol-rich plant communities occur on highly acidic and infertile soils throughout the world suggesting that these communities are adapted to survive under conditions of low nutrient availability. Plant secondary compounds, such as phenolics and tannins, are believed to play an important role in regulating plant–litter–soil and plant–animal interactions and available soil N (Horner et al., 1988; Kuiters, 1990; Scalbert, 1991; Schimel et al., 1996, 1998; Northup et al., 1998; Inderjit et al., 1999). By inhibiting decomposition in acidic infertile soils, polyphenols enhance mor-humus formation, conserve nutrients, and create a more favorable medium for root growth (Northup et al., 1998).

While the importance of polyphenols as inhibitors of organic matter decay and N mineralization has long been recognized (Handley, 1954, 1961), recent studies suggest an additional important role for polyphenols in ecosystem N dynamics. Formation of protein-tannin complexes in litter with high polyphenol concentrations may impede mineralization–immobilization reactions and shift the dominant N cycling pathway from mineral to organic forms (DON). Compared with NH+4 and NO3-, DON is less available and less likely to be lost by leaching or denitrification (Northup et al., 1995b). On the highly acidic, infertile sites of the Ecological Staircase, DON accounts for up to 99% of the total dissolved N in litter leachates (Yu et al., 2002). The DON consists primarily of proteins and peptides in the hydrophobic fraction of dissolved organic matter, suggesting the presence of protein/peptides-polyphenol complexes.

Few studies have examined changes in soil N dynamics during soil evolution on a geologic time scale. The objective of this study was to examine N cycling processes (gross and net rates of N mineralization, immobilization, and nitrification) across the range of litter quality encompassed on the Ecological Staircase in northern California. On the Ecological Staircase, litter from a given species may contain a wide range of polyphenol and nutrient concentrations (Northup et al., 1995a; Northup et al., 1995b). This research investigated the question—is decreased net N mineralization associated with soils of increased age caused by reduced gross mineralization rates or by increased immobilization rates?


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Site
The Ecological Staircase is located in the Jug Handle Reserve on the northern California coast about 200 km north of San Francisco. The Ecological Staircase consists of a series of five marine terraces ranging in age from about 100 000 to 500 000 BP (Merritts et al., 1991). Because all terraces occur within 5 km of each other, they all experience nearly the same Mediterranean-type macroclimate with frequent fog during spring and summer. The mean annual temperature is 12.5°C and the mean annual precipitation is 983 mm (National Oceanic and Atmospheric Administration, 1997-98). About 80% of the precipitation occurs between November and March. Soil and plant communities vary dramatically among terraces. Soils on the youngest terrace (Inceptisols) are slightly acidic and fertile supporting highly productive mixed-conifer forests (Table 1). In contrast, soils on older terraces (Spodosols and Ultisols) are highly acidic and infertile supporting pygmy forests of dwarf (<3 m) conifers and Ericaceae species. This extreme edaphic and biotic gradient provides an ideal opportunity to examine changes in N cycling in soil systems that are progressively older, more acidic and less fertile.


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  		Table 1. Soil and plant characterization data for research sites along the Ecological Staircase.

 
To examine the effects of soil fertility and acidity on N cycling, we chose five vegetation-soil combinations along the Ecological Staircase: Terrace 1 (T1), grassland and tall Bishop pine (Pinus muricata D. Dons) forest; Terrace 2 (T2), tall Bishop pine forest; and Terrace 4 (T4), pygmy forest with sites under Bishop pine and cypress, Cupressus pygmaea [Lemm. Sarg.)] (also referred to as Cupressus goveniana ssp. pygmaea (Lemmon) J. Bartl). Since vegetation and soils on the three oldest terraces are similar, we chose a site on Terrace 4 to represent the low fertility-strong acidity member of the edaphic gradient. The Oa (litter) and the upper 15 cm of the mineral soil were specifically chosen for study because they comprise the dominant rooting zone for fine roots (Northup et al., 1995a).

Net Nitrogen Mineralization
In situ incubations using closed-top tubes (Hart et al., 1994) were conducted at 6-wk intervals between 20 Sept. 1997 and 3 Oct. 1998. Polyvinyl chloride (PVC) tubes (5.2 cm in diameter and 26 cm in length) were driven into the soil to a depth of 20 cm and the top capped to prevent leaching. At the beginning of each incubation, four intact soil cores were established at each of the five sites. Simultaneously, time-zero samples (t0) from the Oa horizon and the 0- to 5- and 5- to 15-cm depth increments of the mineral soil were collected from the immediate vicinity of the incubation cores. A total of eight replicate cores were composited to obtain the t0 sample. After 6 wk, the incubated cores were removed and a new set of cores was established along with collection of new t0 samples. Incubated cores and t0 samples were placed on ice, transported to the laboratory, and stored at 3°C before processing began within 48 h of collection.

In the laboratory, each intact core was divided into three subsamples consisting of the Oa horizon and the 0- to 5- and 5- to 15-cm depth increments of the mineral soil. Individual samples were mixed in a plastic bag, weighed, and then extracted with 60 mL of 2 M KCl over a 6-h period using a mechanical vacuum extractor (Centurion International, Lincoln, NB, Model 24). Following KCl extraction, the extraction was repeated with 60 mL of distilled-deionized (DDI) water over a 6-h period. The KCl and water extracts were combined and analyzed for mineral N (NH+4–N and NO-3–N) and extractable organic N. Throughout the manuscript, we will refer to this extractable organic N as DON. Mineral N was determined conductimetrically (Carlson, 1978; Carlson, 1986), as was organic N following persulfate digestion (Yu et al., 1994). The net N mineralization and DON mobilization rates were calculated by dividing the difference between final and initial (t0) N concentrations by the incubation time.

Soil moisture was determined gravimetrically by drying a subsample to a constant weight at 105°C. At each site, soil temperature was monitored at 5-cm depth using duplicate Optic StowAway temperature loggers (Onset Computer Corporation, Pocasset, MA) that recorded temperature with a 1-h time step. Air temperature and precipitation were obtained from the nearby (7 km) weather station at Fort Bragg, CA (National Oceanic and Atmospheric Administration, 1997-98).

Gross Nitrogen Mineralization Rates by Nitrogen-15 Pool Dilution
Pool dilution of added 15NH+4 was examined twice to measure gross N mineralization–immobilization rates (Hart et al., 1994). A laboratory incubation was performed in January 1998 under controlled environmental conditions and an in situ incubation was performed in June 1998 under field conditions. Acrylic tubes (4.7-cm diam.) were used to make intact soil cores of the Oa horizon and the 0- to 10-cm depths of mineral soil. At each site, four replicate cores were removed for injection of (15NH4)2SO4 solution. The (15NH4)2SO4 solution was injected through an 18 gauge, 15-cm long spinal needle having multiple outlets. To assure an even distribution of (15NH4)2SO4 solution within the soil core, 10 mL of the (15NH4)2SO4 solution (98% 15N enrichment) containing 600 µg N (approximately 4 µg N g-1 soil) was injected into the mineral soil cores using five separate 2-mL injections. For Oa horizons, 3 mL of the (15NH4)2SO4 solution containing 180 µg N (approximately 5 µg N g-1 soil) was injected using three separate 1-mL injections. After injection, cores were incubated for 24 h in the laboratory (15°C) or 36 h in the field (placed in original holes). Field cores were collected; soil was mixed in a plastic bag and processed in the field.

Approximately 15 g of well-mixed Oa horizon and about 40 g of mineral soil were taken for measurement of to NO-3 and NH+4 levels. In the field, these samples were added to 100 mL of 2 M KCl in preweighed specimen containers. Samples were capped tightly, shaken vigorously, and placed on ice for transport to the laboratory. In the laboratory, the samples were reshaken for 15 min followed by gravity filtration through prerinsed, Whatman No. 42 filter paper (Whatman Ltd., Maidstone, England). Laboratory incubated samples were shaken with 2 M KCl for 1 h before filtration. The filtrates were analyzed for mineral N concentrations and isotopic N composition. Soil moisture content was determined using a separate subsample. We determined ambient NH+4 concentrations and {delta}15NH+4 values along with time-zero (within 5 min) recovery of added 15NH+4 on replicate cores (n = 4) from each site. Recoveries ranged from 90 to 94% and there were no differences in 15NH+4 recoveries among sites.

Total mineral N in the filtrates was quantified conductimetrically as described above. For the determination of 15N, mineral N in the extracts was concentrated using the acid trap–diffusion method described by Stark and Hart (1996). Samples spiked with NH+4 indicated recovery of 98 to 100% of the added NH+4 by the diffusion method. The diffused N samples were analyzed for 15N on an isotope-ratio mass spectrometer (Europa Scientific, Crewe, England). Gross N mineralization and immobilization were calculated using the equations given by Hart et al. (1994).

Gross Nitrification Rate by Nitrogen-15 Pool Dilution
Gross nitrification and NO-3 consumption rates were measured in the field (June 1998) in conjunction with the pool dilution study that measured gross N mineralization–immobilization. Ten milliliters of K15NO3 containing 300 µg NO-3–N (49% 15N enrichment) was injected into each of the 0- to 10-cm mineral soil cores (approximately 2 µg N g-1 soil), and 3 mL of K15NO3 solution containing 90 µg NO-3–N was injected into the Oa horizon core (approximately 2.5 µg N g-1 soil). After incubation and extraction with 2 M KCl as described above for 15NH+4, a volume of filtrate containing 40 to 75 µg NO-3–N was placed in open specimen cups with 0.2 g Mg(OH)2 and allowed to react for 6 d to remove NH+4 and NH3. Then an acid trap and 0.4 g of Devarda's alloy was added to convert NO-3 to NH+4 and NH3, which was concentrated using the acid trap–diffusion method and analyzed for isotopic composition on an isotope-ratio mass spectrometer as described above (Stark and Hart, 1996). In spiked samples, the diffusion technique recovered 95 to 99% of the added NO-3. We determined ambient NO-3 concentrations and {delta}15NO-3 values along with time-zero (within 5 min) recovery of added 15NO-3 on replicate cores (n = 4) from each site. Recoveries ranged from 94 to 98% and there were no differences in 15NO-3 recoveries among sites. Gross nitrification and NO-3 consumption were calculated using the equations given by Hart et al. (1994).

Soil Respiration and Microbial Biomass
Soil respiration and microbial biomass were determined on four replicate samples of Oa horizon and mineral soil (0–10 cm) collected from the five sites in conjunction with the field pool dilution studies (June 1998). Each sample was the composite of eight subsamples. Soil materials were mixed and passed through a 4-mm screen to remove coarse materials. For the respiration study, soil moisture was adjusted to 80% of field capacity for Oa horizon soils and 55% of field capacity for the mineral soil. Soils were incubated in sealed Mason jars fitted with septa. Carbon dioxide in the headspace was measured periodically over 29 d using an Infrared Gas Analyzer. In addition, 30-d N mineralization rates were measured on these soil samples by determining concentrations of mineral N (NH+4 and NO-3) at t0 and Day 30. Microbial biomass was measured using the chloroform fumigation extraction method (Horwath and Paul, 1994). Extracts were analyzed for DON following persulfate oxidation as described above (Yu et al., 1994).

Statistical Analyses
Statistical analyses were performed using SYSTAT version 9.0 (SPSS Inc., Chicago, IL). To examine the effects of soil age (fertility–pH gradient), ANOVA was performed for P. muricata data across terraces (T1, T2, T4), followed by a Tukey-Kramer HSD pair-wise comparisons test to determine differences between means. A t-test was used to determine differences between contrasting vegetation types on a given terrace (T1 = grass vs. P. muricata; T4 = P. muricata vs. C. pygmaea). Statistical comparisons for soil characterization data from site-vegetation combinations along the Ecological Staircase were made using ANOVA, followed by a Tukey-Kramer HSD pair-wise comparisons test to determine differences between means. All statistical differences were tested at the p = 0.05 level.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil and Vegetation Characteristics along the Ecological Staircase
Soil and plant characterization data are shown in Tables 1 and 2 (see Yu et al., 1999 for additional details). No Oa horizon was found beneath the grass vegetation on T1. Soil pH, organic C, and nutrient pools all showed a progressive decrease with increasing soil age (T1 > T2 > T4). In contrast, microbial biomass C and N did not show a consistent trend with increasing soil age; however, the mineral soil beneath grass at the T1 site had much higher values than mineral soils from all other sites. Measured C mineralization rates for samples from the T1 and T2 sites were up to twice as large as those measured on the pygmy forest samples (T4). Carbon mineralization rates were about 20 times higher in samples from Oa horizons compared with samples from the underlying mineral soils (0–10 cm). During the 30-d laboratory incubations net C mineralization was less in samples from older soils (T1 > T2 >> T4) and samples collected under P. muricata at T1 exhibited much higher mineralization rates that those collected under grass. Similar to the soil fertility gradient along the Ecological Staircase, foliar nutrient concentrations decreased from T1 to T4 (pygmy forest). Nutrient levels in T2 samples had intermediate values (Yu et al., 1999). All major species in the pygmy forest were polyphenol-rich. Those species that also grow on more than one soil age across the Ecological Staircase have higher concentrations of extractable total phenols and condensed tannins on the older more acidic and infertile sites (T4 > T2 > T1) (Northup et al., 1998).


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  		Table 2. Selected soil characterization data for various vegetation/soil combinations along the Ecological Staircase.

 
Seasonal Dynamics of In Situ Mineral Nitrogen and Dissolved Organic Nitrogen Pools
Ammonium accounted for more than 90% of the mineral N pool and was the dominant form of mineral N at all sites and sampling times. Within Oa horizons, the mineral N pool decreased in older soils (T1 > T2 > T4) (Fig. 1) . A decreasing mineral N concentration (mg N kg-1 soil) and a decreasing Oa horizon thickness (T1 = 18 cm; T2 = 10; T4 = 4) in older soils both contributed to this decreased N pool. In the mineral soil (0- to 5- and 5- to 15-cm soil depths), mineral N pools were generally lowest at the T4 site (pygmy forest) while pools at the T1 and T2 P. muricata sites were similar (T1{approx}T2 > T4). There was no difference in mineral N pools between P. muricata and C. pygmaea on T4, nor were there consistent differences between P. muricata and grass vegetation on T1.



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  		Fig. 1. Soil mineral N (NH+4 + NO-3) pools in time-zero (t0) samples collected for the closed-top core, in situ incubation at sites along the Ecological Staircase.

 
Mineral N pools were generally lowest from late August through January (Fig. 1), and peaked during the spring to early summer (March–July). March to July coincides with plentiful soil moisture and warmer soil temperature (Fig. 2 and 3) . Samples from the Oa horizon under P. muricata at the T1 site had increased NO-3 and NH+4 beginning in December, which may reflect the warmer soil temperatures at this site (Fig. 2).



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  		Fig. 2. Precipitation, air temperature, and soil temperature at the 5-cm depth of the study sites along the Ecological Staircase. Precipitation and air temperature data are from the National Oceanic and Atmospheric Administration station in nearby Fort Bragg, CA.

 


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  		Fig. 3. Gravimetric soil moisture content of the time-zero (t0) samples collected for the closed-top core, in situ incubation at sites along the Ecological Staircase.

 
Dissolved organic N pools (Fig. 4) were 5- to 10-fold greater than mineral N pools (Fig. 1) throughout the year. The pool size of DON increased with respect to soil age (T1 >= T2 > T4) and was similar to trends in mineral N pools. The seasonal pattern of DON pools in the Oa horizon was similar to mineral N pools, except that DON levels increased slightly earlier in the winter and spring and decreased somewhat earlier in the spring and summer.



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  		Fig. 4. Dissolved organic N pools in time-zero (t0) samples collected for the closed-top core, in situ incubation at sites along the Ecological Staircase.

 
Net Nitrogen Mineralization and Dissolved Organic Nitrogen Mobilization Rates
Net N mineralization rates were greatest in samples from the Oa horizons and the 0- to 5-cm depth in the mineral soils (Fig. 5) . Similar to mineral N pools, net mineralization rates in the Oa horizon generally decreased in samples from older soils (T1 > T2 > T4). Net N mineralization rates at the pygmy forest sites (T4) were zero or nearly zero for most of the year. Seasonal changes in net mineralization rates in the Oa horizon samples were closely related to soil moisture content (Fig. 3). Net mineralization rates for all Oa horizon samples declined abruptly in early August to values <=0 mg N m-2 d-1 corresponding with a large decrease in soil water content.



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  		Fig. 5. Net N mineralization determined using the closed-top core, in situ incubation at sites along the Ecological Staircase.

 
Net DON mobilization rates reflect a balance between solubilization of DON and consumption of DON by mineralization and uptake. Net DON mobilization rates (Fig. 6) were generally higher (3 to 5 times) than net mineralization rates (Fig. 5). In contrast to net mineralization rates, net DON mobilization rates did not exhibit any discernable trends with respect to soil age. Net DON mobilization rates in the Oa horizon were generally highest during the February-June period, which coincides with high N mineralization rates. Samples from Oa horizons and from the 0- to 5-cm mineral soil depth had net DON mobilization rates that were generally positive, but negative rates were measured during June-July. This decreased net DON mobilization was preceded by large decreases in the DON pools (Fig. 4) and appeared to coincide with drying of the Oa horizon (Fig. 3).



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  		Fig. 6. Net dissolved organic N mobilization determined using the closed-top core, in situ incubation at sites along the Ecological Staircase.

 
Nitrogen Mineralization and Immobilization Rates by Pool Dilution
To make meaningful comparisons of N cycling rates across sites with large differences in soil organic matter contents, we are reporting rates normalized to both a per unit C and a per unit N basis. Gross N mineralization (GNM) rates ranged between 25 and 40 mg N kg-1 C d-1 for Oa horizons and between 25 and 170 mg N kg-1 C d-1 for mineral soil (0–10 cm depth) (Fig. 7) . Rates of GNM beneath P. muricata generally decreased in older soils across the Ecological Staircase. Decreased GNM rates were especially pronounced in the mineral soil (Table 3). Nitrogen immobilization rates were higher in Oa horizons at T4 sites compared with T1 and T2 sites, but lower immobilization rates were found in mineral soil (0–10 cm) at the T4 site. Net N mineralization (NNM) rates (the difference between GNM and N immobilization) determined by pool dilution decreased on older soils in both the organic (Oa) and mineral horizons (Fig. 7). This trend was consistent with NNM rates determined by the closed-top core incubation method (Fig. 5) and by the 30-d laboratory incubation (Table 2). While NNM rates were positive for all samples during the January laboratory experiments, net mineralization rates were negative for several samples measured during the June field experiments. It should be noted that pool dilution techniques might overestimate immobilization rates because 15NH+4 additions increase NH+4 availability. There were no consistent differences in N mineralization and immobilization rates among samples collected beneath P. muricata and C. pygmaea at the T4 site. Samples of mineral soil collected beneath P. muricata at the T1 site had consistently greater NNM rates than samples collected beneath grass at this site. This difference was consistent with NNM rates determined in 30-d laboratory incubations (Table 2).



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  		Fig. 7. Nitrogen gross mineralization, immobilization, and net mineralization rates reported on a per organic C basis as determined by 15NH+4 pool dilution at sites along the Ecological Staircase. Panels A and B are for the laboratory incubation study (January 1998) and Panels C and D are for the field incubation study (June 1998).

 

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  		Table 3. Statistical analysis comparing 15N pool dilution results reported on a per unit C basis and a per unit N basis. Results are for P. muricata (P) versus grasslands (G) on terrace 1 (T1), P. muricata (P) on terraces 1 (T1), 2 (T2) and 4 (T4), and P. muricata (P) versus C. pygmaea (C) on terrace 4 (T4). Results are presented for both the laboratory incubation and field incubation studies.

 
Mineralization and immobilization rates reported on a per unit N basis provide a measure of N activity (i.e., turnover rate of organic N) in the soil. Rates of GNM were similar on a per unit N basis while immobilization rates were generally higher in samples from the T4 site compared with the T1 and T2 sites (Fig. 8 and Table 3). Similar GNM rates per unit N suggested that N turnover rates were similar despite the large differences in total N pools and litter quality that exist across the edaphic gradient. Consistent with the results reported on a per unit C basis, NNM rates (per unit N) in samples collected beneath P. muricata also decreased in older soils (T1 > T2 > T4). While N cycling rates reported on a per unit C basis were much higher in the 0- to 10-cm mineral soil relative to the organic layer, N cycling rates reported on a per unit N basis are similar in magnitude in mineral and organic horizons.



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  		Fig. 8. Nitrogen gross mineralization, immobilization and net mineralization rates reported on a per nitrogen basis as determined by 15NH+4 pool dilution at sites along the Ecological Staircase. Panels A and B are for the laboratory incubation study (January 1998) and panels C and D are for the field incubation study (June 1998).

 
Gross Nitrification and Nitrate Consumption Rates
Gross nitrification rates determined by 15NO-3 pool dilution were 1.2 to 3.2 mg N kg-1 C d-1 in Oa horizon samples and 3.2 to 23.4 mg N kg-1 C d-1 in samples of mineral soil (0–10 cm) (Fig. 9) . Gross nitrification rates were 5 to 10 times lower than gross mineralization rates (Fig. 7 and 9). These findings were consistent with measured mineral N pools and net mineralization rates (closed-top core incubation method) that found a predominance of NH+4 over NO-3 at all sites. Both gross nitrification and NO-3 consumption decreased as soil age increased. Nitrate consumption was nearly equal to gross nitrification leading to net nitrification values near zero for most sites. There were no significant differences in nitrification and NO-3 consumption rates in soil samples from grasslands and P. muricata vegetation on T1 or between samples collected beneath P. muricata and C. pygmaea at the T4 site. Trends in nitrification and consumption rates were similar across sites when reported on a per unit N basis (data not shown).



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  		Fig. 9. Gross nitrification, nitrate consumption, and net nitrification rates reported on a per organic C basis as determined by 15NO-3 pool dilution at sites along the Ecological Staircase. Results are from the field incubation study (June 1998).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Mineralization Dynamics
Nitrogen mineralization strongly affects many ecosystem processes including N leaching losses and N availability for plant uptake. As demonstrated by both the in situ core incubation and 15NH+4 pool dilution studies (Fig. 5 and 7), NNM rates decreased as soil age increased (T1 > T2 > T4). Rates of NNM at pygmy forest sites (T4) were less than or near zero during most of the year indicating a very low N availability to the vegetation. A gradient in N availability on the Ecological Staircase was demonstrated by Yu et al. (1999). They extracted soil solutions by centrifugation and measured decreased NO-3 concentration in older soils (T1 = 67–203, T2 = 2–44, T4 <= 1µM). Fertilization experiments by Jenny and coworkers also demonstrated that N was the most limiting nutrient in the pygmy forest ecosystem (Jenny et al., 1969).

Because GNM rates on a per unit N basis were similar across the Ecological Staircase, the size of the total N pool appears to be an important factor regulating N mineralization. Total N pools decreased more than two-fold on older soils (T1 = 13.5, T2 = 9.8, T4 = 5.6 Mg N ha-1). These results are consistent with those of Wang et al. (2001) who reported that total organic N was the best predictor of N mineralization across a wide range of soils. At the Ecological Staircase, where erosion and deposition appear to have been minimal over geologic time, soil and ecosystem evolution result in decreased soil N pools and thus decreased N mineralization rates.

Plants may affect N cycling by controlling litter quality (Swift et al., 1979; Aber et al., 1990). An objective of this study was to determine whether litter quality played an appreciable role in regulating GNM, N immobilization, and NNM rates. Large differences in C/N ratio and polyphenol concentrations occur in litter fall across the terraces and present an opportunity to examine N dynamics with respect to litter quality (Tables 1 and 2). Because litter quality has greater impacts on surficial organic layers, this discussion will focus primarily on N dynamics in the Oa horizon. Nitrogen mineralization rates in litter can be affected by the C/N ratio (Berg and Ekbohm, 1983; Prescott et al., 2000), lignin levels, lignin/N ratios (Berg and McClaugherty, 1987; Aber et al., 1990), and concentrations of phenolics, tannins or other secondary compounds (Handley, 1961; Benoit and Starkey, 1968; Palm and Sanchez, 1990, 1991; Gallardo and Merino, 1992; Kalburtji et al., 1999; Mafongoya et al., 1998; Driebe and Whitham, 2000).

In this study, NNM rates decreased dramatically with increasing soil age across the edaphic gradient (Fig. 7 and 8). Rates of GNM determined by 15NH+4 pool dilution techniques on Oa horizon samples collected beneath P. muricata did not differ consistently as a function of soil age, while N immobilization rates were consistently higher at T4 sites. Thus, differences in NNM appeared to relate more strongly to differences in immobilization rates rather than GNM.

Many studies that examined litter quality effects on N mineralization rates focused on C/N; however, this ratio is often a poor predictor of N mineralization (Muller et al., 1988; Carlyle et al., 1990). In this study, the C/N of the Oa horizon increased from 43 at the T1 site to {approx}60 at the T4 site (Table 2). Immobilization rates displayed a similar increase as might be expected if the microbial community was competing more strongly for mineralized N in soils with higher C/N ratios. Thus, decreasing rates of NNM across the edaphic gradient appear to reflect a greater microbial competition for N in older soils.

Nitrogen may limit microbial growth in the older soils along the edaphic gradient. Decreased availability of N was suggested by the low or negative rates of NNM at the pygmy forest sites (T4) compared with the T1 and T2 sites, and was further supported by the ratio of C/N mineralized in the laboratory mineralization study. This ratio indicates the relative extent of C or N limitation for the soil microbial community (Schimel, 1986). A low ratio of mineralized C/N indicates a C-limited community that has excess N relative to its requirements. A high ratio indicates a N-limited community that is either processing N-poor material or immobilizing a large portion of the mineralized N. The ratio of mineralized C/N increased by nearly two-orders of magnitude (T1 = 45, T2 = 105, T4 > 2600) across the edaphic gradient indicating that T1 microbial communities are less N limited than T4 microbial communities (Table 2).

Lignin is believed to be a sink for N during litter decomposition, resulting in formation of humic substances precursors (Stevenson, 1982; Berg and Theander, 1984). In a previous study, we found that lignin concentrations in pine litter (25–35%) did not differ between terraces and that N mineralization rates and litter lignin content were not correlated (Northup et al., 1995b). Thus, differences in lignin concentration do not appear to regulate NNM along the edaphic gradient.

Our previous research suggested that condensed tannin concentrations could partially regulate N mineralization at Ecological Staircase sites (Northup et al., 1995b). Polyphenols have been implicated as regulators of NNM in many previous studies (Palm and Sanchez, 1991; Gallardo and Merino, 1992; Schimel et al., 1996; 1998; Bradley et al., 1997; 2000; Lorenz et al., 2000; Fierer et al., 2001). Phenolics, tannins, and other secondary compounds may limit decomposition and mineralization by affecting microbial activity or by complexing organic compounds (Benoit and Starkey, 1968; Benoit et al., 1968; Swain, 1979; Baldwin et al., 1983; Scalbert, 1991; Field and Lettinga, 1992). Zucker (1983) suggested that protein binding by tannins could lead to N conservation in N-limited ecosystems.

Total phenol and condensed tannin to N ratios in foliage at the T4 site were two to three times greater than at the T1 site (Table 1), suggesting that polyphenols and tannins could affect N dynamics in the pygmy forest (T4). Additionally, polyphenol concentrations in C. pygmaea were 15 to 50% greater than those of P. muricata growing at the same site suggesting a greater potential influence in Oa horizons beneath the C. pygmaea canopy (Table 1). The GNM rates determined by 15NH+4 pool dilution techniques were not appreciably different in Oa horizons beneath P. muricata along the edaphic gradient, despite large differences in polyphenol concentrations in P. muricata foliage. Furthermore, there were no differences in GNM or NNM between soil samples from P. muricata and C. pygmaea sites on T4 even though C. pygmaea foliage had considerably higher polyphenol concentrations. Gross N mineralization rates on a per unit N basis were similar for Oa horizons beneath P. muricata along the edaphic gradient. This suggests that organic N sources are not bound with polyphenols into forms, which limits their availability to microbes. Thus, it appears that protein binding by polyphenols does not play a dominant role in N mineralization regulation at pygmy forest sites.

Nitrification Dynamics
Interpretation of the 15NO-3 pool dilution data indicates that nitrification is active across all sites and that microbial assimilation of NO-3 consumes most of the NO-3 produced (Fig. 9, Table 3). The range of gross nitrification rates in this study (1–23 mg N kg-1 C d-1 or 0.2 –1.7 mg N kg-1 d-1) is similar to values reported for eleven forest ecosystems in New Mexico and Oregon (Stark and Hart, 1997). Moderate gross nitrification rates occur in the pygmy forest despite low soil pH and high litter polyphenol concentrations that have been reported to inhibit nitrification in other soils (Thibault et al., 1982; Baldwin et al., 1983; Olson and Reiners, 1983). Lower gross nitrification rates in the pygmy forest soils (T4) relative to T1 soils may result from a combination of low available substrate (NH+4) and from inhibition of nitrifiers by higher polyphenol concentrations. Gross nitrification rates are higher in the mineral soil than in Oa horizons, and this may relate to lower polyphenol concentrations in the mineral soil because of decomposition or transformations of the polyphenols or because the polyphenols are sorbed on reactive mineral surfaces. Sorption may be especially pronounced in T1 and T2 soils that contain much higher Fe oxide concentrations. Samples from mineral soils at these sites have lower concentrations of soluble polyphenols.

Because microorganisms rapidly assimilated NO-3, net nitrification and gross nitrification were not well correlated. While denitrification may reduce net nitrification, soil moisture was less than field capacity at the time of the study so the bulk soils were well aerated and any denitrification would be confined to microsites. Stark and Hart (1997) suggested that high rates of microbial NO-3 assimilation result from the activity of both saprophytic microorganisms and mycorrhizal fungi. Rapid microbial NO-3 assimilation may be an important mechanism for N retention in highly deficient ecosystems, such as the pygmy forest. Nitrate assimilated by microbial biomass can be released as organic N and NH+4 resulting in conversion of the highly mobile NO-3 into forms that are less susceptible to leaching and denitrification.

Dissolved Organic Nitrogen Versus Mineral Nitrogen
A comparison of mineral N and DON pools indicates that DON exceeds mineral N by a factor of 5 to 10 times (Fig. 1 and 4). This is especially pronounced at the pygmy forest site (T4) where mineral N pools and NNM rates are very low. In fact, measured NNM in the field incubation study was less than or equal to zero for most of the year in samples from both Oa and mineral soil. In situ soil solutions extracted by centrifugation from litter beneath P. muricata and C. pygmaea at T4 indicated that DON accounted for 77 to 99% of the total dissolved N in Oa horizon leachates (Yu et al., 2002). Nitrogen in free amino acids and alkyl amines accounted for 1.5 to 10.6% of the DON fraction, while combined amino acids accounted for 48 to 74% of the DON. These data suggest that proteins and peptides were the main contributor to DON. Most of the DON was found in the hydrophobic fraction, which is consistent with the presence of protein/peptide-polyphenol complexes. We previously suggested that mineralization might be hindered by complexation of proteins or peptides leading to an accumulation of dissolved N in the DON fraction. However, N activity as measured by GNM rates on a per unit N basis was similar across the edaphic gradient. This similarity in GNM rates suggests that N is not preferentially bound in forms with limited microbial availability. Alternatively, the microbial assemblage in pygmy forest soils may be able to utilize N bound to polyphenols. For example, ericoid mycorrhizae produce extracellular enzymes, such as polyphenol oxidases, peroxidases, or tannin carboxyl esterases that degrade polyphenols and result in bound protein becoming available to proteolytic enzymes (Leake and Read 1989; Bending and Read 1996). Direct utilization of organic N results in a short-circuiting of the N cycle by effectively eliminating the mineralization pathway that is often the major bottleneck restricting the supply of N to plants (Northup et al., 1995b; Chapin 1995).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Along the Ecological Staircase, NNM rates decreased in older soils (T1 > T2 > T4). Rates of NNM in soils from the pygmy forest (T4) were less than or near zero most of the year. Rates of GNM in samples collected beneath P. muricata decreased with increasing soil age in mineral soils, while GNM rates in Oa horizons did not consistently differ among soils of different ages. Nitrogen immobilization rates were consistently higher in T4 soils (pygmy forest). Thus, it appears that differences in NNM among soils were primarily attributable to differences in immobilization rates rather than differences in GNM. Decreasing rates of NNM across the edaphic gradient may reflect greater microbial competition for N in older soils. Rates of GNM reported on a per unit N basis (N activity basis) were similar across the edaphic gradient. Thus, the size of the total N pool appears to be the primary factor regulating N mineralization. Because the higher polyphenol levels in litter at T4 site do not affect GNM rates, it seems that complexation of proteins and peptides by polyphenols does not appreciably affect N mineralization. However, microbial respiration rates were lower in pygmy forest soils (T4) compared with the more fertile sites (T1, T2), which indicates that litter quality, N availability and the composition of the microbial community more strongly affect C mineralization. Litter quality or microbial factors appear to have little influence on N turnover rates while these factors have a considerable influence on C turnover rates. Pool dilution studies using 15NO-3 indicate active nitrification and rapid microbial assimilation of NO-3. Rapid microbial NO-3 assimilation may be an important mechanism retaining N in highly deficient ecosystems, such as the pygmy forest. Dissolved organic N is a much larger fraction of dissolved N in the pygmy forest soils (T4) compared with the more fertile sites (T1, T2). There appears to be a change in the dominant N cycling pathway from mineral to organic forms as soils become older and less fertile.


   ACKNOWLEDGMENTS
 
We are grateful for permission to collect samples in the Jug Handle State Reserve. Financial support from the National Science Foundation grant DEB-9527722 made this work possible.

Received for publication January 16, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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