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DIVISION S-7-FOREST & RANGE SOILS

Nitrogen Availability through a Coarse-Textured Soil Profile in the Shortgrass Steppe

^a Dep. of Rangeland Ecosystem Sci., Colorado State Univ., Fort Collins, CO 80523 USA
^b New Zealand Pastoral Agriculture Research Inst. Ltd (AgResearch), Ruakura Research Centre, Private Bag 3123, Hamilton 2001, New Zealand
^c Dept. of Forest Sci., Colorado State Univ., Fort Collins, CO 80523 USA
^d Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523 USA

doddm{at}agresearch.cri.nz

	ABSTRACT

TOP ABSTRACT INTRODUCTION Methods and materials Results Discussion Conclusions REFERENCES

 
We conducted a study of N resources in a loamy sand soil in the shortgrass steppe of northeastern Colorado, on a site dominated by both shrubs and grasses. Our objective was to determine whether the soil N resources were more evenly distributed with depth than is typical for this environment, thereby helping to explain the coexistence of the two plant life forms. We measured total soil C and N, potential net N mineralization, and in situ available inorganic N (using ion exchange resin bags) through the soil profile to a depth of 150 cm. All three measures confirmed that available N was greatest in the surface soil layers (0–10 cm) and decreased substantially with depth. A supplemental watering treatment was imposed on the soil during the spring of 1996 to examine N-leaching potential. The water addition increased soil water content and available NO^-₃ to a depth of 60 cm, indicating that NO^-₃ leaching might occur on this soil type under favorable conditions. To confirm this assertion, we used two simple models to examine the impact of increased soil moisture on in situ mineralization and solute transport processes. The results indicated that NO^-₃ leaching could better account for the observed patterns in available N. This process, by contributing to a more even distribution of resources through a coarse-textured soil profile, could be important for improving the competitive status of shrubs in an otherwise grass-dominated ecosystem.

Abbreviations: CPER, Central Plains Experimental Range • GLM, general linear model • PMA, phenyl mercuric acid • TDR, time domain reflectometry • WFPS, water-filled pore space

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Methods and materials Results Discussion Conclusions REFERENCES

 
SOIL TEXTURE is regarded as an important control on ecosystem structure and function in semiarid grasslands (Epstein et al., 1997a, 1997b). The recent conceptual model of Sala et al. (1997) hypothesizes that the balance between herbaceous and woody plant functional types in such environments is controlled by the interaction between the seasonality of water availability and soil texture. Specifically, in contrast to fine-textured soils, coarse-textured soils allow a greater proportion of incident precipitation to percolate out of the zone of greatest evaporative potential to deep storage (Dodd and Lauenroth, 1997). This moisture resource is more able to be exploited by deep-rooted woody plants than by grasses, given that the bulk of grass root biomass typically occurs in the uppermost layers of the soil profile (Liang et al., 1989; Lee and Lauenroth, 1994). This provides a mechanism explaining why woody life forms are commonly associated with sandy soils in the shortgrass steppe of North America (Kinraide, 1984; Lauenroth and Milchunas, 1991). However, the spatial distribution of water availability is not the only resource that can potentially be influenced by soil texture.

Nitrogen is often regarded as the most important nutrient limiting grassland productivity worldwide (Woodmansee et al., 1978). At the community level, Lauenroth et al. (1978) have shown that both net primary production and species composition respond to N addition when water limitation is alleviated. The higher losses of N associated with coarse-textured soils are likely to be a function of both greater volatilization and leaching losses (Schimel et al., 1986; Delgado et al., 1996). Leaching might lead to a more even distribution of available N through coarse-textured profiles, and more importantly, to greater N availability at depths below the surface soil layers that are dominated by the roots of grasses. Shrubs might have an improved competitive status for such available N at depth, in the same way that they use deep soil moisture resources (Sala et al., 1989; Dodd et al., 1998). This indicates that N and water availability may covary as important controls on ecosystem structure and function in semiarid grasslands (Burke et al., 1997). The objective of this study was to examine the pattern of N availability through the profile of a coarse-textured soil, and to determine whether there was evidence of an available N resource at depths below the zone dominated by grass roots (0–60 cm) (Lee and Lauenroth, 1994). We hypothesized that such a resource could occur through in situ net N mineralization in deeper soil layers, and/or leaching of NO^-₃ from a pool of inorganic N near the surface. To determine potential sources of inorganic N, we estimated potential net N mineralization through the profile using laboratory incubations, and estimated actual soil N availability with depth, using anion–cation exchange resins.

	Methods and materials

TOP ABSTRACT INTRODUCTION Methods and materials Results Discussion Conclusions REFERENCES

 
Location and Site Characteristics
The experiment was conducted at the Central Plains Experimental Range (CPER), located in the shortgrass steppe of northeastern Colorado (40°51' N, 104°43' W, elev. 1650 m). We chose a shrubland area adjacent to an ephemeral stream (Owl Creek) for this study, which began in June 1995. The soil type at the study area is an Ascalon-Vona sandy loam, a deep, well-drained Ustollic haplargid (Crabb, 1982). Kelly et al. (1993) have identified a paleosol at a depth of 1 m at this site. The vegetation is dominated by the C₄ shrub [Atriplex canescens (Pursh.) Nutt.] (four-wing saltbush) with a population density of 0.22 shrubs m^-2; and the C₄ grass [Bouteloua gracilis (H.B.K.) Lag. ex Griffiths] (blue grama) with a basal area of 40% (nomenclature follows GPFA, 1986). We selected a site sufficiently clear of shrubs to excavate a trench 2 m deep, 2 m wide, and 6 m long with a backhoe. We allowed at least 1 m between the nearest shrub stems and the edge of the trench, to take soil samples outside the estimated volume of soil influenced by the shrub roots (40- to 50-cm radius from the shrub stem, Lee and Lauenroth, 1994). During the grazing season (April–October), we erected a temporary electric fence around the site to exclude cattle.

Soil Texture and Bulk Density
To determine the physical characteristics of the soil profile, we inserted small metal cylinders (5-cm diam. and 5-cm length) horizontally into the exposed face of the trench at depths of 0, 5, 10, 30, 60, 90, 120, and 150 cm. No replicates within depths were sampled, since data from other studies at the site (e.g., Dodd and Lauenroth, 1997) had already given us a good understanding of soil physical characteristics. We carefully excavated the tubes and shaved the excess soil from the ends. The soil samples were oven-dried (55°C for 24 h), weighed, and analyzed for sand, silt, and clay percentages using the hydrometer method (Bouyoucos, 1962). Bulk density was calculated from the dry soil weight and the known volume of the cylinders.

Soil Carbon and Nitrogen
We measured both total C and N, and potential net N mineralization on soil samples taken horizontally from the profile in October 1995. At each of the depths noted above, we collected four replicate samples. All samples were collected from soil under Bouteloua plants. From each sample we took one set of subsamples (5 g), ground them in a ball mill, and used automated combustion analysis to determine percent mineral contents of C and N by weight (Nelson and Sommers, 1982). We also used a paired set of subsamples (15 g) to estimate potential net N mineralization in laboratory incubations (Keeney, 1982). We extracted inorganic N from the first set of samples using 50 mL of KCl–PMA (phenyl mercuric acid) solution (2 M KCl) added to the soil, followed by a 30-min agitation. The solutions were decanted through Whatman #2 filter paper and the supernatant analyzed for NH⁺₄ and NO^-₃ on a Perstorp FS3000 autoanalyzer (OI Analytical, College Station, TX). The second set of samples was moistened to field capacity and aerobically incubated in the dark for 30 d at 25°C, after which the inorganic N was extracted in the same way. Potential net N mineralization was estimated as the difference between initial and final NH⁺₄ + NO^-₃ content of the samples. The mineralization data are expressed as mg N m^-2 of a 5-cm deep layer during the 30-d period, based on the conversion of soil sample weight to volume using the bulk density data.

Ion Exchange Resin Bag Sampling
The use of ion exchange resins as a means of measuring plant-available soil nutrients is now well established, with various materials and methods abounding (Skogley and Dobermann, 1996). Binkley and Matson (1983) pioneered the use of resin bags in long-term field studies to measure in situ N availability in a forest soil, but other similar studies have been carried out in grasslands (Gibson, 1986; Hook and Burke, 1995), deserts (Lajtha, 1988), and Arctic tundra (Giblin et al., 1994). The advantages of ion exchange resins over other methods as applied to long-term field studies are considerable (Giblin et al., 1994), but the key feature for this study was their ability to accumulate nutrients moving through the profile via water percolation (Crabtree and Kirkby, 1985).

The exchange resins were made by mixing equal quantities of Ionac C-249 (Sybron Chemicals, Birmingham, NJ) (Na⁺ for cation exchange) and Ionac ASB-1P (Sybron Chemicals, Birmingham, NJ) (Cl^- for anion exchange). For a single resin bag, two level tablespoons of the mixture were placed into a knotted nylon-stocking pouch. To install the resin bags in the soil profile, we excavated 40-cm horizontal shafts into the profile face adjacent to the row of shrubs. Six shafts were excavated at each of the following depths in the profile: 10, 30, 60, 90, and 150 cm. They were all offset along the profile by 10 cm so that each had an undisturbed column of soil above it to the surface. In each shaft we placed a single resin bag, pressed against the upper soil surface and supported by a perforated plastic lid (6-cm diam.) and repacked soil.

The resin bags were installed in the same shafts for four seasonal periods: summer 1995 (June 27–October 25, 120 d); winter 1995–96 (October 25–March 13, 140 d); spring 1996 (March 13–June 28, 107 d); and summer 1996 (June 28–October 25, 119 d). At the end of each period, we removed the bags and analyzed them for NO^-₃ and NH⁺₄ content using KCl extractions in the same manner noted above for the potential net N mineralization measurements. Our calculation of the amount of each N component originally retained by the resin bag included a factor to account for the extraction efficiency of the KCl–PMA solution from the resin. Prior laboratory tests showed this efficiency to be 50% for the range of concentrations occurring in this study (B. Riggle, personal communication, 1996). Results for N retained by the ion exchange resin bags are expressed as a flux rate of mg N m^-2 through the surface of the resin bag for a 30-d period. This calculation is based on the assumption that all ions originated from a cylinder of soil volume above the resin bag having the same diameter as the bag (6 cm).

For the changeover of resin bags in March of 1996, we added an additional facet to the experiment, aimed at determining the effect of a wet spring on N availability. On the soil surface, we inserted an aluminum plate 20 cm vertically into the soil at the midpoint of the trench (i.e., three replicates were now on each side of the plate). One side of the profile received the normal spring precipitation, while the other side received additional water. Specifically, at three times during the spring (mid-April, mid-May, and mid-June) we added the equivalent of 15 mm rainfall every second day for 8 d, followed by the equivalent of 45 mm rainfall. In the late afternoon of each day, we applied the additional water evenly by hand, using a watering can to a distance of 1 m back from the edge of the trench.

To monitor soil water content before, during, and after the additions, we used the time domain reflectometry (TDR, Topp et al., 1980) function of a Tektronix 1505B cable tester (Tektronix, Wilsonville, OR). Two 30-cm stainless-steel welding rods placed 3 cm apart were inserted 0.27 m into the soil profile next to each shaft at the same depths as the resin bags. Values for the soil dielectric constant (K_a) obtained from the TDR wave form were converted to volumetric soil water content (_v), using a calibration equation developed in a separate experiment on a nearby sandy loam soil type using mini-lysimeters (Wythers, 1996):

(1)

Statistical Analyses
We analyzed the results for potential net N mineralization using the general linear model (GLM) procedure of SAS (SAS Institute, 1996), testing for the main effect of depth and including multiple comparisons between depths using Fishers LSD test. The soil water content data and resin bag N data were analyzed with a GLM nested model design (depth effects nested within water treatment effects). A natural log transformation of the mineralization and resin bag data was necessary to stabilize the large degree of variation for different depths in the profile. An important limitation of the single excavation site was that the design of the supplementary watering treatment represented a form of pseudoreplication (Hurlbert, 1984) through lack of spatial independence of the replicates (more correctly subsamples). The comparison of resin data from both sides of the profile both before and after treatment application was necessary to mitigate this problem. Unless otherwise indicated, use of the term significant indicates a P-value of 0.10. This level was considered appropriate for the small experimental size (Steel et al., 1997).

Modeling
Although changes in mineral N availability resulting from the watering treatments would be demonstrated by the field data collected, this information could not identify the underlying process, that is, whether changes in mineral N availability were the result of leaching or increased in situ mineralization. To examine this question we used two modeling approaches: The first involved modeling the impact of soil moisture on mineralization rates to estimate likely increases of in situ nitrification as a result of increased soil water content, comparing them with the observed accumulation in the resin bags. Doran et al. (1988, cited in Parton et al., 1996) present a relationship between the water-filled pore space (WFPS) of the soil and the fractional effect on nitrification. We used our information on soil physical characteristics and the soil water data collected from the wet and dry profiles to calculate the WFPS from April to June 1996. The model then allowed us to calculate values for the fractional effect of WFPS on nitrification.

In the second approach, we used a leaching model to examine the potential for NO^-₃ movement under the conditions imposed in this experiment. Rose et al. (1982) present a simple model of solute transport (their equation 24) that enabled us to estimate the change in the mean depth of NO^-₃ ions in this soil:

(2)

Where = volumetric soil water content, = mean depth of solute (mm), I = irrigation amount (mm), I = time (d), E = evapotranspiration (mm), and D_r = rooting depth (mm). Subscripts: fc = field capacity, and n = time step.

We parameterized the model for _fc using the data on volumetric soil water content before water addition and the data on the depth of the wetting front, applying equation 13 of Rose et al. (1982):

(3)

We performed the Rose model calculation for the addition of 45 mm of supplementary water over a 1-d period, assuming that evapotranspiration was not completed before percolation (in which case ). We then used the same equation in an iterative process to calculate _n for the subsequent water additions made in May and June (adjusting _n-1 and _n-1) to estimate the mean depth to which the first solute pulse travelled.

To investigate likely differences in leaching potential under the experimental regime across different soil types, we performed the same calculations for a sandy clay loam and a sandy clay. Previous studies of these two soil types at the CPER have given us good information on field capacity and soil water content responses to precipitation (Dodd and Lauenroth, 1997), which we used as estimates for _fc and _n-1 in the calculations.

	Results

TOP ABSTRACT INTRODUCTION Methods and materials Results Discussion Conclusions REFERENCES

 
Soil Texture and Bulk Density
Soil texture was relatively uniform throughout the profile, which was categorized as a loamy sand. Sand content varied between 81 to 88%, except for the 120-cm layer, where it was 73%. Silt content varied little: between 2 and 6% throughout the profile. Clay content reflected the difference: between 10 to 13% throughout most of the profile, except for the 120-cm layer, at which it was 19%.

Bulk density was also consistently between 1.45 to 1.55 Mg m^-3 throughout the profile, except for the surface layer (1.17 Mg m^-3) and the 120-cm layer (1.35 Mg m^-3).

Soil Carbon and Nitrogen
Levels of total soil C in the top two 5-cm layers were not significantly different, but were significantly greater than those in all other depths (Fig. 1) . From the surface to the 60-cm depth, soil C decreased from 770 to 240 g C m^-2, but below this increased significantly to 370 to 410 g C m^-2 in the 90- through 150-cm depths. Total soil N patterns largely followed those of total C, except for the deepest layers that we measured (Fig. 1). Soil N was highest in the top two layers (70 g N m^-2), diminished with depth down to 60 cm (27 g C m^-2), and increased significantly (36 g N m^-2) in the 90-cm depth. At all measured depths down to and including 90 cm, the C/N ratios in the soil were relatively consistent, ranging from 9 to 12, and there were no significant differences among depths. However, this ratio increased to >15 at the 120-cm depth, and >20 at the 150-cm depth, which was significantly greater than all other depths. This was due to the decrease in soil N relative to soil C: at 150-cm depth soil, N was the lowest of any layer (18 g N m^-2).

View larger version (46K): [in this window] [in a new window]   Fig. 1 Total soil C and N in a loamy sand soil profile (bars denote standard errors)

View larger version (34K): [in this window] [in a new window]   Fig. 2 Potential net N mineralization in a loamy sand soil profile, based on 30-d aerobic laboratory incubations (bars denote standard errors)

Seasonal and Depth Effects
Inorganic N content of the ion exchange resin bags was generally lowest during the winter (5.8–18.5 mg N m^-2), greatest during the spring (6.7–32.4 mg N m^-2), and intermediate during the summer–fall (7.6–30.0 mg N m^-2) (Table 1) . The largest differences among seasons occurred at the 10- and 30-cm depths.

View this table: [in this window] [in a new window]   Table 1 The effect of season and soil depth on the N accumulation of resin bags in the soil profile (significant differences between depths at the P 0.10 level are indicated by different letters)

Water Treatment Effects
The addition of water to one side of the profile (wet treatment vs. dry control) had the desired effect of increasing the volumetric soil water content to a substantial depth in the profile. At the beginning of spring (during which the treatment was applied) there were no significant differences in water content between the soils in each half of the profile, both in terms of a main effect (wet vs. dry treatment) and between treatments at individual depths. At the end of the April and May series of water additions, the soil water content was significantly greater in the wet treatment at the 10-, 30-, and 60-cm depths; and at the end of the June series, this was also true of the 90-cm depth.

The effect of each application series on soil water in the profile is exemplified by the results of the May series of water additions (Table 2) . Applying 15 mm of additional simulated rainfall every other day for a week only increased soil water in the uppermost layer, since there was a significant difference between the wet and dry treatments at 10 cm only by May 22 (P 0.001). Two days after the application of 45 mm of simulated rainfall on May 22, the wet treatment had significantly greater soil water at 10, 30, and 60 cm (P 0.001).

View this table: [in this window] [in a new window]   Table 2 The effect of supplementary water addition during spring 1996 on volumetric soil water content in the soil profile (significant differences between depths and treatments at the P 0.05 level are indicated by different letters)

During the spring, the total amount of inorganic N retained by the resin bags (NO^-₃ + NH⁺₄ summed over all depths) increased significantly with supplemental watering. This main effect of the water treatment was significant for NO^-₃ but not for NH⁺₄. Supplemental water appeared to increase available N of both inorganic forms at all individual depths (Table 3) , although only one of the comparisons of watered vs. unwatered treatments within depths was significant for either form of inorganic N. The greatest relative difference in available NO^-₃ between the two water treatments occurred at the 60-cm depth: 37 vs. 9 mg N m^-2 for the wet and dry treatments, respectively, representing a >300% increase in available NO^-₃, a difference that was significant at P 0.10.

View this table: [in this window] [in a new window]   Table 3 The effect of supplementary water addition on the N accumulation of resin bags in the soil profile (significant differences between depths and treatments within periods at the P 0.10 level are indicated by different letters)

During the summer period, following the supplemental watering treatments, both NO^-₃ and NH⁺₄ levels in the resin bags were slightly greater in the previously watered treatment at almost all depths. However, neither main effect or within-depth comparisons indicated significant water treatment effects for either NO^-₃ or NH⁺₄ content of the resin bags.

Modeling
The fractional effect of WFPS on nitrification ranged between 0.18 and 0.23 for the dry profile and between 0.24 and 0.31 for the wet profile, depending on depth. The calculated percentage increases in fractional effect (from dry to wet) were 36, 35, and 26% for the 10-, 30-, and 60-cm depths, respectively. The equivalent increases in N accumulation (from dry to wet) that we observed in the resin bag data were 90, 70, and 300%, respectively.

The results of the Rose model calculations are shown in Table 4 . The model estimated the total potential penetration of NO^-₃ ions during the 3-mo period at 496 mm in the loamy sand soil type used in the field experiment. Changing the value of field capacity in the model (for a different soil type) had a substantial effect on the predicted total penetration of the solute: 262 mm in the sandy clay loam and 206 mm in the sandy clay.

	Discussion

TOP ABSTRACT INTRODUCTION Methods and materials Results Discussion Conclusions REFERENCES

Semiarid environments are generally characterized by a dominance of the NO^-₃ form of inorganic N (Charley and West, 1977; Klopatek, 1987; Reuss and McNaughton, 1987; Burke, 1989). Several studies have indicated that NO^-₃ is the predominant form of inorganic N in the shortgrass steppe (Schimel and Parton, 1986; Hook et al., 1991; Hook and Burke, 1995; Hook and Burke, submitted). Our measurements of the inorganic N pools in the sandy soils of this shortgrass site provide more support for these observations. The conclusion that NO^-₃ is the dominant form of inorganic N in this environment suggests the potential for NO^-₃ leaching to be an important factor controlling the vertical distribution of available N, since NO^-₃ is much more mobile in soil solution than ammonium. Epstein et al. (1998) have recently demonstrated significant NO^-₃ leaching at a local site during periods of high precipitation.

The supplementary water treatment during the spring did have a large effect on NO^-₃ adsorption by the resin bags. The pattern of NO^-₃ accumulation by the resin bags through the profile (Table 3) corresponded well with the pattern of soil water content through the profile (Table 2). The lack of significant differences in available N and soil water content between the two sides of the profile before applying the watering treatment confirms that the treatment was effective and helps to mitigate concerns with pseudoreplication. This result indicates that soil water dynamics can have a significant impact on the distribution of the available N resource through this soil profile, and that water availability and N availability are closely linked. The key question is whether the important process is NO^-₃ leaching or merely increased in situ mineralization through more favorable local soil moisture conditions. It is worth noting that the quantitative increase in available N resulting from supplemental water addition is well within the amount potentially mineralized in situ at all depths (Fig. 2). This observation suggests that the source of increased available N at any depth could be solely attributable to in situ nitrification rather than leaching. However, there was no significant effect of supplemental watering on ammonium adsorption by the resin bags, which might be expected if in situ mineralization processes were controlling local inorganic N availability. This suggests that NO^-₃ leaching was impacting N availability, although NH⁺₄ concentrations are notably low, and nitrification rates do not necessarily lag mineralization rates, so there is still some possibility that the increased NO^-₃ was generated in situ.

The results of the calculations of fractional effect of WFPS on nitrification show much lower percentage increases (from dry to wet) in fractional effect than were observed in the soil profile. This indicates that increased in situ nitrification is unlikely to have accounted for the observed increases in NO^-₃ accumulation with supplementary watering.

Applying the Rose leaching model indicated that in the loamy sand soil type, the potential mean depth of NO^-₃ ion penetration through the profile during the 3 mo of the supplemental watering was almost 50 cm. This value is reasonably close to the depth at which a significant increase in accumulation of NO^-₃ ions was measured (60 cm), supporting the conclusion that NO^-₃ leaching was occurring in this system. The discrepancy could be accounted for by the fact that these calculations ignore the impact of normal rainfall for the 3-mo period. For example, 2 d after the series of water additions in May, 60 mm fell on the site during a 4-d period. This would have enhanced the effect of the supplemental water and perhaps contributed to the relatively high levels of NO^-₃ captured by the resin bags placed at 30 cm in the dry profile.

The Rose model calculations for the other two soil types showed that finer soil textures are likely to be associated with reduced solute penetration. We currently lack the data to test these model predictions. However, this exercise provides a theoretical basis for the assertion that leaching could be an important determinant of the vertical distribution of soil N resources in the shortgrass steppe environment. An extension of the field experimentation described here into other soil types would provide evidence to support the idea that the distribution of soil N in the profile can be linked to vegetation structure.

In applying the water treatment during the spring, we attempted to create near-ideal conditions for N mineralization in the upper soil layers and for the leaching of that inorganic N resource to the lower layers. The conditions under which both of these limitations are alleviated probably occur relatively infrequently in the shortgrass steppe. A long-term soil water simulation that we have conducted at this site (Dodd and Lauenroth, 1997) provides some indication of the likely frequency of the soil moisture conditions that we imposed in this study. The simulation indicated that an increase in soil water content of 2% at the 60-cm depth (as observed in this experiment; see Table 2) could have occurred in 27 of the 50 yr during the period 1940 to 1989. The estimates for a 2% increase in soil water content in the deeper layers were for 16 of the 50 yr at 90 cm and 4 of the 50 yr at 150 cm. Despite the relatively low frequency of significant water penetration in this environment, infrequent pulses of available N that can be used by shrubs without competition from grasses may be sufficient to support the shrub population. To maintain adequate tissue N concentrations, low inputs may be supplemented by root and stem storage, or balanced by low losses, through a more conservative nutrient-reallocation strategy at senescence. Dead A. canescens leaves have been shown to contain about 39% of the tissue N concentrations of green leaves, compared with 48% for grasses (D.G. Milchunas, unpublished data, 1988).

Another feature of shortgrass steppe vegetation dynamics that could result in increased NO^-₃ leaching is associated with the long-term role of animal disturbances (e.g., ant mounds, fecal deposits, and gopher mounds). These disturbances cause gaps in the vegetation through the death of individuals of the dominant species (B. gracilis), which by definition create patches of enhanced water and nutrient availability (Coffin and Lauenroth, 1990). For the time that such gaps remain uncolonized by new individuals and by roots of adjacent species (Hook et al., 1994), the potential exists for increased relocation of nutrients by leaching.

Even under the most ideal conditions for redistribution of available N in favor of deep-rooted plants, the bulk of the available N still resided in the zone accessible to the roots of B. gracilis (i.e., 0–90 cm) (Lee and Lauenroth, 1994). On this basis, we might conclude that greater NO^-₃ leaching in very coarse-textured soils would provide no real advantage for N acquisition by deep-rooted shrubs over shallow-rooted grasses. However, what this hypothesized mechanism provides for is a demonstrable shift in the spatial distribution of resources through the soil profile. A pool of available water and N in a zone less intensively exploited by the dense root structures of bunchgrasses such as B. gracilis is potentially more accessible to the extensive root-foraging pattern typical of shrub species. Soil microbial populations, which represent a further competitive element for inorganic N in these soils, are also likely to be lower in deeper soil layers.

Within the context of a suite of contrasting plant adaptations (e.g., root architecture and nutrient allocation), shrub and grass life forms could differentially exploit the altered pattern of resource availability that results from heterogeneity in soil texture across the shortgrass steppe landscape. This competitive balance is embodied in the Sala et al. (1997) model, which focuses on spatial and temporal variability in soil water as the key factor controlling community structure. This idea is supported by a number of other studies (Soriano and Sala, 1983; Sala et al., 1989; Dodd and Lauenroth, 1997; Dodd et al., 1998), but this study demonstrates that an equivalent hypothesis for soil N resources deserves further examination.

	Conclusions

TOP ABSTRACT INTRODUCTION Methods and materials Results Discussion Conclusions REFERENCES

 
The main soil N resource at this loamy sand site occurred in the top 10 cm of the soil profile, based on estimates of total soil N, potential net N mineralization, and ion exchange resin bag N adsorption data. There was little evidence from these measurements to suggest the occurrence of any substantial soil N resource below the surface layers. However, available N was significantly increased to a depth of 60 cm by adding supplementary water to the soil during the spring. This increase was mainly attributable to increased NO^-₃ accumulation, indicating that water and N availability may be linked through leaching in this soil type. Modeling of water effects on in situ mineralization and on solute transport supports the conclusion that leaching was occurring under the conditions imposed. The leaching effect is probably infrequent in this system, and therefore its importance as an N resource for the shrubs at this site remains unclear.Hook in press)

	ACKNOWLEDGMENTS

 
This research was supported by the National Science Foundation, Shortgrass Steppe Long-Term Ecological Research project (BSR–9011659), and the Colorado State University Agricultural Experiment Station (1–57661). The CPER is administered by the Great Plains Systems Research Unit of the USDA–ARS. During the course of this study, M.B. Dodd was on paid study leave from AgResearch (New Zealand). We wish to thank Becky Riggle, Mark Lindquist, and Nicolas Trautner for valuable assistance with laboratory and fieldwork. Comments from three anonymous referees were helpful in improving the manuscript.

Received for publication June 8, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION Methods and materials Results Discussion Conclusions REFERENCES

 

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• Charley J.L., West N.E. Micro-patterns of nitrogen mineralization activity in soils of some shrub dominated semi-desert ecosystems of Utah. Soil Biol. Biochem. 1977;9(5):357-365.
• Coffin D.P., Lauenroth W.K. A gap dynamics simulation model of succession in a semiarid grassland. Ecol. Modell. 1990;49:229-236.
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