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Forest, Range & Wildland Soils

Resin Adsorption of Carbon and Nitrogen as Influenced by Season and Time Since Fire

Dep. of Ecosystem and Conservation Sciences, The Univ. of Montana, Missoula, MT 59812. M.D. MacKenzie, current address: 442 Earth Science Building, Dep. of Renewable Resources, Univ. of Alberta, Edmonton, AB, Canada T6G 2E3. T.H. DeLuca, currently at The Wilderness Society, 503 West Mendenhall Bozeman, MT 59715

^* Corresponding author (tom_deluca{at}tws.org)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Natural disturbance, such as fire, represents a long-term displacement of ecosystem dynamics, while seasonal changes, such as spring thaw and summer drought, represent a short-term perturbation superimposed on the longer disturbance cycle. In spite of the importance in determining forest ecosystem function of these two forms of perturbation, no previous studies have simultaneously investigated the effects of season and fire exclusion on soil C and N dynamics in the inland Northwest. Fire exclusion has resulted in decreased N mineralization due to changes in stand structure and decreased litter quality. However, C and N dynamics are also affected by seasonal fluctuations in soil temperature and moisture. We used ionic and non-ionic resin capsules to quantify indices of labile C and N including ninhydrin reactive N (NR-N–amino N), ammonium (NH₄⁺), nitrate (NO₃^–), anthrone reactive C (AR-C–hexose sugars), and phenols (low molecular weight aromatics). Repeated measures ANOVA showed that most soil biochemical properties varied significantly with time, season, and their interaction. However, only a few of the variables displayed linear trends with increasing time since fire (TSF), including NR-N, NH₄⁺–N, and NO₃^––N for winter and spring measurements, and AR-C for the spring measurement, all of which exhibited decreasing levels. Scatter plots showed that NR-N and AR-C were sequentially higher with each season, but decreased with TSF and that inorganic N was sequentially lower for each season and also decreased by TSF, except for spring NO₃^– levels on recently burned sites. None of the variables exhibited a significant difference during the summer measurement, likely as a result of seasonal drought, which emphasizes the importance of sampling time.

Abbreviations: AR-C, anthrone reactive carbon • DON, dissolved organic nitrogen • NH₄⁺, ammonium • NO₃^–, nitrate • NR-N, ninhydrin-reactive nitrogen • SOM, soil organic matter • TSF, time since fire

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
SOIL C AND N AVAILABILITY are important drivers of ecosystem processes including both above ground dynamics, such as plant competition and productivity, as well as belowground dynamics, such as microbial community structure and decomposition rates (Nilsson and Wardle, 2005; Schlesinger, 1997; Vitousek and Farrington, 1997; Wardle, 2002). Natural disturbances, such as fire, add a stochastic element to the equation, representing a long-term disruption to ecosystem dynamics (Chapin et al., 2002); whereas, seasonality, such as snowmelt and summer drought, adds a somewhat predictable factor, representing a short-term fluctuation on top of the long-term disturbance dynamics (Groffman et al., 2004). However, it is not clear from the literature how C and N pools change with the confounding influences of season and TSF. Studies on N cycling and secondary succession are generally contradictory, where some studies from conifer dominated ecosystems show N availability, and specifically nitrification, to decrease with increasing time since disturbance (Brais et al., 1995; DeLuca et al., 2002; Lodhi and Killingbeck, 1980; MacKenzie et al., 2004; Rice and Pancholy, 1974); other studies show N availability to increase, with no specific inhibition of nitrification or high gross nitrification with low net nitrification (Davidson et al., 1992; Robertson, 1982; Robertson et al., 1988). There is a plethora of information on soil C sequestration in primary and secondary succession (Bhatti et al., 2002; Falkowski et al., 2000; Schlesinger and Andrews, 2000), but these do not help us to understand ecosystem function in terms of soil organic matter (SOM) mineralization, where it may be more useful to examine the pools of available C that drive microbial activity. Many studies have also examined how changing seasons affect fluxes of soil C and N pools (Cain et al., 1999; Jaeger et al., 1999), but these studies focus on research plots in the same stage of plant community development. To date, few studies have effectively reported the simultaneous effects of season and secondary succession (or time since disturbance) on labile C and N pools.

Historically, low elevation ponderosa pine (Pinus ponderosa Doug. ex. laws) forests of western Montana burned every 10 to 50 yr with low severity (Arno and Allison-Bunnell, 2002; Arno et al., 1995, 1997; Barrett et al., 1997; Brown et al., 1994). This fire regime promoted an open canopy of ponderosa pine trees with an understory of grasses and forbs. Fire has also been shown to promote N mineralization in the short-term (Smithwick et al., 2005; DeLuca and Zouhar, 2000; Kaye and Hart, 1998; White et al., 1991), which increased N availability in this ecosystem. However, Euro-American inhabitation of the west has drastically altered the historic fire return interval resulting in the exclusion of fire from many of these forests for over 130 yr. To study the effect of increasing TSF on plant community dynamics, forest floor, and mineral soil N cycling, we established a chronosequence of sites in western Montana. We showed that the extended absence of fire has caused N availability (collected with ionic exchange resins) to decrease with time (Hart et al., 2005) and believe it to be due to a concomitant increase in tree basal area and total shrub cover (MacKenzie et al., 2004). Seasonal fluctuations may affect pools of labile C and N, but there are few studies that address C and N flux with changing season in this ecosystem. This is a noted oversight, because these forests experience extreme fluctuations of temperature and precipitation with season. Ponderosa pine forests of western Montana receive most precipitation as winter snow pack, with cold, wet spring months, hot, dry summer months and cold, wet fall months (NCDC, 2005; Nimlos, 1986). Both labile C and N pools are influenced by periods of wetting-drying and by cycles of freeze-thaw in strongly seasonal systems (DeLuca et al., 1992; Fierer and Schimel, 2002; Grogan et al., 2004), primarily due to the adverse effect of these abiotic factors on the microbial community and microbial mediated SOM transformations.

In this study, we integrate field conditions over a 1-yr period by using ionic and non-ionic resin capsules to characterize labile C and N pools along a fire chronosequence. Resin adsorption of C and N compounds represent the accumulation of bioavailable substrate for both plants and microbes, as the resins simulate uptake and immobilization processes and minimize their availability for further transformations (Binkley and Matson, 1983; Friedel et al., 2000). The purpose of this study was to investigate how labile pools of C and N are simultaneously influenced by season and TSF, and to examine the importance of season when investigating nutrient flux following fire. Specifically, we examined three different pools of labile N and two different pools of labile C.

Resin-sorbed NR-N is an index of the most labile and readily bioavailable portion of the dissolved organic N (DON) pool (Jones et al., 2004; Sculten and Schitzer, 1998). The ninhydrin reagent reacts with polyamides, amino acids, amino sugars, and ammonium (DeLuca and Keeney, 1993), but not cyclic or heterocyclic N compounds. Resin-collected NH₄⁺ and NO₃^– represent a good index of plant available N because levels of resin-collected N provide an estimate of N mineralization that is somewhere between gross and net N mineralization rates as much of the adsorbed inorganic N is unavailable for microbial transformation (Binkley and Matson, 1983; Friedel et al., 2000). Resin-sorbed AR-C is an index of labile C compounds that are positively correlated to short-term microbial respiration and biomass (DeLuca, 1998), where the anthrone reagent reacts with hexose sugars. Resin-sorbed soluble phenols are an index of C compounds that have the potential to form polyphenol protein complexes (PPC) and reduce the availability of N monomers (Hattenschwiler and Vitousek, 2000). However, these low molecular weight compounds may also be mineralized as microbial energy sources before they form more recalcitrant compounds (Stark and Hyvarinen, 2003).

The purpose of this work was to simultaneously investigate the influence of season and TSF on the accumulation of inorganic and soluble amino N on ionic resins. We hypothesize that the seasonal availability of NR-N and inorganic N will decrease from winter to summer due to the onset of hot, dry weather, as well as with increasing time since fire as indicated by our previous work. We are also measuring important pools of labile C that could influence N availability, but that have not been well studied to date. We expect these indices of labile C to decrease with season and TSF, as microbial activity ceases during hot, dry periods.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Site Description
Seasonal measurements were made at 10 sites in western Montana of varied TSF (2, 24, 45, 78, 87, 92, >120 (two sites), >130 (two sites) years since fire). Maps of historic fire polygons were provided by the local Forest Service Ranger Stations, going back to 1880 and 1870, and we selected sites from these that had no record of harvesting since the time of disturbance. These sites were located in the Bitterroot National Forest, the Lolo National Forest, and the Blackfoot Clearwater Game Reserve, all within a 100-km radius of Missoula, MT. All sites had similar slope (10–15°), aspect (S to SE), elevation (1200–1350 m), and pH (4.22–4.85) as can be seen in Table 1. The dominant plant species at these sites were ponderosa pine and Douglas-fir [Pseudotsuga menziesii (Mirbel) Franco] in the overstory, with an understory dominated by graminoides [Carex geryii Boott, Agropyron spicatum (Pursh.) Scrinb. & J. G. Sm.] and ericaceous shrubs [Arctostpylos uva-ursi (L.) Spreng., Vaccinium mirtyllus L.]. Douglas-fir basal area and total shrub cover increased significantly with TSF, but not to the exclusion of ponderosa pine or graminoids and there was no trend observed with N-fixing plants at these sites (MacKenzie et al., 2004). Soils were classified as Typic Dystrocryepts formed in metasedimetary or granitic colluvium or glacial fluvial deposits and were generally skeletal with fine sandy loam to silt loam soil textures. Further details on site selection, vegetation, and soil properties can be found in MacKenzie et al. (2004, 2006).

Ionic and non-ionic resin capsules were installed, left to incubate for several months and recovered for analysis at three different times from November 2001 to September 2002. Climatic conditions of western Montana are characterized by cold winters with moderate precipitation, wet springs with rapid warming, hot summers with little precipitation and wet falls with rapid cooling (NCDC, 2005; Nimlos, 1986). Therefore, the winter sampling period was from November 1st to May 1st, the spring sampling period was from May 1st to July 1st and the summer sampling period was from July 1st to September 1st, based on reported and observed climatic conditions for the area. Resins were collected, placed on ice for transport to the laboratory and then immediately placed into –20°C controlled climate chamber until analysis.

Chemical Analysis
Ionic resin capsules were extracted by three consecutive 10 mL rinses of 2 M KCl, each shaken for 30 min (Kjønaas, 1999). After each rinse, the KCl was decanted into a centrifuge tube and after all three rinses the extractant was centrifuged at 1280 x g (3000 RPM) for 10 min to remove any remaining soil particles. Ionic resin extracts were analyzed for NH₄⁺ and NO₃^– on a Technicon III Auto Analyzer (Bran-Luebbe, Chicago, IL) using the salicilate-nitroprusside method for NH₄⁺ and the NH₄Cl-Cd reduction method for NO₃^–, and reported as NH₄⁺–N and NO₃^––N (Mulvaney, 1996). These extracts were also used to determine the amount of amino N by the colorimetric NR-N method (Moore, 1968).

Non-ionic resin capsules were placed under a 2-mm nozzle connected by silicone tubing to a partitioning pump that delivered 0.8 mL min^–1 of extractant for 35 min and were extracted with two different eluents in succession (Morse et al., 2000). Deionized water was used as the first eluent for analysis of AR-C and 50% MeOH was used second for the analysis of soluble phenols (Morse et al., 2000; Yu and Dahlgren, 2000). Although XAD-7 is only moderately effective in the adsorption of simple sugars, these sugars are readily desorbed with deionized water (Morse et al., 2000). The XAD-7 resins are highly effective in the sorption of simple phenolic compounds and these are not readily desorbed by deionized water, while the 50% aqueous methanol eluent effectively desorbs the majority of these low molecular weight phenolic compounds (Morse et al., 2000; T.H. DeLuca, unpublished data, 2000).

Statistical Analysis
The SYSTAT 9.0 statistical package was used to conduct all statistical analysis for this study (SSI, 2002). Repeated measures of ANOVA were used to test the effect of season and TSF on individual biochemical variables that were resampled at the same location for three consecutive seasons. The repeated measures ANOVA test statistic was calculated by contrasting the variation between time factors (where TSF sites were averaged across season), with the variation within season factors (where seasons were averaged by TSF sites). Finally, repeated measures ANOVA contrasts the variation within season and time factors, where each season and TSF site was considered separately, to produce an interaction term (Underwood, 1997; Wilkinson, 1999). We also used linear regression analysis to determine if individual biochemical variables changed systematically with TSF for each season. In all cases, assumptions of normality, constant variance, and error independence were met (Wilkinson, 1999) or the data were transformed to meet the assumptions, including log transformation of the TSF data for linear regression. An alpha of 0.10 was used to determine significant differences among seasons, or with TSF, and scatter plots were produced by Sigma Plot 8.0 (SSI, 2002) to demonstrate how soil biochemical variables interact with season and TSF.

	RESULTS

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Nitrogen Pools
All nitrogen pools, both organic and inorganic, were greatly influenced by both season and TSF. Resin-sorbed NR-N, the labile fraction of the DON pool, exhibited significant changes with TSF and season, but there was no TSF x season interaction as indicated by repeated measures of ANOVA (Table 2). Ninhydrin reactive N exhibited significant linear decreases with TSF for both winter and spring measurements, but not for the summer measurement (Fig. 1a , b, and c). The scatter plot also showed the variability associated with NR-N levels that increased with season (winter to summer) and decreased with TSF (winter and spring). Recently burned sites had more labile organic N present through out the year, but there was a steady increase in NR-N levels on the fire excluded sites with seasonal progression.

View larger version (20K): [in this window] [in a new window]   Fig. 1. Scatter plots of seasonal ionic resin capsule data, including winter (•), spring () and summer () for ninhydrin reactive N (NR-N) (a, b, and c), NH4+–N (d, e, and f) and NO3––N (g, h, and i) concentration (µg capsule–1) from a chronosequence of sites (n = 10) with increasing time since fire in western Montana. R2 and p-values are included for linear regression analysis of natural logarithm transformed time since fire data.

Carbon Pools
All pools of labile C measured were influenced by season and TSF, although only one pool varied systematically with TSF. Anthrone reactive C varied significantly with TSF, season, and the TSF x season interaction (Table 2). However, only the spring measurement of resin-sorbed sugars exhibited a significant decrease with TSF (Fig. 2b ). The scatter plots (Fig. 2a, b, and c) showed that AR-C levels fluctuate with season, increasing in concentration from winter to summer measurements, but do not consistently change with TSF except during the spring period.

View larger version (23K): [in this window] [in a new window]   Fig. 2. Scatter plots of seasonal ionic resin capsule data, including winter (•), spring () and summer () for anthrone reactive C (AR-C) (a, b, and c) and soluble phenols (d, e, and f) concentration (µg capsule–1 and mg capsule–1, respectively) from a chronosequence of sites (n = 10) with increasing time since fire in western Montana. R2 and p-values are included for linear regression analysis of natural logarithm transformed time since fire data.

	DISCUSSION

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Effect of Time Since Fire
Our results demonstrate that TSF influenced all indices of C and N availability (Table 2), but that only N pools exhibited linear declines with TSF, which is similar to results from previous studies (Hart et al., 2005; MacKenzie et al., 2004, 2006). These results suggest that in this ecosystem litter quality and the rate of N mineralization decrease with increasing time since disturbance. We believe that this is due to an increase in Douglas-fir basal area and total shrub cover on these sites (MacKenzie et al., 2004), which contributes to an increase in forest floor depth and resistance to decomposition, most likely as a result of increasing lignin content.

Plant available N has been shown to decrease with time since fire in some conifer ecosystems (DeLuca et al., 2002; Lodhi and Killingbeck, 1980; Rice and Pancholy, 1974) and increase or remain unchanged in others (Davidson et al., 1992; Perez et al., 2004; Robertson, 1982; Robertson et al., 1988). In this study, a decrease in soluble organic and inorganic N over time indicates that resource competition will become more intense as the system shifts from high to low resource availability (Schimel and Bennett, 2004). Plant reliance on organic N sources is unknown in this ecosystem, but it seems unlikely that there will be a shift toward organic N uptake with increasing time and low resource availability, as both NR-N and inorganic N appear to decrease at similar rates (Fig. 1).

Labile C indices did not consistently follow the same decrease with increasing TSF, except for spring AR-C levels (Fig. 2b). Decreasing AR-C levels over time indicate that the amount of substrate available to microbes for energy transformations and biomass building is decreasing with TSF. The concurrent decrease in available N discussed above might be due to increased plant uptake of N in the recovering ecosystem. However, the fact that labile C is also decreasing while total C increases (Hart et al., 2005) suggests that the organic matter accumulated may actually reduce microbial degradation rates as discussed further in the next sections.

Effect of Season
All resin-sorbed C and N pools were affected by seasonal fluctuations of temperature and moisture (Table 2) for the single year of observation. Although it is impossible to extrapolate from a single year to a broad range of climate extremes that might occur year to year, we are confident that the given year of study was within the regional climatic norms. Levels of organic and inorganic N exhibited significant trends for both the winter and spring measurement periods, but not for the summer period (Fig. 1). Although winter microsite conditions at the forest floor/mineral soil interface are dominated by low temperature and moisture availability, it is possible for some microbial activity and N cycling to continue in the subnivial zone (Brooks and Williams, 1999). This is represented here by the presence of available NH₄⁺–N levels in winter that are almost double those observed in the spring (Fig. 1d and e). It is possible that the transition between winter and spring, which is dominated by significant fluctuations in diurnal temperatures and large amounts of snow melt and precipitation, is driving N availability at these sites. The flush of inorganic N that occurs during the winter and spring periods (Fig. 1d, e, g, and h) may represent the majority of plant available N for the entire year in this xeric ecosystem. Climate data for the area indicates strong warming trends and high precipitation during the late winter and early spring measurement periods (NCDC, 2005; Nimlos, 1986). Increases in soil moisture and temperature are known to be the main drivers of N mineralization (DeLuca et al., 1992; Fierer and Schimel, 2002; Grogan et al., 2004).

The transition between spring and summer months in this region is characterized by high temperatures, low precipitation and, soil drying (NCDC, 2005; Nimlos, 1986). Microbial cells are disrupted during episodes of drying, which has been shown to result in significant increases of labile C and N (Fierer and Schimel, 2002), and may explain the summer increase in NR-N and AR-C (Fig. 1c and 2c). This increase might also be explained by low plant uptake levels with ongoing microbial transformations. A study of alpine ecosystem dynamics in the Colorado Front Range found that plants took up more available N early in the season, while microbes immobilized more N late in the season (Jaeger et al., 1999). This may also explain increasing AR-C levels with seasonal progression, as AR-C is positively correlated with microbial biomass and respiration (DeLuca, 1998). Jaeger et al. (1999) also found decreasing resin-collected N levels with progression through spring and summer, corroborating the results from this study and indicating that as the soil matrix dries, plant available N becoming increasingly depleted due to reduced microbial activity. One major caveat that must be considered when interpreting the resin data is that when soils are extremely dry, resins may not directly represent C and N dynamics as soil solution mass flow and diffusion rates decline with reduced soil moisture, however, we feel confident that our results appropriately represent transformation rates by season.

Soluble phenol levels had a distinct seasonal pattern with spring levels much higher than winter and summer levels (Fig. 2e). Soluble phenols were also an order of magnitude higher than our other index of C availability, indicating that they are potentially a very important substrate for microbial activity. The fact that soluble phenols were higher during the spring measurement and extremely high for several of the 80- to100-yr-old sites may indicate a seasonal accumulation. This accumulation of substrate may be due to either reduced microbial mineralization (Stark and Hyvarinen, 2003) or as a by-product of the decomposition of other C substrates (Hattenschwiler and Vitousek, 2000). It is not clear from this data set if resin-collected soluble phenols will be a good indicator of microbial activity, but given that some phenols have been implicated in allelopathy (Bais et al., 2004; Nilsson et al., 1998), it is clear that this group of organic C compounds merits further investigation.

Interaction of Time Since Fire and Season
The interaction between season and TSF was reflected in the labile pools of C and N examined here (Table 2). Many of these pools decreased linearly with TSF (Fig. 1 and 2), but only for certain seasons. Some variables did not change systematically with TSF in any season, but rather the levels of different biochemicals fluctuated by season. Similarly, Cain et al. (1999) found that N levels decreased along a successional sand dune gradient, but were only able to measure seasonal values at one seral site where they found no spatial autocorrelation for resin-collected N levels. The interaction between season and time is often overlooked in forest, range, and wildland soils research as it is difficult to sample soil properties at remote sites, multiple times throughout the year. Although chronosequence research is rife with limitations, it is one of the only means by which scientists can examine the long-term interactions between time since disturbance and seasonal change.

Soils are often sampled regardless of season or with techniques that define one pool of N well, but not another (Hartley et al., 1999; Schimel and Bennett, 2004; Vervaet et al., 2002). Data from studies that use techniques that measure microbial available N, such as the buried bag method, instead of plant available N, such as resins or foliar N, may produce results that do not represent the changes associated with plant community dynamics and seasonality. Consistent with our original hypotheses, we found a decrease in plant available organic and inorganic N and hexose sugars, indicating that litter quality decreases with TSF. Seasonal data indicate that NR-N and AR-C levels increase from winter to summer, and that there is separation between plant and microbial acquisition of N, which is contradictory to our original hypotheses. Finally, these results indicate that season takes on a dominating influence on various environmental variables when water becomes limiting during the hottest, driest part of the year, thus obscuring any trends generated by TSF.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The importance of studying the relationships between state factors, subsurface variables and ecosystem function has been discussed in the literature (Chapin et al., 2002; Groffman et al., 2004). In this study we have described a relatively simple method using ionic and non-ionic resin capsules to follow the influence of TSF and seasonal progression (state factors) on labile pools of C and N (ecosystem function). Natural catastrophic disturbance in this region represent a long-term shift in subsurface processes from high to extremely low (fire exclusion) labile C and N availability. Seasonal progression from winter to summer represents an annual oscillation from low to high C availability and from high to low N availability, which is superimposed on the long-term development trend. By integrating time since fire and seasonal dynamics we are able to produce a better understanding of nutrient and energy flux in fire prone forest ecosystems of the Inland Northwest.

	ACKNOWLEDGMENTS

 
This research was funded by a USDA McIntire-Stennis grant, provided by the College of Forestry and Conservation at The University of Montana. We thank Pete Grum, Eric Sawtelle, Nathan Hilliard, and Tricia Burgoyne for their assistance in sample collection and analysis, and three anonymous reviewers for their useful comments.

Received for publication December 14, 2005.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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