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a Suleyman Demirel Univ., Faculty of Forestry, 32260 Isparta, Turkey
b Adirondack Watershed Institute, Paul Smith's College, Routes 86 & 30, P.O. Box 265, Paul Smiths, NY 12970
c Dep. of Forestry, 3108 Jordan Hall, College of Natural Resources, North Carolina State Univ., Raleigh, NC 27695
* Corresponding author (keltind{at}paulsmiths.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: CO, control • FR, fertilization • TSP, triple super phosphate • VC, vegetation control • VC + FR, vegetation control + fertilization
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Fertilization typically increases the concentration of N in the forest floor and the mineral soil and has the potential to increase net mineralization (Maimone et al., 1991; Polglase et al., 1992; Carlyle, 1995; Hart and Stark, 1997). In radiata pine (Pinus radiata D. Don) stands of Australia, fertilization with 200 kg N ha–1 doubled net N mineralization rates for at least 2 yr (Carlyle, 1995), and fertilization with 400 kg N ha–1 increased net N mineralization rates two to three fold for at least 4 yr (Raison et al., 1992). This continued higher rate of N mineralization was attributed to remineralization of immobilized fertilizer N and decomposition of fine roots having higher N content. However, in a similar study by Goncalves and Carlyle (1994), N addition had no lasting effect on N mineralization 5 to 6 yr after the treatment. Although NH+4–N usually dominates soil N pools, net nitrification can increase after fertilization (Raison et al., 1992; Hart and Stark, 1997), and this increase seems to be a linear function of the quantity of net mineralization (Goncalves and Carlyle, 1994).
Vegetation control may significantly change the soil environment and plant community composition, which, in turn, may alter soil nutrient supply. Decreased canopy cover after vegetation control lets more sunlight reach the forest floor, which may increase temperatures in the forest floor and mineral soil. Effects on soil moisture are less clear, although moisture content may also increase due to decreased precipitation interception and evapotranspiration after the reduction in canopy cover. While temperature controls microbial and enzymatic activities, moisture controls movement of substrate via mass flow and diffusion (Powers, 1990; Goncalves and Carlyle, 1994; Sierra, 1997; Zak et al., 1999); therefore increased temperature and moisture content may result in more rapid mineralization. Based on laboratory studies, soil net N mineralization appears to be at its maximum when soil moisture content is near field capacity and soil temperature is around 25°C (Stanford et al., 1973; Cassman and Munns, 1980; Kladivko and Keeney, 1987; Goncalves and Carlyle, 1994).
Along with these changes in the physical environment, eliminating nutrient-rich hardwoods and herbaceous plants from pine stands could also affect the chemistry of soil organic matter. Substrate quality, described as soil C and N concentrations, C/N ratio (Powers, 1990; Bauhus and Khanna, 1999; Piatek and Allen, 1999) and lignin/N ratio (Scott and Binkley, 1997) plays an important role in organic matter decomposition and mineralization of N. As long as there is a sufficient amount of labile organic C available in the soil as an energy source for microbes, mineralized N will mostly be used within the microbial population, and net mineralization will occur when the mineral-soil C/N ratio falls below 20 to 25 (Morris and Campbell, 1991). Wood et al. (1992) reported that loblolly pine stands under hardwood and herbaceous control had lower soil substrate quality (i.e., organic N concentration), which resulted in reduced net N mineralization. Polglase et al. (1992) suggested that litter from understory vegetation could be a better supply for available N than overstory litter; however, they also reported an increase in net N and P mineralization after sustained weed control and complete fertilization in southern pine plantations.
This research study was initiated to understand how soil N dynamics are affected by fertilization and vegetation control in a mid-rotation loblolly pine stand. Specific objectives of this study were (i) to assess the effects of these treatments on soil temperature and moisture, (ii) to compare net N mineralization under field and laboratory conditions, and (iii) to examine the effects of substrate quality by comparing field and laboratory results.
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MATERIALS AND METHODS |
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The pretreatment stand was a 14-yr-old second-rotation loblolly pine plantation with a site index (25 yr) of 18 m. The site was chopped and burned during site preparation, and no other silvicultural treatments had been applied. Before treatment, the stand had 971 pines ha–1 with a basal area of 24 m2 ha–1. Competing vegetation mainly consisted of Virginia pine (Pinus virginiana Mill.) (1.2 m2 ha–1) and several hardwood species, including sweetgum (Liquidambar styraciflua L.), yellow poplar (Liriodendron tulipifera L.), red maple (Acer rubrum L.), southern red oak (Quercus falcata Michx.), white oak (Quercus alba L.), and black cherry (Prunus serotina J.F. Ehrh.), with a combined basal area of about 9 m2 ha–1.
Experimental Design and Treatments
The experimental design was a two by two factorial arranged in a randomized complete block design with three blocks. Twelve 0.16-ha treatment plots were established in the winter of 1997-1998. Measurement plots of 0.04 ha, centered within the treatment plots, were used for further sampling. Plots were assigned to one of three blocks to minimize within-block variation in pretreatment stand vegetation characteristics (height, basal area, stand density). Treatments included two levels of vegetation control (none and complete vegetation control) and two levels of fertilization (none and a fertilizer application of 224 kg N ha–1 applied as Urea plus 56 kg P ha–1 applied as triple superphosphate). Complete vegetation control was performed in March 1998 by chainsaws, eliminating all hardwoods and naturally regenerated Virginia pines. Cut-stumps were then sprayed with a 50% solution of Garlon 3A [(3,5,6-trichloro-2-pyridinyl) oxyacetic acid, Dow Elanco, Indianapolis, IN] to minimize sprouting. Slash was left on the ground to decay. Roundup [N-(phosphonomethyl) glycine, Monsanto Co., St. Louis, MO] was applied as needed at recommended rates to eliminate any resprouting or growth of herbaceous plants throughout the study period. Fertilizer application was performed by hand in March 1998, immediately after vegetation control was completed. Hereafter, no-vegetation control + no-fertilization treatment will be called the control treatment (CO), complete vegetation control + no-fertilization will be vegetation control (VC), no-vegetation control + fertilization will be fertilization (FR), and complete vegetation control + fertilization will be combination (VC + FR).
Field Mineralization
All soil measurements were taken from the surface 0- to 10-cm layer of mineral soil. This layer was chosen because a study done in a mid-rotation loblolly pine stand on the same soil type, but with a thicker A horizon, was able to close the N budget by sampling the surface 15 cm of soil (Piatek and Allen 1999). The in situ incubation method described by Raison et al. (1987) was used to estimate field net N mineralization. Mineral soil samples were collected monthly at five stratified random locations (each plot was divided into five equal areas and one random sample was collected from each area) in each 0.04-ha measurement plot starting in July 1998 (four months after the treatments were applied) until July 2000 for a total of 25 mo. The July 1998 incubation lasted for about two months, therefore, there was a total of 24 collection dates. Each month, five pre-incubation soil samples (0- to 10-cm layer) were collected from each plot with 3.8-cm (i.d.) polyvinyl chloride (PVC) tubes, then were composited by plot. Five more tubes were driven into the soil, then capped to prevent leaching and the soil in the tubes was left intact for field incubation. During this soil sampling process, a temperature probe was inserted to a 10-cm depth at five locations in each plot to measure soil temperature. These five readings were averaged to estimate mean soil temperature for each plot at each sampling date. After an incubation period of one month (except for July 1998), incubated samples were collected and also composited by plot. Composite pre-incubation and post-incubation samples were taken to the laboratory and refrigerated at 4°C until extraction. Soil moisture content was measured gravimetrically for each collection date using 10-g subsamples dried at 105°C for 24 h.
Laboratory Mineralization
Starting in June 1999 (15 mo after treatments were applied), monthly aerobic laboratory incubations were conducted for a total of 14 mo. The same pre-incubation composite soil samples, described above, were used for the laboratory incubation. Duplicate 10-g soil samples were weighed into 50-mL centrifuge tubes, and placed in an incubator set at 28°C. Moisture contents of these samples were not statistically different by treatments at the time of incubation for most sampling dates (significantly different only on one date), so they were left at field moisture. Two 500-mL beakers of deionized water were also placed in the incubator to help minimize the amount of water lost from the samples during incubation. Measurements of pre- and post-incubation water content indicated a minimal amount of water loss, with samples still being moist at the end of the incubation. Samples were removed from the incubator for analysis at the same day that field-incubated samples were collected.
Laboratory Analysis
Duplicate 10-g subsamples were extracted in 35 mL of 2 M KCl. The soil-KCl suspension was shaken for 1 h and then centrifuged for 15 min. The supernatant was filtered and kept frozen if not analyzed immediately. Extracts were analyzed colorimetrically for NH+4–N and NO–3–N using a Lachat autoanalyzer (QuikChem 8000, Zellweger Analytics, Inc., Milwaukee, WI). Potassium chloride extractable N was estimated as the sum of NH+4–N and NO–3–N from pre-incubation samples. Monthly field net N mineralization was estimated by subtracting pre-incubation soil N (NH+4–N plus NO–3–N) from field post-incubation soil N. Similarly, laboratory net mineralization was estimated by subtracting pre-incubation soil N from laboratory post-incubation soil N. Positive periods of monthly net mineralization values were added to estimate cumulative net mineralization for field and laboratory studies. Concentration values from the field study were converted to a per hectare basis using a bulk density of 1.2 g cm–3, a value reported for the 0- to 10-cm layer of a nearby Cecil soil (Gent et al. 1984), and percentage of coarse fragments (>2 mm) determined for composite samples (see below).
Soil samples from three different collection dates (11–15 mo after treatments) were composited and mixed well to determine total soil C and N for each plot. Mixed soil samples were air-dried for several days and passed through a 2-mm screen to separate fine- and coarse-fractions. Fine-earth fractions (<2 mm) were then passed through a 60-mesh screen to further improve the uniformity of the samples. Subsamples of 1 mg were taken from these 60-mesh samples and analyzed for C and N using a CHN elemental analyzer (CE Instruments-NC 2100, CE Elantech Inc., Lakewood, NJ).
Statistical Analysis
All statistical analyses were performed using SAS statistical software (SAS Institute, 1999). Analyses of variance were conducted using the mixed models procedure to test the effects of treatments, sampling dates, and their interactions on monthly estimates of extractable soil N, and on field and laboratory mineralization. In addition, treatment effects on annual field and laboratory mineralization, and mean soil temperature, moisture content, and total C and N contents were analyzed by analyses of variance. All within plot samples were averaged by plot and the plot was used as the experimental unit in all analyses of variance. Relationships between field and laboratory estimates of net mineralization and nitrification from the same period were examined by linear regression analysis. Slopes of the regression lines were also compared among treatments to determine if individual treatment responses in the field were similar to those in the laboratory. Significance was accepted at p 
0.10 for all analysis, since the soil parameters were expected to be highly variable (Mudano, 1986; Adams et al., 1989).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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On average, the soils contained 29.4 Mg C ha–1 and 1.5 Mg N ha–1 in the fine earth (<2 mm) fraction of the surface 0- to 10-cm layer, with a C/N ratio of approximately 20 (Table 2). Although fertilized soils had 180 kg ha–1 more N and lower C/N ratios, differences relative to unfertilized soils were not statistically significant (Table 2).
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pools ranged from 1.2 to 2.2 kg ha–1 in the C, 2.6 to 11.0 kg ha–1 in the FR treatment, 1.3 to 4.4 kg ha–1 in the VC treatment, and 4.2 to 18.4 kg ha–1 in the VC + FR treatment (Fig. 2a)
. Both treatments and their interaction had a significant effect on the extractable mineral N pool (Table 1). In July 1998, VC had 0.75 times and FR had 4.5 times greater extractable mineral N than the control. The combination of VC + FR was greater than additive, with 7.5 times greater extractable mineral N than the control. There was a significant interaction between fertilization and sampling date (Table 1), with the fertilization effect decreasing over time. The interaction between VC and FR was consistent through time. In June 2000, 2 yr following treatment, the extractable mineral N in the VC + FR treatment was still about three times greater than the control.
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Field Mineralization
Monthly net N mineralization ranged from –5.0 (net immobilization) to 14 kg ha–1 across all treatments (Fig. 3a)
. There was a significant effect of sampling date on monthly net mineralization (Table 1), with net mineralization being higher in summer months than winter months for all treatments. As was observed with monthly extractable mineral N, there was also a statistical interaction between VC and FR treatment effects on net mineralization (Table 1). The VC + FR treatment showed consistently greater monthly net mineralization than the sum of the VC and FR treatments. The separate VC and FR treatment responses were more variable, but net immobilization did occur more frequently in the FR treatment compared with the VC treatment during winter months in both years. The interaction between VC and FR is more readily seen on an annual basis, wherein FR increased net mineralization by 24% over CO, VC increased net mineralization by 58% over CO, and VC + FR increased net mineralization by 330% over CO (Table 3).
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Laboratory Mineralization
Monthly net mineralization ranged from –1 to 24 mg N kg–1 soil across all treatments throughout the laboratory study period of 14 mo (Fig. 4)
. Mineralization was lower in soils collected in winter months from the CO, FR, and VC treatment plots and higher in those collected in the summer, also indicated by a significant date effect and treatment x date interaction (Table 4). The VC + FR treatment showed a similar seasonal pattern; however, monthly mineralization was always higher than 6 mg N kg–1 soil. On an annual basis, FR applied alone had no treatment effect on net mineralization, while VC and VC + FR increased net mineralization by 53 and 473%, respectively, compared with CO (Table 5).
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Field-Laboratory Comparison
Field and laboratory assays yielded similar treatment effects and exhibited the same seasonal pattern (Fig. 3 and 4), with summer months having greater mineralization than winter months. The monthly laboratory and field estimates of net N mineralization were positively correlated. Net mineralization from the laboratory incubation explained 25% of the variation in field net mineralization (Fig. 5a)
, and net nitrification from the laboratory incubation explained 53% of the variation in field net nitrification (Fig. 5b). Treatments had no effect on the relationship between field and laboratory net N mineralization (result not shown). The goodness of the fit was improved to R2 value of 0.90 and 0.94 for net mineralization and nitrification, respectively, when annual values for each plot were used, instead of monthly values (result not shown).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In undisturbed forest soils, NO–3–N pools are generally very small; however, they are usually elevated after disturbance (Vitousek and Matson, 1985; Raison et al., 1992; Carlyle, 1995; Hart and Stark, 1997). The increase in NO–3–N observed for several months after fertilization was expected (Fig. 2), however, the continued elevation in NO–3–N in the VC + FR treatment was not expected. The net balance between N production and consumption regulates the mineral N pool, and the pool would be very small when consumption equals or exceeds production. This has been shown for mature conifer forests where the NO–3–N consumption to production ratio can be >1 (Davidson et al., 1992). Typically, microbial populations have a preference for NH+4–N, and under high and steady inputs of labile C, most NH+4–N would be immobilized and little NO–3–N would be produced (Davidson et al., 1992; Hart et al., 1994). The continued elevated NO–3–N observed in the VC + FR treatment may be explained by the combination of increasing the size of the mineral N pool with fertilization and decreasing immobilization with vegetation control, as inputs of labile C may be reduced with vegetation control.
Field Mineralization
The annual net N mineralization rate of about 22 kg ha–1 yr–1 estimated in control plots (Table 3) was similar to rates of 19 to 34 kg ha–1 yr–1 reported for a nearby mid-rotation loblolly pine stand growing on the same soil type (Piatek and Allen, 1999). Our estimate of net mineralization in control plots is on the low end of the 16 to 100 kg ha–1 yr–1 range for annual N uptake reported for mid-rotation loblolly pine stands (Ducey and Allen, 2001), but it is consistent with the low site index estimate of 18 m. In contrast, the annual net N mineralization rate of about 96 kg ha–1 yr–1 in the VC + FR treatment was close to the maximum uptake value of 100 kg ha–1 yr–1 reported by Ducey and Allen (2001). We only measured net N mineralization in the surface 0- to 10-cm layer; therefore, additional N may come from mineralization lower in the profile. Additional plant available N may also come from mineralization in the forest floor and atmospheric deposition. However, the forest floor has been shown not to be a source of available N in mid-rotation loblolly pine plantations (Piatek and Allen, 2001). Atmospheric deposition supplies about 10 kg N ha–1 yr–1 in this region (Richter and Markewitz, 1996).
A combination of redistribution of total soil N and higher microbial immobilization may explain low rates of net N mineralization in mid-rotation loblolly pine stands. Total soil N declines as N accumulates in the vegetation and forest floor (Richter et al., 2000), while soil C increases slowly (Richter et al., 1995). The gradual decline in total soil N and increase in the soil C/N ratio suggest that the potential for N immobilization increases with stand development. The significance of microbial immobilization in C-rich forest soils has been demonstrated by several researchers (Vitousek and Matson, 1984; Vitousek and Matson, 1985; Raison et al., 1992; Hart et al., 1994; Bauhus and Khanna, 1999). Bauhus and Khanna (1999) reported that about 800 kg C ha–1, 150 kg N ha–1 and 45 kg P ha–1 could be stored by microbial populations found in the forest floor and top horizons of the mineral soil in an average forest ecosystem.
Nitrogen additions through fertilization can increase net N mineralization (Carlyle, 1995; Hart and Stark, 1997). This was the case at our site, where fertilization alone increased annual net N mineralization by about 19% over the control (Table 3). We don't know how much of this increase is associated with transformations of fertilizer versus native soil N, but speculate that the additional mineralization is largely from fertilizer N that had been immobilized. In the fertilizer treatment, net N immobilization occurred at five dates in the first year and two dates in the second year (Fig. 3), following a similar immobilization pattern as observed by others (Vitousek and Matson, 1985; Mudano, 1986; Raison et al., 1992). Raison et al. (1992) reported that 37% of 400 kg N ha–1 applied as ammonium sulfate was immobilized during the initial 8 mo in radiata pine stands of Australia. After the initial immobilization, net N mineralization in fertilized soils was two to three folds greater than non-fertilized soils, and remained elevated above non-fertilized soils for at least 4 yr. Laboratory studies with 15N ammonium sulfate have demonstrated that soils have the potential for high immobilization of added N (e.g., Vitousek and Matson, 1985; Schimel and Firestone, 1989; Johnson et al., 2000). The added N cycles rapidly, with studies showing that >90% of residual fertilizer N can be found in organic forms in soils within a year of application (Allen et al., 1973; Smith et al., 1978). In an aerobic laboratory incubation study, Smith et al. (1978) found that organically bound fertilizer N contributed proportionally more to net N mineralization potential than native soil N, and suggested this was associated with the organically bound fertilizer N containing greater concentrations of amino sugars and amino acids. It is also possible that a significant fraction of the immobilized fertilizer N was initially retained in organic matter via abiotic processes, and subsequently mineralized. Abiotic immobilization can be significant, with estimates ranging from 6 to 90% of total N immobilization in laboratory studies (Johnson et al., 2000; Barrett et al., 2002). There is also the "N priming effect," wherein added fertilizer N stimulates mineralization of native organic N, but the interpretation of this effect is unclear (Jenkinson et al., 1985; Molina et al., 1990). Still another explanation for the fertilizer effect on net N mineralization was offered by Raison et al. (1992); which was, the fertilizer may have dissolved some of soluble organic C, which increased the microbial population and net N immobilization of added N: net N mineralization of added N ensued after the pulse in available C was depleted. These same dynamics may have happened in our study, though to a lesser magnitude and, perhaps, duration owing to the lower dose of fertilizer applied.
Vegetation control increased annual net N mineralization more than fertilization relative to the control (Table 3). Other studies have reported increased net N mineralization with vegetation control (Andariese and Vitousek, 1988; Smethurst and Nambiar, 1989; Li et al., 2003), and have attributed this to increases in soil temperature and reductions in microbial immobilization and vegetation uptake. Assuming a Q10 of two for net N mineralization (Stanford et al., 1973), then the approximate 2°C higher temperature measured in summer in the vegetation control plots (Fig. 1a) would explain about 30% of the increase in net N mineralization with vegetation control. Microbial immobilization of N can be reduced with vegetation control in response to decreased above and belowground C inputs. Vitousek and Matson (1985) showed that microbial immobilization of applied 15N ammonium sulfate was reduced from 90 to 70% when harvest residues were removed and regrowth was suppressed with herbicides in a loblolly pine stand. We did not measure belowground litter inputs, but vegetation control reduced aboveground litterfall by about 750 kg C ha–1 yr–1 compared with the control (Gurlevik, 2002). Assuming that microbial biomass has a C/N ratio of 8 and a C-use efficiency of 50% (Paul and Clark, 1989), we estimate that about 47 kg ha–1 yr–1 less N would have been immobilized by microbial biomass after vegetation control. If the reductions aboveground are indicative of what occurred belowground, then reduced immobilization probably explains most of the increase in net N mineralization with vegetation control. The inverse probably explains why net N mineralization is lower with fertilization compared with vegetation control. Fertilization increases C inputs as a result of greater productivity, which should result in more immobilization. This is supported by more frequent occurrences of net immobilization with fertilization compared to vegetation control (Fig. 3a).
We hypothesize that the strong positive interaction between vegetation control and fertilization was likely caused by a reduction in bioavailable C with vegetation control and an increase in mineral N with fertilization. The VC + FR plots had higher levels of extractable NO–3–N (Fig. 2c) and net mineralization (Fig. 3a), with net nitrification accounting for about 83% of annual net mineralization (Table 3). Hart et al. (1994) suggested that there may be a critical NH+4–N pool size for substantial net nitrification to occur, and this pool size is controlled largely by C availability. Under reduced C availability, heterotrophic microbial demand for N would be reduced and more NH+4–N would become available for autotrophic nitrifiers (Davidson et al., 1992; Hart et al., 1994). In addition, an increase in nitrifier populations or reduced microbial assimilation of NO–3–N in the presence of high NH+4–N pools may also result in higher net nitrification (Hart and Stark, 1997). Therefore, at this site, reduction of C input into soil due to vegetation control and increased available soil N (mainly NH+4–N) due to fertilization may have resulted in greater net nitrification. These effects should diminish with time as the stand responds positively to the treatments, which should increase C inputs, the size of the microbial population, and N immobilization.
Monthly net N mineralization in the field was generally higher in warm summer months than cool winter months for all treatments (Fig. 1 and 3), suggesting increased temperatures may have stimulated net N mineralization in the summer. Strong positive relationships between temperature and net N mineralization were shown by Goncalves and Carlyle (1994) and Sierra (1997) under laboratory conditions. In our study, soil temperature accounted for <10% of the variation in monthly field net N mineralization (data not shown). Piatek and Allen (1999) found no relationship between soil temperature and monthly field net N mineralization in mid-rotation loblolly pine stands.
Interestingly, the field and laboratory results from our study showed similar treatment responses for annualized data (Tables 3 and 5) and similar seasonal dynamics in mineralization (Fig. 3 and 4), even though the laboratory study was done under constant temperature. This suggests that other factors in addition to temperature may drive the seasonal dynamics of net N mineralization. The laboratory study was conducted at field moisture content, so some of the seasonal variation in net N mineralization could be attributed to differences in soil moisture content. However, with the exception of three sampling dates, soil moisture content was relatively constant during the laboratory study (Fig. 1b). This lack of variation in soil moisture content perhaps explains why we found no correlation between soil moisture content and net N mineralization (data not shown).
It is possible that net N mineralization may also vary in response to seasonal changes in labile C inputs. Though we did not measure labile C inputs in this study, other researchers have documented seasonal variation in potentially important sources of bioavailable C such as the light fraction (e.g., Spycher et al., 1983; Boone, 1994), potentially mineralizable C (e.g., Franzluebbers et al., 1995), dissolved organic C (e.g., Currie et al., 1996), and fine root turnover (e.g., Kelting et al., 1995). Microbial biomass has also been shown to vary seasonally (e.g., Buchanan and King, 1992; He et al., 1997; Piao et al., 2000). Studies have also shown that increased bioavailable C inputs can result in greater immobilization of N (e.g., Gallardo and Schlesinger, 1995; Whalen et al., 2000; Compton and Boone, 2002). In particular, the light fraction has been shown in 15N addition studies to be an important short-term sink for mineral N (Whalen et al., 2000; Compton and Boone, 2002). Boone (1994) showed an inverse correlation between monthly variations in net N mineralization potential and light fraction organic matter. Thus, it seems reasonable to hypothesize that a large part of the monthly variation in net N mineralization may be explained by seasonal variation in bioavailable C inputs to the mineral soil.
In conclusion, given the similarity of treatment and seasonal effects observed for the field and laboratory studies, and considering that the laboratory study was done under constant temperature, the results of this study suggest that vegetation-derived labile C is an important driver for net N mineralization.
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ACKNOWLEDGMENTS |
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Received for publication May 1, 2002.
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