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a USDA-FS, Pacific Northwest Research Stn., Olympia Forestry Sciences Lab., 3625 93rd Ave., Olympia, WA 98512-9193 USA
b Dep. of Forestry, North Carolina State Univ., Box 8008, Raleigh, NC 27695 USA
kpiatek/r6pnw_olympia{at}fs.fed.us
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMethodsResultsDiscussionConclusionsREFERENCES |
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50% of this decline. At mid-rotation, harvest intensity, but not site preparation intensity, affected N mineralization, with stem-only harvest plots mineralizing 11 kg N ha-1 more than whole-tree harvest plots during the seven months. Chop–burn–no herbicide plots mineralized 34(±3) kg N ha-1, chop–burn–herbicide: 30(±3) kg N ha-1, shear–pile–disk–herbicide: 28(±3) kg N ha-1, and shear–pile–disk–no herbicide: 19(±3) kg N ha-1 in the seven months. Mid-rotation mineralization was positively correlated with soil temperature and negatively correlated with soil P and soil C:N ratio. The effect of harvest on N mineralization was probably exerted through P nutrition, whereas the lack of site preparation effects suggested that large nutrient removals that occurred with shearing and piling did not have lasting and negative effects on N availability in this plantation. |
INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMethodsResultsDiscussionConclusionsREFERENCES |
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Mechanical preparation of harvested sites for tree planting can further increase mineralization (Burger and Kluender, 1982; Burger and Pritchett, 1984; Vitousek and Matson, 1985; Fox et al., 1986; Vitousek et al., 1992), although the effects of site preparation are usually confounded by the effects of harvesting (Johnson, 1992). Mixing of organic matter with mineral soil (Burger and Kluender, 1982) improved soil aeration (Vitousek and Matson, 1985), and exposure of mineral soil to direct solar radiation during site preparation has been found to stimulate mineralization.
During the 1970s and 1980s in the southeastern USA, harvested sites were commonly prepared for tree planting by shearing of residual stems, piling of slash into windrows, then disking in the inter-windrow areas (Haines et al., 1975). Operational shearing and piling, done in large tracts, often led to the removal of topsoil along with slash and debris, which decreased organic matter and N content in the interwindrow areas comprising the planting surface (Pye and Vitousek, 1985). Estimated displacement of N during this operation varied but was as large as 300 to 650 kg N ha-1 (Morris et al., 1983; Tew et al., 1986, respectively). Despite such large N losses, N mineralization rates were elevated for as long as five years following this method of site preparation (Vitousek et al., 1992).
Limited information on longer-term effects of harvesting and site preparation on N mineralization is available. Several investigators have postulated that losses of N during site preparation may result in N limitations to forest productivity at mid-rotation when N demand is greatest (Burger and Kluender, 1982; Neary et al., 1984; Morris and Lowery, 1988; Fox et al., 1989; Allen et al., 1990; Thornley and Cannell, 1992). Net N mineralization rates in clayey kaolinitic soils (0–15 cm depth) under loblolly pine in the Piedmont of North Carolina were 80 to 90 kg N ha-1 yr-1 at age 1 yr (Vitousek and Matson, 1984; 1985), declining to 40 to 50 kg N ha-1 yr-1 at age 5 yr (Vitousek et al., 1992). Similarly, the top 15 cm of a podzolized sandy soil under Monterey pine (Pinus radiata D. Don) aged 1, 2, and 3 yr mineralized 73, 52, and 45 kg N ha-1 yr-1, respectively (Smethurst and Nambiar, 1995). Data on N mineralization from soil organic matter at later stand ages are fragmented, but they indicate a further decline in mineralization rates. For example, mineralization in fine loamy soil under some mixed coniferous forests in the Sierra Nevada at 0 to 14 cm depth was 49 kg N ha-1 yr-1 at age 5 yr, 31 kg N ha-1 yr-1 at age 17 yr, and 12 kg N ha-1 yr-1 at age 100 yr (Frazer et al., 1990).
The objectives of this study were to (i) determine the extent of net N mineralization decline in a mid-rotation loblolly pine plantation, (ii) assess the effect of nutrient removal during harvesting and site preparation on net N mineralization 15 yr after treatment, and (iii) correlate mid-rotation mineralization rates with soil nutrient and environmental conditions. By examining N mineralization rates across a range of treatments, we tested the hypothesis that N availability at mid-rotation is negatively impacted by increasing intensity of N removal.
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Methods |
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TOPNOTESABSTRACTINTRODUCTIONMethodsResultsDiscussionConclusionsREFERENCES |
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Previous Work
Net N mineralization was examined as part of a larger study to determine impacts of forest management practices on long-term site productivity. Previous work at this site has documented harvest and site preparation effects on nutrient budgets (Pye and Vitousek, 1985; Tew et al., 1986), soil physical conditions (Gent et al., 1984; Stewart, 1995), environmental conditions and planted pine response to competition (Byrne et al., 1987; Nusser and Wentworth, 1987; Byrne and Wentworth, 1988; Fredericksen et al., 1991), pine plantation growth (Allen et al., 1990), and N transformations (Vitousek and Matson, 1985; Vitousek et al., 1992). Detailed site descriptions are provided in these earlier articles.
Treatments
In 1981, a 22-yr-old loblolly pine plantation of the previous rotation was clearfelled. The study was installed as a split-plot, randomized complete block design with three replications (blocks) within 1 km of each other. The treatments were a combination of harvesting and site preparation assigned to main plots and vegetation control assigned to subplots. Two types of harvest were implemented: whole-tree and stemwood-only removal. Whole-tree harvest resulted in an estimated removal of 180 kg N ha-1, 19 kg P ha-1, 89 kg K ha-1, 178 kg Ca ha-1, and 35 kg Mg ha-1. Stem-only harvest resulted in an estimated removal of 57 kg N ha-1, 5 kg P ha-1, 35 kg K ha-1, 51 kg Ca ha-1, and 14 kg Mg ha-1 (Tew et al., 1986).
Following harvest, two types of site preparation were applied. In the lower intensity chop–burn, large pieces of logging debris were fragmented with a roller drum chopper and burned in a low-intensity, incomplete controlled burn. Chopping and burning resulted in an estimated displacement of 46 kg N ha-1, and 0 kg ha-1 P, K, Ca, and Mg (Tew et al., 1986). In the high intensity shear–pile–disk treatment, stumps were sheared with a KG blade, and slash was piled away from the planting site into windrows 47 m apart. Along with slash, piling removed some topsoil. The area between the windrows was disked to a depth of 7 to 12 cm (Gent et al., 1984). Shearing and piling displaced an estimated 591 kg N ha-1, 34 kg P ha-1, 92 kg K ha-1, 363 kg Ca ha-1, and 64 kg Mg ha-1 (Tew et al., 1986). Vegetation control was implemented in subplots, measuring 30 m by 15 m. In the herbicide treatments, an application of hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-Triazine-2,4(1H,3H)-dione] at the time of tree-planting in 1982 was followed by a broadcast application of glyphosate [N-(Phosphonomethyl)glycine] in late August, once a year during the next 2 yr, and as-needed in the next 3 yr (Vitousek and Matson, 1985). In the no herbicide treatments in 1995, pines were mixed with hardwood species. In the chop–burn–no herbicide treatment, pine basal area was 11 m2 ha-1, while hardwood basal area reached 9 m2 ha-1. Hardwood tree species in the chop–burn–no herbicide treatment were present in the following order of abundance: oaks (Quercus alba L., Q. rubra L., Q. falcata Michx.), red maple (Acer rubrum L.), hickories (Carya glabra (Mill.) Sweet, C. tomentosa Poir. Nutt.), dogwood (Cornus florida L.), cherry (Prunus serotina Ehrh.), sweetgum (Liquidambar styraciflua L.), and small quantities of other southern hardwoods, such as tulip poplar (Liriodendron tulipifera L.), sourwood (Oxydendrum arboreum DC.), and white ash (Fraxinus americana L.) (Mellin, 1995). Shear–pile–disk–no herbicide treatments contained 23 m2 ha-1 pine basal area and 2 m2 ha-1 of hardwood basal area in the following order of abundance: tulip poplar, oaks, red maple, black gum (Nyssa sylvatica Marsh.), cherry, and small quantities of sweetgum, dogwood, hickories, and white ash. There were 24 subplots in the main study. Additionally, one unharvested control plot remained in each block for a total of three plots. Pine trees on these plots were 36 yr old at the inception of the current study in 1995.
Field Sampling
Net N mineralization was assessed at plantation age 15 yr by the sequential, in situ soil incubation method (Raison et al., 1987). Soil samples were collected monthly from early May to early December. Each plot of the stem-only and whole-tree harvest treatment
was sampled, as were three unharvested control plots. The forest floor layer was pushed aside, and the A horizon was sampled to a depth of 0 to 15 cm by taking four soil cores per plot. Samples were composited per plot and transported to the laboratory for extraction. Additionally, four PVC tubes (5-cm in diam.) were inserted vertically into the mineral soil to a depth of 15 cm to incubate a soil volume in the absence of plant uptake. Incubation lasted an average of 31 d (minimum 26, maximum 35 d) in capped tubes to prevent leaching with rain. After incubation, these samples were also collected, composited by plot, and transported to the laboratory for extraction. Soil temperature was measured with a temperature probe inserted to a depth of 10 cm. Temperature readings were taken at the time of soil sampling between 0900 and 1400 h, and the sequence of entry into blocks and plots varied. Four temperature readings were averaged per plot.
Laboratory Analyses
Duplicate 10-g subsamples of soil from each plot were extracted with twenty-five mL of 2 M KCl (Raison et al., 1987). The soil with solution was shaken mechanically for one hour, and then centrifuged for 15 minutes. Supernatant was collected with a pipette and unfiltered extracts were frozen for later analysis. Extracts were analyzed colorimetrically for NH+4 and NO-3 (Method 12-107-06-2-A and 12-107-04-1-B; Quickchem, Lachat Instruments, Mequon, WI). Soil KCl-extractable N was obtained by adding the values of KCl-extracted ammonium and nitrate. Net N mineralization in the absence of plant uptake or leaching was estimated by subtracting preincubation soil N (initial) from incubated soil N (Raison et al., 1987). Monthly rates of net N mineralization were summed for the May through December collection period.
Soil water content was determined gravimetrically for each initial and end-of-incubation composite soil sample. Duplicate 10-g subsamples of soil from each plot were oven-dried for 24 h at 105°C. Gravimetric soil water content was converted to volumetric water content by multiplying by treatment-specific bulk densities from Stewart (1995).
Soil N, P, and C were determined for each plot on a soil sample composited from three different collection dates. Each sample was oven-dried at 70°C and ground in a Wiley mill to pass through a 2-mm screen. Duplicate 0.5-g subsamples from each plot were wet-digested in a sulfuric acid and hydrogen peroxide mix (Parkinson and Allen, 1975), and total soil N and P (total organic P and polyphosphates) were determined colorimetrically (Method 13-107-06-2-D and 13-115-01-1-B for N and P, respectively; Quickchem, Lachat Instruments). Additionally, soil total C and N concentrations were quantified with a PE 2400 CHN Elemental Analyzer (Perkin-Elmer Corp., Norwalk, CT).
Statistical Analyses
Treatment effects on net N mineralization, soil temperature, soil moisture, total soil N, C, P, and ratios of N:P, C:N, and C:P were tested by analysis of variance (ANOVA) for a split-plot design (SAS, 1988) with whole plots of harvesting and site preparation, and subplots of vegetation control
. To determine whether net N mineralization in unharvested control plots was statistically different from that in the treatment plots, the following contrast statement was constructed in a randomized complete block design: T1 + T2 + T3 + T4 + T5 + T6 + T7 + T8 - 8 (T9) = 0, where T = treatment combination. Nine treatment combinations were assigned as follows: T1 through T8 for each of the eight combinations of harvesting, site preparation, and vegetation control; the ninth treatment was the unharvested control.
Soil moisture content after the 31-d incubation was compared to soil moisture content of bulk soil outside of the PVC tube to test whether soil water content was affected inside the capped incubators, thus changing N mineralization conditions from those of the bulk soil. ANOVA for the average soil water content inside and outside the incubators was conducted every month.
The effects of soil nutrients and soil environmental factors on N mineralization rates were explored. For this purpose, soil temperatures were summed across the measurement period. Soil N, P, C, and N:P and C:N ratios, soil water, and the sum of soil temperatures were correlated with the sum of monthly N mineralization rates. Significance was accepted at P 
0.05 for all analyses.
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Results |
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TOPNOTESABSTRACTINTRODUCTIONMethodsResultsDiscussionConclusionsREFERENCES |
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Soil Nitrogen, Phosphorus, and Carbon
There were no treatment effects on soil N. Harvesting effects on soil P were significant, with whole-tree harvested plots having 18% higher P content than stem-only plots (Table 3)
. Carbon was affected by site preparation; where organic matter was removed 15 yr prior in the shear–pile–disk treatments, soil C was 30% lower than on the chop–burn plots, in which organic matter was left in place or lightly burned. Ratios of soil N:P and C:N were significantly lower on the shear–pile–disk treatments than on the chop–burn treatments. None of the treatment interactions were significant.
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, but positively with N:P
and C:N ratios
and with the sum of soil temperature
(Fig. 4)
. On a monthly basis, net N mineralization was not correlated with any of the soil nutrient or environmental variables.
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Discussion |
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TOPNOTESABSTRACTINTRODUCTIONMethodsResultsDiscussionConclusionsREFERENCES |
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Decline in Nitrogen Availability Since Plantation Establishment
Net N mineralization in the A horizon (0–15 cm depth) was almost three times greater at plantation establishment than at mid-rotation. In Years 1 and 2 after treatment, net N mineralization was 77 and 62 kg N ha-1, respectively, measured from May to November (Vitousek and Matson, 1985; Vitousek et al., 1992). At mid-rotation, plantation age 15 yr, average net N mineralization was 28 kg N ha-1 during the same time period. Mineralization in the unharvested control plots, 36-yr-old in 1995, was 13 kg ha-1 higher than that reported for these same plots at age 22 yr in 1982 (Vitousek and Matson, 1985). The methods used in 1995 differed from those used by Vitousek and Matson (1985) in that we incubated intact soil cores rather than mixed soil in buried bags. The difference in methods notwithstanding, it has been shown that a decline in N availability with age of forests (Frazer et al., 1990; Smethurst and Nambiar, 1995), is a general phenomenon, so the decline observed on our site could not be attributed exclusively to the differences in methodology.
The observed reduction in net N mineralization between plantation establishment and mid-rotation cannot be attributed to increased plant uptake in older stands because the soil core method used here tested N mineralization in the absence of plant uptake (i.e., roots were severed during insertion of PVC incubation tubes). Reduction of N mineralization rates may be due in part to the decrease in growing-season soil temperatures. Measured at 15-cm depth between July and September, average soil temperatures were 25°C immediately after treatment, and 20°C in 1995 at 10-cm depth. Soil temperatures at 15 cm can be expected to be lower than at 10 cm, so that the actual drop in soil temperatures since treatment was probably greater than 5°C. However, taking our conservative estimate of the difference in soil temperature of 5°C and assuming a doubling of the reaction rate for every 10-degree increase in temperature (Q10 of 2), we can account for 38 to 67% of N mineralization rates observed directly after treatment. A decrease in decomposition and N mineralization rates has been observed upon lowering of soil temperatures (Theodorou and Bowen, 1983; Bonan and Van Cleve, 1992; Kirschbaum, 1995). Other factors, such as changes of moisture conditions and substrate quality since plantation establishment, may contribute to the decrease in N mineralization (Burger and Kluender, 1982; Theodorou and Bowen, 1983; Waide et al., 1988). These factors were not assessed at this site in the earlier years. Unharvested control plots exhibited higher N mineralization rates in 1995 than in 1982 or 1983, and that may be due to higher soil temperatures at stand age 36 vs. 22 yr that result from taller canopy and greater solar penetration to the forest floor. These differences could not be explored for lack of sufficient data.
Nitrogen Availability at Mid-Rotation
At mid-rotation, plantation age 15 yr, effects of harvesting were thought to be no longer significant, given the far greater nutrient removal and changes in soil bulk density due to site preparation (Gent et al., 1984; Tew et al., 1986; Stewart, 1995). Contrary to expectations, net N mineralization was significantly reduced in plots with higher harvest intensity. Reduction in N mineralization in whole-tree harvest plots was also observed during the first 2 yr after treatment establishment and was attributed to the removal of N-rich substrate in foliage and branches (Vitousek and Matson, 1985). At plantation age 15 yr, however, there were no treatment differences in total soil N in the surface soil, and N mineralization rates and total N were not correlated.
Of the soil nutrients and their ratios examined at mid-rotation, soil P was significantly affected by harvesting, and net N mineralization and soil P were negatively correlated. The influence of soil P on N mineralization is uncertain; DiStefano and Gholz (1989) reported no effects of P additions on net N mineralization in a slash pine plantation in Florida, while Ryan et al. (1972) reported that in anaerobic conditions, P decreased ammonia formation through greater microbial activity and N immobilization. Yanai (1991) observed that an increase in P leaching from the forest floor 2 yr following forest harvest was less than could be predicted by a reduced plant P uptake. She concluded that mechanisms such as a decrease in P mineralization rates or an increase in microbial P immobilization probably help reduce P loss from the forest floor after disturbance. In our study, the differences in the amount and, possibly, character of logging residue left on the forest floor in the stem-only and whole-tree harvests warrant the creation of different microbiology and chemistry, including P nutrition, in the underlying A horizon 15 years later. The 15-yr dynamics of such events are not possible to reconstruct without actual data. Environmental variables did not explain the difference in N mineralization between harvest treatments. In 1995, significantly higher soil temperatures were observed in the stem-only harvest plots in July and November; however, N mineralization was not significantly higher in these plots in those two months. Thus, apart from a suspected P-involvement, variables measured in this study could not explain the effect of harvest intensity on net N mineralization at midrotation.
A strong site preparation effect was expected because piling of slash generally removes more nutrients from the planting surface than either harvest treatment. In fact, shear–pile–disk treatment did not result in a significant mid-rotation decrease in net N mineralization, as hypothesized (Burger and Kluender, 1982; Morris and Lowery, 1988; Allen et al., 1990). Higher N mineralization rates observed in the chop–burn–no herbicide plots were expected because hardwood leaf litter generally decomposes and mineralizes more readily than conifer needle litter (Nadelhoffer et al., 1982; Zak et al., 1986; Stump and Binkley, 1993; Piatek and Allen, unpublished data, 1997). However, the shear–pile–disk–no herbicide plots also contained hardwoods but exhibited the lowest N mineralization rates. The hardwood component on the two non-herbicide treatments differed in species composition and in hardwood biomass relative to pines. Oak species dominated the hardwoods in the chop–burn–no herbicide plots, in contrast to a mix of species including red maple, hickory, tulip poplar, sweetgum, and oak in the shear–pile–disk–no herbicide plots (Mellin, 1995). Hardwood species composition, in addition to their presence, may have affected N mineralization rates. Nadelhoffer and others (1982) reported that N mineralization in predominantly oak sites was higher than on conifer and other hardwood sites. On the other hand, Zak and others (1986) observed lower rates of N mineralization in oak-dominated as compared to sugar maple-dominated ecosystems. Hardwood foliar biomass production constituted 13% of the total dry foliar weight collected on the shear–pile–disk–no herbicide plots, and 45% on the chop–burn–no herbicide (Piatek and Allen, unpublished data, 1997). Such differing ratios of hardwood to pine foliar litter may have contributed to the differences in N mineralization by affecting soil chemistry and microbial activity. Soil P availability, for example, may modify the relationship between hardwood litter chemistry and N mineralization (Pastor et al., 1984). On the other hand, the small hardwood component on the shear–pile–disk–no herbicide plots simply may not be large enough to impart stimulating effects of hardwood foliar litter on N mineralization.
Why did mid-rotation rates of N mineralization vary with harvest intensity rather than site preparation intensity, despite a far greater nutrient displacement and changes in soil physical characteristics associated with the shear–pile–disk site preparation? We found no clear answer. Harvesting may have affected different ecosystem components than site preparation and, as a result, produced different conditions for N mineralization. Evidence to support this hypothesis comes from soil nutrient analyses and from studies of soil physical properties. Harvest effects, but not site preparation effects, were significant for soil P. Site preparation effects, but not harvest effects were significant for soil C and for C:N ratio. Harvesting, relative to site preparation, caused little disturbance to the soil structure. In contrast, site preparation either compacted the soil during chopping or alleviated compaction by disking (Gent et al., 1984), resulting early on in different aeration conditions which, however, did not persist (Stewart, 1995). Harvesting also affected the amount of nutrient-rich foliage that could be incorporated into the soil as substrate for N mineralization. Site preparation affected the amount of nutrient-poor forest floor, as well as green foliage on stem-only harvest plots. In terms of nutrient removals, whole-tree harvesting removed 4% of total ecosystem N (aboveground vegetation, forest floor, and mineral soil from 0–60 cm depth included; data from Tew et al., 1986) and 29% of total P, while stem-only removed 1.3% N and 7.6% P. Site preparation, on the other hand, removed 1% total ecosystem N and no P in chopping and burning, and 13% N and 52% P in shearing, piling, and disking.
Chopping and burning removed less nutrient capital than even stem-only harvest. On the chop–burn plots at plantation establishment, the forest floor most likely immobilized site N resources for several years (Piatek and Allen, unpublished data, 1997) while microbial biomass utilized the large C pool. At mid-rotation, that forest floor was likely incorporated into the soil organic matter and may have contributed to the observed N mineralization. Under this scenario, the shear–pile–disk plots, where large amounts of organic matter were removed, should exhibit lower N mineralization rates due to increasingly limiting amounts of easily decomposable organic matter. We have not observed this effect statistically, but the much lower rate of N mineralization on the shear–pile–disk–no herbicide plots may be an indication of future conditions, as was predicted by many (Burger and Kluender, 1982; Neary et al., 1984; Morris and Lowery, 1988; Fox et al., 1989; Allen et al., 1990; Thornley and Cannell, 1992). In fact, a 16-yr-old loblolly pine stand in the Piedmont of Durham County, North Carolina, exhibited a tree-growth decline attributed to a decrease in soil nutrients following windrowing (Fox et al., 1989). Exactly when in a lifetime of a plantation this potential N limitation occurs probably depends on the chemistry and total amount of substrate for decomposition.
Treatment effects alone explained 33% of the variability in the rate of net N mineralization. Addition of block effects increased r2 of the model to 91%, suggesting that variability in N mineralization is very dependent on location and associated conditions. Even at a microsite level, enrichment in C availability, such as from a fallen log, seems to drive the rates of N transformations (Hart et al., 1994).
Higher sums of soil temperatures generally resulted in higher overall N mineralization
. However, within a given month, mineralization rates at mid-rotation were not correlated with monthly soil temperatures nor with monthly soil water content. In contrast, temperature and moisture seemed to be the primary controls in the seasonal trend of N mineralization in the first 2 yr after treatment, in 1983 and 1984 (Vitousek and Matson, 1985). This might be expected since our temperature had a narrower range across the season (8–21°C), while Vitousek and Matson (1985) observed a much wider range (0.4–36°C). After treatment effects, the seasonal sum of soil temperature explained an additional 42% of the variability in net N mineralization.
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Conclusions |
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TOPNOTESABSTRACTINTRODUCTIONMethodsResultsDiscussionConclusionsREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMethodsResultsDiscussionConclusionsREFERENCES |
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Received for publication May 18, 1998.
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