Growth response and recovery of 15N-fertilizer one and eight growing seasons after application to lodgepole pine in British Columbia

Forest Ecology and Management ELSEVIER

Forest Ecology and Management 65 (1994) 219-229

Growth response and recovery of 15N-fertilizer one and eight growing seasons after application to lodgepole pine in British Columbia C.M. Preston *'a, D.J. Mead b aPacific Forestry Centre, Natural Resources Canada, 506 West Burnside Road, Victoria, B.C., F8Z 1MS, Canada bDepartment of Plant Science, P.O. Box 84, Lincoln University, Canterbury, New Zealand

(Accepted 27 September 1993 )

Abstract

This paper reports plant and soil distribution of ~5N 1 and 8 years after fertilizer application near Spillimacheen in the British Columbia interior. The experiment was originally established to test the efficacy of fertilization on snow; ~SN-urea, ~SNH4NO3 and NH~SNO3 were applied at 100 kg N h a - t to l l-year-old lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm. ) in January 198 I. After one growing season (October 1981 ), eight of the 16 plots were destructively sampled. Total recovery of 15N in plot trees was low, from 1.9 to 10.1%. Recovery in understorey was comparable (2.4-3.4%), and 30.6-73.2% of ~SN was retained in the soil in organic form. The remaining eight plots were sampled in August 1988. There was a significant growth response to fertilization, amounting to a 34% increase in stem volume for fertilized (ammonium nitrate or urea) versus control trees after eight growing seasons. Approximately two-thirds of the 15N recovered in 1981 could still be accounted for in plant biomass and soil. There had been little additional ~SN uptake by plot trees, but more continuing uptake by understorey. About one-fifth of ~SN recovered in 1988 was found outside the plot boundaries. The results are consistent with the hypotheses that (i) tree response to fertilization is largely the result of the increase in photosynthetic capacity generated by the first year of uptake, (ii) fertilizer N, once immobilized in the soil, has low availability to crop trees, and (iii) the N mineralized is subject to losses, presumably by leaching and denitrification. Strategies are needed for maximizing the uptake of N in the first growing season. Further research is recommended to determine what factors limit the uptake of available N by trees, and to quantify natural levels of leakage of mineral N from the ecosystem due to denitrification and leaching. Keywords: Growth; Recovery; Fertilization; Nitrogen; Pinus

1. Introduction Forestry studies with 15N-fertilizer (Hulm and Killham, 1990; Preston et al., 1990, and references therein) indicate that uptake by crop trees in one growing season is often a small percent of

*Corresponding author.

the applied fertilizer N, in the range of 5-15%. Some 10-50% may be lost through denitrification, leaching, or volatilization of ammonia, while up to 70% may remain in the soil. It is well known from agricultural research that fertilizer N, once immobilized in the soil in organic forms, has low availability to plants (Preston, 1982). However, little is known on the longterm fate of this residual fertilizer N in forestry

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CM. Preston, D.J. Mead / Forest Ecology and Management 65 (1994) 219-229

situations. Where fertilization does produce a growth response, it is usually sustained over several years before growth rates return to control or pre-fertilization levels. This may be due to the initial increase in foliage biomass and photosynthetic capacity, which then sustains an enhanced growth rate for several years (Miller and Miller, 1976; Fagerstr6m and Lohm, 1977; Brix, 1983). Internal cycling of the fertilizer N also contributes to this long-term response (Proe et al., 1992). It has thus been generally assumed that little fertilizer uptake occurs in forest systems after the first season (see e.g. Mead and Pritchett, 1975; Hulm and Killham, 1990; N f m m i k and Larsson, 1992). This hypothesis can only be tested by long-term studies using 15N in the field because studies relying on total N are unable to distinquish beteen N applied as fertilizer and that from other sources. A field study on 15N uptake by lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) near Spillimacheen in interior British Columbia provided a unique opportunity to investigate the long-term fate of fertilizer 15N. In this study, originally established to investigate the efficacy of winter application of fertilizer N on snow, two of the four replicates of four treatments (15Nurea, 15NH4NO3, N H 4 NO3 and control) were destructively sampled after one growing season (Preston et al., 1990) to determine the recovery of ~SN in trees, understorey and soil. The remaining two replicates were destructively sampled in August 1988 after eight growing seasons. This paper reports fertilizer response and longterm recovery and distribution of ~5N in the ecosystem, and compares the results with those obtained after one season. Other results from the 1988 sampling are presented elsewhere (Mead and Preston, 1992, 1994; Preston and Mead, 1993).

brief. The site is about 32 km west of Spillimacheen (50°53'N, 116°51'W) in the Rocky Mountain Trench. The site is a level to moderately sloped kame terrace with humps and depressions of _ 10 m. The terrace runs in a NW to SE direction with a NE to SE aspect and a mean elevation of 1370 m. The Dystric Brunisol (Agriculture Canada Expert Committee on Soil Survey, 1987 ), corresponding to a Dystric Cambisol (FAO) or Dystrochrept (USA), is developed on till derived from shale, conglomerates and granites, and is 25-50 cm deep with a gravelly clay loam texture. A fire in 1967 resulted in a dense regeneration of lodgepole pine. The study area was thinned in 1976 to a spacing of 3.6 × 3.6 m 2, and the trial established in the winter of 1980-1981. At that time the trees were about 11 years old and had an average height of 4.2 m (SD 0.53) and DBH of 6.7 cm (SD 1.10). The forest floor was a thin layer of moss and material from the thinnings. Understorey vegetation was sparse, but included willows (Salix spp.), birch leaf spirea (Spirea betulifolia Pall. ), Sitka alder (Alnus sinuata ) and

2. Materials and methods

2.1. Site and treatments As these have been previously described (Preston et al., 1990 ), the treatment here will be

Table 1 Recovery of fertilizer N after one growing sesasona Recovery (% of total applied) J5N_urea

ISNH4N03

NH415NO3

Soil Organic Inorganic Total

63.5 17.3 80.8

73.2 13.8 87.0

30.6 8.5 39.1

Plant biomass Tree Understorey Total

10.1 2.4 12.5

5.3 2.9 8.2

1.9 3.4 5.3

Total recovery

93.5

95.2

44.4

aData from Preston et al., 1990.

C.M. Preston, D.J. Mead / Forest Ecology and Management 65 (1994) 219-229

bunch berry (Comus eanadensis L. ). In 1988 plot trees ranged from 7.5 to 9.4 m in height, while alders were 2 to 2.5 m in height and in variable amounts from plot to plot. The single-tree circular microplots of 2 m radius, without barriers, were fertilized on snow on 29 January 1981. The trial had four treatments (control, 15N-urea, 15NH4NO3' and NH4~5NO3 ) replicated four times in a completely randomized design. Nitrogen was applied at 100 kg N ha- 1, with 15N enrichments above natural abundance (15Nex) of 2.705% for ~SN-urea, 2.606% for 15NH4NO3 and 1.894°/0 for NH4~SNO3. Two of the four replicates were harvested in October 1981 after one growing season (Preston et al., 1990); results are summarized in Table 1.

2.2. 1988field sampling The remaining two replicates were destructively sampled in August 1988 after eight growing seasons. This included the trees, understorey, litter and soil, both within the 2 m radius and from areas surrounding the plots. Detailed height, growth, and volume measurements were also made on the sample trees. Within the 2 m radius of the sample trees, all understorey vegetation was cut at ground level, and separated into herbs, shrubs and alder. Total fresh weights were measured, and subsamples taken and weighed. Next, all litter was collected, weighed, subsampled and the subsample weight recorded. After removal of understorey and litter, the tree was felled, divided into components, weighed and subsampled in the field. The annual heights of the trees back to the the winter when the fertilizer was applied were determined by studying the branching and stem scale patterns. These data were recorded for use in the growth assessment, and to divide the crown into three portions for field sampling; the lower crown which was formed prior to 198 l, the mid-crown (1981-1984) and the upper crown (19851988). For each section of the crown, a subsample of several branches was retained, while the main stem was subsampled by cutting discs of 23 cm thickness at 1-m intervals. For sampling of coarse roots ( > 2 ram), the

221

diameter of all lateral roots was measured close to the stump. For each tree, two laterals were completely excavated, weighed, and subsampled in the field. These samples were used to obtain a regression of dry weight versus diameter squared (R 2= 0.93 ) from which the biomass of the other laterals could be estimated. The whole stump and tap roots were also excavated, weighed and subsampled. For sampling outside the plots, representative samples of understorey (herbs, shrubs and alder) were taken to 0.5 m outside the plot boundaries. The uptake into nearby trees was estimated from the atom% 15N in current foliage in their lower crown. The linear equation used for these estimates was determined from a relationship between uptake and the level of labelling based on the plot trees; this equation explained 90% of the variation. Soil (0-5, 5-15 and 15-30 cm) was sampled from three pits within each microplot. For each microplot, one pit was used to determine bulk density, proportion of coarse fragments ( > 4 m m ) , and amount of fine roots ( < 4mm) by excavating a measured soil volume of 1000-1500 c m 3 for each sampling depth. To check for movement of l SN outside the plots, four cores (5 cm diameter) were taken to 20 cm depth, two at 25 cm and 50 cm outside the plot on the upslope side, and two similarly on the downslope side.

2.3. Sample processing and data analysis Soil samples were air dried and sieved to 4 mm, the coarse fraction discarded, and fine roots collected for analysis. (It was not possible to separate fine roots of lodgepole pine from other species. ) Air-dry soil samples ( 15 g) were extracted with 100 ml of 0.1 M K2SO4 solution containing 5 mg 1-1 phenylmercuric acetate and vacuum filtered through No. 41 Whatman filter paper, followed by analysis with specific ion electrodes (Orion model 95-12 ammonia gas-sensing electrode and Orion 93-07 nitrate electrode using model 90-02 double-junction reference electrode with 0.5 M K2SO4 as the outer filling solution). The soil samples which had been extracted were then oven dried at 70°C, ground to 50/zm

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C M. Preston, D.J. Mead / Forest Ecology and Management 65 (1994) 219-229

in a Siebteknik mill and analysed for total organic N and ~SN enrichment using standard methods of Kjeldahl digestion and distillation followed by mass spectrometry with a Vacuum Generators Sira 9 mass spectrometer (Preston et al., 1990). The lodgepole pine branches were separated into current and non-current needles and branches and the dry weights of all components determined after oven-drying at 70 °C for 48 h. The stem discs and large root and fine root samples were dried in the same way and their dry weights recorded. Subsamples of the stem, branch, needle and root samples were then ground to 20 mesh (850/~m) in a Wiley mill and analysed for total N and lSN enrichment as previously described. Samples of understorey vegetation (herbs, shrubs, alder and litter) were dried in the same way as the tree components, and the dry weights for the within-plot samples only were recorded. Material was not further separated into plant components. Subsamples were ground and analysed for total N and t SN as previously described. To determine growth response, volumes were calculated from ring and height measurements using the conic integral method (Whyte, 1971 ). The stump disc, 1981 disc and breast height disc were used to obtain the growth rings. On each disc, two diameters were marked, and measurements of the outside bark diameter, inside bark diameter, ring widths back to 1976 (to mearest 0.5 m m ) and pith diameter were recorded. Values of ~SN enrichment (atom%~SNex) in soil and plant samples were calculated by subtracting values for control samples of the same type. The recovery of 15N as organic N on a plot basis was calculated using the analytical data for total N and atom%lSNex of the extracted soils, together with the site average bulk density from the eight microplots. Data were subject to analysis of variance techniques. Where appropriate, data were first transformed to overcome heterogeneity of variances and orthogonal, single degree of freedom comparisons were used to test for differences between treatments (Steele and Torrie, 1960).

3. Results

3.1. Tree growth response Figure 1 shows individual tree growth response (volume inside bark) to the 100 kg N ha -~. The data were adjusted by covariance analysis using the increment in the year prior to fertilization as the covariate. There was a statistically significant response to fertilization at P < 0.05 after the third growing season, but there was no difference between the urea and ammonium nitrate fertilizers. This response amounted to a 34% increase in volume growth after 8 years. Height growth was also improved ( P = 0.033 ) by the application of fertilizers. At the end of the trial the control, ammonium nitrate and urea treatments averaged 7.7 m, 8.5 m and 9.0 m in height, respectively. 3.2. Overall 15N recovery As shown in Table 2, the total amount of 15N found in the ecosystem after eight growing seasons, including inside- and outside-plot recoveries (as percentage of total 15N applied) was 61.6%, 60.7% and 28.5% (SE 10.43) for 15Nurea, 15NH4NO3 and NH/5NO3, respectively. The single degree of freedom comparison between the nitrate-labelled N and other forms was significant at P=0.083. Recovery by plot trees was only a small component of the total recovery 70-

u~a

60A

50-

~

40-

~

30-

NH4NO3

j

e o 20 10

81

82

83

84

85

86

87

88

Year Fig. 1. Growth response determined on the eight individual trees.

C.M. Preston, D.J. Mead / Forest Ecology and Management 65 (I 994) 219-229

223

Table 2 Recoery of 15N after eight growing seasons Recovery (% of total applied)

Inside plot Soil N (organic) Trees Understorey Litter Fine roots Inside-plot total

15N.urea

l SNH4NO3

NH~ s NO3

SEa

33.0 8.4 1.8 4.1 2.6 49.9

33.9 8.9 1.4 3.3 2.1 49.7

15.8 3.1 1.4 2.1 1.0 23.4

6.01 0.75 0.68 1.09 1.07 7.81

8.1

3.6

2.2

0.74

Outside plot Soil Trees plus u nderstory plus litter Outside-plot total

3.7

7.5

2.8

2.44

11.8

11.1

5.1

3.05

Inside plus outside plot Soil Plant Total

41.1 20.5 61.6

37.6 23.2 60.7

18.0 10.4 28.5

6.42 4. l0 10.43

aStandard error.

(8.4%, 8.9% and 3.1%, respectively) with the difference between nitrate and other forms being highly significant ( P = 0.009). In October 1981, there had been little indication of movement of ~SN outside plot boundaries. This was based on a hydrological study (E. Hetherington, personal communication, 1981 ), as well as sampling of soil outside the plots. In addition, the trees were much smaller, so that feeding of roots from trees outside the plot would have been much less important. Therefore, outside-plot recovery of 15N was considered unimportant in 1981, and was not assessed in any detail. By contrast, of the ~SN recovered in plant biomass and soil in 1988, approximately one-fifth was found outside the plot boundaries for all three 15N forms. No attempt was made to distinguish what proportion of this outside-plot ~SN was the result of outside roots feeding into the plots as opposed to N transport by hydrological forces or deposit of ~SN-endched litter outside the plot boundaries.

3.3. 15N in plant biomass For within-plot tree uptake, 15N uptake data from 1981 and 1988 were compared by ANOVA after log transformation which was used to overcome heterogeneity of variances. This analysis showed that uptake in the nitrate treatment was significantly lower ( P = 0.0004) than the urea or ammonium labelled treatments. The difference between the latter two was not significant (P=0.2160); nor was the time effect or treatment by time interaction significant (P=0.0967 and P=0.2215, respectively). The average recovery of 15N by the trees in the 1981 sampling was 5.8% of that applied; the corresponding amount for the 1988 sampling was 6.8%. The recovery of l SN in tree components in 1988 for the three treatments and the probabilities of significant differences are shown in Table 3. It can be seen that ~SN was found throughout the trees, with the largest part being in the lower crown. The important significant differences were found for mid-crown foliage, lower crown

224

C.M. Preston, D.J. Mead / Forest Ecology and Management 65 (1994) 219-229

Table 3 Total tree and component recovery of ~SN, standard error of the mean, and probability (P) for significant single degree of freedom treatment constrast Treatment

Component

Recovery (% of ~SN applied) 15N-urea 15NH4 15NO3 Std. error Probabilities for contrasts lSNO3 vs. lSN-urea and lSNH4 15N-urea vs. I~NH4

Upper crown

Middle crown

Lower crown

Stem

Large roots

Total

FoI. a

Br. b

Fol.

Br.

Fol.

Br.

0.29 0.38 0.16 0.182

0.07 0.18 0.07 0.079

0.79 0.76 0.41 0.094

0.49 0.39 0.17 0.120

3.35 2.64 0.99 0.516

1.87 1.46 0.51 0.279

0.81 1.52 0.28 0.067

0.71 1.60 0.54 0.228

8.38 8.93 3.13 0.750

0.488 0.752

0.620 0.367

0.050 0.846

0.173 0.584

0.050 0.406

0.043 0.374

0.002 0.005

0.112 0.071

0.009 0.641

aFol., foliage. bBr., branches.

Total N

15N

Total N

100%

15N

NN Illl[[lllll[I

[lllIJ/llIJt

50 Y.

25*/.

0% 1981

l

1988

Roots

[~

Stem

~

CurrentBranches

~

NonCurrantFoliage

[]~]~ CurrentFoliage

Non CurrentBranches

Fig. 2. Distribution of total N and 15N in tree components in 1981 and 1988.

components and stem; there was substantially lower recovery of nitrate in these components, and generally there was no difference between urea and the ammonium treatments. However for the stems (and large roots, although these were more variable) there was a greater recovery of 15N in the ammonium than in the urea treatment. Nevertheless, total uptake was very similar between urea and ammomium at 8.4% and 8.9%, respectively; this may be compared with 3.1% recovery for the nitrate treatment.

Because the trees took up little additional ~SN after the first growing season, an opportunity was available to study retranslocation of 15N within the trees; this is presented elsewhere (Mead and Preston, 1994) with much more detailed information on within-tree distributions of 15N. However, Figure 2 is presented to indicate the mobility of the ~SN and its similarity to the distribution of total N in tree components for both sampling dates. Between 1981 and 1988, there was a large decrease in the proportion of ~5N and

C.M. Preston, D.J. Mead / Forest Ecology and Management 65 (1994) 219-229

N in current foliage and branches and a large increase in their proportion in non-current branches; changes in the relative proportion of ~SN and N in other components were minor. The recovery of ~5N by herbs, shrubs, alder and fine roots within the plots was a relatively small part of the total recovery, and showed no significant treatment effects ( P > 0.3). On average, 3.16% (SE 1.085%) of the ]SN was recovered in litter. The recovery in the aboveground parts of the understorey was 1.52% (SE 0.681%), of which the shrubs accounted for 88%. There was almost no ~SN recovery by alders in the 1988 sampling.This was due to biological nitrogen fixation and consequent isotope dilution of the ~SN uptake from soil (Mead and Preston, 1992). 3.4. 15N in soil Inside the plots, soils were analysed from three depths down to 30 cm and for all treatments the recovery was highest in the top 5 cm of mineral soil (Fig. 3). A split-plot analysis of variance showed a strong depth effect (P=0.0001) and treatment by depth interaction (P=0.0011 ),

225

whereas the main treatment effect was not significant (P=0.3133). This was a result of the labelled nitrate showing a much lower recovery (6.7% with a standard deviation of 3.57% ) in the 0-5 cm horizon than the other two treatments (17.6% and 20. 4% with standard deviations of 2.73% and 4.70% for the 15N_ureaand ~5NH4NO3 treatments, respectively). Below 5 cm the recovery was similar for all treatments. This pattern indicates that the nitrate initially moved rapidly down the profile but once it became immobilized changes were similar to the other treatments. Table 4 compares the distributions of ~SN by depth in 1981 and 1988, as percentages of ~SN recovered within the soil column. For 1981 both inorganic and organic ~SN data are presented; in 1988 all ~SN recovered was in organic form. For organic ~SN in both 1981 and 1988, recoveries by depth were similar for ~SN-urea and l SNH4NO3 but organic ~SN derived from nitrate had lower overall recovery, as well as higher proportions at the two deeper levels. In 1981, the distributions of inorganic and organic N were similar for urea but skewed to greater depths for

Recovery of'SN by depth in soil

15N0 ~

ISNH 4

lSN.urea

5

10

15

Recovery ( Percent of total applied )

Fig. 3. Recovery of I~N in the soil in 1988.

20

25

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CM. Preston, D.J. Mead / Forest Ecology and Management 65 (1994) 219-229

Table 4 Recovery and distribution of soil 15N in 1981 and 1988 1981

1988 Organic

Organic

Inorganic

Total

Recovery (% of ISN applied) Total soil recovery

30.6

8.4

39.0

15.9

As % of total ~SN in soil column 15N-urea 0-5 cm 5-15 15-30

57.0 25.9 17.1

50.9 29.9 19.2

55.7 26.7 17.6

59.2 26.2 14.6

15NH4NO 3

0-5 cm 5-15 15-30

64.7 25.4 9.9

26.4 28.1 45.5

58.6 25.8 15.6

64.4 23.2 12.4

NH~SNO3

0-5 cm 5-15 15-30

45.3 30.2 24.5

28.9 29.3 41.8

41.8 29.9 28.3

45.7 31.2 23.1

ammonium and nitrate sources. For total soil 15N recoveries in 1981, 15N-urea and ISNH4NO3 had similar distributions by depth, but NH415NO3wa S skewed to greater depths, consistent with the greater mobility of nitrate in the spring after application.

3.5. 15N recoveries in 1981 and 1988 Table 5 shows the average rate of loss of soil I S N a s a percentage of that immobilized in organic forms in the soil in 198 I, based on data for within-plot soil organic 15N. On average, some 7% of the residual soil 15N was lost each year, corresponding to 4.4 kg N ha-1, 5.6 kg N ha-1 and 2.1 kg N ha- 1 for 15N.urea ' 15NH4NOa and NH15NO3, respectively. When this calculation was attempted in several different ways, such as including outside-plot 15N, or comparing total site recoveries in 1981 and 1988, the average annual rates of loss all fell in the range of 5-8% per year based on October 1981 15N amounts. In summary, comparison of within-plot recoveries of 15N in soil and plant biomass after 1 and 8 years showed little or no increase in 15N recovered in crop trees, a somewhat greater increase in 15N recovered in understorey plus litter, and a decrease (about 50%) of 15N recovered

Table 5 Comparison of total site tSN recoveries in 1981 and 1988 and estimation of mineralization rate of within-plot soil organic ~SN 15N_ urea Total recoveries (% of tSN applied) 1981 93.3 1988 61.7

tSNH4NO3

95.2 60.8

Total 1988 recovery ( as %of total 1981 recovery) 66.1 63.9 Soil organic tSN (within-plot only) as kg N ha- 1 1981 63.5 73.2 1988 33.0 34.0 Change -30.5 -39.2 Average annual loss AskgN ha -~ As % of 1981 amounts

4.4 6.9

5.6 7.7

NH~SNO3

44.4 28.4

64.0

30.6 15.8 - 14.8

2.1 6.9

in plot soil. These results were similar for all three original forms of 15N.

4. Discussion It is generally found that 2-6% of native soil N is mineralized each year in temperate agricultural soils. The rate of mineralization of residual fertilizer N is initially higher, but declines expo-

C.M. Preston, D.J. Mead / Forest Ecology and Management 65 (I 994) 219-229

nentially to approach that of native soil N, as the N becomes incorporated into more complex, more highly condensed and less readily available macromolecules and organomineral complexes (Smith and Power, 1985; Webster and Dowdell, 1985; Kelley and Stevenson, 1987 ). In three longterm studies, the annual rate of loss or mineralization of residual ~SN fertilizer (or incorporated 15N plant material) was approximately 2% after the initial period of rapid loss (Broadbent and Nakashima, 1974; Webster and Dowdell, 1985; Ladd et al., 1985). The present estimates of an average mineralization of 5-8% per year of the immobilized 15N would presumably include higher rates in the years immediately following fertilization) and are thus in reasonable agreement with other studies of immobilization and remineralization of N in agricultural soils. These annual average rates of loss in the field can also be compared with those from a pot trial of the availability of the residual fertilizer ~SN (Preston and Mead, 1993). Briefly, surface soil ( 0 - l 0 cm ) was sampled in 1988 from four of the tSN plots at this site (one each of lSNH4NO3, NH4~5NO3 and both ~5N-urea plots). Lodgepole pine seedlings grown from seed in pots for 9 months took up an average of 8.5% of the total ~SN per pot. This is very close to the average annual loss in the field (7.1%), and most likely due to the more favourable temperature and moisture status of the seedlings, as well as their very high rooting density and lack of leaching loss. While the amounts of residual ~5N presumably mineralized each year were small, in the range of 2-6 kg N ha-1 year-~, they are similar to the amounts of ~5N taken up in the first growing season. Based on the comparison of tree recoveries in 1981 and 1988, it does not appear that much of this ~SN was taken up by the trees. It is possible that 15N was mineralized, as indicated by the pot trial, but was not taken up by microbial or plant biomass, and was subject to nitrification followed by leaching or denitrification. In the pot trial of these t SN-labelled soils, nitrate was found in the pots without trees. Recent work in agricultural soils has indicated that significant amounts of ~5N may be mineralized and nitrified during the winter, then lost by

227

denitrification (rather than leaching) in the spring before significant plant uptake can occur (Mahli and Nyborg, 1986; Heaney and Nyborg, 1988 ). The potential for denitrification losses is especially high when snowpack is retained until the soil begins to thaw (Heaney et al., 1992). Such a scenario may also be occurring in this forest soil, resulting in a 'leakage' of the N pool even where no fertilizer has been applied (Macdonald et al., 1989 ). It appears that the relevant N transformations can continue even when the soil is frozen (Heaney and Nyborg, 1988 ). This work also demonstrates the similarity in behaviour of the residual soil 15N, regardless of its original form. While there were large differences in the behaviour of t SN-urea, ~SNH4NO3 and NH15NO3 in the first season after application, once immobilized in the soil in organic form, they lost similar proportions of t SN from the system, and the relative distributions of ~SN inside- versus outside-plots were remarkably similar after 8 years. Again, this result should not be surprising, as it has been demonstrated in agricultural soils, that once N (or C) has been processed through the microbial pathways, the resuiting humified soil organic matter shows little variation in chemical structure regardless of the chemical form of fertilizers or the type of plant litter (Oades et al., 1988).

5. Conclusions Comparison of results after one and eight growing seasons indicated that uptake of fertilizer N by crop trees was largely complete after 1 year at this site. It is inferred from the soils data that residual ~SN in the soil continued to be mineralized but the amounts taken up by trees were too small to have any significant effect on growth. While some of the residual t SN was taken up by the understorey, and some was found outside the plot boundaries, the overall losses amounted to some one-third of the ~5N found in October 1981. Estimates of the average rate of mineralization of the residual soil 15N were in the range of 58%. This is probably somewhat higher than the native soil N, and similar to values found for t 5N

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C.M. Preston, D.J. Mead / Forest Ecology and Management 65 (I 994) 219-229

immobilized in agricultural soils. It was also consistent with results from a pot trial of the availability of the residual ~SN. Both the 1- and 8-year results from this site demonstrate the importance of maximizing uptake of N fertilizer during the first growing season. This can be done to some extent through timing of application and choice of fertilizer form to avoid losses through leaching, denitrification and volatilization of ammonia. However, other factors may limit fertilizer N uptake, such as short growing season, limitations related to water or other nutrients, the low fine root density of many pine species, or other physiological factors. Even in the first year, the amount of 15N found as inorganic N in October 1981 was greater than the total plant uptake by trees and understorey (Table 1 ), and it appears that much of the 15 N mineralized over the subsequent 7 years was lost from the site. The factors controlling initial fertilizer loss by leaching, denitrification or ammonia volatilization are now well understood. Low utilization of fertilizer N by conifer plantations also appears to be a widespread phenomenon. The results of the present work and other recent studies of 15N fertilizer uptake in conifers suggest some questions for further research. First, why do the trees compete so poorly for N, or fail to take it up even when it is available in the soil, especially in the first season after fertilization? Second, what is the seasonal pattern of mineralization in forest soils of British Columbia, and is there a loss of nitrate in late winter and spring, before the main period of plant uptake?

Acknowledgements This study was funded by Canada-British Columbia Forest Resource Development Agreement (FRDA) Federal Direct Delivery, Extension, Demonstration, Research and Development Sub-program, Project F52-41-116 to Dr. G.F. Weetman (Faculty of Forestry, UBC). We thank Rick Fournier, Kathy Marek, Elizabeth Schnorbus-Panozzo, Morag McDonald and Dave Dunn for assistance in the

field, and Kevin McCullough and Ann Rusk for the 15N analyses. Travel assistance for D.J. Mead was provided by the Lincoln College Foundation, Pacific Forestry Centre and Western Forest Products.

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