Plant and Soil 157: 147-150, 1993. (~ 1993 Kluwer Academic Publishers. Printed in the Netherlands. PLSO 3180

Short communication

lSN Abundance of forests is correlated with losses of nitrogen

P E T E R H O G B E R G and CHRISTIAN JOHANNISSON Section of Forest Soils, Department of Forest Ecology, Swedish University of Agricultural Sciences, S-901 83 Ume& Sweden

Received 26 April 1993. Accepted in revised form 9 July 1993

Key words: ~SN, forest soils, N deposition, N losses

Abstract

A direct correlation was found between fractional losses of added N and the change in 6 ~SN%o during 19 years in an experiment with annual additions of N at three rates to a Scots pine (Pinus sylvestris L.) forest in northern Sweden. This confirms that processes leading to losses of N discriminate against ~SN, and opens possibilities to conduct retrospective studies of the N balance in forests.

Introduction

High levels of nitrogen deposition have increas- ingly been discussed as a major threat to large areas of northern temperate forests (Abet et al., 1989; Schulze, 1989; Skeffington and Wilson, 1988). Forests subject to high loads of N ulti- mately reach the state of N saturation, which can be defined as when N inputs exceed the com- bined demand of plants and microorganisms (Aber et al., 1989). When this happens, large losses of N, especially through leaching of ni- trate, will take place and accelerate losses of base cations. To predict the development of effects of N deposition on initially N-limited systems complex dynamic models (Aber et al., 1991; ~g ren and Bosatta, 1988) and simpler static models (Gundersen, 1991) have been pro- posed. It is clear that immobilization of N in soils is particularly difficult to predict, but there is a lack of understanding of other processes as well.

There are very few experimental data against which models could be tested. Long-term forest fertilization experiments like the Swedish Forest Opt imum Nutrition Experiments (SFONEX; Tamm, 1989a,b, 1991), with applications of N once a year do not mimic perfectly N deposition

(Johnson, 1992; Miller and Miller, 1988), but are among the best materials available for testing of effects of N-loading on forests (Table 1). The SFONEX are useful since they are located in areas with low deposition of N and S, have been run for two decades, and have resulted in various distinct levels of impact of N additions (Tamm, 1989a, b, 1991).

Earlier work at the SFONEX Norrliden and Str~san showed that forests with large losses of N accumulate 15N (H6gberg, 1990, 1991; H6gberg et al., 1992). This should be so because processes

Table 1. Amounts of ammonium nitrate or urea (kgN ha l y r - I ) applied to plots in the fertilization experi- ment at Norrliden. All treatments were treated with 2.5 kg B ha ~ in 1980, because of incipient B deficiency on N2-N3 plots. Plots treated with P and K received those treatments every third year; the total amounts applied 1971- 1989 amounted to 280 and 540 kg ha ~, respectively

Period (years) Nitrogen addition rate

NO N1 N2 N3

1971-1973 0 60 120 180 1974-1976 0 40 80 120 1977-1989 0 30 60 90 Total(kg N ha -~ ,

1971-1989) 0 690 1380 2070

148 Short communication

leading to loss of N, e.g. ammonia volatilization, nitrification followed by leaching or denitrifica- tion, and denitrification in itself, discriminate against the heavier 15N (Shearer and Kohl, 1986). In SFONEX, samples of current needles have been collected from the trees for conven- tional foliar analyses every year. With time, trees in plots receiving high loads of N developed a high 15N abundance, while trees in plots given low amounts of N showed a constant or declining 15N abundance (H6gberg 1991; H6gberg et al., 1992), presumably because the fertilizers were depleted in 15N, as shown by analyses of the few batches available (H6gberg, 1990). In general, fertilizers derived from atmospheric N 2 other- wise have a ~SN abundance close that of its source (Shearer et al., 1974).

Interestingly, declining German forest stands, which probably lose large quantities of N, have also been found to have elevated 15N abundance (Gebauer and Schulze, 1991). We report here that there is, indeed, a direct correlation be- tween loss of N and the ~SN abundance of the forest at Norrliden.

Materials and Methods

Norrliden is located in a Scots pine, Pinus sylvestris L., forest (38-yr-old in 1989) on a gently sloping glacial till soil in N Sweden (64°21 ' N, 19o46 ' E, 260 mal t . ) . Further details about experimental lay-out, soils, climate etc. are given by Tamm (1989a, b, 1991) and H6gberg (1990, 1991). The part of the experi- ment studied here, E55, consists of forty 30 × 30 m plots, and comprises three levels of N given as NH4NO 3 or urea, alone or in combination with PK, and with additional control (NO) and PK (NOPK) plots (Table 1). The treatments N0-N3 had three replicate plots each, whereas the treatments NOPK-N3PK had two replicate plots each. Treatments started in 1971. In the N3 treatments, the growth response culminated within 5-10 years (Tamm, 1989a)

Samples of current needles have been col- lected annually, starting in 1970, i.e. one year before treatments commenced. Needles have been taken from ten trees per plot, and mixed

into one composite sample per plot. We have analysed samples from 1970, 1971, 1975, 1980, 1985 and 1989.

Analyses of 15N abundance were carried out by high-precision isotope ratio mass spec- trometry on Kjeldahl digests (Haystead, 1983), and are reported in delta (3) units:

15N = 1000 x (Rsample - Rstandard)/Rstandard(%~ )

where R = mass 29/mass 28, and the standard used was pure N2, which had a 615N = -2.6%0, i.e. lower than atmospheric N 2 (Mariotti, 1983).

Fractional loss of added N was estimated by subtracting the amount of added N (cf. Table 1) and the store of N in soils (down to 20 cm in the mineral soil) plus stands of the respective NO and PK plots from the store of N of plots treated with N and NPK (CO Tamm, A Aronsson and B Popovic, pers. comm.; data on soils only are found in Tamm, 1991). Sampling of soils was carried out in 1988, while the stands were sam- pled in 1984. Nitrogen in soils plus stands on NO and PK plots amounted to 913-1140 and 135- 140kgNha -1, respectively, while soils plus stands on N-treated plots contained at most 706 additional kgN ha -1. Thus, the amounts of N added, 690-2070 kg ha -1 (Table 1), were com- paratively large. We assume that the stores of N in the two compartments did not increase much 1984-1989, because the annual retention of N should have decreased gradually during the ex- periment.

Results and discussion

In 1970, the year before treatments started, the 615N (%0) of current needles varied between -1.54 and 2.89 (~ = 0 . 1 2 - 0.14, n = 37: three samples were lost). We have, therefore, chosen to correlate the change in 615N (%0) from 1970 to 1989 with loss of N. Separate regression analyses were made for the NH4NO 3 and urea treat- ments. The regressions were weighted by num- bers of replicates, i.e. n = 3 in the case of N1- N3, while n = 2 in the case of N1PK-N3PK.

Despite few degrees of freedom, the correla- tion between the change in 615N (%0) 1970-1989 and the fractional N loss was obvious (Fig. 1). A

,~ ±0

!

"6

7 Z u~ ~o .c

g g 6

-1

+0

/ 0

~o /

/ /

/

r 2 =0 .94 °*

0 100

I

/ /

t /

I • o / /

/ /e

/ /

/o / r 2 =0.75"

/ e

5O

N lost in % of added

Fig. 1. The relation between the fractional loss of added N from plots fertilized with N or NPK and the change in ~SN abundence of current needles of Scots pine during the experiment at Norrliden 1970-1989. a, plots fertilized with NH4NO3; b, plots fertilized with urea (cf. Table 1). Each data point represents the mean for 2-3 plots.

correlation between 6 ~SN (%0) of current needles collected in 1989 and fractional N loss was also found, but was weaker, p = 0.023 and p = 0.057 for NH4NO 3 and urea plots, respectively. This is probably because of memory effects reflecting large pretreatment differences, which ranged 4.4 615N (%0) as mentioned above. We only tested linear regressions, but realize that the relation should be more complex over a larger interval of N loss. For example, there should be no frac- tionation of added N if all added N was lost (e.g. Peterson and Fry, 1987). However, the role of exchange reactions with and priming of endogen- ous N pools (Jenkinson et al. 1985) is not resolved. The intercepts with the y-axes, -0 .9 and -9 .6 615N (%~), were, nevertheless, not far from the -3 .0 and -10.8, reported for the few analyzed batches of NH4NO 3 and urea, respec- tively (H6gberg, 1990). The larger isotope effect in urea treatments is because of volatilization of ammonia and higher rates of nitrification (H6gberg, 1990, 1991).

Hence, we conclude that the ~SN abundance of forests is strongly correlated with loss of N from

Short communication 149

the ecosystem. Differences in the 15N abundance of N inputs (Heaton, 1986; Virginia and Del- wiche, 1982; cf. Fig. 1), soil N transformations (Shearer and Kohl, 1986; cf. Fig. 1), sources of N used by plants and their assimilation pathways (Handley and Raven, 1992; Shearer and Kohl, 1986) superimpose variations on this relation. In experiments like SFONEX, where needle sam- ples have been collected regularly, we have a unique tool which enables retrospective studies of the development of N saturation.

Acknowledgements

We would like to thank C-O Tamm, A Aronsson and B Popovic for access to their unpublished materials, and J ,~gren and L Bondesson for useful comments. Funding was provided by the Swedish Council for Forestry and Agricultural Research, the Swedish Forestry Research Foundation and the Swedish Environmental Protection Board. The study is partly a contribu- tion to the EC project NiPhys.

References

Aber J D et al. 1989 BioScience 39, 378-386. Aber J D et al. 1991 Ecol. Applicat. 1, 303-315. A.gren G I and Bosatta E 1988 Environ. Pollut. 54, 185-197. Gebauer G and Schulze E - D 1991 Oecologia 87, 198-207. Gundersen P 1991 For. Ecol. Manage. 44, 15-28. Handley L L and Raven J A 1992 Plant Cell Environ. 15,

965-985. Haystead A 1983 In Soil Analysis: Instrumental Techniques

and Related Procedures. Eds. K A Smith, pp 377-406. Marcel Dekker, New York.

Heaton T H E 1986 Chem. Geol. 59, 87-102. H6gberg P 1990 Oecologia 84, 229-231. H6gberg P 1991 Soil Biol. Biochem. 23, 335-338. H6gberg P e t al. 1992 Plant and Soil 142, 211-219. Jenkinson D S e t al. 1985 J. Soil Sci. 36, 425-444. Johnson D W 1992 J. Environ. Qual. 21, 1-12. Mariotti A 1983 Nature 303, 685-687. Miller H G and Miller J D 1988 Environ. Pollut. 54, 219-

231. Peterson B J and Fry B 1987 Annu. Rev. Ecol. Syst. 18,

293-320. Schulze E-D 1989 Science 244, 776-783. Shearer G B and Kohl D H 1986 Aust. J. Plant. Physiol. 13,

699-757.

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Shearer G B e t al. 1974 Soil Sci. 118, 308-316. Skeffington R A and Wilson E J 1988 Environ. Pollut. 54,

159-184. Tamm C-O 1989a Ambio 18, 184-191. Tamm C-O 1989b Comm Norw. For. Res. Inst. 42, 179-196.

Tamm C-O 1991 Nitrogen in Terrestrial Ecosystems. Spring- er Verlag, Berlin, 115 p.

Virginia R and Delwiche C C 1982 Oecologia 54, 317-325.

Section editor: H Lambers