Journal of Soil Science, 1990,41,279-293

Nutrient changes in decomposing beech leaf litter assessed using a solution flux approach

R. G. J O E R G E N S E N & B. M E Y E R Institute of Soil Science, von Siebold-Strasse 4 ,0-3400 Goettingen, Federal Republic of Germany

S U M M A R Y

The decomposition of beech (Fagus sylvatica L.) leaf litter was examined in lysimeters. The experiment allowed comparison of data from mass changes of bioelements in leaf litter and the solution flux balance of throughfall input and lysimeter output over a two-year period (1983 and 1984). The annual C loss from the leaf litter was 19%. Na and Kconcentrations in this leaf litter decreased in the first year and remained constant in the second, while those of Ca and Mg showed no significant changes. Nand S amounts per ha increased during the first year by about 7 kg N ha-' and 0.5 kg S ha-'. Ash, Fe, Mn and Al amounts per ha increased to 470, 130 and 240% of their initial levels. These net increases in the first year of decomposition are discussed.

The mean annual water and element input by throughfall during the experiment corresponded approximately to the long-term average. Thirty to 40 mm more water was evaporated from the litter layer in the drier year (1983) than in 1984. Total N, NO,, Na and C1 flux rates of throughfall inputs and lysimeter outputs were equal. NH, input rates were greater and organic N smaller than the output rates of the lysimeters. Water balance data indicated that the lysimeter output of K, Mg, Ca and SO, exceeded throughfall input. Probable reasons for these differences are discussed.

I N T R O D U C T I O N

In forest ecosystems, dead material such as leaf litter decays by biotic and abiotic processes. Biotic litter decomposition, ending in mineralization of organic material, is the most important process for the return of photosynthetically bonded CO, into the global carbon cycle and for the release of nutrient elements from plant debris into bioelement cycles. Following nutrient release by biological decomposition, a certain loss of bioelements can be caused by leaching. The contribution of this process to the total flux rate depends on the kind of chemical bonding (covalent, complex or electrostatic) and on the microbial metabolic requirement for an element. During the first period of litter decomposition, essential elements are retained by the decomposing organisms. Not only a retention but also a net increase was observed for N and other elements (e.g. Gilbert & Bocock, 1960; Anderson, 1973; Gosz et al., 1973; Howard & Howard, 1974; Staaf, 1980).

The mechanisms of the net increase or the retention of nutrient elements are only partly under- stood, especially for N, the main element in regulating decomposition processes. Beech (Fagus sylvatica L.) leaf litter initially contains a large amount of lignin and only a little N. Consequently, the annual decomposition rate is very small in comparison with other species of leaf litter. Different layers can be built up on the soil surface depending on the decomposition pattern. The mor type of humus is developed in acid beech forests without earth worms, and the mull form is found in calcareous beech forests with large and active earthworm populations. The C/N ratio of such a N-limited substrate decreases without N release during a certain period of decomposition until a definite 'critical' level is reached. Staaf (1980) explained his finding of a net N increase by additional N coming from the underlying old litter or soil. This explanation can be tested by carrying out a

279

280 R. G. Joergensen & B. Meyer

decomposition experiment with beech leaf litter in lysimeters. In the experiment reported here, data from mass changes of bioelements in leaf litter are compared with the solution flux balance of throughfall input and lysimeter output.

S T U D Y AREA

The leaf litter and soil samples were collected from the experimental area of the research project 'Ecosystems on Limestone' (Special Research Project SFB 135 of the German Research Foundation, DFG) in the Goettingen Forest, Lower Saxony, FRG. The location on the German topographic map (1:25000) is No. 4526, R/H: 35270/571098. The range in elevation above sea level is from 400 to 420 m. The mean annual temperature is 7.9"C and the mean precipitation 720 mm. The climate is classified as submontanic. The bedrock is lower shell-lime (wave-lime). The vegetation is mature beech forest, 100 to 115 years old, with a large number of understory herb species (Melico fagetum hordelymetosum). This ecosystem is considered to be stable, unlike the acid beech forests (Luzulofagetum) on red sandstone in the Solling (Ellenberg et al., 1986). This calcareous beech forest is characterized by large and active earthworm populations. The soils of the experimental area vary greatly over short distances, between Rendzina (Mueckenhausen, 1977; = Rendzic Leptosol (FAO) or Lithic Rendoll (USDA)) and Terra fusca Rendzina (Mueckenhausen, 1977; =Transition Chromic Cambisol-Leptosol (FAO) or Lithic Eutrochreptic Rendoll (USDA)). Terra fusca Rendzinas are the dominant soil type, covering more than 50% of the total area. Maximum carbon accumulation is obtained in this soil type, partly because the soil is deep and partly because it is heavily buffered against acidification; thus i t retains its ability to stabilize organic matter.

M A T E R I A L S A N D M E T H O D S Throughfall Throughfall was collected from 1 January 1983 in two 5 1 polypropylene bottles, with a poly- propylene funnel of 25.48 cm diameter (Fig. I). In the funnel, a coarse pvc sieve held back leaves, branches, insects etc. Samples were collected weekly, filtered (Schleicher & Schuell 595- 1/2) and analysed.

An assumption made in this work is that the throughfall solution entering the lysimeters has the same composition as that falling into the throughfall collectors. I t is not the aim of this paper to give data on the whole spatial variability of water and bioelement input caused by the heterogeneity of

Funnel for throughfall

L y s i m e t e r type I L y s i m e t e r t y p e II x I Glass fibre PVC-mesh , Nylon m e s h 1 mm Natural litter layer . - - - - -. .

Fig. 1. Schematic layout of the lysimeters.

Decomposition of beech leaf litter 28 1

covering by the canopy. This aspect was considered for water cycling by Gerke (1987) and for element flux rates by Kues (1984) and by Meiwes & Beese (1988). Throughfall collectors and lysimeters were situated in chosen locations that represented the average situation of the litter layer under the canopy.

Lysimeters The lysimeter installation consisted of four pvc tubs with a surface area of 0.2475 m2 (45 x 55 cm) and with a depth of 12 cm (Fig. 1). The free-draining lysimeters stood 10 cm apart on the soil surface. The solution passing through the litter was sampled in 5 1 polypropylene bottles standing in a covered trench. Four 12 1 jars were used as an overflow in the event of heavy precipitation. The leaf litter was laid on a glass-fibre floor over a 1.5 cm pvc mesh. The tops of the lysimeters were covered with a nylon gauze, 1 mm mesh, so that macrofaunal organisms were excluded. The effluent solution was collected weekly, filtered (Schleicher & Schuell595-1/2) and analysed.

Lysimeter type I contained a layer of 300grn-’ air-dried fresh leaf litter (145gCrn-,). Lysimeter type I1 contained the same layer of 300 g rn-, above the remains of the upper layer of the previous year from which 5 g was taken for chemical analysis. The bottom layer of the previous year was completely removed. Fresh leaf litter of autumn 1981, 1982 and 1983 was introduced into the lysimeter experiment. The balance periods for this study were the years 1983 (21 December 1982 to 13 December 1983) and 1984 (21 December 1983 to 17 December 1984). The lysimeter experiments have run since April 198 1.

Water analyses Nitrogen. In a 75cm3 sample, NH, and NO, contents were analysed by steam distillation

(Bremner & Keeney, 1966). The sample was then transferred to a Kjeldahl flask, 10cm3 concen- trated H,SO, and 500 mg Se-catalyst mixture (Merck) were added and the flask was heated until the digest cleared. Thereafter, the mixture was boiled gently for 3 h. The cooled sample was then transferred to a distillation flask and after addition of40 cm3 10 M NaOH, NH,content was analysed by steam distillation. All three distillates were collected in Erlenmeyer flasks containing 5 cm3 2% H,BO, and an indicator. NH,-N was determined with standard 0.005 M H,SO, by automatic pH-titration.

Cations. Na, K, Mg and Ca were determined by atomic absorption spectrophotometric analysis (Varian AA-775).

Chloride. A sample of defined volume acidified with HNO, was placed in a beaker and chloride measured potentiometrically by addition of 0.01 M AgNO, by automatic titration.

Sulphate. From a sample of defined volume, the cations were eliminated with a strong acidic cation exchange resin. Seven cm3 0.005 M BaCI, and 2 cm3 0.005 M H,SO, were added and mixed for 10 min on a magnetic stirrer to precipitate BaSO,. The excess of BaCl, was measured potentiometrically by addition of 0.005 M Na2-EDTA by automatic titration.

Leaf Litter Analyses After collection, all leaf litter samples were air-dried and homogenized by passing through a centrifugal mill with a 0.25 mm mesh.

Dry matter. A sample was dried at 105°C to constant weight.

Ash. A sample was heated at 700°C to constant weight.

Cations. After heating at 550°C to constant weight, the sample was dissolved in 50% HNO, and filtered (Schleicher & Schuell, 595-1/2). Na, K, Mg, Ca, Fe, Mn and Al were determined by atomic absorption spectrophotometric analysis (Varian AA-775).

Sulphate. Five cm3 0.1 M NaOH was added to a 0.5 g sample in a porcelain crucible and heated until all water had evaporated. The crucible was then heated at 480°C for 10 h. After cooling 15 cm3 0.1 M HCI were added to the residue and allowed to stand for 3 h. After filtration (Schleicher & Schuell589-3), the solution was made up to 100 cm3 before determination of SO,.

282 R. G . Joergensen & B. Meyer

Total S. After dry combusion at 1 IOO"C, SO, was determined by gas chromatography (Carlo Erba EA 1106).

Total C and N . After dry combustion, N, and CO, were separated by gas chromatography (Carlo Erba ANA 1400).

R E S U L T S A N D DISCUSSION

Changes in leaf litter Organic carbon. The interpretation of nutrient release data requires information about C loss

rates. Meaningful comparisons from litter decomposition experiments can be obtained by using mathematical models. A detailed review was given by Wieder & Lang (1982). For the C loss of the recalcitrant beech leaf litter, a single exponential function was shown to be a fairly correct math- ematical approximation of the natural decomposition processes. The model In (CJC,") = a - bt used in this paper was that of Jenny et al. (1949).

The mathematical model corresponded quite well with measured decomposition rates for each layer in the three experimental years (Table 1). The decomposition rates were almost identical, although climatic conditions in the three years were very different. 1982 and 1983 had hot and dry summers with considerable vapour pressure deficits in the atmosphere. In 1984, the annual precipi- tation rate was higher and the temperature lower than average (Joergensen, 1987). Mean annual C loss was 19%, not much less than the 23% measured by Joergensen (1987) in his litter bag exper- iment with exclusion of macrofauna. The latter experiment was performed in the same experimental area, yet varied in that the organisms (< 1 mm) could enter the litter bags from the soil and all other sides and not only from the top, as in the current lysimeter experiment. In an acid beech forest, Staaf (1980) measured annual C loss rates of approximately 20%, which corresponds to the present data.

Table 1. Linear relationships of the C loss of fresh leaf litter of autumn 1981, 1982, and 1983 and comparison between measured and calculated C loss in the lysimeters

Remaining after Regression Correlation 1 year

Intercept coefficient coefficient Year a b r Measured Model

1981 4.597 -0.000506*** -0.997 80.7 82.5 1982 4.591 -0.000607*** -0.983 77.5 79.0 1983 4.605 -0.000577*** -0.992 81.2 81.2

Mean 4.596 -0.000562*** -0.980 79.8 80.7

Remaining after 2 years No.

of Measured Model samples

69.3 68.6 6 65.8 63.3 10

65.6 8

67.6 65.8 24

***P < 0.00 1.

The results in Table 2 were obtained using air-dried leaf litter samples, for which the moisture content was measured (typically 9-1 1 %). The concentrations of organic carbon were about 50 to 60 mg gg' higher compared to other data (e.g. Gosz et al., 1973). Oven-dried leaf litter must be stored in a desiccator if it is not to revert rapidly to the 9-1 1 % equilibrium water content.

Nitrogen. In the first year of litter breakdown, the N content increased from 12.1 to 17.6 mg g-'. In the second year there was a further increase to 20.0 mg g-' (Table 2). This increase in N content has always been found in investigations of beech leaf litter decomposition (e.g. Staaf, 1980; Joergensen, 1987). Under the environmental conditions of the lysimeter, with complete exclusion of macrofauna and restricted admittance of other faunal organisms, a net N increase of 7 kg ha-' a- ' was measured during the first year. In the second year, the amount ofN remained constant (Table 3).

Decomposition of beech leaf litter Table 2. Bioelement content in the leaf litter and change (YO) compared to initial content

283

Time C N,,,,, S,,,,, SO,-S Na K Mg Ca Fe Mn Al Ash (years) (mg g-' dry material)

0 534 SD 5

I 512 SD 3

2 490 SD 3

I 96 2 92

***

12.1 1.4 0.7 0.3 2.5 1.1 17.3 0.5 0.4 0.2 52.9 I .6 0.2 0.1 0.1 0.2 0.1 1.8 0.3 0.2 0.1 1.9

17.6 2.0 1.0 0.2 1.1 0.9 19.3 1.9 0.5 0.6 82.5 1.7 0.2 0.2 0.0 0.1 0.1 2.6 0.2 0.1 0.1 4.3

20.0 2.0 1.3 0.2 1.2 1.1 18.2 3.1 0.7 0.9111.9 0.6 0. I 0.0 0.0 0.1 0.2 2.5 0.7 0.1 0.2 16.8

Change (YO) compared to time 0 145 143 143 67 44 82 112 380 125 300 156 165 143 186 67 48 100 105 620 175 450 212

Significance of the regression coefficient b(time) ** * *** - * - - *** * *** ***

*P <0.05; **P<O.OI; ***P< 0.001,

Table3. Bioelement amounts in the leaf litter layer and change (YO) compared to the initial amount, corrected for the calculated mean loss

Time C N,,,,, S,,,,, SO,-S Na K Mg Ca Fe Mn A1 Ash years (kg ha-')

0 1460 33.2 3.9 1.9 0.7 6.9 2.9 47.3 1.3 1.0 0.7 145 1 1180 40.5 4.6 2.4 0.4 2.5 2.0 44.3 4.3 1.2 1.3 190 2 960 39.3 3.9 2.4 0.4 2.4 2.1 35.6 6.1 1.3 1.7 219

Change (YO) compared to time 0 1 81 122 I I8 126 57 36 69 94 331 120 186 131 2 66 118 I00 126 57 35 72 75 469 130 243 152

Significance of the regression coefficient b(time) *** - - - - ** * * *** - ** **

*P<0.05; **P<O.OI; ***P<O.OOl.

Decomposition in the litter layer led to only a slight redistribution of the N fractions during the first year. Amino acid N fell from 60.3 to 56.4% of total N and amino sugar N increased from 0.8 to 4.3%. The sum of these fractions, i.e. the sum of defined N-compounds, remained constant. No further changes of N fractions occurred in the second year. The 16.9% of non-hydrolysable N did not change significantly during 2 years of decomposition (Joergensen, 1987).

Sulphur. Like N, the S concentration increased by 50% over the initial level during the first year. In the second year the concentration remained constant (Table 2). Fifty to 60% of total S was sulphate. If all sulphate was bonded to Ca, 5% of Ca would have been CaSO, initially, 6% after 1

284 R. G . Joergensen & B. Meyer

year and 8% after 2 years. Most organic S is bonded in amino acids such as methionine and cysteine (Freney, 1967). There is no net loss of S, either as organic S or as sulphate during the first 2 years of litter decomposition. It is known from other experiments that S is retained during the initial period of decomposition (Staaf, 1980).

Sodium and Potassium. In plants and plant residues, these two elements are only electrostatically bonded to cell membranes; thus, they are readily leached when these membranes are destroyed. After autumnal leaf fall was complete a t the end of November, beech leaf litter contained only 2.5 mg K g- ' (Table 2), approximately half of the content measured at the end of October (Meiwes & Beese, 1988). In the second year, Na and K concentrations increased again, because cation cxchange capacity (CEC) increased owing to accumulation of silicate particles. Thus, the amounts remained constant at the level of the first year (Table 3).

Magnesium and Calcium. In plant tissues Mg is present in chlorophyll and enzymes, but also to a great extent in salts. Ca is mainly structurally bonded and generally less soluble than Mg. In beech leaf litter the release rate Mg and Ca followed the rate of C loss in both years with a considerable delay. The concentrations of Mg and Ca remained constant (Table 2). The relative first-year loss was slightly greater for Mg than for Ca, though not significantly. Staaf (1980) found that after 2 years the relative loss for both elements was similar. In comparison with beech leaf litter from acid forests, the concentration of calcium is doubled (Staaf, 1980; Ulrich et al., 1986). The concentration of all the other elements is in the range ofpublished data (Nihlgard & Lindgren, 1977; Meiwes & Beese, 1988).

Ash. In 2 years the ash concentration increased from 5.3 to 11.2% (Table 2). Under the decomposition conditions of the lysimeter, with an annual C loss rate of only 19%, this was a net accumulation of more than one-third from 145 to 219 kg ha-' (Table 3). In his litter bag experiment with exclusion of macrofauna, Joergensen (1987) observed a similar increase in the ash concen- tration from 4.6 to 7.5% in I year and also a net accumulation from 130 to 155 kg ha-&. Thus, the concentration increase is largely a result of ash input and only to a small extent a result of ash enrichment due to C loss. An estimate of silicate material was made by subtraction of the cation sum from the ash amount. The cation sum only increased slightly from 22.2 to 25.3mgg-'. Consequently, the increased ash concentration was mainly due to input of silicate material.

Iron, Manganese and Aluminium. The concentration (Table 2) and amount (Table 3) of these three elements increased remarkably. After 2 years, the amounts of Fe, Mn and Al had increased to 470, 130 and 240%, respectively, of the initial level. It seems reasonable to relate this increase to the ash input, especially the input of silicates and oxides. This increase was divided by the increase in Fe, Mn and Al. The ratios obtained were almost identical in both experimental years; the ratios were approximately 20, 200 and 100 corresponding to Fe, Mn and Al contents of 5 , 0.5 and 1 YO of the silicate/oxide input (Table 4). The correlation between ash or silicate/oxide input rates and those of

Table 4. Estimate of silicate material (SIM) and ratios of the annual additional amount of silicate material (ASIM) and iron (AFe), manganese (AMn), and aluminium (AAI)

Silicate material "Z cations (Ash-Z cations) ASIM AFe AMn AAl

Time ASIM/ ASIM/ ASIM/ (years) (mg g ' dry material) AFe AMn AAl

0 22.2 30.7 ( 1-0) 27.5 1.38 0.14 0.34 20 I97 81 I 24.3 58.2 (2-1) 28.4 1.26 0.15 0.29 23 I89 98 2 25.3 86.6

"Z cations=Na+ K + Mg+Ca + Fe+ Mn + Al

Decomposition of beech leaf litter 285

the Fe, Mn and Al indicates that they originate from the same source, i.e. intercepted dust from the environment.

In the Solling, the average (1969-1985) Fe, Mn and Al concentrations in the throughfall were 0.190,0.611 and 0.326 mg dm-3, respectively (Matzner, 1988). Assuming the sameconcentrations in the Goettingen Forest, the following annual input rates could be calculated: 1.1 kg ha-' for Fe, 3.4 kg ha-' for Mn, and 1.8 kg ha-' for Al. Data for the Goettingen Forest from Meiwes & Beese (1988) showed that Mn input into soil was just 20% that of the Solling. Al and Fe were not measured by these authors.

Only 3 to 6%, or 15 to 30%, as estimated by Meiwes & Beese (1988), of the Mn input by throughfall, but 20 to 30% of the A1 input was retained by the litter layer. The estimated Fe input by throughfall could explain only 40 to 60% of the increased amounts of Fe in the litter layer. The remainder is thought to originate from silicate particles deposited on the litter layer. These dust particles may also be an important source of Mn and Al, assuming throughfall has little or no importance, because the residence time of the passing solution is too short for chemical immobiliz- ation reactions to take place. The necessity of balancing ash, together with Fe, Mn and Al was not foreseen at the start of the experiment. However it is an important finding of this experiment that input and transport processes of these elements need elucidation by further investigations.

Solution flux Water flux into a beech forest ecosystem from precipitation can be divided into throughfall (T) , stemflow ( S ) and intercepted water. Throughfall is the dominant pathway for element input into the litter layer. Input by stemflow is restricted to very small areas of the surface, and is, therefore, not considered in this paper.

The total element input to the canopy is the sum of several individual input processes. The main contributors are:

(i) wet deposition by rainfall, snowfall, fog etc.; (ii) dry deposition of particulate matter; (iii) interception of aerosols and gaseous substances like SO,, NH, and nitrous oxides in water

films covering plant surfaces. Stemflow and throughfall are loaded with the element input of these three processes, plus leached metabolites and salts from the vegetation, minus the amounts assimilated by the canopy or the canopy inhabiting organisms.

The flux rates of these input processes can be calculated using the following equations (Ulrich et d., 1986; Matzner, 1988). Total deposition (D) of an element x is interception ( I ) plus precipitation in the open ( P ) :

D, = I , + P ,

I, = Ig, + IP,

I, = S, + T, - P , - Ax

( 1 )

where interception is made up of gas and particulate contributions:

(2)

Some elements are leached from or assimilated into the canopy:

(3)

where A is the sink/source term, for elements that are leached or assimilated. Leaching, assimilation and gaseous dry deposition can be neglected for Na:

I N 1 = + - (4) If the interception of Na is known, the particle interception of H, K, Ca, Mg, Al, Fe, and S could be calculated:

'px = INa/PNd 'x ( 5 )

If gaseous deposition of NH, and HNO, can be neglected, Equation (5) can be used to estimate the interception of N.

286 R . G . Joergensen & B. Meyer

Water. Rainfall in the open was 635 mm in 1983 (Gerke, 1987). 471 mm (75.0%) fell as through- fall on the surface of the litter layer. This is slightly more than the 7 1.3% given by Matzner (1 988) for a beech forest in the Solling (1969-1985). 1983 loss by interception was 76mm (12%) of total rainfall, 88 mm (14%) was stemflow, both values were close t o the Solling data, which were 15.7% and 13.0%, respectively (Matzner, 1988; see also Ulrich et a/ . , 1984). In 1984, total rainfall and interception were not measured. Throughfall was 30% greater and lysimeter output 40% more in 1984 (Table 5). The average throughfall of 1983-84 (Table 6) was very close to the 6-year average from 1983 to 1988 (Table 7) which was 558 mm. Theestimated annual amount of rainfall in the open is 744 mm. In 1983, the loss by evaporation was 39 mm in lysimeter type I and 50 mm in lysimeter type I1 and in 1984,15 mm and 28 mm, respectively. In 1983, the difference between throughfall and lysimeter output was significant, but not in the wetter year 1984. Looking at the 2-year period 1983/ 84, the difference is again significant. Data from throughfall and lysimeters based on the weekly sampling period were rarely significantly different for the flux rates of water and bioelements. The mean coefficients of variation for this short period were between 14 and 61%, but sometimes exceeded 100% for all elements. Prolonging of the observation periods to 1 year or 2 years decreased the coefficient of variation and led to significant differences. A similar effect was observed by Seufert (1988).

Five per cent and 7% of the throughfall was lost by evaporation from lysimeter types I and I1 respectively. The differences between the two lysimeter types were not significant for either water or any of the other element fluxes measured. However, there was a tendency for the output rates of lysimeter type I1 to be slightly higher than those of type I. In most cases the results of the differ- ent lysimeter types were not compared with each other, but averaged for comparison with the through fall.

The upper part of the litter layer is more important for evaporation. After dry periods, water precipitation on litter will gradually increase litter moisture levels to field capacity from the top. Additional water will then percolate very rapidly through to the bottom of the litter and infiltrate into the soil unless the water capacity of the soil is exceeded. Additional rainfall can then enhance litter moisture to saturation (Waring et al., 1981). Water movement into litter and percolation may be restricted if the litter is initially very dry and hydrophobic conditions occur (Debano & Rice, 1973). Helvey & Patrick (1965) reported a mean water content for leaf litter of approximately 160% dry weight a t field capacity and 220% at saturation. Dunger (1958) measured field capacities for oak (130%), beech (160%) and ash leaves (230%).

Hydrogen. The input rates in throughfall in 1983 were 50% higher than in 1984 (Table 5). In the next 4 years the hydrogen input showed less variation and was at a lower level (Table 7). The hydrogen flux in the throughfall from 1981 to 1983 measured by Kues (1984) was higher, approxi- mately 0.24 kg ha-' a- ' , more than double the amount measured in this experiment. The hydrogen input from rainfall in the open was approximately 0.3 kg ha-' a - ' (Meiwes & Beese, 1988).

Approximately 60 to 75% of the hydrogen input by throughfall was lost in passing through the litter lysimeters (Table 6). A certain percentage was lost by reaction with carbonates and a further amount by reaction with NH,. The eastern side of the experimental area of the Goettingen Forest is close to arable land that is fertilized with manure. In warm, dry periods, such as the summer of 1988, a pH above 7 was sometimes measured in the throughfall. The same effect was observed in the throughfall of a forest in the Wingst area (near Cuxhaven) by Buettner et a/. (1986), who measured NH, input rates between 28.2 and 46.0 kg ha-' a- ' .

Sodium andchloride. For only these two elements were the throughfall and lysimeter output rates of 1983 and 1984 significantly different (Table 5). Marine aerosols are more important sources for CI and Na than for the other elements. The molar CI/Na ratio in seawater is 1 .O. In the area of the Goettingen Forest, a molar CI/Na ratio of 1.2 was measured in the rainfall in the open, which increased after canopy passage to 1.6 (Meiwes & Beese, 1988). This relative enrichment in the throughfall could be caused by interception of CI, or HCl from combustion sources or by under- estimation of leaching. The ratios for the Solling are 1.4 and 1.6, respectively (Ulrich et al., 1986). In comparison to the 6-year average (Table 7), the amounts of Na were higher and the amounts of CI

Tab

le 5

. Ann

ual f

lux

rate

s of w

ater

and

bio

elem

ents

in th

roug

hfal

l and

lysi

met

ers

H*O

H

N

,,,,,

N,,

NH

,-N

NO

,-N

SO,-S

(mm

) (k

g ha

-')

1983

Th

roug

h fal

l Ly

sim

eter

I Ly

sim

eter

I1

1984

Th

roug

hfal

l Ly

sim

eter

I Ly

sim

eter

I1

Thro

ugh f

all

Lysi

met

er I

Lysi

met

er I1

47 1

432

42 1

620

605

592 **

**

**

0.12

5 0.

038

0.02

4

0.08

8 0.

043

0.03

5

16.6

4.

1 5.

4 7.

1 17

.0

14.9

4.

4 3.

9 6.

6 22

.5

16.8

5.

2 3.

7 8.

0 25

.6

19.1

4.

7 5.

1 8.

7 22

.8

18.3

6.

3 4.

2 7.

7 25

.7

19.4

6.

3 3.

6 9.

5 26

.5

Sign

ifica

nce o

f diff

eren

ces b

etw

een

1983

and

1984

-

-

-

-

-

* * -

-

-

**

**

-

-

-

Y .f; 3

21.4

8.

5 26

.7

4.8

36.5

3

%

5.4

38.3

s$ %

**

**

**

-

-

5

19.6

8.

5 15

.9

2.7

17.9

20

.2

8.5

26.2

4.

6 32

.1

8. h 0-

rp

26.5

11

.4

15.2

3.

3 26

.7

27.7

11

.6

18.7

5.

1 33

.0

rp

27.3

11

.4

20.3

2

2

h

-

-

-

**

**

-

-

**

**

**

*P

i 0.0

5; **P

< 0.

01.

h)

00

00

Tabl

e 6. A

vera

ge a

nnua

l flu

x ra

tes o

f wat

er an

d bi

oele

men

ts in

thro

ughf

all a

nd ly

sim

eter

s in

the

year

s 19

83 an

d 19

84

H*O

H

N

,,,,,

No,

, N

HiN

N

O,-N

SO

,-S

C1

Na

K

Mg

Ca

(mm

) (k

g ha

-’)

Thro

ughf

all

‘CV

-A _+

Yo

CV

-RA

f Y”

CV

-RW

%

Lysi

met

er 1

C

V-A

7”

C

V-R

A _+

Yo

CV

-RW

k Yo

Lysi

met

er I1

C

V-A

& Yo

C

V-R

A &

Yo

CV

-RW

k %

1983

19

84

1983

+ 19

84

546 19 2 6

519 24 2 10

507 24 1 11

* -

*

0.10

7 17

.9

4.4

5.5

7.9

19.9

23

.0

25

10

11

4 14

21

21

8

2 8

2 1

6 3

42

16

29

34

18

26

14

0.04

1 16

.6

5.4

4.0

7.2

24.1

23

.9

9 14

26

6

19

10

22

26

3 2

9 4

19

2 45

22

31

50

30

34

18

0.03

0 18

.2

5.9

3.6

8.7

26.5

24

.3

26

10

14

2 12

5

17

36

5 8

9 6

7 6

43

21

37

61

26

27

20

Sign

ifica

nce o

f diff

eren

ces b

etw

een

thro

ughf

all (

n =

2) a

nd ly

sim

eter

s (n =

4)

-

-

-

**

-

-

**

**

-

**

**

**

-

**

**

-

-

-

-

* *

10.0

15

.5

21

3 10

5

19

31

10.1

22

.5

22

24

2 2

21

17

10.0

23

.5

23

18

1 11

21

24

-

** *

-

-

**

3.0

15

11

19 4.9

8 2 17 5.1

8 5 21 **

**

**

22.3

28

6 B

19

9

3 32

.9

1 17

1 16

37.4

R.

3 B

22

e 2 rp 5

12

;s rp 3

* -

**

‘Abb

revi

atio

ns u

sed

in th

is T

able

and

in T

able

7: C

V-A

=po

oled

coe

ffici

ent o

f var

iatio

n be

twee

n ye

ars,

CV

-RA

=poo

led

coef

ficie

nt o

f var

iatio

n of

the

repl

icat

es o

n an

an

nual

bas

is, C

V-R

W =

pool

ed c

oeffi

cien

t of v

aria

tion

of th

e re

plic

ates

on

a w

eekl

y ba

sis;

*P<

O.O

5; *

*P<O

.Ol.

Tabl

e 7.

Ann

ual f

lux

rate

s ofw

ater

and

bio

elem

ents

in th

roug

hfal

l in

the

year

s 19

85 to

198

8, av

erag

e oft

he ye

ars

1983

to 1

988

b

H

N,,,,

Nor

p N

H,-N

N

O,-N

S0

4-S

c1 N

a K

Mg

Ca

0

rp 8

H2O

(mm)

(kg h

a-')

b

1985

I9

86

1987

I9

88

-. ..- 16

.8

E'

0.06

8 22

.6

5.2

8.2

9.2

24.6

31

.8

6.4

20.2

3.

1 26

.7

%

483

0.08

0 11

.3

2.0

4.4

5.0

23.1

30

.6

6.9

13.5

2.

3 54

3 0.

070

21.6

6.

0 6.

6 9.

0 17

.9

32.9

9.

4 17

.8

2.6

19.2

3

697

533

0.07

4 21

.8

5.9

8.5

1.4

17.5

25

.0

5.6

13.4

2.

4 19

.0

(D

cp

Q- s 6 2

Ave

rage

198

3-88

55

8 0.

084

18.8

4.

7 6.

4 7.

8 20

.5

27.7

8.

0 16

.0

2.7

20.5

"C

V-A

& Yo

16

25

23

32

25

21

16

18

C

V-R

A

Yo

2 10

3

9 5

3 II

6

4 12

10

11

27

17

14

21

%

5 ..\

%ee

foot

note

to T

able

6

290 R. G. Joergensen & B. Meyer

lower in the two experimental years. The mean molar Cl/Na ratio of 1983/84 was 1.5 (Table 6), much lower than the 6-year average of 2.2.

After passage through the litter layer, no further enrichment was measured in the output of the lysimeters. Na was the only cation where equivalent amounts could be measured from input/output rates and mass changes of the decomposing leaf litter (Table 8).

Table 8. Comparison of mass balance in leaf litter (AL) and solution flux balance ( A W)

Element

Litter layer Solution flux Initial AL o u t AW A L - A W

(kg ha-’)

N LysimeterI

S Lysimeter I

Na Lysimeter I

K Lysimeter I

Mg Lysimeter I

Ca Lysimeter I

I1

I1

I1

I1

I1

11

31 72 4 8 0.8 I .2 7

10 3 5

50 92

+ 7 + 6 + I

0 0 0

-4 - 5 - 1 -1 -3 - 12

17 18 24 27 10 10 23 24

5 5

33 37

+ I 0

-4 -7

0 0

-7 -8 -2 -2

-11 - 15

+ 6 +6 f 5 + 7

0 0

+ 3 + 3 + I + I +8 + 3

Potassium, magnesium, calcium and sulphur. The throughfall input rates were not significantly different between 1983 and 1984 for these four elements (Table 5). Neither were significant differ- ences measured in the output of the lysimeters, with the exception of K, when a significantly higher output was observed in 1983. Dust from the immediate environment and combustion processes are more important input sources than marine aerosols. Again, the average of the two experimental years (Table 6) corresponded quite well with the average of the 6-year period (Table 7).

The output rates of the lysimeters were significantly higher than the input rates from throughfall. The additional amounts could not have been released from decomposing leaf litter. The sources, therefore, must be particle deposition on the canopy, transferred, but not dissolved in the throughfall, held in the leaf litter layer and subsequently dissolved there.

The significantly higher K output in the leachates in 1983 was especially due to higher flux rates in the three winter months January, February and March. This suggests deposition of dust from arable land that is located 500 m to the east. During dry periods, the wind usually comes from the east.

The increased output of magnesium is the same in both years. A strong correlation is known between deposition of Ca and Mg (Ulrich et al., 1986). Carbonates are likely to be an important source of both elements, especially because the surrounding soils contain relatively large amounts of carbonate. Dust from the paths to the experimental area may be another source, especially in dry years. The difference between lysimeter leachates and throughfall were less significant for Ca, especially in 1984, owing to higher variability.

Looking at a balance period of 1 year, the sulphate output rates of the lysimeters were not significantly different from the input rates from throughfall. Only the 2-year period produced a significant difference. However, there was a pronounced tendency for the litter lysimeter leachates to contain more sulphatt: than the throughfall. Input of sulphate in rainfall in the open during the winter (October to April) was equivalent to the summer period, but throughfall and stemflow contained much higher amounts during the winter, when dry deposition of sulphate from

Decomposition of beech leaf litter 29 1

combustion sources was much higher (Meiwes & Beese, 1988). The leachates of the litter lysimeters also contained the highest additional amounts of sulphate during this period (Joergensen, 1987).

Dry deposition of SO, direct to the litter layer could cause some of the additional amounts in the lysimeter outputs, especially in the winter period when the canopy leaves are missing. Measurements of flux rates considering this effect were not made. The average SO, concentration in the experimen- tal area of the Goettingen Forest is not known, but an average concentration of approximately 0.03 mg rn-, was measured in a forest near Stuttgart (Seufert, 1988). In aerosols of the Solling, 30 to 35% lower concentrations of Mn, Fe, NH,, NO,, SO, and CI were measured below the canopy than above (Hoefken, 1983). In addition, deposition velocity decreased from 20 m s-l to approximately 0.002 m s - ' and 0.006 m s-I (Fowler, 1980). Assuming a concentration of 0.02 mg SO, m-3 under the canopy and a deposition velocity of 0.002 m s-', the SO,-S input into the lysimeters would be 6.3 kg ha-' a - *. The estimated amount is approximately equal to the SO,-S excess measured in the lysimeter output, i.e. dry deposition of sulphate would be under-estimated using the normal method of calculating deposition (Equation 3). The SO, under the canopy could be reacting with CaCO, dust to produce CaSO,. However, it is difficult to understand how this SO, input affected the rates of lysimeter output and not the rates of the throughfall input. SO, must react in the atmosphere below the canopy with CaCO, on dust particles that are retained by the filtration of the throughfall before analysis.

Nitrogen. The lysimeter output rates of 1984 were significantly different from 1983 for organic N and total N, but not for ammonium and nitrate. There were no significant differences for throughfall (Table 5) . The rates of N input by throughfall were close to the 6-year average for all three N forms analysed (Table 6). The rates of NH, input were rather low, especially in the years 1983 to 1985, in comparison with the data of the Solling (Ulrich et al., 1986).

The input rate of 18 kg ha-' a- ' total N in the throughfall was equal to the output rate of the lysimeters. The N input by litter into the soil was approximately 90 kg ha-' ax'. The flux rate reached 20% of the transfer rate by litter fall.

The amount of nitrate did not change in passing through the litter layer. Nitrate is rejected as a N source by microorganisms in the presence of NH,-N or organic N. Denitrification and nitrate ammonification would not be important owing to the aerobic conditions in the small litter layer. The importance of anion exchange effects on nitrate movement in the litter layer is unknown.

The percentage of NH, in leachates decreased significantly in both years, indicating the possi- bility of microbial utilization. The percentage of organic N rose significantly in 1984. It is not known to what extent N in leachates is residual throughfall N which has not been taken up. Incoming ammonium may exchange onto sites or organic N may be adsorbed and subsequently used by microorganisms. After mineralization of litter or microbial biomass, it may reappear in the lysimeter leachates. However, litter-decomposing microorganisms only slightly change the N com- position of the throughfall, in contrast to the organisms of the underlying mineral soil, in which almost all dissolved N is converted to nitrate (Joergensen, 1987; Meiwes & Beese, 1988).

The necessary time of access for assimilation of dissolved N into biomass seemed to be too short. Only in dry periods when evaporation would occur was the duration of water contact long enough for microorganisms to utilize dissolved nutrients such as N. With a maximum evaporation rate of 40 mm a- ' and an average concentration of total N for the whole period of 3.3 mg drn-,, the throughfall can be calculated as providing 1.3 kg ha-' a-I. Considering that the concentration in dry periods is two to three times higher, a maximum of 4 kg ha-' a-l could be calculated. Thus, approximately 5 to a maximum of 20% dissolved N in the throughfall could be utilized by leaf litter-inhabiting organisms.

The net increase of 6 to 7 kg N ha-' a - ' in the litter (Tables 3 and 8) provided evidence that respiration or leaching of N-free substances, such as carbohydrates, is insufficient to explain the enhanced N concentration in the litter. Also, invading fungal mycelia were of little or no importance in the thin litter layer of a calcareous beech forest. No differences could be observed between the N concentrations of the litter in the lysimeters and in the litter bag experiment on the soil surface (Joergensen, 1987). The net increase must be a result of input processes that were not measured in this experiment. Estimates of the amounts involved in these processes are as follows.

292 R. G . Joergensen & B. Meyer

1. Aeolic sedimented Ah-horizon contains approximately 0.25% organic N. Thus, this process could supply 0.09 kg N ha-l a-I. A probable mechanism is adsorption of NH, onto the cation exchange sites of silicate or organic dust.

2. Asymbiotic bacterial N-fixation has been observed on all aerial parts of the forest (Jones & Bangs, 1985). Todd et al. (1978) estimated a N-fixation rate in oak-hickory litter of 0.63 kg ha-' a-I. The most effective species can assimilate 20 mg N per g metabolized sugar (Child, 1981). If litter supplied the same energy yield per g and 10% of the annual litter loss were used for N-fixation, 1.2 kg ha-' a-I could be assimilated.

3. Dry deposition of NH, and HNO, could be estimated by assuming an annual mean air concentration of 100 ng rn-, and a deposition velocity of 0.01 m s- ' for both gases. Thus, dry deposition would give 0.33 kg N ha- I a-I. The actual concentrations ofeither gas could be at least 10 times greater. Consequently, rates of 3 kg N ha-' a-I could be reached. Knowledge about concentrations of NH, and HNO, in the air is limited. In the usual approach explained in Equations (1) to (5) input of gaseous N is not considered; thus, the real input is underesti- mated. High interception rates of gaseous HNO, were measured in experiments with artificial surfaces in the Solling (Matzner, 1988). There is increasing interest in obtaining data for NH, concentration and input rates (Buettner et a/ . , 1986).

4. Faecal pellets of canopy-inhabiting phyllophageous insects could transfer N to the litter layer. In the Solling, the mean loss rate of green leaf tissue as a result of these organism was 5% (Funke, 1971). A similar result was observed by Nielsen (1978) in the Danish beech forest of Hestehave. If 5% of the canopy were consumed or cut by these organisms, assuming a N amount of 60 kg ha-' in the canopy, approximately 3 kg ha-' a - I N could be supplied to the litter layer.

All four input processes could have contributed to the net increase measured. The most important process seemed to be the adsorption of NH, onto cation exchange sites of silicate dust or organic particles that are retained by filtration of the throughfall before analysis. This was confirmed by the observation that many dark-coloured substances were retained by the filter. Some of this adsorption could occur in the atmosphere below the canopy. More efforts should be made to measure the estimated rates of asymbiotic bacterial N-fixation, gaseous interception of NH, and NO, and deposition of undissolved particles.

A C K N O W L E D G E M E N T S

We thank Karin Schmidt, Heike Nuhn, Angelika Reitz, Martha Bergschneider and Frauke Wehmann for help with the experiments and Dr J . Schauermann for S analysis of the litter samples. We would also like to thank the German Research Foundation (DFG) for its financial support.

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Decomposition of beech leaf litter 293

(Received 14 April 1988; accepted 31 December 1989)