Impact of land-use change on nitrogen mineralization in ... - CiteSeerX

Biol Fertil Soils (2000) 31 : 441–448

Q Springer-Verlag 2000

ORIGINAL PAPER

Valentina Zeller 7 Michael Bahn 7 Meinhard Aichner Ulrike Tappeiner

Impact of land-use change on nitrogen mineralization in subalpine grasslands in the Southern Alps

Received: 8 January 1999

Abstract A field study was conducted to investigate the effect of abandonment of management on net N mineralization (NNM) in subalpine meadows. NNM, soil microbial biomass N (SMBN), fungal biomass and physicochemical characteristics (total C, total N, dissolved organic carbon (DOC) and pH) were determined in surface (0–10 cm) soil from May to October 1997 in an intensively managed and an abandoned meadow at 1770 m a.s.l.. The cumulative NNM was lower in the abandoned area and ranged from 150 to 373 and from 25 to 85 mg N g –1 soil in the intensively managed and the abandoned areas, respectively. The total organic C increased in the abandoned area, while total N showed no difference between abandoned and managed meadow. SMBN showed no difference between the investigated sites, whereas ergosterol, a measure of fungal biomass, increased significantly at the abandoned site. The cumulative NNM was negatively correlated with total organic C, C : N ratio, DOC and ergosterol content, and positively correlated with the NHc 4 -N content of soil. The decrease in NNM at the abandoned site may be explained by the lower availability of N in substrates characterized by a high C : N ratio which, together with a decrease in pH in the litter layer, may have increased fungal biomass.

V. Zeller (Y) 7 U. Tappeiner European Academy of Bolzano, Domplatz 3, 39100 Bozen, Italy e-mail: vzeller6dnet.it Tel.: c39-0471-306031 Fax: c39-0471-306099 V. Zeller Biologisches Labor der Landesagentur für Umwelt und Arbeitsschutz, Unterbergstrasse 2, 39155 Leifers, Italy M. Bahn, U. Tappeiner Institute of Botany, Department of Ecology, University of Innsbruck, Sternwartestrasse 15, 6020 Innsbruck, Austria M. Aichner Research Centre for Agriculture and Forestry Laimburg, Pfatten, 39040 Auer, Italy

Key words Land-use change 7 Subalpine meadows 7 Net N mineralization 7 Soil microbial biomass N 7 Ergosterol

Introduction Traditionally managed meadows are an essential element in forming the cultural landscape of the Alps. Over the past few decades, land-use change has led to the abandonment of many traditionally managed subalpine meadows. As a result, plant species composition has changed, and an invasion of dwarf shrubs has occurred in previously managed grasslands (Cernusca et al. 1992; Tappeiner et al. 1999). These changes in plant species composition may be associated with changes in the availability of N for plants (Cernusca et al. 1999b). Besides N inputs by fertilizer applications, plant-available N depends mainly on the net N mineralization (NNM) activity of soils (Runge 1983). In this process the quality of substrates, as well as the size and community structure of soil microbial biomass, seems to play a key role in determining the NNM activity in soils (Olff et al. 1994; Scott and Binkley 1997; Burket and Dick 1998). Thus, changes in management practice, which affect substrate quality and soil microbial biomass, are assumed to significantly alter the NNM activity of soils. NNM may be altered with abandonment of management due to changes in litter quality caused by a shift in plant species composition. In the abandoned area the presence of dwarf shrubs, containing large amounts of lignin and polyphenols, and reduced leaf N concentrations (Bahn et al. 1994, 1999b) may cause a decrease in litter quality. Changes in litter quality lead to changes in soil organic matter composition, which is linked to N mineralization (Hassink 1994). Rasmussen et al. (1998) reported that N mineralization increased with total soil N content of soil organic matter. However, GonzalezPrieto et al. (1992) found that N mineralization was not necessarily linked with total soil N, as only specific frac-

442

tions of soil organic matter are involved (Hassink 1995; Monaghan and Barraclough 1997). Abandonment of management has been found to result in a decrease in the pH of soil (Bitterlich et al. 1999). The pH regulates the activity and composition of soil microflora (Blagodatskaya and Anderson 1998) and, consequently, of NNM. Curtin et al. (1998) showed that N mineralization markedly increased when the pH of acidic soils was increased. Despite this, N mineralization is often reported to be relatively insensitive to changes in pH, occurring over a wide range (Paul and Clark 1996). Several authors have shown that organic fertilizer application, as well as continuous defoliation, a feature common to managed grasslands, increases microbial biomass in grasslands (Hopkins and Shiel 1996; Mawdsley and Bardgett 1997). In contrast, abandonment of agricultural management of grasslands may reduce soil microbial biomass (Bardgett et al. 1997; Garcia et al. 1997). Furthermore, reductions in the intensity of management have been found to cause shifts in the species composition of soil microbial biomass (Bardgett et al. 1996). Brunner (1987) reported an increase in fungal species diversity and Stahl et al. (1999) found an increase in fungal biomass in abandoned fields. Soil microbial biomass itself is an important pool of readily mineralizable organic N in soils (Bonde et al. 1988) and is therefore a critical component for assessing potentially mineralizable N (Burket and Dick 1998). Temporal variations in NNM may be linked to changes in soil microbial biomass N (SMBN) since microbial N released after death is readily mineralized by the surviving microorganisms (Lethbridge and Davidson 1983). It is reasonable to believe that N mineralization is also related to the species composition of the soil microbial community: Fungi generally have higher C : N ratios ranging from 15 : 1 to 4.5 : 1, compared with bacteria, which range from 3 : 1 to 5 : 1 (Paul and Clark 1996), and are therefore suggested to contain less potentially mineralizable N. Fungal hyphae have been reported to be decomposed slowly in soils, since they contain substances (e.g. chitin) of low decomposability in their tissues (Stahl et al. 1999). Furthermore, N mineralization may be affected by species composition of soil microorganisms, as species differ in their ability to degrade various organic compounds (Swift et al. 1979; Paul and Clark 1996). However, it has been shown that changes in soil environmental conditions, e.g. changes in substrate quality followed by abandonment of management, lead to the increase of the species, which are adapted to utilize the changed substrate (Schinner and Gstraunthaler 1981). Several studies, especially in lowland sites, have shown that NNM of grasslands is affected by different intensity of management (Olff et al. 1994; Zeller et al. 1997a). However, little is known about NNM and its relation to the soil microbial community and physicochemical properties in abandoned meadows of the Alps. In view of this, the present study was designed for

the ECOMONT project (Cernusca et al. 1999a,b) to determine NNM in field conditions in abandoned areas with respect to NNM in traditionally managed meadows. Moreover, NNM measurements were related to soil characteristics (total C, total N, C : N ratio, DOC and pH), as well as to SMBN and fungal biomass, in order to improve our understanding of the effects of abandonment of management on NNM in mountain grasslands.

Materials and methods Study sites The field study was conducted at the ECOMONT-Project area, Passeier Valley in northern Italy (467 49–50b N, 117 15–19b E), at two locations with contrasting management (intensively managed meadow, abandoned area). The experimental areas are situated 1770 m above sea level and have an average annual rainfall of 1041 mm and an average annual temperature of 3.5 7C (Tappeiner et al. 1999). The intensively managed hay meadow is cut each year between the end of July and the beginning of August and fertilized with cattle dung in spring or autumn every year (Tappeiner et al. 1999) (Table 1). The term, intensively managed, is based on the highest intensity of management possible in subalpine meadows of the Alps and is not comparable to intensively managed meadows of lowland sites. Yields and the C : N ratio of the dung applied in spring 1996 are shown in Table 1. The sward is dominated by grasses and has been classified as Festuco–Agrostietum (Tasser et al. 1998). The characteristic species are Festuca rubra, Phleum alpinum, Trifolium repens, Ranunculus acris, Rumex acetosa, Taraxacum officinalis, Deschampsia cespitosa, Leontodon hispidus, Rhinanthus aristatus, Crepis conyzifolia, Alchemilla vulgaris and Trifolium pratense (Tasser et al. 1998). Management of the abandoned area ceased about 10 years ago and since then it has received no fertilizer nor has it been cut. The predominant species of the area are dwarf shrubs of the species Vaccinium myrtillus, V. vitis-idaea, V. uliginosum, Calluna vulgaris and Arctostaphylos uva-ursi (Tasser et al. 1998). Other important species of this vegetation type (Caricetum sempervirentis with dwarf shrubs) are Carex sempervirens, Avenella flexuosa and Juncus trifidus (Tasser et al. 1998). The dry matter contents and nutrient concentrations of aboveground phytomass, as well as the dry matter contents of belowground phytomass, are shown in Table 1. Compared with the intensively managed meadow, the abandoned area had higher C and lower N concentrations in the above-ground phytomass. Intensively managed meadows contain only small quantities of undecomposed litter over the mineral soil, whereas the abandoned area was characterized by a large amount of persistent, partly undecomposed litter (Bitterlich et al. 1999). As shown in Table 1, the amounts of total C increased, and total N and litter pH decreased with abandonment of management. Soil in both the intensively managed meadow and the abandoned area is classified as Cambisol (FAO classification system) (Tappeiner et al. 1999). The 0–40 cm soil layer of the intensively managed meadow had the following characteristics (Bitterlich et al. 1999): bulk density 0.6 g cm –3, sand 38%, silt 41%, clay 21%. The characteristics of the soil of the abandoned area were: bulk density 0.6 g cm –3, sand 43%, silt 39%, clay 18%.

Experimental design The experiment consisted of 12 replicate plots for each type of land-use (intensively managed meadow and abandoned area) arranged as 1!1.2 m plots in four rows and three columns in an

443 Table 1 Management practice(fertilizer application, removal of above-ground phytomass) and characteristics of the aboveground and below-ground phytomass and litter layer

Management Cattle dung (g dry matter m –2 year –1) a Cattle dung (g C kg –1) a Cattle dung (g N kg –1) a Cattle dung C : N a Yield 1997 (g dry matter m –2 year –1) b Yield 1997 (g N m –2 year –1) b Above-ground phytomass (g dry matter m –2) c Herbs (g dry matter m –2) c Grasses (g dry matter m –2) c Dwarf shrubs (g dry matter m –2) c Dead plant material (g dry matter m –2) c Ash content (g kg –1) b Acid detergent fibre (g kg –1) b Total N (g N kg –1) b C : N ratio b,d Below-ground phytomass (g dry matter m –2) c Litter layer Total C (g C kg –1) a Total N (g N kg –1) a C : N ratio a pH a

Abandoned area

18 June, 23 July, 27 August and 16 October. For ammonium and nitrate measurements, soil was sieved (~5 mm) and stored for a maximum of 4 days at 4 7C. SMBN, dissolved organic C (DOC), ergosterol, total soil C, total soil N and pH were determined for five replicate plots for each land-use by dividing soil samples taken near the tubes into subsamples. SMBN and DOC were measured on samples taken in June, July, August and October. Ergosterol was measured on samples taken in June and October. Subsamples were sieved (~2 mm) and stored at –20 7C until analysis. Total soil C, total soil N and pH were measured on air-dried samples taken in June. All analyses were made on duplicate soil subsamples.

Intensively managed meadow Mean B SD

Mean B SD

146.7B105.4

Absent

426.8B16.7

Absent

24.4B1.7

Absent

Methods

17.5B1.2 558B185

Absent Absent

NNM and mineral-N

12.3B2.9

Absent

787B386

1090B482

187B85

94B46

397B110

117B40

Absent

518B305

203B41

360B91

75.4B12.5

51.1B4.4

297.8B14.4

237.0B5.2

22.6B2.8

15.0B0.7

41.4B5.6

63.2B3.3

601B217

914B318

350.9B16.4 23.3B1.1 15.1B0.9 3.8B0.2

442.0B30.9 15.7B2.2 28.6B3.9 3.4B0.1

NNM was determined by the resin core method (Zeller et al. 1997b), which is a modification of procedure proposed by Raison et al. (1987) and Hübner et al. (1991). The methodology is based on sequential soil coring and in situ exposure of largely undisturbed soil columns confined within polyethylene cores (diameter 8 cm, length 15 cm) and an ion exchange resin layer at the bottom (10 g of DOWEX 1!8, 20–50 mesh, Cl – form; 10 g of DOWEX 50 W, 20–50 mesh, Na c form and 10 g of glass spheres). Within the tubes, uptake of mineralized N by plants is prevented and the anion- and cation-exchange resin-bags at the bottom of the tube prevent leaching. Exposed resin bags were washed with deionized water and – dried at room temperature prior to elution of NHc 4 and NO3 . The – elution of NHc 4 and NO3 from resin bags followed a batch procedure: The resin bags were mixed with 1 M NaCl solution (resin : NaCl 1 : 5 w/v) and shaken at 200 rpm for 1 h. This procedure was repeated with fresh NaCl solution until elution was complete. – NHc 4 and NO3 were extracted from the soil by shaking 50 g of moist soil for 1 h with 200 ml 0.0125 M CaCl2 solution. This weak salt solution is commonly used to extract the plant-available mineral N (Schinner et al. 1996). However, the 0.0125 M CaCl2 solution is less effective in extracting NHc 4 compared to other commonly used solutions of higher salt concentration (e.g. 2 M KCl, 0.5 M K2SO4) (Schinner et al. 1996). The NH4–N and NO3–N concentrations of soil extracts and the resins were analysed using an autoanalyser (Braun & Lübbe GmbH, Noerderstett, Germany). The soil moisture was measured gravimetrically (Schinner et al. 1996). NNM and cumulative NNM were calculated as described in Eqs. 1 and 2, respectively:

a

(S. Gamper, E. Tasser and U. Tappeiner., unpublished data) (A. Kasal, A. Cassar and E. Dallagiacoma, unpublished data) c (I. Kaufmann, M. Bahn, U. Tappeiner and A. Cernusca, unpublished data) d C measured as loss on ignition at 550 7C b

area of 6!4 m (Bahn et al. 1999a). Each replicate plot was divided into four subplots of 0.5!0.5 m, which were subsequently used for soil sampling and NNM measurements. For determination of NNM, two polyethylene tubes (Ø 8 cm) with resin bags at the bottom were inserted (0–10 cm depth) into the subplots 1, 2, 3 and 4 at each land-use replicate plot on 16 May, 18 June, 23 July and 27 August, respectively. Soil cores within polyethylene tubes and ion exchange resin bags were collected after field exposition for 4–6 weeks, on 18 June, 23 July, 27 August and 16 October, respectively. Soil cores and resin bags of individual land-use replicate subplots were bulked for NO3–-N and NHc 4 -N measurements. Soil mineral-N was determined in undisturbed soil next to the polyethylene tubes exposed for NNM measurement. Bulked soil samples were taken at a depth of 0–10 cm by taking ten randomly located soil cores (Ø 3.5 cm) at each land-use replicate plot. Sampling was performed on 15 May,

c P c NNMp(NO P 3 corecNH 4 corecNO 3 resincNH 4 resin) P c P(NO 3 begcNH 4 beg)

(1)

where (NO cNH ) denotes the mineral-N content at the c beginning of the field incubation, and (NO P 3 corecNH 4 core) and c (NO P 3 resincNH 4 resin) denote the accumulated N in the soil cores and in the resin bags at the end of the incubation period, respectively. Mineral N contents were expressed relative to dry matter of soil (mg N g –1 soil). P 3 beg

c 4 beg

cumNNNpNNMp1cNNMp2cNNMp3cNNMp4

(2)

where cumNNM denotes the cumulative NNM mineralized over four sequential measurement periods from 15 May to 16 October on each land-use replicate plot (np24), and NNMp1, NNMp2, NNMp3 and NNMp4 denote the N mineralized over periods 1 (15 May–18 June), 2 (18 June–23 July), 3 (23 July–27 August) and 4 (27 August–16 October), respectively. Soil microbial biomass N SMBN was determined as ninhydrin-reactive N after a fumigation-extraction method as described by Schinner et al. (1996). Soils were fumigated for 24 h with chloroform. Ninhydrin-reac-

444 tive N was extracted by shaking 10 g of fumigated and unfumigated soil with 50 ml of 2 M KCl solution for 30 min. Soil extracts were immediately filtered (filters Machery and Nagel, type MN619). The ninhydrin-reactive N was determined as reported by Amato and Ladd (1988). The ninhydrin-reactive N was multiplied by the factor of 3.1 to calculate SMBN (Amato and Ladd 1988). Fungal biomass As an indicator of fungal biomass, ergosterol was measured in ethanol extracts without saponification (Djajakirana et al. 1996). Ergosterol was extracted by overhead shaking of 5 g of moist soil with 50 ml of 96% ethanol for 30 min. Soil extracts were immediately filtered (filter type Whatman 540) to reduce re-adsorption by soil colloids and filtrates were evaporated to dryness at low daylight in a vacuum rotary evaporator at 40 7C. The residues were collected in 2 ml of ethanol and filtered again (Millipore Millex GV syringe filters, 0.22 mm). Quantitative determination of ergosterol was performed by HPLC [main column: LiChroCART 125-3, Purospher RP-18 endcapped, pre-column: LiChroCART 4-4, LiChrospher 100 RIP-18 (5 mm), mobile phase: 98% methanol / 2% water, flow rate 1.0 ml min –1] at a constant temperature of 30 7C using a UV/Vis wavelength detector (Milton Ray SM4000) at a wavelength of 282 nm. Dissolved organic C DOC was extracted by shaking 10 g of moist soil samples in prerinsed (10% HCl) plastic bottles with 100 ml high purity water (Milli-Q, water purification system, Millipore Corp.) for 2 h (180 rpm) (ÖNORM L1092 1993). Extracts were immediately centrifuged for 10 min (4000 rpm). The supernatant was filtered (Millipore, Millex HV syringe filters, 0.45 mm) and the DOC content of the filtrate was analysed by a TOC analyser (TOC-5000A; Shimadzu, Kyoto, Japan) in the following way: 5 ml of the extract was acidified with 100 ml of 10% HCl and purged with purified air (TOC gas generator; Whatman, Haverhill, Mass.) for 10 min to eliminate inorganic C, then DOC was oxidized to CO2 by catalytic combustion at 680 7C and quantified by non-dispersive infrared gas analyser (NDIR). Standard solutions were freshly prepared from 100 ppm of organic C as potassium hydrogen phthalate (Nacalai Tesque, Kyoto, Japan). Total soil C and N Total soil C and N were determined with an elemental analyser (CHNS 932; LECO Instruments, St. Joseph, Mich.). Soil standard no. 502–308 was used as reference material.

Results NNM and soil mineral-N The cumulative NNM of the four measurement periods from 15 May to 16 October was significantly lower in the abandoned area than in the intensively managed meadow (Fig. 1). The cumulative NNM at a depth of 0–10 cm ranged between 150 and 373 and between 25 and 85 mg N g –1 soil in the intensively managed and the abandoned area, respectively. Compared to the intensively managed meadow, NNM was significantly reduced in the abandoned area at each of the sampling dates (Fig. 2a). NNM in the abandoned area was correlated with the NNM in the managed meadow (rp0.357; P~0.05; np96) and in both the maximum NNM occurred in the period from 18 June to 23 July. With the exception of 23 July and 27 August, the amount of soil NHc 4 -N was significantly lower in the abandoned area than in the intensively managed meadow (Fig. 2b). The time course of soil NHc 4 -N concentration was very pronounced and showed a maximum on 23 July, which coincided with the maximum for NNM. NHc 4 -N of the intensively managed meadow and the abandoned area were significantly correlated (rp0.612, P~0.001, np96). Amounts of NO3–-N were very low in both of the experimental areas and not detectable on 18 June in the intensively managed meadow and on 23 July and 18 June in the abandoned area (Fig. 2b). Total C, total N, pH and DOC of soils The total C content of the surface soil (0–10 cm) was significantly higher in the abandoned than in the intensively managed meadow, while the total soil N did not show any significant difference between management treatments (Table 2). As a consequence, soil C:N ratio was significantly greater in the abandoned area than in the intensively managed meadow. The pH value of the

Statistical analysis Statistical analysis was performed with the software program SPSS for Windows (1994). Data are given as arithmetic means with standard deviations in tables and as arithmetic means with standard errors in figures. To test the effect of abandonment, significant differences between the means of the measured variables were calculated by t-tests. To detect significant variation over time, a one-way ANOVA was performed. Significant differences between the means were calculated by the Bonferroni test for each sampling date. If the data showed no variance homogeneity, the one-way ANOVA was replaced by the Kruskal-Wallis test and the Bonferroni test was replaced by the Mann-Whitney Utest. The significance level was Pp0.05 for all tests. Correlations between the variables were calculated by the Pearson’s correla– tion coefficient at Pp0.05. For correlation of soil NHc 4 , NO3 , DOC, SMBN and ergosterol with cumulative NNM means of values at different sampling dates were calculated for each land-use replicate plot.

Fig. 1 Cumulative net N mineralization (NNM) (15 May–16 October 1997) in the intensively managed subalpine meadow and in the abandoned area

445 Fig. 2 NNM of the surface (0–10 cm) soil of the abandoned and intensively managed meadows throughout four measurement periods (a). – NHc 4 -N and NO3 –N content of surface (0–10 cm) soil in the abandoned and intensively managed meadows (b). Significant differences between the experimental areas are indicated by asterisks (*) Table 2 Total C, total N, pH and dissolved organic C (DOC) of surface (0–10 cm) soil in the abandoned and intensively managed meadows. Significant differences between the experimental areas are indicated by asterisks (*)

Total C (%) Total N (%) C : N ratio of soil pH (0.01M CaCl2) DOC

Abandoned area

Date

Intensively managed meadow Mean B SD

Mean B SD

18 18 18 18 18 23 27 16

9.45B0.6 0.65B0.3 14.5 B1.0 4.5 B0.1 170.8 B61.3 203.8 B20.6 225.0 B57.1 202.4 B42.9

11.36B0.79 ** 0.66B0.02 17.2 B0.3*** 4.3 B0.1 * 222.8 B62.7 246.3 B54.6 271.6 B64.4 276.3 B71.0

Jun Jun Jun Jun Jun Jul Aug Oct

*P~0.05, **P~0.01, ***P~0.001

surface soil (0–10 cm) showed a slight, but significant, decrease with abandonment. The DOC content ranged from 95 to 257 and from 143 to 372 mg g –1 soil in the managed meadow and abandoned area, representing 0.21% and 0.22% of the total C, respectively. The differences between DOC contents in the two experimental areas were not statistically significant at each of the sampling dates (Table 2). DOC showed no significant variation over time in both of the experimental areas. SMBN and ergosterol SMBN ranged from 103 to 236 and from 125 to 217 mg N g –1 soil in the managed and the abandoned area, representing 2.6% and 2.3% of the total N, respectively. Differences between SMBN in the managed meadow and the abandoned area were not statistically significant (Fig. 3a). With the exception of October, when a significant increase in SMBN occurred in the intensively managed meadow, the temporal variation of SMBN was not statistically significant. Fig. 3 Soil microbial biomass N (SMBN) of surface (0–10 cm) soil in the abandoned and intensively managed meadows (a). Ergosterol concentration in the surface (0–10 cm) soil in the abandoned and intensively managed meadows (b). Significant (P~0.001) differences between the experimental areas are indicated by asterisks (*)

Ergosterol contents ranged from 0.2 to 10.6 and from 5.0 to 15.8 mg g –1 soil in the intensively managed and the abandoned area, respectively. Thus, in contrast to SMBN, the fungal component ergosterol was found to be positively affected by abandonment (Fig. 3b). In both experimental areas, the ergosterol content increased significantly from June to October. Correlation between variables NNM was significantly correlated with NHc 4 -N (rp0.528; P~0.001, np96), DOC (rp–0.437; P~0.05, np40) and ergosterol contents (rp–0.594; P~0.05, np20). When calculated separately for the abandoned area and the managed meadow, correlations were significant only between NNM and NHc 4 -N (abandoned area, rp0.963; P~0.000, np48; managed meadow, rp0.683; P~0.001, np48). In contrast, no relationship was found between NNM and soil nitrate or microbial biomass N. The cumulative NNM was significantly correlated to mean soil NHc 4 -N, total soil C, soil C : N ratio and mean DOC (Table 3). The data provide evidence

446 Table 3 Correlation matrix (r values) for NNM and different soil and microbial characteristics. Pearson’s correlation coefficient, np10

a NHc 4 –N – NO3 –N a Ct Nt Ct:Nt DOC a pH SMBN a Ergosterol a

Cum. NNM

NHc 4 –N

NOP 3 –N

Ct

Nt

Ct : Nt

DOC a

pH

SMBN a

0.842** n.s. –0.754* n.s. –0.787** –0.658* n.s. n.s. –0.874**

n.s. –0.708* n.s. –0.732* n.s. 0.636* n.s. –0.732*

n.s. n.s. n.s. n.s. n.s. n.s. n.s.

n.s. 0.963*** 0.655* n.s. n.s. 0.860**

n.s. n.s. n.s. n.s. n.s.

0.728* n.s. n.s. 0.917***

n.s. n.s. n.s.

n.s. –0.675* n.s.

*P~0.05, **P~0.01, ***P~0.001, n.s. not significant a Means of measurements from different sampling dates for each land-use replicate plot

Fig. 4 Relationship between the cumulative NNM and the C : N ratio of the soil (a) or between the cumulative NNM and the ergosterol content of the soil (b)

that with increasing soil C : N ratio and reduction of pH the ergosterol content increased, whereas the cumulative NNM declined (Table 3). The relationships between the cumulative NNM and soil C : N and between the cumulative NNM and ergosterol content are shown in Fig. 4. The cumulative NNM was not related to mean SMBN pool (Table 3). Related to the SMBN pool, the NNM was significantly lower in the abandoned area compared to the intensively managed meadow. The ratio of cumulative NNM : SMBN was 1.5 and 0.3 in the intensively managed and the abandoned area, respectively.

Discussion Our data showed that, with the abandonment of subalpine meadows, the release of plant-available mineral-N decreased dramatically. Our findings of a reduced NNM in a dwarf shrub-dominated area agree with results reported by Van Vuuren and Berendse (1993), who showed lower NNM in dwarf shrub than in grassdominated heathland. Olff et al. (1994) found that the cessation of fertilizer application decreased NNM in grasslands with continuous removal of above-ground phytomass. However, this decrease in NNM was accompanied by a decrease in soil organic matter content, whereas abandonment resulted in an increase in soil organic matter at our investigated site.

Although the amount of NNM differed greatly in managed and abandoned grassland, the time course of NNM, as well as of soil NHc 4 -N, was consistent in the intensively managed meadow and the abandoned area. Our findings of a summer maximum in NNM are consistent with findings by Morecroft et al. (1994), who investigated the N mineralization in limestone and acidic grasslands in the UK. The consistent time course of NNM and NHc 4 -N may reflect the linkage between NNM and NHc 4 -N in soil, which represents the difference between NNM, plant uptake and N losses (Runge 1983). The very low amount of NO3–-N of soil in both areas indicate a low nitrification activity in these low pH soils. Our data suggest that the very low NNM in the abandoned area is related to changes in substrate quality. The C : N ratio was markedly higher in aboveground phytomass, litter and soil in the abandoned area compared to the managed area, this increase being most pronounced in phytomass. The increase in the C : N ratio in the abandoned area is attributed to the absence of fertilizer application and to changes in plant species composition, i.e. the presence of dwarf shrubs with a high proportion of C and lignin-rich material and low leaf N concentrations (Bahn et al. 1994, 1999b). With the mineralization of substrates containing low N concentrations, small quantities of mineralized N are released into soil compared with substrates of high C : N ratio, because relatively more N must be immobilized

447

to satisfy microbial requirements (Janssen 1996). The enhanced presence of polyphenolic structures may have additionally depressed NNM in abandoned meadows due to the inhibition of the decomposition process (Müller et al. 1988; Mafongoya et al. 1998). In contrast, the application of organic fertilizers in the intensively managed meadow may have contributed to higher NNM, since the mineral fraction of N contained in cattle dung stimulates NNM (Paul and Clark 1996). In addition, cattle dung with C : N ratios ranging between 19 and 21 (Zeller et al. 1997a) contain higher concentrations of N than litter from dwarf shrub-dominated grassland. Soil acidification has been shown to take place in abandoned grasslands at the soil surface (Bitterlich et al. 1999). However, our data suggest that pH may not be responsible for the decrease in NNM in the abandoned area, since differences between the pH in the surface (0–10 cm) soil from the abandoned and managed areas were not pronounced. Differences in NNM between the abandoned area and the intensively managed meadow were not related to SMBN, since no significant differences in SMBN between abandoned and managed grassland were found. The finding that SMBN did not significantly change with abandonment of management is in accordance with findings of Klein et al. (1998) in the successional series of a shortgrass steppe. In contrast to the abandonment of management, the removal of fertilizer application and grazing led to a significant reduction in SMBN in upland grasslands (Bardgett and Leemans 1995). Our findings of an increased fungal biomass with abandonment of management are consistent with those of Bardgett et al. (1993) and Stahl et al. (1999), who found an increase in the proportion of fungi relative to bacteria in grasslands after the cessation of agricultural management. The relatively higher ergosterol content in the abandoned area may reflect a shift in species composition of soil microorganisms caused by changes in soil environment (C : N ratio of soil, pH) resulting from the cessation of fertilizer inputs and changes in plant species composition. Schinner and Gstraunthaler (1981) reported that the diversity of fungal species is related to plant species composition in alpine soils and that the microbial community might adapt to changing environmental conditions by selection of certain species. We suggest that fungi are favoured in abandoned areas because they generally persist at lower pH better than bacteria (Swift et al. 1979) and have the ability to degrade lignin-rich litter efficiently. Moreover, the mycelial growth of the fungi allows the transportation of nutrients (e.g. N) over a distance into the nutrient-poor substrate (Hammel 1997). Fungal cytoplasm is only maintained if nutrients are available and with a decreased nutrient availability, hyphal extension occurs with reduction of the cytoplasm (Paustian and Schnürer 1987). In addition, the increased ergosterol contents may also be attributed to the occurrence of mycorrhiza,

which are known to be associated with dwarf shrubs in abandoned meadows (Haselwandter 1989). In conclusion, the decrease in NNM with the abandonment of management is linked to a reduced substrate quality in terms of a low C : N ratio in the abandoned area. Changes in NNM seem to be associated with an increase in the fungal component of soil microbial community, whereas the N content of soil microbial biomass was not related to the change in NNM. The influence of plant–microbe interaction on NNM in abandoned areas, as well as the interaction of changes in substrate quality with species composition of soil microbial community, should be addressed in further studies. Acknowledgements This study was part of the ECOMONT Project (ENV4-CT95-0179) and was funded by the EU (Framework 4, Environment and Climate, TERI). Determination of mineral-N was carried out by O. Andreaus at the Research Centre for Agriculture and Forestry Laimburg. We thank F. Michelini for giving permission to use laboratory facilities at the Biological Laboratory, G. Mich and G. Scalzer for assistance with the sampling procedure, D. Tait and J. Santer for their helpful suggestions on improving laboratory procedures and A. Siegwolf for correcting the English text. We gratefully acknowledge R.D. Bardgett and R. Kiem for their comments on the manuscript.

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