Directory UMM :Data Elmu:jurnal:S:Soil Biology And Chemistry:Vol32.Issue6.Jun2000:

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Biochemical properties of acid soils under climax vegetation

(Atlantic oakwood) in an area of the European temperate±humid

zone (Galicia, NW Spain): general parameters

M.C. LeiroÂs

a,

*, C. Trasar-Cepeda

b

, S. Seoane

a

, F. Gil-Sotres

a

Departamento de EdafologõÂa y QuõÂmica AgrõÂcola, Universidad de Santiago de Compostela, E-15706 Santiago de Compostela, Spain b

Departamento de BioquõÂmica del Suelo, Instituto de Investigaciones AgrobioloÂgicas de Galicia, Consejo Superior de Investigaciones CientõÂ®cas, Apartado 122, E-15080 Santiago de Compostela, Spain

Accepted 6 October 1999

Abstract

The concept of sustainable development suggests that soil quality should be measured on the basis of the most environmentally sensitive properties of native soils under climax vegetation. The most relevant properties are biochemical. We describe the general biochemical parameters of the O and Ah horizons of 40 native Umbrisols under climax Atlantic oakwood in Galicia (NW Spain). The properties studied were: microbial biomass C (O horizons 19352450 mg C kgÿ1, Ah horizons 7812

253 mg C kgÿ1), N ¯ush (68220 and 26213 mg kgÿ1), soil respiration (12.924.5 and 2.620.8mg CO2-C gÿ1hÿ1), ATP (8.91

23.20 and 2.7721.38mg gÿ1), dehydrogenase activity (5552205 and 207258 nmol INTF gÿ1hÿ1), catalase activity (3.921.1 and 2.020.9 mmol H2O2consumed gÿ1hÿ1), N mineralization capacity (113266 and 30213 mg N kgÿ110 dÿ1), and arginine

ammoni®cation rate (11.125.9 and 4.922.2mg N-NH4+gÿ1hÿ1). The values reported are generally within the ranges found in

the literature. The correlations between biochemical parameters and chemical variables show in these soils microbial population size and activity directly related to both organic matter and available nutrient contents.72000 Elsevier Science Ltd. All rights reserved.

Keywords:Soil biochemical properties; Soil microbial activity; Soil nitrogen mineralization; Temperate forest soils

1. Introduction

There is intense interest in the identi®cation of soil quality markers and the de®nition of soil quality indi-ces. As regards the latter, many approaches seek more or less complex combinations of physical, chemical and biochemical properties that jointly evaluate the three basic functions de®ning sustainable soil quality (Pankhurst et al., 1997), these qualities are production (the capacity to yield healthy, abundant crops), ®l-tration (the capacity of the soil to remove any

pollu-tant from waters that pass through it) and degradation (the capacity of the soil to function properly as part of a mature, self-sustaining ecosystem). It is recognized, however, that a simpler approach is to use only bio-chemical properties, which are the most sensitive to en-vironmental stress (Vanhala and Ahtiainen, 1994) and play the greatest role in degradation (Yakovchenko et al., 1996).

According to Visser and Parkinson (1992), the biochemical properties of the soil can be studied at three dierent levels: microbial populations, biotic communities, and the properties involved in organic matter and nutrient cycles. In spite of the ecological interest of the ®rst two levels, their immediate rel-evance to soil quality evaluation is doubtful; much more relevant is the characterization of the soil

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* Corresponding author. Tel.: +34-981-963-100, ext. 15042; fax: +34-981-594-912.

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properties involved in the transformation of organic matter. The biochemical properties corresponding to this third level can be divided in two groups (Nan-nipieri et al., 1995): general parameters, which include all the variables directly related to microbial activity (microbial biomass C and N, respiration, nitrogen mineralization capacity, etc.), and speci®c parameters, which include the activities of extra-cellular hydrolytic enzymes that are involved in the carbon, nitrogen, sulphur and phosphorus cycles and are to some extent independent of microbial population dynamics because of their stabilization by soil colloids (Burns, 1982).

The estimation of soil quality on the basis of bio-chemical properties alone is currently limited by the lack of studies considering the simultaneous variation of a wide range of biochemical properties. Further-more, comparisons among the results of dierent pub-lished studies are hindered by the wide disparity of analytical methods that have been used, especially as regards determination of enzyme activities (Dick, 1997). Although total standardization of analytical methods for biochemical properties may well be im-possible because the validity of a given method varies from one type of soil to another, the development of biochemical soil quality indices requires, at least, the availability of data obtained by the same methods for a signi®cant number of soils of similar characteristics.

The scarcity of comparable data of biochemical properties is particularly restrictive in the case of native soils: according to the philosophy of sustainable development, these soils should be used as standards for soil quality evaluation because they have developed freely to attain an equilibrium between their environ-ment and their physical, chemical and biological prop-erties (Doran et al., 1994). What is needed, if biochemical properties are to realize their potential as markers of soil quality, is the compilation of compre-hensive data bases recording the biochemical proper-ties of native soils in dierent regions of the world (Dick, 1997; Pankhurst et al., 1997).

Galicia (NW Spain) is a region of the European temperate±humid zone in which there still exist many areas under the climax vegetation, Atlantic oakwood. The soils of these woodlands are Umbrisols (ISSS Working Group RB, 1998), and their biochemical properties have hitherto been studied as little as those of the native soils of other regions. We have therefore collected data on a wide range of the biochemical properties of their organic layers and of the 7 cm of their Ah horizons. In this paper we summarize our results concerning general parameters; speci®c par-ameters, i.e. the hydrolytic enzymatic activities, are the subject of a companion paper (Trasar-Cepeda et al., 1999b).

2. Material and methods

2.1. Soils

We studied 40 soils developed under climax veg-etation dominated byQuercus robur L. orQ. pyrenaica

L. at sites distributed throughout Galicia, NW Spain (Fig. 1 and Table 1). All these soils are Umbrisols (ISSS Working Group RB, 1998); their general physi-cal and chemiphysi-cal properties are summarized in Table 2. At all sites, the sampling area displayed little disturb-ance of human origin, and its tree vegetation was com-posed mainly of healthy mature specimens of the above-mentioned species.

At each site, samples were taken at 10±15 points dis-tributed uniformly over an area of about 1 ha. After removal of the litter, samples were taken with a trowel from the O horizon (which was distinguished by its morphology and was between 1 and 10 cm deep, depending on the sampling point) and from the top 7 cm of the Ah horizon. The 10±15 samples for a given site and layer were pooled in the ®eld to obtain a com-posite sample representative of that layer at that site. All samples were collected shortly after the ®rst autumnal rains, a time of year at which the biochemi-cal properties of these soils are generally close to their annual means (Trasar-Cepeda et al., 1999a). The O and Ah pools were transported to the laboratory in isothermal bags and sieved through 4 mm meshes, and their moisture contents were determined on the day of collection prior to storage at ÿ208C pending further analyses. When withdrawn from the freezer they were thawed for 1 week at 48C and were maintained at this temperature until all analyses had been carried out (always within 15 d).

2.2. Analytical methods

2.2.1. Soil chemical and physical properties

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reten-tions at 33 and 1500 kPa (1/3 and 15 bar, respectively), which were measured with a Richards plate-and-mem-brane apparatus using undisturbed soil samples (Gui-tiaÂn and Carballas, 1976). Table 2 summarizes the results of these analyses.

2.2.2. Biochemical properties

2.2.2.1.C and N ¯ushes. C and N ¯ushes were

deter-mined by the fumigation±extraction method (Vance et al., 1987), fumigating for 24 h at 258C. The carbon contents of fumigated and unfumigated samples were determined by oxidation with potassium dichromate, and the dierence was converted to biomass C by

dividing by Kec0:45: Nitrogen ¯ush was calculated

as the dierence between ninhydrin-reactive N in fumi-gated and nonfumifumi-gated samples as determined by the colorimetric method of Joergensen and Brookes (1990). In each case, C and N were extracted in tripli-cate and determined in each extract in duplitripli-cate; results shown correspond to the means of all six deter-minations and are expressed in mg kgÿ1.

2.2.2.2.Soil respiration. Soil respiration was

deter-mined by static incubation (GuitiaÂn and Carballas, 1976); the CO2produced during a 10-d period by 25 g soil samples incubated at 258C with optimal moisture content (i.e. the water retained at 33 kPa) was

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lected in 10 mL of a 1 M NaOH solution, which was then titrated using thymol blue as indicator. Two samples of each soil were incubated, and the CO2trap of each was titrated in duplicate; results shown corre-spond to the mean of the four values, and to facilitate comparison with the literature are expressed as mg CO2-C produced gÿ1 hÿ1, i.e. as the average rate of CO2production during the whole 10-d incubation.

2.2.2.3.Nitrogen mineralization capacity. To determine

nitrogen mineralization capacity, duplicate 10 g soil samples were extracted for 30 min with 50 ml of 2 M KCl before and after incubation for 10 d at 258C with optimal moisture content (see Soil respiration), and

ammoniacal N and total inorganic N were determined in the extracts by Kjeldahl distillation (Bremner, 1965). Nitrogen mineralization capacity (mg kgÿ110 dÿ1) was calculated from the dierence between the values obtained before and after incubation. Results shown correspond to the mean of two values.

2.2.2.4.Arginine ammoni®cation rate. Arginine

ammo-ni®cation rate was determined by the method described by Alef and Kleiner (1986). After addition of 0.5 ml of a 2 g lÿ1arginine solution to two of four replicate 2 g soil samples, all four were incubated for 3 h at 308C and then extracted with 8 ml of 2 M KCl. The am-monium in each extract was determined

colorimetri-Table 1

Location and site characteristics of the soil samples studied Sample No. Longitude

(W)

Latitude (N)

Altitude (m.a.s.l.)

Slope (%)

Parent material Predominant tree species Depth of the O horizon (cm)

7 7819'150 42819'400 530 9 granites Q. pyrenaicaL. 2

8 7826'100 42821'300 780 15 granites Q. pyrenaicaL. 2

10 7818'030 42820'500 830 33 granodiorites Q. roburL. 1

12 7836'450 41858'580 810 3 granites Q. roburL. 2

13 7825'220 42806'310 690 11 migmatites Q. roburL. 5

14 7856'480 42810'350 370 18 schists Q. roburL. 2

15 7852'100 42839'380 670 7 granodiorites Q. roburL. 3 16 8830'400 42836'150 530 12 granodiorites Q. pyrenaicaL. 3

17 7812'350 42839'530 1080 50 slates Q. pyrenaicaL. 2

18 8804'200 42853'000 400 17 slates and quarzites Q. pyrenaicaL. 2 19 8819'300 43801'420 600 13 granodiorites Q. roburL. 5

22 7855'100 42852'450 460 5 granodiorites Q. roburL. 4

23 7843'510 43815'000 420 0 schists Q. roburL. 10

24 7808'180 43826'430 165 22 quarzites and slates Q. roburL. 7

27 8804'100 43825'040 15 50 orthogneisses Q. roburL. 3 28 8841'420 42853'320 170 13 granodiorites Q. roburL. 5 29 8851'000 42852'290 255 43 granodiorites Q. roburL. 7

33 8846'500 43810'040 230 23 metagabres Q. roburL. 3

34 8801'550 43821'200 380 13 schists Q. roburL. 5

35 8801'380 43824'080 290 45 slates and schists Q. roburL. 7 36 8827'050 43814'500 265 0 granodiorirtes Q. roburL. 5

37 8821'300 42837'000 665 6 schists Q. roburL. 5

39 8814'150 42822'350 370 30 granodiorites Q. roburL. 5 40 8809'070 42818'530 150 29 granodiorites Q. roburL. 5

46 8806'010 42838'500 650 10 schists Q. roburL. 5

47 8810'440 42851'150 335 20 schists Q. roburL. 3

48 8801'050 43802'550 550 10 paragneisses Q. roburL. 7

50 8807'330 43807'100 540 5 orthogneisses Q. roburL. 5

51 8822'490 42858'130 260 14 schists Q. roburL. 5

52 8839'110 43800'510 240 25 gneisses Q. roburL. 5

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cally by the phenol and sodium nitroprusside method of Dorich and Nelson (1983), and arginine ammoni®-cation rate (mg N-NH4+ gÿ1hÿ1) was calculated as the dierence between the mean value of the two arginine-treated samples and the mean value of the two untreated samples.

2.2.2.5.ATP.To two of four replicate 2 g soil samples

was added 20ml of a solution of 1 mg of pure ATP in 10 ml of bidistilled water. Each of the four samples was then extracted for 30 min with 10 ml of a cold sol-ution of 0.5 M trichloroacetic acid and 0.5 M Na2HPO4(Jenkinson and Oades, 1979), 100ml samples of the ®ltered extracts were added to 900ml of neutra-lizing buer (0.25 M Tris±HCl+4 mM EDTA, pH 7.5), and ATP was quanti®ed by the luciferine±lucifer-ase method, the light evolved in the 10 s following ad-dition of the enzyme system being determined from OPTOCOM1

luminometer measurements with the aid of a calibration line. The results for the spiked samples and the results for the unspiked samples were then each averaged. Finally, ATP losses due to sorption by the soil and degradation during extraction, which were assumed proportional to original ATP content, were corrected for using the constant of proportionality cal-culated from the results for the spiked and unspiked samples. Results are shown asmg ATP gÿ1.

2.2.2.6.Dehydrogenase activity.Dehydrogenase activity

was determined using a modi®cation of the method of von Mersi and Schinner (1991) described by CaminÄa et al. (1998). After addition of 1.5 ml of Tris±HCl buer and 2 ml of a 0.4% solution of INT (2-(p -iodophe-nyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride),

triplicate 1 g soil samples were incubated in the dark for 1 h in a shaking water bath at 408C, mixed thoroughly with 10 ml of 1:1 ethanol/dimethylforma-mide, and left at room temperature for 10 min before ®ltration. Iodonitrotetrazolium formation (INTF) was determined spectrophotometrically in the ®ltrate by measuring absorbence at 490 nm and reading the cor-responding INTF concentration from a calibration line constructed using samples treated as above except for the addition of various concentrations of INTF instead of INT. Results shown (nmol INTF gÿ1 hÿ1) corre-spond to the mean of the three values.

2.2.2.7.Catalase activity. Catalase activity (EC

1.11.1.6) was determined according to the method of Johnson and Temple (1964). Triplicate 0.5 g soil samples were suspended in a mixture of 40 ml of dis-tilled water and 5 ml of 0.3% H2O2. After 10 min stir-ring at room temperature, 5 ml of 3 M H2SO4 was added and residual H2O2 was determined by titration against 0.1 M potassium permanganate. Results shown (mmol H2O2 consumed g

ÿ1

hÿ1) correspond to the mean of the three values.

2.3. Expression and analysis of results

All values reported are expressed on an oven-dry soil basis (1058C). Statistical analysis were performed using Statistics 4.5 for Windows (StatSoft, Inc., 1993).

The values, means and standard deviations of the biochemical parameters measured in the O and Ah horizons studied are listed in Tables 3 and 4, respect-ively.

Table 2

Maxima, minima, means and standard deviations (S.D.) of the physical and chemical properties, in the O and Ah horizons of the studied soils O horizonsn40) Ah horizonsn40)

minimum maximum mean2S.D. minimum maximum mean2S.D.

pH KCl 2.58 4.66 3.3220.52 2.77 4.10 3.4920.32

Total C (%) 14.62 49.97 30.5328.56 5.20 18.34 10.75 123.10

Total N (%) 0.77 2.17 1.5420.35 0.32 0.99 0.6520.18

Available Ca2+_{(mg 100 g}ÿ1₎ ₃₆ ₃₅₂ ₁₂₄₂₇₀ ₆ ₁₀₃ ₂₄₂₂₀

Available K+_{(mg 100 g}ÿ1₎ ₁₇ ₆₁ ₃₆₂₉ ₆ ₂₄ ₁₂₂₁₄

Labile Pi (mg 100 gÿ1₎ ₁₉ ₁₂₄ ₆₃₂₂₅ ₈ ₄₃ ₂₀₂₉

Water retained at 1500 kPa (%) 37 151 84226 8 47 2924

Available water (%) 11 103 54218 11 132 39221

Al2O3(%) 0.23 1.77 0.7020.39 0.11 2.88 1.0920.72

Fe2O3(%) 0.11 1.94 0.5920.35 0.25 1.97 0.9320.51

Sand (%) ND ND ND 13 80 52214

Silt (%) ND ND ND 12 60 29211

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Table 3

Values of the general biochemical parameters studied, in O horizonsn40of climax soils under oakwood in Galicia (NW Spain) Sample %

Total C

% Total N

Microbial biomass Ca

N ¯ushb

CO2-C

evolvedc qCO2

d

ATPe Dehydrogenasef Catalaseg NH4

mineralizedh Total inorganic N mineralizedh

Arginine ammoni®cationi

7.1 31.2 1.57 1916 43.9 9.7 5.1 6.14 571 4.4 109 114 3.7

8.1 28.6 1.76 3286 134.6 1.1 3.5 14.05 1320 4.6 106 181 13.0

10.1 14.6 0.95 1840 66.2 8.0 4.4 9.06 657 7.1 22 52 14.5

12.1 21.2 1.19 1798 47.9 16.4 9.1 11.95 550 2.2 148 148 13.2

13.1 21.0 1.16 1924 53.6 12.1 6.3 9.64 649 4.7 90 92 10.2

14.1 18.3 0.97 2205 54.3 6.3 2.9 12.86 722 2.6 ÿ65 ÿ53 11.3

15.1 17.3 1.07 1271 40.4 8.7 6.9 9.38 674 4.4 46 56 12.8

16.1 27.8 1.51 2455 73.7 12.5 5.1 7.10 369 2.9 89 100 8.2

17.1 23.1 1.29 1986 71.7 14.1 7.1 13.58 923 6.6 38 78 14.2

18.1 17.8 1.10 1503 52.1 11.7 7.8 5.69 752 3.0 84 84 11.0

19.1 32.3 1.57 2094 46.6 18.3 8.7 4.69 739 6.4 97 100 7.1

20.1 26.9 1.56 1655 55.7 12.5 7.6 6.18 786 5.0 101 168 5.0

22.1 18.2 1.52 1396 43.9 8.6 6.2 3.77 637 4.9 74 78 3.2

23.1 50.0 1.92 2260 58.2 20.6 9.1 7.87 628 4.7 141 166 0.0

24.1 46.2 2.07 1968 70.7 18.0 9.2 8.57 636 3.9 132 136 13.0

25.1 37.8 1.60 1764 54.5 11.0 6.3 8.82 635 3.8 74 85 1.7

27.1 19.2 1.02 1356 39.7 8.0 5.9 8.57 335 3.8 47 47 10.4

28.1 36.2 1.66 1663 50.0 13.9 8.4 5.61 544 3.8 129 132 8.3

29.1 32.4 1.92 2029 81.9 10.1 5.0 5.88 242 2.4 108 157 6.2

31.1 40.0 2.17 2136 67.5 23.2 10.9 7.68 332 2.9 332 335 10.3

33.1 28.5 1.50 1437 61.2 10.4 7.3 10.35 471 3.7 10 54 10.6

34.1 31.0 1.83 1756 74.0 9.4 5.3 10.16 474 3.5 6 73 13.3

35.1 39.8 1.76 1998 80.5 13.6 6.8 12.07 558 4.1 43 57 13.2

36.1 39.2 1.83 2691 60.6 17.8 6.6 11.57 347 5.0 149 157 1.8

37.1 26.6 1.51 1285 90.4 10.5 8.2 7.50 522 3.1 29 36 15.1

39.1 35.7 1.71 1346 49.6 13.7 10.2 4.62 404 3.5 10 34 8.4

40.1 36.5 1.83 2422 91.6 24.1 10.0 14.13 494 4.6 126 209 13.8

41.1 39.3 1.95 2734 101.3 16.5 6.0 6.96 432 3.4 152 152 18.6

42.1 31.5 1.59 2582 74.9 12.6 4.9 7.88 302 3.7 20 95 6.6

43.1 39.6 1.86 1697 52.7 11.8 7.0 4.01 398 2.7 119 119 10.5

44.1 33.4 1.58 2418 90.9 15.8 6.6 11.93 672 3.4 171 210 13.7

45.1 38.8 1.81 2099 83.8 16.9 8.1 15.26 294 3.3 137 180 12.8

46.1 26.6 1.26 1586 60.7 12.6 7.9 14.42 413 4.3 20 31 14.4

47.1 31.0 1.75 1786 91.6 12.6 7.1 11.57 918 3.1 125 147 23.6

48.1 36.8 1.88 1837 78.6 13.7 7.5 6.03 556 3.9 107 177 26.0

49.1 38.6 1.80 2324 93.6 12.7 5.5 10.50 398 2.8 157 157 6.3

50.1 34.5 1.68 1791 68.3 11.9 6.6 12.41 459 4.0 88 117 25.0

51.1 19.4 0.95 1484 53.5 7.3 4.9 7.21 553 2.2 91 93 8.2

52.1 25.3 0.77 2186 89.7 11.4 5.2 2.41 471 3.0 47 59 5.9

53.1 28.8 1.28 1430 74.2 14.2 10.0 8.20 439 3.7 101 110 18.3

Mean 30.5 1.54 1935 68.2 12.9 6.9 8.91 557 3.9 90 113 11.1

S.D. 8.6 0.35 450 20.0 4.5 1.9 3.29 205 1.1 65 66 5.9

a_{mg C kg}ÿ1_. b_{mg N kg}ÿ1_.

c_m_{g CO}

2-C gÿ1hÿ1.

d

mg CO2-C mgÿ 1

C biomass hÿ1

.

e

mg ATP gÿ1.

f

nmol INTF gÿ1hÿ1.

g

mmol H2O2consumed g ÿ1

hÿ1.

h

mg N kgÿ110 dÿ1.

i

mg N-NH4 +

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3. Results and discussion

3.1. Biochemical parameters

3.1.1. Microbial biomass C

In the O horizons, microbial biomass C ranged from 1271 to 3286 mg kgÿ1(mean 1935 mg kgÿ1, coecient of variation CV 23%); 38% of the soils had values in the range 1600±2000 mg kgÿ1. In the Ah horizons, values ranged from 250 to 1483 mg kgÿ1 (mean 781 mg kgÿ1, CV 32%); most values (63%) were in the range 600±1000 mg kgÿ1. These data are not dissimilar to those reported for other soils, for which values in the ranges 2000±10,000 and 200±1500 mg kgÿ1 have been reported for O and Ah horizons respectively (Vance et al., 1987; Sparling et al., 1994; Joergensen et al., 1995). On average, the proportion of total C con-tent constituted by microbial biomass C was 0.68% in the O horizons and 0.78% in the Ah horizons; in both layers, most values lay in the range 0.4±0.6%. These values are similar to those reported in the literature (Wardle, 1992).

3.1.2. Microbial biomass N

The fumigation-induced ninhydrin-reactive N ¯ush was 68.2220.0 mg kgÿ1 (CV 29%) in the O horizons and 25.6212.6 mg kgÿ1 (CV 49%) in the Ah hor-izons. These values are within the ranges reported in the literature (Sparling et al., 1994; Joergensen, 1996). There was close correlation between microbial biomass C and the ninhydrin-reactive N ¯ush in both the O horizons r0:98, P < 0.001) and the Ah horizons r0:93, P< 0.001); the value of the corresponding

regression coecient, 27 in both cases, is practically identical to the value of 28.2 reported by Badalucco et al. (1992).

The literature is not consistent as regards the con-version factor that should be used to transform N ¯ush data into microbial biomass N values. Biomass N values calculated in this study using a conversion fac-tor of 3.5 (the mean of the 3.10 reported by Amato and Ladd (1988) and the 3.75 and 3.50 reported by Sparling et al. (1994)) imply that the proportion of total N content constituted by microbial biomass N was 1.6020.57% in the O horizons and 1.4020.59% in the Ah horizons. Both ®gures lie towards the lower ends of the ranges reported in the literature (Wardle, 1992; Joergensen et al., 1995).

3.1.3. Microbial biomass C-to-N ratio

The microbial biomass C-to-N ratio, calculated as the ratio between the microbial biomass C and mi-crobial biomass N values obtained as above, was 8.52

1.9 for O horizons and 9.823.2 for Ah horizons. To calculate the microbial biomass C-to-N ratio directly from the C and N ¯ushes, total N ¯ush must be

esti-mated from ninhydrin-reactive N ¯ush by multiplying the latter by a factor that, as in the case of the N ¯ush to biomass N conversion factor, is controversial. Pub-lished values include 1.4 (Sparling and Zhu, 1993), 1.8 (Joergensen and Brookes, 1990), to 1.9±2.3 (Inubushi et al., 1991). In this study the value 1.85 (the mean of the above) aorded average microbial biomass C-to-N ratios of 7.2 for O horizons and 8.3 for Ah horizons; these values are in acceptable agreement with the values noted above, suggesting that these two methods of calculating microbial biomass C-to-N ratios are equally valid. The fact that for both O and Ah hor-izons both estimates are greater than 5 shows that in these soils the microbial biomass is mainly fungal rather than bacterial, since the C-to-N ratio ranges from 3 to 5 for bacteria and from 4 to 15 for fungi (Paul and Clark, 1989).

3.1.4. Soil respiration

CO2 production ranged from 1.1 to 24.1 mg CO2-C gÿ1 hÿ1 in the O horizons (mean 12.9 mg CO2-C gÿ1 hÿ1, S.D. 4.5mg CO2-C gÿ1hÿ1) but were signi®cantly lower in the Ah horizons, ranging from 1.4 to 5.2 mg CO2-C gÿ1hÿ1(mean 2.6 mg CO2-C gÿ1hÿ1, S.D. 0.8

mg CO2-C gÿ1 hÿ1). These values are within the reported ranges for woodland soils (Wardle, 1993; Gregorich et al., 1994).

3.1.5. Metabolic quotient (qCO2)

The metabolic quotient (the quantity of CO2-C pro-duced per unit of microbial biomass C per unit time; Anderson and Domsch, 1985) has been considered as indicative of the maturity of a soil ecosystem, although severe limitations on its use have recently been stressed (Wardle and Ghani, 1995). In this study its value was 6.921.9 in O horizons and 3.521.3 in Ah horizons. The Ah values are similar to those reported in the lit-erature (Anderson and Domsch, 1993; Wardle, 1993), but the O horizons values are considerably greater than in most reports (Ross and Tate, 1993; PoÈhnacker and Zech, 1995), although they coincide with theqCO2 values observed by Dilly and Munch (1995) in a study of the decomposition of Alnus glutinosa (L.) Gaestn. litter. The fact that in the Ah horizons qCO2 is on average only about half its value in the O horizon may be attributed to the lower readily degradable C content of the Ah horizons, which determines the replacement of the predominantly zymogenous micro¯ora of the O horizon by an energetically more ecient autochtho-nous micro¯ora. Rosenbrack et al. (1995) have reported that changes in fungal population structure have similar eects on the qCO2 of decomposing A.

glutinosa(L.) Gaestn. litter.

3.1.6. ATP

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Table 4

Values of the general biochemical parameters studied, in Ah horizonsn40of climax soils under oakwood in Galicia (NW Spain) Sample %

Total C

% Total N

Microbial biomass Ca

N ¯ushb

CO2-C

evolvedc

qCO2d ATPe Dehydrogenasef Catalaseg NH4

mineralizedh

Total inorganic N mineralizedh

Arginine ammoni®cationi

7.2 10.9 0.61 1483 28.3 2.4 1.6 2.40 208 1.9 18 19 4.9

8.2 10.0 0.70 1129 43.9 3.0 2.6 6.34 298 1.6 40 55 0.9

10.2 5.7 0.39 802 19.1 2.4 3.0 1.88 185 1.9 19 19 5.8

12.2 9.5 0.51 695 13.6 3.6 5.2 2.06 163 0.8 35 35 5.8

13.2 6.3 0.37 699 12.1 1.8 2.5 2.42 193 1.7 20 20 7.0

14.2 5.2 0.32 840 15.0 2.2 2.6 6.74 96 0.9 1 1 4.6

15.2 7.8 0.56 690 15.6 2.4 3.5 2.98 269 2.5 22 22 6.0

16.2 12.3 0.76 1020 24.1 2.9 2.9 3.77 107 1.0 6 21 4.1

17.2 8.3 0.55 817 14.7 2.8 3.4 1.57 340 3.4 11 15 6.3

18.2 6.6 0.40 663 15.8 2.3 3.5 1.42 236 1.1 17 17 1.8

19.2 12.3 0.65 589 10.9 2.9 4.9 2.06 278 3.6 8 11 4.4

20.2 9.5 0.57 566 13.0 1.9 3.3 4.18 235 2.0 16 30 3.1

22.2 8.9 0.59 434 12.5 1.8 4.1 1.40 222 1.7 14 14 3.2

23.2 14.5 0.81 832 28.8 3.1 3.8 3.60 247 3.2 31 31 6.3

24.2 9.5 0.68 431 15.8 1.6 3.7 2.26 213 1.5 23 32 0.9

25.2 12.5 0.57 250 7.7 1.4 5.6 1.46 106 0.8 27 27 1.6

27.2 11.1 0.54 606 17.1 1.9 3.1 1.46 161 2.1 27 27 3.5

28.2 9.4 0.44 673 19.9 2.2 3.2 2.06 216 1.8 28 28 3.7

29.2 14.8 0.93 1102 41.2 3.0 2.7 2.72 136 2.0 59 60 8.0

31.2 11.6 0.79 982 35.1 2.3 2.4 3.96 147 1.7 37 69 2.5

33.2 10.3 0.68 600 24.1 1.7 2.8 2.30 162 3.9 8 24 3.5

34.2 14.6 0.96 975 43.4 3.2 3.2 2.24 187 3.3 11 41 5.8

35.2 12.2 0.77 644 32.3 2.2 3.4 2.19 170 3.5 14 35 3.8

36.2 18.3 0.99 1333 32.3 3.6 2.7 4.25 117 2.2 39 48 7.0

37.2 11.8 0.85 567 31.0 2.5 4.5 3.86 257 4.2 7 21 3.9

39.2 11.5 0.74 386 14.4 3.3 8.5 2.24 216 1.9 8 15 3.8

40.2 6.6 0.40 639 18.7 1.9 2.9 4.08 198 0.6 12 21 4.4

41.2 13.1 0.79 858 25.2 3.0 3.5 0.82 187 1.9 28 28 3.2

42.2 9.7 0.63 851 20.8 2.1 2.5 0.83 152 2.1 16 43 4.8

43.2 10.2 0.63 755 26.6 2.6 3.5 2.16 170 1.3 26 26 4.9

44.2 7.8 0.43 682 33.7 2.3 3.4 4.29 217 0.6 40 49 6.3

45.2 15.1 0.85 666 36.6 3.3 5.0 3.76 239 1.7 40 54 6.7

46.2 14.7 0.76 882 31.7 4.6 5.2 2.71 307 3.4 21 25 6.0

47.2 9.3 0.72 786 28.7 3.3 4.2 1.87 307 1.6 33 42 7.0

48.2 13.2 0.92 952 30.5 2.3 2.4 5.63 210 2.2 6 47 7.2

49.2 10.1 0.63 1041 27.7 1.9 1.9 1.92 174 1.3 18 18 4.5

50.2 15.2 0.98 514 39.8 2.1 4.1 2.90 234 2.2 34 48 8.7

51.2 6.7 0.41 740 23.7 2.4 3.3 1.83 240 1.3 28 33 2.7

52.2 7.9 0.48 950 24.0 1.4 1.5 1.69 211 1.3 18 18 7.0

53.2 15.1 0.69 1130 76.5 5.2 4.6 2.31 262 2.9 27 32 11.4

Mean 10.8 0.65 781 25.6 2.6 3.5 2.77 207 2.0 22 30 4.9

S.D. 3.8 0.18 253 12.6 0.8 1.3 1.38 58 0.9 12 15 2.2

a

mg C kgÿ1.

b

mg N kgÿ1.

c

mg CO2-C g ÿ1

hÿ1.

d

mg CO2-C mgÿ1C biomass hÿ1. e

mg ATP gÿ1.

f

nmol INTF gÿ1hÿ1.

g_{mmol H}

2O2consumed gÿ1hÿ1. h_{mg N kg}ÿ1_{10 d}ÿ1_.

i

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recovered, and in the Ah horizons 43213%. These ®gures are lower than those reported by Contin et al. (1995), but similar to those found by Webster et al. (1984). The lower recovery in the organic layer suggests that ATP losses were due mainly to hydrolytic processes mediated by soil biomass, which was greater in the organic layer than in Ah. The ATP values in the O and Ah horizons, once corrected for losses, were 8.9123.29 and 2.7721.38 mg gÿ1 respectively. Com-parisons with the results of other authors are limited by the wide variety of methods that have been used for soil ATP determination, but the amounts found in this work are similar to those reported by Bolton et al. (1993) and Contin et al. (1995). It is noteworthy that the mean ratios between microbial biomass C and ATP, 220 for O horizons and 280 for Ah horizons, are close to the values obtained with pure cultures (200± 250; Martens, 1995), which suggests that the microbial populations of these soils consist predominantly of active individuals.

3.1.7. Dehydrogenase activity

Dehydrogenase activity was 5572205 nmol INTF gÿ1 hÿ1 in the O horizons (range 242±1320 nmol INTF gÿ1 hÿ1) and 207258 nmol INTF gÿ1 hÿ1 in the Ah horizons (range 96±340 nmol INTF gÿ1 hÿ1). In both cases the measured dehydrogenase activities had highly symmetric bell-shaped frequency distri-butions. Comparison with the literature is impossible given the absence of reported woodland soil dehydro-genase activity data obtained with INT as substrate.

3.1.8. Catalase activity

Catalase activity was 3.921.1 mmol H2O2 con-sumed gÿ1hÿ1in the O horizons (range 2.2±7.1 mmol H2O2 consumed gÿ1 hÿ1) and 2.020.9 mmol H2O2 consumed gÿ1hÿ1 in the Ah horizons (range 0.6±4.2 mmol H2O2 consumed gÿ1 hÿ1). As in the case of de-hydrogenase activity, the scarcity of published catalase data and the diversity of methods used to obtain such data as have been reported together make meaningful comparison with the literature impossible.

3.1.9. Nitrogen mineralization capacity

Preincubation inorganic N content ranged from 11 to 235 mg kgÿ1 in the O horizons and from 6 to 74 mg kgÿ1 in the Ah horizons. In both layers, about 90% of inorganic N was ammoniacal. Both the inor-ganic N contents and the predominance of ammonia-cal forms are normal for Galician soils (GonzaÂlez-Prieto et al., 1992). Incubation increased total inor-ganic N content by 113266 mg kgÿ1 in the O hor-izons and by 30215 mg kgÿ1in the Ah horizons, with increases in ammoniacal N (90265 and 22212 mg kgÿ1, respectively) that show the predominance of ammoni®cation over nitri®cation in these soils. These

values are very similar to those reported in the litera-ture (Ross and Tate, 1993; Gregorich et al., 1994).

3.1.10. Arginine ammoni®cation

The arginine ammoni®cation rate was 11.125.9 mg gÿ1 hÿ1in the O horizons and 4.922.2mg gÿ1hÿ1 in the Ah horizons. These ®gures are well above the few values reported in the literature (Alef and Kleiner, 1986; Franzluebbers et al., 1995) and, as with the N mineralization results reported above, show the high ammoni®cation capacity of Galician climax soils.

3.2. Correlations between general biochemical parameters and chemical variables

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3.3. Correlations within the general biochemical parameters

The mutual correlations among biochemical par-ameters within each type of layer are weak, apparently because of the relatively small range of most variables in each layer. However, when the data for the O and Ah horizons are analysed jointly, most of the investi-gated properties are highly correlated (P< 0.001) with each other (Table 6). In keeping with the ®ndings of Sparling et al. (1994), Joergensen et al. (1995) and Franzluebbers et al. (1995), those showing closest cor-relation with others are in general microbial biomass C, microbial biomass N and CO2 production. How-ever, in keeping with the observations of Engels et al. (1993), catalase activity is most closely correlated with dehydrogenase activity, which together with the close relationship of both these properties with CO2 pro-duction and ATP corroborates the conclusion of Bol-ton et al. (1993) that both catalase and dehydrogenase can be regarded as potential markers of soil microbial activity. This is a potentially useful ®nding in that both catalase and dehydrogenase are more easily and rapidly determined than CO2 production, and may therefore be useful for the rapid estimation of soil mi-crobial activity (Nannipieri et al., 1990). It is also worth noting that arginine ammoni®cation rate is most closely correlated with microbial biomass N and ATP, which shows the relationship between the mineraliz-ation of organic matter and both the size and activity of the microbial population.

As regards parameters derived from biomass C and N, neither the biomass-C-to-total-C ratio nor the bio-mass-N-to-total-N ratio exhibit very close correlation with any of the other variables considered. Nor does the biomass C-to-N ratio, but it is noteworthy that this property is negatively correlated with measures of nitrogen dynamics such as microbial biomass N, N mineralization capacity and arginine ammoni®cation rate. Finally, it is also worth noting that qCO2 is clo-sely correlated with most of the primary properties considered, CO2 production especially, which is in keeping with the ®ndings of Joergensen et al. (1995).

4. Conclusions

The data reported above show that in native Gali-cian soils (Umbrisols under climax vegetation of Atlantic oakwood) the values of general biochemical parameters vary widely but are generally within pre-viously published ranges for temperate woodland soils and are much higher in the O horizon than in the Ah horizon. Derived indices (qCO2, microbial biomass C-to-total C ratio, microbial biomass C-to-ATP ratio, etc.) suggest that active individuals predominate in the

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microbial communities of these soils, and that fungi predominate over bacteria. The fact that qCO2 is sig-ni®cantly lower in the Ah horizons than in the O layers suggests that the opportunistic organisms typical of the latter are replaced in the Ah horizons by popu-lations that are better adapted to the soil medium. The ®nding that the general soil properties most closely correlated with the biochemical properties are the total C, total N and available nutrient contents suggests that the number and activity of soil microorganisms depend mainly on the quantity of mineralizable sub-strate and the availability of nutrients.

Acknowledgements

This work was ®nanced by the Xunta de Galicia. The authors thank Ana Isabel Iglesias-Tojo for her help with the analysis of the samples.

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