Biochemical properties of acid soils under climax vegetation
(Atlantic oakwood) in an area of the European temperate±humid
zone (Galicia, NW Spain): speci®c parameters
C. Trasar-Cepeda
a, M.C. LeiroÂs
b,*, F. Gil-Sotres
ba
Departamento de BioquõÂmica del Suelo, Instituto de Investigaciones AgrobioloÂgicas de Galicia, Consejo Superior de Investigaciones Cientõ®cas, Apartado 122, E-15080 Santiago de Compostela, Spain
b
Departamento de EdafologõÂa y QuõÂmica AgrõÂcola, Facultad de Farmacia, Universidad de Santiago de Compostela, E-15706 Santiago de Compostela, Spain
Accepted 6 October 1999
Abstract
The general and speci®c biochemical parameters of soils are highly sensitive to disturbance of the environment, but their use for diagnosis of soil degradation is limited by lack of comparable published data and lack of accepted methodological standards. With a view to establishing an appropriate data base for the soils of Galicia (NW Spain), we investigated the biochemical properties of the O and Ah horizons of 40 native Umbrisols under climax Atlantic oakwood in this region. We report here our results on speci®c biochemical parameters (i.e. extracellular hydrolytic enzyme activities) characterizing the phosphorus, nitrogen, carbon and sulphur cycles. The enzymes studied were phosphomonoesterase (23.51210.37 and 6.6223.29mmolp-nitrophenol gÿ1 hÿ1, values for O and Ah horizons, respectively), phosphodiesterase (3.60
21.95 and 0.9620.51 mmol p-nitrophenol gÿ1
hÿ1), casein-protease (2.9720.83 and 0.9420.32mmol tyrosine gÿ1 hÿ1), BAA-protease (23.74211.35 and 15.2628.91mmol NH3 gÿ1 hÿ1), urease (24.90213.60 and 16.59210.61 mmol NH3 gÿ1 hÿ1), CM-cellulase (0.5920.17 and 0.2320.10mmol
glucose gÿ1hÿ1), invertase (12.6622.75 and 6.9322.14mmol glucose gÿ1hÿ1),b-glucosidase (8.4325.14 and 1.5520.89mmol p-nitrophenol gÿ1hÿ1), and arylsulfatase (0.6720.30 and 0.4620.20mmolp-nitrophenol gÿ1hÿ1). For the variables for which comparable data are available, the values observed are generally within previously published ranges. Principal components analysis of the combined biochemical, physical and chemical data for these soils shows ®ve factors, of which the three most important concern microbial activity and its logical dependence on nutrient content, the accumulation of soil organic matter and the mineralization of soil organic matter.72000 Elsevier Science Ltd. All rights reserved.
Keywords:Soil biochemical properties; Soil enzymes; Temperate forest soils
1. Introduction
It is widely accepted that soil quality can be expressed in terms of the capability of a soil to accept, store and recycle the water, minerals and energy required for optimal crop production while at the
same time preserving a healthy environment (Arshad and Cohen, 1992; Parr et al., 1992). Thus good-quality soils should carry out the following functions: crop production, ®ltration and degradation. The most im-portant of these three functions for the evaluation of soil quality is perhaps degradation, as it de®nes the ca-pacity of a soil to mineralize soil organic matter and degrade exogenous plant material and anthropogenic inputs such as organic wastes, pesticides, hydrocar-bons, etc. (Dick, 1997). The degradation function
Soil Biology & Biochemistry 32 (2000) 747±755
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* Corresponding author. Tel.: +34-981-563-100, ext. 15042; fax: +34-981-594-912.
depends to a large extent on the biochemical properties of the soil (Visser and Parkinson, 1992), which are usually divided into two groups (Nannipieri et al., 1995): general biochemical parameters, which are directly related to the number and activity of soil microorganisms, and speci®c parameters, the activities of extracellular hydrolytic enzymes stabilized by being bound in complexes with clay minerals and humic col-loids.
Hydrolytic extracellular soil enzymes make nutrients available to plants and microorganisms by converting them from unassimilable to readily assimilable forms (Saratchandra et al., 1984). However, although this implies that they exert a very important part in the degradation function, this potential for characterising soil quality has often been ignored. This is partly due to there having been relatively few studies in which the activities of a large number of enzymes have been stu-died simultaneously, and partly to the diculty in comparing data obtained for dierent soils, by dier-ent workers using dierdier-ent incubation conditions, sub-strates and substrate concentrations and buer systems (Saratchandra et al., 1984). What is needed, if the po-tential of enzymatic activities and other biochemical properties for soil quality assessment is to be realized, is an extensive database of the biochemical properties of soils from dierent parts of the world. Moreover, since the goal of sustainable development implies that soil quality should be determined with reference to undisturbed native soils (Doran et al., 1994), the com-pilation of such a database should give priority to cli-max soils in so far as is possible.
The above considerations have led us to compile an extensive database of the biochemical properties of cli-max soils in Galicia (NW Spain), a region situated in the European temperate±humid zone in which exten-sive areas of climax vegetation (Atlantic oakwood) still exist. The general biochemical parameters of these cli-max soils (all Umbrisols; ISSS Working group RB, 1998) were reported by LeiroÂs et al. (1999). Here we report the activities of a variety of hydrolytic enzymes involved in the carbon, nitrogen, phosphorus and sul-phur cycles, including enzymes with substrates of both high and low molecular mass, and we discuss their re-lationships with each other and with the physical, chemical and general biochemical parameters of the soils.
2. Material and methods
2.1. Soils
We studied 40 soils developed under climax veg-etation dominated byQuercus roburL. orQ. pyrenaica
L. at sites distributed throughout Galicia, NW Spain.
At all sites, the sampling area displayed little if any disturbance of human origin, and its tree vegetation was composed mainly of healthy adult specimens. All the soil are Umbrisols (ISSS Working Group RB, 1998); their locations, the methods used for sampling and for determination of their physical, chemical and general biochemical parameters, and the values of these parameters, have been described by LeiroÂs et al. (1999).
2.2. Analytical methods
The activities of urease (EC 3.5.1.5) and of proteases (EC 3.4.4) hydrolysing benzoylargininamide (BAA-protease) and casein (casein-(BAA-protease) were determined as described by Gil-Sotres et al. (1992). Brie¯y, urease activity was determined using urea as substrate, incu-bating for 1.5 h at 378C and pH 7.1 (phosphate buer 0.2 M) and measuring the NH4+ released with an am-monium electrode. BAA-protease activity was deter-mined using the same incubation conditions and the same method to determine NH4+ but with a
-benzoyl-N-argininamide (BAA) as substrate. In both cases en-zymatic activity is expressed in mmol NH3 gÿ1 hÿ1. Casein-hydrolysing activity was determined with casein as substrate, incubating for 2 h at 508C and pH 8.1 (Tris±HCl buer 0.05 M) and determining the amino acids released by the Folin colorimetric method; enzy-matic activity is expressed in mmol tyrosine gÿ1hÿ1.
Acid phosphomonoesterase (EC 3.1.3.2), b -glucosi-dase (EC 3.2.1.21), phosphodiesterase (EC 3.1.4.1) and arylsulfatase (EC 3.1.6.1) activities were determined by incubating the soils with a substrate containing a
p-nitrophenyl moiety and spectrophotometrically
measuring the amount of p-nitrophenol liberated by enzymatic hydrolysis. In these cases the enzymatic ac-tivity is expressed in mmolp-nitrophenol gÿ1 hÿ1. Acid phosphomonoesterase activity was determined with
p-nitrophenyl phosphate as substrate, incubating at pH 5.0 (Modi®ed Universal Buer) and 378C. After 30 min 2 M CaCl2was added (to stop the reaction and to avoid the brown coloration caused by organic matter) and the liberated p-nitrophenol was extracted with 0.2 M NaOH (Tabatabai and Bremner, 1969; Saa et al., 1993). b-glucosidase activity was determined as described for phosphomonoesterase activity except that the substrate was p-nitrophenyl-b-glucopyranoside and the p-nitrophenol released was extracted with THAM±NaOH 0.1 M of pH 12 (Eivazi and Tabata-bai, 1988). Phosphodiesterase activity was determined with bis-p-nitrophenyl phosphate as substrate, incubat-ing at pH 5.0 (THAM buer 0.05 M) and 378C for 1 h (Bowman and Tabatabai, 1978). Arylsulphatase ac-tivity was determined with p-nitrophenyl sulphate as substrate, incubating at pH 5.8 (acetate buer 0.5 M) and 378C for 1 h (Tabatabai and Bremner, 1970).
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Table 1
Values of the speci®c biochemical parameters studied, in O horizons n40of climax soils under oakwood in Galicia (NW Spain)
Sample Total C (%) Total N (%) Phosphomonoesterasea Phosphodiesterasea Arylsulfatasea Casein-proteaseb BAA-proteasec Ureasec CM-cellulased b-glucosidasea Invertased
7.1 31.2 1.57 10.49 1.65 0.20 2.76 11.93 12.66 0.62 7.45 12.44
8.1 28.6 1.76 40.36 4.33 1.03 4.04 18.33 11.37 0.88 15.26 15.56
10.1 14.6 0.95 9.17 1.21 0.47 2.88 20.54 18.34 0.31 5.31 11.21
12.1 21.2 1.19 24.59 1.72 0.27 3.66 8.90 13.89 0.54 6.71 12.40
13.1 21.0 1.16 12.75 3.50 0.67 3.54 29.90 28.27 0.46 4.36 9.39
14.1 18.3 0.97 36.91 2.73 0.60 2.84 24.98 16.44 0.51 7.31 14.31
15.1 17.3 1.07 9.47 2.37 0.88 3.15 30.86 28.50 0.30 4.43 11.48
16.1 27.8 1.51 30.03 2.90 0.70 4.33 13.02 11.43 0.83 11.64 16.92
17.1 23.1 1.29 10.79 2.58 0.80 3.47 36.21 29.31 0.45 9.05 13.41
18.1 17.8 1.10 12.38 2.22 0.47 2.43 18.22 19.62 0.58 6.56 13.76
19.1 32.3 1.57 19.20 7.44 0.69 4.11 39.14 19.80 0.71 2.05 15.77
20.1 26.9 1.56 15.51 3.88 0.73 3.71 48.97 40.10 0.54 7.13 12.93
22.1 18.2 1.52 11.93 2.36 0.41 2.69 29.85 28.18 0.70 5.89 14.59
23.1 50.0 1.92 16.74 3.61 0.53 4.48 23.81 34.57 0.88 13.86 16.20
24.1 46.2 2.07 24.05 4.81 0.83 4.08 24.99 24.12 0.93 8.20 16.40
25.1 37.8 1.60 18.17 3.08 0.35 3.04 22.23 30.71 0.75 12.05 14.35
27.1 19.2 1.02 11.76 3.47 0.53 1.52 21.87 14.47 0.27 4.00 13.39
28.1 36.2 1.66 15.90 2.53 0.34 1.78 14.12 16.94 0.57 6.42 18.83
29.1 32.4 1.92 9.35 2.09 0.29 1.15 5.91 12.04 0.31 4.23 10.19
31.1 40.0 2.17 14.38 3.62 0.22 2.43 10.07 9.35 0.40 7.07 10.41
33.1 28.5 1.50 33.89 5.56 0.86 3.60 34.48 29.29 0.67 11.41 13.70
34.1 31.0 1.83 24.29 1.67 0.72 2.13 26.17 38.70 0.80 6.29 15.44
35.1 39.8 1.76 38.19 2.49 0.55 2.17 10.47 22.68 0.78 5.68 16.93
36.1 39.2 1.83 20.78 3.02 0.40 3.99 21.99 31.77 0.74 5.08 17.72
37.1 26.6 1.51 27.47 5.20 1.12 2.41 41.57 65.60 0.50 4.93 11.52
39.1 35.7 1.71 19.20 4.02 1.58 1.15 19.30 20.15 0.56 6.98 9.03
40.1 36.5 1.83 34.40 8.01 0.61 3.81 42.61 35.19 0.56 4.21 8.47
41.1 39.3 1.95 35.49 5.48 1.06 3.30 14.67 20.08 0.41 29.63 10.08
42.1 31.5 1.59 12.47 2.16 0.52 2.96 28.81 20.81 0.48 3.77 10.19
43.1 39.6 1.86 30.68 2.91 0.62 2.90 12.52 14.52 0.79 17.09 12.96
44.1 33.4 1.58 47.45 3.02 0.56 2.58 27.88 66.26 0.68 17.00 11.33
45.1 38.8 1.81 34.02 11.82 0.67 2.09 16.92 20.01 0.64 4.34 9.62
46.1 26.6 1.26 32.84 3.15 0.63 1.85 33.05 33.03 0.52 8.76 9.36
47.1 31.0 1.75 18.52 3.17 1.50 3.50 23.66 21.00 0.44 14.15 12.19
48.1 36.8 1.88 34.95 3.19 0.65 3.19 40.05 33.57 0.53 13.79 9.95
49.1 38.6 1.80 31.97 3.06 0.53 3.32 5.84 10.81 0.86 9.04 12.81
50.1 34.5 1.68 33.03 4.55 0.81 2.83 44.93 47.12 0.61 7.61 11.31
51.1 19.4 0.95 25.59 3.10 0.81 3.16 16.17 15.12 0.52 7.73 7.92
52.1 25.3 0.77 21.22 3.40 0.90 2.85 8.13 1.76 0.51 3.83 10.64
53.1 28.8 1.28 30.00 3.05 0.50 3.02 26.44 28.32 0.53 6.71 11.28
Mean 30.5 1.54 23.51 3.60 0.67 2.97 23.74 24.90 0.59 8.43 12.66
S.D. 8.6 0.35 10.37 1.95 0.30 0.83 11.35 13.60 0.17 5.14 2.75
ammolp-nitrophenol gÿ1hÿ1. bmmol tyrosine gÿ1hÿ1. cmmol NH
3gÿ1hÿ1. dmmol glucose gÿ1hÿ1.
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Table 2
Values of the speci®c biochemical parameters studied, in Ah horizons n40of climax soils under oakwood in Galicia (NW Spain)
Sample Total C (%) Total N (%) Phosphomonoesterasea Phosphodiesterasea Arylsulfatasea Casein-proteaseb BAA-proteasec Ureasec CM-cellulased b-glucosidasea Invertased
7.2 10.9 0.61 3.25 0.45 0.14 0.83 3.63 7.38 0.26 1.13 5.96
8.2 10.0 0.70 8.64 0.97 0.68 1.19 6.75 5.94 0.33 1.92 8.67
10.2 5.7 0.39 2.23 0.40 0.21 0.95 16.06 10.21 0.06 1.02 4.75
12.2 9.5 0.51 6.72 0.56 0.07 1.44 3.73 5.96 0.25 1.09 5.76
13.2 6.3 0.37 2.43 0.64 0.42 0.63 15.94 14.53 0.10 0.77 7.29
14.2 5.2 0.32 7.87 1.16 0.25 1.15 5.50 6.59 0.09 1.48 4.42
15.2 7.8 0.56 2.73 0.81 0.54 1.34 15.86 17.14 0.11 1.16 9.27
16.2 12.3 0.76 6.95 0.76 0.31 1.67 7.10 10.33 0.26 2.36 5.36
17.2 8.3 0.55 4.10 1.12 0.63 1.19 25.44 19.94 0.11 1.63 10.95
18.2 6.6 0.40 3.99 0.72 0.25 0.84 8.33 10.55 0.15 1.20 7.22
19.2 12.3 0.65 6.56 1.68 0.66 0.62 25.60 26.08 0.23 1.71 7.53
20.2 9.5 0.57 2.24 0.97 0.38 0.61 28.96 22.50 0.18 3.86 7.49
22.2 8.9 0.59 3.20 0.92 0.37 0.66 25.86 18.43 0.18 0.93 6.69
23.2 14.5 0.81 4.64 1.62 0.56 0.91 21.94 30.03 0.26 1.42 8.47
24.2 9.5 0.68 5.90 1.96 0.46 0.96 13.16 12.83 0.36 2.72 4.57
25.2 12.5 0.57 3.12 0.44 0.07 0.36 5.03 6.64 0.22 1.03 2.00
27.2 11.1 0.54 4.33 1.02 0.58 0.71 14.74 6.99 0.06 0.95 9.37
28.2 9.4 0.44 3.50 0.63 0.34 0.69 8.52 3.92 0.21 1.41 9.12
29.2 14.8 0.93 6.04 0.84 0.41 0.87 13.80 18.12 0.32 1.46 8.58
31.2 11.6 0.79 5.26 0.86 0.35 0.51 5.88 10.28 0.26 1.12 9.91
33.2 10.3 0.68 9.97 0.84 0.56 0.69 23.44 17.74 0.19 0.94 8.35
34.2 14.6 0.96 8.99 0.72 0.51 0.68 12.31 28.50 0.47 1.62 5.66
35.2 12.2 0.77 8.46 0.88 0.44 0.64 12.43 20.33 0.33 1.22 2.97
36.2 18.3 0.99 3.76 0.54 0.23 1.02 8.02 23.24 0.24 0.67 4.43
37.2 11.8 0.85 12.57 1.24 0.91 1.17 27.64 47.14 0.19 1.10 10.89
39.2 11.5 0.74 8.98 1.23 0.43 1.13 14.73 9.47 0.30 1.98 6.49
40.2 6.6 0.40 6.76 0.62 0.54 0.78 11.69 6.46 0.17 1.07 3.75
41.2 13.1 0.79 12.04 2.28 0.85 1.40 8.80 13.53 0.22 1.97 7.85
42.2 9.7 0.63 3.70 0.82 0.41 0.99 24.81 15.47 0.06 0.73 9.85
43.2 10.2 0.63 6.64 0.86 0.56 1.28 12.11 13.67 0.33 1.72 4.46
44.2 7.8 0.43 6.93 0.38 0.33 0.72 13.21 49.78 0.22 0.77 6.50
45.2 15.1 0.85 13.82 0.30 0.52 0.70 14.78 11.98 0.33 1.56 6.82
46.2 14.7 0.76 11.31 1.56 0.70 0.84 30.10 28.56 0.39 4.58 8.76
47.2 9.3 0.72 6.23 1.02 0.85 1.75 18.69 18.59 0.13 2.02 8.02
48.2 13.2 0.92 6.66 0.76 0.43 0.66 33.21 26.58 0.28 1.58 6.87
49.2 10.1 0.63 8.22 0.88 0.27 1.40 2.67 3.17 0.42 0.95 5.58
50.2 15.2 0.98 8.64 0.83 0.65 0.88 34.33 25.27 0.14 0.89 8.43
51.2 6.7 0.41 6.75 0.85 0.67 1.12 8.36 14.58 0.16 1.45 5.53
52.2 7.9 0.48 5.00 0.69 0.57 0.93 4.48 3.60 0.14 0.83 7.64
53.2 15.1 0.69 15.76 2.69 0.45 0.82 22.79 21.36 0.34 4.06 4.91
Mean 10.8 0.65 6.62 0.96 0.46 0.94 15.26 16.59 0.23 1.55 6.93
S.D. 3.1 0.18 3.29 0.51 0.20 0.32 8.91 10.61 0.10 0.89 2.14
ammolp-nitrophenol gÿ1hÿ1. bmmol tyrosine gÿ1hÿ1. cmmol NH
3gÿ1hÿ1. dmmol glucose gÿ1hÿ1.
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Invertase (EC 3.2.1.26) activity was determined with saccharose as substrate, incubating for 3 h at 508C and pH 5.5 (acetate buer 2 M) and determining reducing sugars as per Schinner and von Mersi (1990). Carboxy-methylcellulase (CM-cellulase) activity was determined similarly, except that the substrate was carboxymethyl-cellulose and the incubation time was 24 h (Schinner and von Mersi, 1990). The enzymatic activities are expressed inmmol glucose gÿ1hÿ1.
2.3. Expression and analysis of results
All determinations were performed in triplicate, and all values reported are averages of triplicate determi-nations expressed on an oven-dried soil basis (1058C). Statistical analyses were performed using Statistics 4.5 for Windows (StatSoft Inc., 1993)
3. Results and discussion
The values, means and standard deviations of the enzyme activities in the O and Ah horizons are listed in Tables 1 and 2.
3.1. Enzymatic activities
3.1.1. Acid phosphomonoesterase
Mean acid phosphomonoesterase activity in the O horizons was 23.51 mmol p-nitrophenol gÿ1 hÿ1 (S.D. 10.37, range 9.17±47.45, coecient of variation CV 44%). In the Ah horizons it fell to 6.62 mmol p- nitro-phenol gÿ1 hÿ1 (S.D. 3.29, range 2.23±15.76, CV 50%). Although phosphomonoesterase has been one of the most extensively studied soil enzymes (Burns, 1978), data for the organic layers of woodland soils are relatively scarce. The values observed in this study lie within the range found in woodland soils in the Apennines (Nannipieri et al., 1980), in subarctic regions (Neal, 1982) and in Canada (Pang and Kolenko, 1986). Similarly, the Ah horizon values obtained in this study fall within the range reported in the much more abundant literature on phosphomo-noesterase in mineral horizons (Speir, 1977; Franken-berger and Dick, 1983).
3.1.2. Phosphodiesterase
Mean phosphodiesterase activity in the O horizons was 3.60mmol p-nitrophenol gÿ1hÿ1(S.D. 1.95, range 1.21±11.82, CV 54%), between six and seven times less than phosphomonoesterase activity; in the Ah horizons activity ranged from 0.30 to 2.69 mmol p-nitrophenol gÿ1 hÿ1 (mean 0.96, S.D. 0.50, CV 52%). References to phosphodiesterase activities in the literature are scarce and generally unsystematic. Rastin et al. (1988, 1990) mention very low values for both soil layers
(between 0 and 0.02 mmol p-nitrophenol gÿ1 hÿ1). For Ah horizons, Frankenberger and Dick (1983) reported values similar to those observed in this work, but Ross et al. (1995a,b) found much higher values of between 7 and 22mmolp-nitrophenol gÿ1hÿ1.
3.1.3. Arylsulfatase
Mean arylsulfatase activity was 0.66 mmol p- nitro-phenol gÿ1hÿ1in the O horizons (S.D. 0.30, CV 45%, range 0.20±1.58) and 0.45 mmolp-nitrophenol gÿ1hÿ1 in the Ah horizons (S.D. 0.20, CV 43%, range 0.07± 0.91). Although the values observed in this study in both the O and Ah horizons are within the range reported in the literature for natural ecosystems, the maximum values are considerably lower than those found in organic layers by authors such as Baligar and Wright (1991) and Saratchandra et al. (1984), who reported maxima of 3.89 and 5.69 mmolp-nitrophenol gÿ1 hÿ1, respectively. Although this discrepancy may be partly due to methodological dierences, it suggests that in native Galician soils the sulphur requirement of soil microorganisms is satis®ed, at least in the top few centimetres of the soil, with ensuing downregulation of arylsulphatase. This implies the possibility of these soils developing an arylsulfatase de®ciency, the conse-quences of which for the sulphur cycle are dicult to predict.
3.1.4. Casein-protease
Casein-protease activity ranged from 1.15 to 9.08 mmol tyrosine gÿ1 hÿ1 in the O horizons (mean 3.10, S.D. 1.26, CV 41%), and from 0.36 to 1.75mmol tyro-sine gÿ1hÿ1in the Ah horizons (mean 0.94, S.D. 0.32, CV 43%). The values observed in Ah horizons are within the range reported by several authors for native soils (e.g. Nannipieri et al., 1980; BonmatõÂ et al., 1991; Perucci, 1992). We have been unable to ®nd references to casein-protease activity in O horizons; nevertheless, the values of 5.50±38.67 mmol tyrosine gÿ1 hÿ1 found by Dilly and Munch (1995) in rotting Alnus litter are much higher than those found in this study. The observed depthwise variation suggests that proteolysis of substrates of high molecular weight diminishes as the decomposition of plant debris advances, i.e. in the order litter, O layer, humus, at least when the results are expressed as activity per unit mass of soil.
3.1.5. BAA-protease
In the O horizons BAA-protease activity ranged from 5.84 to 48.97 mmol NH3 gÿ1 hÿ1 (mean 23.50, S.D. 11.54 CV 49%), and in Ah horizons from 2.67 to 34.33 mmol NH3 gÿ1 hÿ1 (mean 15.26, S.D. 8.91, CV 58%). No other values for temperate zone soils appear to be available for comparison, in spite of the proposal of Ladd and Buttler (1972) that BAA be used to deter-mine the activity of proteases acting on small peptides.
It may be worth mentioning that the values reported for three soils from the Apennine Mountains by Nan-nipieri et al. (1980) are within the range found in this work.
3.1.6. Urease
Urease activity ranged from 1.76 to 66.26mmol NH3 gÿ1 hÿ1 in O horizons (mean 24.90, S.D. 13.60, CV 55%) and from 3.17 to 49.78mmol NH3gÿ1hÿ1in Ah horizons (mean 16.58, S.D. 10.67, CV 64%). Although urease, like phosphomonoesterase, has been one of the most extensively studied soil enzymes (Burns, 1978), most published work refers to agricultural soils, urea being one of the most widely used nitrogenated fertili-zers in many parts of the world. Research on urease activity in woodland soils has been relatively scant. The values observed in this study are similar to those found by Nannipieri et al. (1980) in soils from the Apennine Mountains, but between 2 and 20-times higher than those reported by Speir (1977), Speir et al. (1980), Saratchandra et al. (1984) and Deng and Taba-tabai (1996a).
3.1.7. CM-cellulase
In the O horizons CM-cellulase activity ranged from 0.27 to 0.93 mmol glucose gÿ1 hÿ1 (mean 0.59, S.D. 0.17, CV 29%), and in the Ah horizons from 0.06 to 0.47 mmol glucose gÿ1 hÿ1 (mean 0.23, S.D. 0.10, CV 43%). The Ah horizon values are comparable with those reported in the literature, but those observed in O horizons are slightly lower than reported values (Kanazawa and Miyashita, 1987; Ohtonen, 1994).
3.1.8.b-Glucosidase
Mean b-glucosidase activity was 8.42 mmol p- nitro-phenol gÿ1 hÿ1 in the O horizons (S.D. 5.14, range 2.05 to 29.63, CV 61%) and 1.55 mmol p-nitrophenol gÿ1 hÿ1 in the Ah horizons (S.D. 0.89, range 0.67 to 4.58, CV 57%). The Ah horizon values can be con-sidered normal, but the O layer values are quite high in comparison with those found by Batistic et al. (1980), Kanazawa and Miyashita (1987) and Deng and Tabatabai (1996b).
3.1.9. Invertase
Invertase activity ranged from 2.27 to 18.83 mmol glucose gÿ1 hÿ1 in the O horizons (mean 12.06, S.D. 3.41, CV 28%) and from 2.00 to 10.95 mmol glucose gÿ1hÿ1in the Ah horizons (mean 6.85, S.D. 2.22, CV 28%). Both sets of values are similar to those found by Batistic et al. (1980), Frankenberger and Dick (1983), Speir et al. (1984), Schinner and von Mersi
(1990) and Ross et al. (1995a,b). Ta
ble
C. Trasar-Cepeda et al. / Soil Biology & Biochemistry 32 (2000) 747±755
3.2. Correlations between speci®c biochemical parameters
As in the companion study of the general biochemi-cal parameters of these soils, and for the same reasons as were discussed by LeiroÂs et al. (1999), correlations among dierent biochemical properties were sought considering the data for the O and Ah horizons jointly. Almost all the activities reported in this paper are signi®cantly correlated with each other (P< 0.001 in most cases; see Table 3). As in related studies (Bon-matõÂ et al., 1991; Tate et al., 1991; Perucci, 1992), the enzyme correlating best with other enzymes is phos-phomonoesterase, which correlates especially closely with CM-cellulase and b-glucosidase, followed by phosphodiesterase and casein-protease. Since phospho-diesterase is also closely correlated with casein-protease and CM-cellulase, there is clearly a strong inter-relationship among enzymatic processes involved in the carbon, nitrogen and phosphorus cycles. By con-trast, arylsulfatase is not very highly correlated with any other enzymatic activity; this apparent lack of coupling between the sulphur cycle and the other major nutrient cycles, which contrasts with the results of Speir et al. (1980, 1984), Frankenberger and Dick (1983) and Saratchandra et al. (1984), may be due to the downregulation hypothesized above.
Among the nitrogen cycle enzymes there is close cor-relation between urease and BAA-protease, but neither of these two enzymes correlates highly with casein-pro-tease, which contrasts with the ®ndings of Speir et al. (1980). This pattern suggests that in Galician soils the degradation of proteins and the degradation of smaller nitrogenated compounds such as peptides and urea are subject to dierent regulatory mechanisms.
Among the carbon cycle enzymes, CM-cellulase is highly correlated withb-glucosidase and invertase, and these latter are also moderately well correlated with each other. This re¯ects the synergism among these enzymes in the degradation of the carbon compounds received by the soil (Panda and Sharma, 1994; Deng and Tabatabai, 1996a,b).
Similarly, the phosphorus cycle enzymes phospho-monoesterase and phosphodiesterase are likewise clo-sely correlated, which is in keeping with the ®ndings of Frankenberger and Dick (1983) and Rastin et al. (1988) and shows the coupling between the enzymes of this cycle.
3.3. Relationships within chemical, physical and biochemical properties
The enzyme activities we measured also correlate closely with physical and chemical properties related to the availability of water and nutrients (results not shown) and with many of the general
biochemi-cal parameters discussed by LeiroÂs et al. (1999). To clarify the structure of these interdependences, we performed a joint principal components analysis (PCA) of our data on the physical, chemical and biochemical properties of the O and Ah layers of these soils, including both general and speci®c bio-chemical parameters. The ®ve main factors identi®ed together account for 76% of the variance; the load-ings of the 27 soil characteristics considered on each factor are listed in Table 4.
Factor I, which accounts for 28% of the total variance, exhibits close positive correlation with
de-hydrogenase, microbial biomass C and available
Ca2+ and K+, and somewhat lower positive corre-lation with ATP, microbial biomass N, catalase, inver-tase, casein-protease and available P, and may thus be regarded as related to the size and activity of the mi-crobial community. The presence of available elements among its de®ning variables may be attributed to the logical dependence of microbial activity on nutrient contents. The high loadings of invertase and casein-protease suggest that, contrary to what has been suggested by Skujins (1978), Nannipieri et al. (1982) and Speir and Ross (1990), the activities of both these enzymes depend more on the microbial community than on their extracellular accumulation, in these soils at least.
Factor II accounts for 16% of the total variance. At its positive pole it is de®ned mainly by phospho-monoesterase, although CM-cellulase, the C-to-N ratio and total C and N contents also have high loadings, and at its negative pole by pH. Though it is not easy to interpret, these associations suggest that it is related to the accumulation of hydrolytic enzymes and of organic matter that is poorly hu-midi®ed because of the acidity of the medium. The relationship of phosphomonoesterase and CM-cellu-lase to the accumulation of organic matter has been mentioned by Harrison (1983) and Sinsabaugh and Linkins (1988), among others.
Factor III accounts for 15% of the total variance. Its main de®ning property is nitrogen mineralization capacity as shown by total inorganic N, although soil respiration and ammoni®cation also contribute to it. Together with the relatively high loadings of the total N and C contents, these properties suggest that Factor III represents the mineralization of organic matter.
Factor IV, which accounted for only 9% of the total variance, may be regarded as re¯ecting the nature and weathering of the parent material, being de®ned by Al and Fe oxide contents. Factor V is de®ned by BAA-protease and urease; though it accounts for only 8% of the total variance, its emergence as a separate factor con®rms the independence of the degradation of low molecular weight nitrogenated compounds, in keeping with the correlation results discussed above.
4. Conclusions
The climax soils of Galicia, a region in the Euro-pean temperate humid zone, are generally acid Umbri-sols (ISSS Working Group RB, 1998) with high organic matter contents. The values of their biochemi-cal properties are in general within the ranges reported in the soil literature for soils from other geographical areas, and exhibit close mutual correlation, the chief exceptions being indicative of a lack of coupling between enzymes involved in the sulphur cycle and those involved in the other nutrient element cycles. The main determinants of the values of biochemical properties in these soils are the activity of their mi-crobial communities and the processes involved in the accumulation and transformation of organic matter.
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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