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Post-fire soil microbial biomass and nutrient content of a pine forest soil from a dunal Mediterranean environment
Soil Biol Biochem. Vol. 28. No. IO/l I. DD. 1467-1475. 1996 Copyhght 0 i996’&evicr Scienk Ltd
PII: soo384:17(%)00160-5
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POST-FIRE SOIL MICROBIAL BIOMASS AND NUTRIENT CONTENT OF A PINE FOREST SOIL FROM A DUNAL MEDITERRANEAN ENVIRONMENT S. DUMONTET,’ H. DINEL,Z A. SCOPA,’ A. MAZZATURA’ and A. SARACINO’* ‘Dipartimento di Produzione Vegetale, Universita della Basilicata, 85100 Potenza, Italy and ‘Centre for Land and Biological Resources Research, Agriculture and Agri-Food Canada, Ottawa, Canada (Accepted 23 May 1996)
Summary-A chronosequence of forest fires (from 1981 to 1992) in a homogeneous Mediterranean pine forest ecosystem was investigated. The effect of fire on soil microbial biomass was evaluated in the O5 cm soil layer utilizing the substrate induced respiration (SIR) method (Anderson and Domsch, 1978) and the fumigation extraction (FE) method (Sparling and West, 1988). The C, N and P contents of soil surface layer were greater 1 y after a fire and declined in soils as time since the last fire increased. After 11 y, the soil nutrient content and the size of the microbial biomass were still lower than the neighbouring unburned site suggesting that forest fires have a long-term effect on soil microbiological properties. Copyright 0 1996 Elsevier Science Ltd
MATERIALSAND METHODS
INTRODUCTION
The growing concern about forest wildfires has stimulated an impressive number of scientific papers dealing with fire ecology ~en~u lute. Most of them are devoted to the understanding of the mineral nutrient dynamics in burned soils (DeBano and Conrad, 1978; Christensen, 1987; Kutiel and Naveh, 1987a,b; DeBano, 1991; Klopatek et al., 1991). Little information is available on the modifications imposed by fire on soil microflora, as well as on microbial recolonization of burned soil (Margaris, 1977; Chandler et al., 1983; Takahiro et al., 1989). In addition, no information is available for the Mediterranean region, and more so for the pine forest ecosystems. Most of the papers dealing with the effect of fire on Mediterranean pine forest are devoted to the study of vegetation dynamics (Trabaud et al., 1985; Moravec, 1990; Ne’eman et al., 1992; Vita et al., 1993; Saracino and Leone, 1993). As regards the effects of fire of different intensities on nutrient dynamics, in the Aleppo pine forests of the Mediterranean basin, the work of Kutiel and Inbar (1983), Kutiel and Naveh (1987a,b) and Gillon and Rapp (1989) can be cited. Our aim was to describe nutrient availability and quantitative aspects of microbial biomass over a chronosequence of burned Aleppo pine (Pinus hulepensis Miller) forest soils. The research was carried out in a homogeneous pine forest ecosystem where sites were selected to reflect various post-fire periods. *Author for correspondence.
Study urea The study area was a natural Aleppo pine (P. hulepensis) forest located in a dunal environment facing the Ionian Sea (Southern Italy). Understory vegetation is made up by evergreen sclerophyllous shrub species, mainly Phillyreu ungustifoliu, Rosmurinus oficinulis, Pistuciu lentiscus, and Cistus sulvifolius (Francini, 1953). The forest had not been utilised for logging since 1965. The topography of the area is dominated by fossil dunes running parallel to the coastline and ranging from 1 to 18 m in height. The soil, originated from the recent Quaternary period, is constituted mainly from loose siliceous-calcareous sands. The soil is characterised by a light colour (10YR 6/3 pale brown), according to the Munsell soil color charts (Anonymous, 1990), a high degree of reflectance and a very low waterholding capacity. The Mediterranean climate, semiarid type, is characterized by relatively warm summers (July mean temperature: 24.9”C) and relatively cool winters (January mean temperature: 8.2”C) with an annual mean rainfall of 535 mm, 67% of which falls in the autumn-winter period, 23% in spring and 10% in summer (Fig. 1). Sites selection and sampling Five sites were selected from the Italian Forest Service records and represented five major fire events that occurred from August 1981 to May 1992. All sampling sites were located between 0.5 and 1 km from the sea and along a 10 km transect running roughly parallel to the coastline. The
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50
100 Marina di Ginosa (5 m a.s.1.) (26, 53)
15.8”C
534.7 mm
40
0
- 80
1 J
I
F
I
I
I
I
I
I
I
I
I
(
0
MAMJJASOND Month
Fig. 1. Climate diagram from the meteorological observatory of Marina di Ginosa, located in the site studied. Top: name of observatory (elevation) (numbers of years of observation for temperature and precipitation respectively) (mean annual temperature) (mean annual precipitation). Thick line: mean monthly temperature. Thin line: mean monthly precipitation. Hatched area: relative humid season. Bar area: relative dry season.
control site (no. 1) was an unburned site located on the same transect. All the burned sites had been affected by high intensity fires and the burned areas were between 38 and 110 ha. At sites 2 and 3 the burned stand was removed after fire, while sites 4, 5 and 6 were left undisturbed. A general description of the sites is reported in Table 1. Samples were collected in the dune hollow at each site in June 1992. They were randomly taken in three replicates in the O-5 cm soil layer under the canopy covered zone (or in the zone which was covered by canopy before fire) and in the proximity of tree stumps at sites 5 and 6. The LFH and ash layers were not included in the samples. The three replicates of each sample were pooled and subsequently three sub-samples were obtained and individually processed both for chemical and microbiological analysis.
Analytical methods
Chemical analysis were carried out on air-dried soil samples. Physical and chemical soil characteristics (Table 2) were determined according to MacKeague (1978). Soluble organic C, N and P were measured in filtered solutions obtained after 40 g (air-dried basis) sample was shaken for 1 h in a 0.05 N KzS04 solution. Total organic C analysis was carried out using the potassium dichromate oxidation method, total N using the Kjeldahl method (with selenium as catalyst) (Sparling and West, 1988) and total P using the ammonium molybdate method (MacKeague, 1978).
Microbiological analysis
Microbiological analysis were carried out on soil samples equilibrated for 5 d at 22°C at 60% of their W.H.C. Both physiological and fumigation methods were used to assess fire effects on soil microbial biomass. Microbial biomass C was evaluated using a modified (200 mg glucose kg-’ dry soil) substrate induced respiration (SIR) method (Anderson and Domsch, 1978). Microbial biomass-C and -N were determined using the fumigation extraction (FE) method (Sparling and West, 1988). Counts of nonactinomycete bacteria (B), actinomycete bacteria (A) and fungi (F) were carried out according to Wollum (1982). Statistical
analysis
Simple linear regression was used to evaluate relationships between soil chemical properties and microbial activities. Data were processed utilising the ANOVA procedure and Duncan’s multiple range test of means by the SAS statistical package (SAS Institute (1988)). Principal component analysis was utilised to find the most significant descriptors for the post-fire evolution; this analysis was performed with SASPRINCOMP procedure and interpretation was in accord with the conceptual approach of Jolicoeur and Mosiman (1960) and Gabriel (1971). RESULTS
Climate (Fig. 1) and soil characteristics, such as particle size and pH, confirmed that the different
Post-fire soil microbial biomass and nutrient content
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Table 1. General description of sampling sites Sites
Geographical co-ordinates
Fire dates
General description
1
Lat 40”28’44 N Long 16”57’26 E
Unburnt
45 y-old dense pine forest with patchy shrubs understorey chiefly constituted by Phil/yrea angustifolia and Rosmnrinus offieinalis The litter was constituted by a 2 cm undecomposed needles layer.
2
Lat 40”30’42 N Long 17”01’12 E
May 1992
40 y-old pine forest with charred trees. On top soil were present black, white ashes and sparse evergreen sclerophyllous (chiefly P. mgus~i&dia). Experimental plot. The burnt stand was left undisturbed at this station.
3
Lat 40”26’25 N Long 16”53’21E
August 199 1
Old pine forest stand of charred trees with shrub understorey chiefly constituted by P. lentiscus, P. angustifolia, and C. salvifolius The top soil was covered by a 0.5 cm layer of ashesExperimental plot. The burnt stand was left industurbed at this site.
4
Lat 40”30’20 N Long 17”00’08 E
July 1990
Garrigue constituted chiefly by resprouters P. angusfifolia, Pistacia lentiscus, Myrtus commwtis and meseeders C&us salvifolius and Dorycnium hirsurum.ExperimentaI plot. The burnt stand was left undisturbed at this site.
5
Lat 40”28’37 N Long 16”57’29 E
June 1986
As site no. 2
6
Lat 40”27’24 N Long 16’55’07 E
August 198 1
Garrigue constituted chiefly by R. of~cinalis and P. angustifolin. The burnt stand was logged after tire.
sites were pedologically homogeneous (Table 2). The main differences among sites were the date of fire, density of canopy and the degree of shrubs and grass covering in the oldest burned sites. Unburned soil characteristics and microbial biomass
The soil of the unburned site was low in organic matter with an organic C and total N content of 1.1 and 0.07%, respectively (Table 2). The P-Olsen was also low at 12.6 PgP g-‘. The low content of organic matter and mineral nutrients was also reflected in the amount of soluble organic C, soluble total N and soluble P (Table 3). The microbial biomass C for the unburned soil measured by SIR and FE methods was 138 and 176 mg C kg-’ dry soil, respectively, while the N-biomass was 27 mg N kg-’ dry soil, representing approximately 3.8% of the soil N. Burned soils characteristics and microbial biomass
Total organic C, total N and P-Olsen content of the sites sampled are shown in Table 2. It is worth noting that organic C and total N contents are similar for all sites, except for site 3 where the organic C and total N were 5.8 and 5.4 times larger than
those measured in the unburned site. The P-Olsen content varied amongst sites: the largest amount was found in soil 3, the lower in site 1. Soil 2 had a P-Olsen content close to that of soils 6 and 5. The amounts of soluble C, N and P and the total C, total N and P-Olsen of the studied sites are presented in Table 3. Even though site 3 had a solubleC content statistically similar to that of all sites (unburned and burned, except site 2), the percentage of soil C present as soluble-C was 4.6 times lower than the unburned site. The same was observed for the mineral nitrogen (NH: + NO;) content, which was at site 3, 4.5 times higher than the control (which showed the lowest content), but when expressed as percentage of total N it fell to the lowest value (1.2 times lower than the control). The soluble-P content was much more homogeneous and did not show statistically significant differences between sites. In Table 4 biomass-C and -N values obtained by SIR and FE methods are reported. It is interesting to note the consistency of these two methods: the ANOVA test showed that they were not statistically different at P < 0.05. The lowest values in microbial biomass were obtained for sites 6 and 5 (burned in
Table 2. Soil characteristics Site
Sand W)
Silt W)
Clay W)
PH
Organic C’ (g kg-’ dw)
Total N’ (g kg-’ dw)
1 2 3 4 5 6
95.9 94.8 95.0 95.1 96.0 95.9
0.6 0.2 0.7 0.9 0.0 0.7
3.5 5.0 4.3 4.0 4.0 3.4
1.9 7.8 7.3 8.0 1.5 7.9
I l.OC(kO.3)
0.7b 0.6b 3.8a 0.7b 0.7b 0.7b
9.0’ (+0.8) 63.9” (k2.0) 17.gb (kO.5) I I .4’ (kO.4) I I .5c (kO.3)
(kO.08) (kO.08) (H.22) (kO.14) (~0.00) (kO.14)
’ (&standard error). Values in the same column followed by the same letter are not different at P < 0.05 (Duncan’s multiple range test).
P-Olsen’ (mg kg-’ dw) 12.6e (50.49) 18.0k (kO.22) 32.9’ (kO.51) 16.2d (f0.22) 18.9b (kO.22) 17.7c (H.22)
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S. Dumontet et al. Table 3. Soluble C, N and P content Soluble-C’ (mg C kg-’ dw)
Soluble-C (% of soil C)
in burned and unburned
Soluble-N’
Site N-NH;
I
2 3 4 5 6
15.1”~ @l.l3) 12.9’ (kl.95) 17.7” (il.03) 17.5’h (kO.06) 16.6ab (kO.69) 14.5”k (f0.77)
0.14 0.14 0.03 0.10 0.15 0.13
+
3.3e 8..ib 14.v 7.1’ 4.7d 4Y
’ (*standard error) Values in the same column followed
(f0.06) (kO.75) (f0.53) (kO.09) (kO.03) (_+O.lO)
12.5b (kO.23) 10.5’ (+0.23) 11.1’ (iO.21) 14.7” (kO.23) 14.3M(iO.20) 12.5” (irO.29)
by the same letter are not different
soil chemical
properties
and
Soil chemical properties (pH, organic C, soluble C, total N, mineral N, organic-N, P-Olsen and soluble-P) and microbial variables (SIR biomass, BIOSIR), FE biomass (BIOFE), N biomass Table 4. C and N mxrobial Biomass-C’(mg
C kg-’ dw)
Site SIR I 2 3 4 5 6
138’ 172b 290’ 20Sb 79* IOld
(fll.7) (kl2.0) (k8.1) (zt6. I) (zk7.8) (rtl7.4)
Biomass-C’ (mg C kg-’ dw) pooled SIR and FE values
FE 176b 132’ 274’ 197b 75d 7sd
(k15.5) (i3.0) (k21.2) (*6.8) (*9.0) (*9.0)
156’ (kl2.2) 152’ (f12.6) 282a (f10.7) 201 b (k4.4) 77* (f5.4) 8gd (*lo.5)
N-NH; + N-NO: (X of total soil N)
Soluble-P’ (mg P kg-’ dw)
Soluble-P as % of P-Olsen
0.47 1.39 0.39 I .02 0 67 0.64
0.50”b (kO.00) 0.35b (kO.05) 0.75” (kO.15) 0.70” (f0.10) 0.55ab (kO.05) 0.50db (+o.oo)
4.0 2.2 2.4 4.3 2.6 2.8
N-NO; OrgatWN (mg N kg-’ dw)
August 1981 and June 1986, respectively), while the highest one (higher than the unburned site) was measured in site 3 (burned on August 1991) (Table 4). Biomass-C at site 2, which was burned less than 4 weeks before sampling, was similar to those measured in the unburned site. Even the values for sites I and 2 are statistically different for both SIR and FE, the mean values obtained from the two methods did not confirm such a difference. Despite the fact that site 3 had the highest biomass value, the soil had the lowest biomass-C content when expressed as a proportion of total soil organic C. A similar pattern was also observed for biomass-N (Table 4). The biomass immobihsation of N at site 3 was almost twice the value of the unburned soil and the biomass-N value was onethird that of the control when expressed as a percentage of soil total N. On the other hand, the other burned sites showed a decreasing trend of biomass-C and -N as a function of increasing time since the date of the fire. Relationship between microbial activit,v
soils
Biomass-C
at P < 0.05 (Duncan‘s
multiple
range test).
(BION), C-to-N ratio of the biomass (BIOCN), BIOSIR as % of soil organic C (CSIR), BIOFE as % of soil organic C (CFE), N biomass as % of soil total N (NBION) and non-actinomycete bacteriato-actinomycete bacteria + fungi ratio (B/A + F) were tested for their degree of association. The correlation matrix (Table 5) shows that only BIOSIR had a significant positive correlation with organic C and mineral N, while BION was positively and significantly correlated with mineral N. There were other strong correlations among variables which appear to be random in some cases. In contrast, there was no significant correlation amongst total N, P-Olsen and soluble C. In order to determine underlying interrelationships amongst variables, the data were subjected to principal component analysis. The PRINI vs PRINZ explained 79.6% of the total variance (Fig. 2) and revealed that there was a close association between other microbial variables (BIOFE) and soil chemical properties (P-Olsen, total N, organic C and soluble C). The relative position of sites 6 through 4 in Fig. 2 suggests that these sites may have reached a similar state of development after the fire, while positions for sites 3 and 2 indicate that these sites were affected differently by fire and the effect of the fire in site 3 was still present.
biomasses
in burned and unburned
(% of soil C)
SIR
FE
1.3 1.9 0.5 I.2 0.7 0.9
1.6 1.4 0.4 I.1 0.6 0.6
Biomass-N’ (mgd;kK’
27b (k5.0) 37b (k5.0) 508 (i5.4) 30b (k2.9) 13c (kl.2) 14c (*2. I)
’ (*standard error). * The calculation is made taking into account the Biomass-C FE values. 3 Non-actinomycete bacteria/actinomycete bacteria + fungi. Values in the same column followed by the same letter are not different at P < 0.05 (Duncan’s
soils
Biomass-N (% of soil N)
3.8 3.7 1.3 4.3 1.9 2.0
multiple
Biomass C/N2
B/A + F3
6.5 3.6 5.5 6.6 5.8 5.4
9.6 18.6 8.8 9.6 8.6 11.2
range test).
Post-fire soil microbial biomass and nutrient content
1471
Fig. 3 shows the trends of the microbial biomassC and -N (expressed as mg kg’ of dry soil and as % of soil C and N) along the studied chronosequence. It is interesting to note that biomass-C and -N were increasing from site 2 to 3 and decreasing afterwards, while biomass-C and -N were decreasing from site 2 to 3, increasing from site 3 to 4 and decreasing afterwards.
DISCUSSION
Little is known about the effect of fire on soil biomass-C and -N and on the soil nutritional status in Mediterranean environments. Several authors reported an increase of soil mineral nutrients following fires, in different non-Mediterranean ecosys1984; tems (Chandler et al., 1983; Pyne, Christensen, 1987; DeBano, 1991; Klopatek et al., 1991). Such an increase was explained by an enrichment of soil by ash, which would represent one reservoir for mineral N, P, Ca, K and Mg (Christensen, 1987). Klopatek et al. (1991) stated that the soil mineral enrichment by ash, and specifically for N, is strictly related to the fire intensity. From direct observations of the sites investigated and from the records of the Italian Forest Service, it can be assumed that all fires studied here were of high intensity. Klopatek et al. (1991) also stated that one of the greatest initial effects of fire is the reduction of the soil C-to-N ratio resulting from an increase in mineralization processes; however, our data do not support such a statement. The soil Cto-N ratio (Table 2) ranged from 15 to 25 for sites affected by the most recent fires, including the unburned site. The variation of nutritional status of soils along the chronosequence remained unchanged, except for site 3 which was burned 1 y before the sampling. Kutiel and Naveh (1987b) found that the concentration of N-NH: in a brown rendzina soil under a pine forest was more than twice that of an unburned site 8 months after fire, and the concentration of total P increased 3-fold after 2 months following the fire. Furthermore, the characterisation of the soil profiles showed that such a soil enrichment was associated with a decrease of the mineral content of the ash layer. Prieto-Fernandez et al. (1993) also observed 1 month after fire in a HumicCambisol under Pinus pinuster an enrichment in total N and N-NH:, while N-NO; was found to be increased only in the 5510cm soil layer. Saa et al. (1993) reported a meaning 5-fold increase of inorganic soluble P in the O-5 cm layer of a humiccambisol under Pinus pinaster and Ulex europaeus 1 month after wildfire, by comparison with the unburned sites. In our study the substantial unchanged amounts of mineral nutrients in site 6 (Table 2), by comparison with site 1, could be explained by the semi-arid climate of the area stu-
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et al.
B/A+F Organic N
z”
$ 1
0
.
x
Soluble C
-1
-1
0
1
PRIN 1
Fig. 2. Orthogonal distribution of the principal component (PRIN) analysis for the postfire evolution of the dunal sandy soils. PRIN 1 vs PRIN 2 explained 79.6% of the total variance. BIOSIR and CSIR represent microbial biomass obtained by the substrate induced respiration method and expressed as mg C kg-’ dw and as % of total soil C, respectively. BIOFE and CFE represent microbial biomass obtained by fumigation-extraction method and expressed as mg C kg-’ dw-and as % of soil total C, respectively. BION and NBION represent the microbial biomass expressed as mg N kg-’ dw and % of soil total N respectively. BIOCN represents the C-to-N ratio of soil microbial biomass while B/A + F represents the non-actinomycete bacteria-to-actinomycete bacteria + fungi ratio. The numbered black dots represent the studied sites.
died, characterised by relatively low annual precipitation (535 mm), summer drought and mesic temperature (158°C). After fire events mineral nutrients may temporarily remain in the ash layer, but eventually they may be solubilised, brought downward through the soil profile and utilised by the growing plants and soil microorganisms. Thus, they could be leached by rain from the sandy soil upper layer during the following months. In comparison with other ecosystems in which the fire causes a short term flush of mineral elements (Pyne, 1984; Kutiel and Naveh, 1987a; Klopatek et al., 1991), a relevant soil enrichment was observed only for site 3 which was burned 1 y before sampling (Table 2). This enrichment should not be associated with the condensation of fire-vapourised organic matter downward in the soil profile as proposed by DeBano (1991) due to time elapsed since the fire at this site. If we consider soluble-C, -N and -P content, site 3 behaves in a contrasting manner to the others (Table 3). In fact, although its soluble-C content was comparable with the other sites, it was only 23.3% that of the unburned site when expressed as % of total C. This finding could be explained by the higher values of biomass-C (Table 4), indicating a higher rate of use and immobilisation of the readily available C. In addition, in the O-5 cm layer of site 3 the amount of CHCls-extractable lipids
was 20 and 10 times higher than those found in the unburned soil and in site 2, respectively (Dine], unpublished data). These findings highlight the multiple effects of fire on soil and remaining vegetation, which could also have a direct consequence on some biotic soil characteristics. It was observed at site 3 a relatively well-established herbaceous vegetation (mainly Oryzopsis miliacea), probably due to the soil profile enrichement in mineral N (Table 3) by the rain-solubilized ashes. Kutiel and Naveh (1987b) observed that a rapid compensation of Ndepletion occurred during fire and suggested that it resulted from Na fixation by the free-living diazotrophs. In our study the N content of site 3 was more than 5 times higher than the control (Table 2) but it is unlikely that it can only be explained by the stimulation of bacterial nitrogen fixation. The observed N enrichment may be responsible for the stimulation of microbial biomass growth resulting in an increase of both soil organic matter content and mineralisation. Although sites 3 and 2 showed the lowest soluble-N organic content, site 3 had the higher amount mineral-N (NH: + NOT), which represents only 0.38% of the soil total N (Table 3). These data suggest that a consistent amount of ash mineral-N at this site was immobilised in microbial cells or transformed into another insoluble organic form.
Post-fire soil microbial biomass and nutrient content
z
250
2 if
200
1
150
0 a g
.d $
-O-
Biomass-C (FE)
+
Biomass-N
-..O---
Biomass-Cas%ofsoilC
---A--.
Biomass-N as 46 of soil N
‘& ____.__.__._.___...... -.-..A
: :’
loo
1473
50
OIf
1
0
I
,
I
2
I
4
’
I
6 Time afterfire (years)
’
I
8
I
I
10
I
i
0
12
Fig. 3. Postfire evolution of biomass and carbon expressed as mg C kg-’ dw (mean f standard error) and as % of soil C and N. The observed behaviours are probably due to the pedological characteristics of the soil which make it unable to limit the leaching of mineral nutrients from the surface ash. Lewis (1974) stated that the importance of nutrient losses due to leaching vary considerably in relation to the absorptive characteristics of soils. The expected low cation exchange capacity of the sandy soil studied may be responsible for the nutrient losses; although leaching was limited in time and intensity by the semi-arid climate, the mineral nutrients could have run off during the years following fire, with a consequent reduction of microbial and plant biomass production. The biomass-C values (Table 4) of sites 1 (unburned) and 2 (burned 4 wk before sampling) are very close to each other considering both SIR and FE values, but they are not statistically different (P < 0.05) if the results of SIR and FE methods are pooled. Biomass-N values of these sites are also not statistically different (P < 0.05). These results are in contrast with those reported by Takahiro et al. (1989), which showed a strong decrease of the soil microbial biomass in a burned Japanese red pine forest 2 and 6 months after fire. Despite similar microbial biomass values for sites 2 and 1 the C-to-N ratio of microbial tissues suggests a fire-stress (Table 4). Biomass C-to-N ratio ranged between 5.4 and 6.6, except for site 2 which had a value of 3.6. These data are confirmed by the non-actinomycete bacteria-to-actinomycete bacteria + fungi ratio which ranged for all soils from 8.8 to 11.2, except for soil 2 which had a value of 18.6. These results are consistent with those of Powlson (1975) who observed in pasteurised greenhouse soils that fungi are more sensitive to heat, fol-
lowed by actinomycete bacteria and nonactinomycete bacteria. Similarly, Margaris (1977) reported that Discomycetes, Pyrenomycetes and Basidiomycetes decreased considerably just after fire and then increased their biomass. Thus, the heat sensitivity of fungi could be responsible for the lowest C-to-N ratio in site 2. The C and N content of the microbial biomass (Fig. 3) showed an increase during the first year after fire and a decrease in subsequent years. The oldest burned sites (site 6 and 5, burned 11 and 6 y before sampling, respectively) still had the lowest microbial biomass values. It was evident that the interference of fire with the microbial commmunities in sandy soil studied lasts for a long period. The lack of information about the size of microbial biomass of pine forests in Mediterranean ecosystems prevents comparison with our data. any Nevertheless, it is worth noting that the size of microbial biomass reported in this work was extremely low as compared to the values reported by other authors who studied the effect of fire on the size of soil microbial biomass. In fact, Takahiro et al. (1989) found values from 737 to 895 mg C-biomass kg-’ dry soil in a O-2 cm layer of an immature clay-loam soil under Pinus densiJora stand, while Margaris (1977) found in the Greek Mediterranean phryganic ecosystem an unlikely microbial biomass value as high as 7150 mg C kg-’ dry soil. CONCLUSIONS
The effects of fire, on the chemical characteristics of the sandy soil studied, Seem to follow a chronosequence which roughly shows the same pattern of
1474
S. Dumontet
the already described soil microbial biomass variations (Fig. 3). Organic C, total N, P-Olsen and mineral-N contents are, in fact, higher in site 3 burned 1 y before sampling than in the other sites, including site 2 which was burned a few weeks before sampling. This figure is reversed if soluble-C and mineral-N, expressed as % of soil organic C and total N, are considered. In this case site 3 shows the lowest values. This means that the sandy soil is significantly enriched in C, N and P 1 year after fire, but a small portion of these elements are in a soluble inorganic form. Even though fire seems to affect soil chemical properties hardly at all, except at 1 y after the pine forest was burned (Tables 2 and 3) its effects become evident if microbial variables are considered. Soil microbial biomass values show, in fact, a high degree of relation with the fire date. The most relevant in features in this respect are:
(i) Even 11 y after fire the microbial biomass is far from recovery of the same value of the unburned site; (ii) the increase in microbial biomass recorded 1 y after fire is, under the climatic and pedological characteristics of our study, rather ephemeral; (iii) the increase in microbial biomass observed 1 y after fire (soil 3) is not consistent with the increase of soil organic C. In fact microbial biomass expressed as % of soil organic C and total N at this site accounts for less than half of the control value. These data confirm that fire exerted a strong and long-lasting effect on the microbial communities of our sandy soil. More investigations are needed to fill the gap of information which prevents us from comparing the effects of fire on microbial biomass in similar Mediterranean environments.
REFERENCES
Anderson J. P. E. and Domsch K. H. (1978) A physiological method for the quantitative measurement of microbial biomass in soils. Soil Bioiogy and Biochemisrry 10, 215-221. Anonymous (1990) Munsell Soil Color Charts. Munsell Color, Baltimore, Maryland. Chandler C., Cheney P., Thomas P., Trabaud L. and Williams D. (1983) Fire effects on soil, water, and air. In Fire in Forestry. Vol. I: Forest Fire Behaviour and Eficrs, pp. 171-202. Wiley, New York. Christensen N. L. (1987) The biogeochemical consequenses of fire and their effects on the vegetation of the coastal plain of the southeastern United States. In The Role of Fire in Ecological Systems (L. Trabaud, Ed.), pp. I-21. SPB Academic, The Hague. DeBano L. F. (1991) The effect of fire on soil properties. In Proceedings of Management and Productivity of Western-Montane Forest Soils (A. E. Harwey and L. F. Neuenschwander, Compilers), pp. 151-l 56. 1990, Boise,
Idhao,
General
Technical
Report
INT-280,
LJ. S.
et al.
Department of Agriculture, Forest Service, Intermountain Research Station. DeBano L. F. and Conrad C. E. (1978) The effect of fire on nutrients in a chaparral ecosystem. Ecology 59, 489497. Frdncini E. (1953) II pino d’Aleppo in Puglia. Annali Facoita Agraria Universitd Bari 8, 3099416. Gabriel K. R. (1971) The biplot graphic display of matrices with application to principal component analysis. Biometrika 58, 453-467. Gillon D. and Rapp M. (1989) Nutrient losses during a winter low-intensity prescribed fire in a Mediterranean forest. Plant Soil 120, 69977. Jolicoeur P. and Mosiman J. E. (1960) Size and shape variation in the painted turtle: A principal component analysis. Growth 24, 339-354. Klopatek J. M., Klopatek C. C. and DeBano L. F. (1991) Fire effects on nutrient pools of woodland floor materials and soils in a pinyon-juniper ecosystem. In Fire and Environmenf. rhe Ecological and Cultural Perspectives (S. C. Nodvin and T. A. Waldrop, Eds), 154-159. Proceedings of an International PP. Symposium, 1990, Knoxville, Tennessee. General Technical Report SE-69, USDA Forest Service, Southeastern Forest Experimental Station. Kutiel P. and Inbar M. (1983) Fire impacts on soil nutrient and soil erosion in a Mediterranean pine forest plantation. Catena 20, 1299139. Kutiel P. and Naveh Z. (1987a) Soil properties beneath Pinus halepensis and Quercus calliprinos trees on burned and unburned mixed forest on Mt Carmel, Israel. Foresr Ecology and Management 20, 1 I-24. Kutiel P. and Naveh Z. (1987b) The effect of fire on nutrients in a pine forest soil. Plant Soil 104, 269-274. Lewis W. M. Jr. (1974) Effects of fire on nutrient movement in a South Carolina pine forest. Ecology 55, 112& 1127. MacKeague J. A. (1978) Manual of Soil Sampling and Methods of Analysis. Soil Research Institute, Research Branch, Agriculture Canada, Ottawa. Margaris N. S. (1977) Decomposers and the fire cycle in Mediterranean-type ecosystems. In Proceedings of the Symposium on the Environmental Consequences of Fire and Fuel Management in Mediterranean Ecosystems (H. A. Mooney and E. C. Conrad, Tech. Coordinators), pp. 3745. 1977, Palo Alto, California. USDA Forest Service General Technical Report WO-3, Washington, DC. Moravec J. (1990) Regeneration of N. W. African Pinus halepensis forests following fire. Vegetatio 87, 29-36. Ne’eman G., Lahav H. and Izhaki I. (1992) Spatial pattern of seedlings 1 year after fire in a Mediterranean pine forest. Oecoiogia 91, 365-370. Powlson D. S. (1975) Effects of biocidal treatments on soil organisms. In Soil Microbiology (N. Walker, Ed.), pp. 193-224. Butterworths, London. Prieto-Fernandez A., Villar M. C., Carballas M. and Carballas T. (1993) Short-term effects of a wildfire on the nitrogen status and its mineralization kinetics in an Atlantic forest soil. Soil Biology & Biochemistry 25, 1657-1664. Pyne S. J. (1984) Introduction to Wiidland Fire. Wiley, New York. Saa A., Trasar-Cepeda M. C., Gil-Sotres F. and Carballas T. (1993) Changes in soil phosphorus and acid phosphatase activity immediately fofiowing forest fires. -Soil Bioloav & Biochemistrv 25. 1223-1230. Saracino A. and Leone-V. (1993) Natural regeneration 2 and 4 years after fire of Pinus halepensis Miller in dunal environment. In Fire in Mediterranean Ecosystems (L. Trabaud and R. Prodon, Eds), pp. 141-150. 1992,
Post-fire soil microbial bioinass and nutrient content Banyuls sur Mer, France, Commission of the European Communities, Ecosystem Research Report no 5. SAS Institute (1988) SAS/STAT user’s guide. Cary, NC, U.S.A. Sparling G. P. and West A. W. (1988) Modification of the fumigation-extraction technique to permit simultaneous extraction and estimation of soil microbial C and N. Comunications in Soil Science 327-344.
and Plant Analysis 19,
Takahiro Tateishi, Takao Horikoshi and Hiroyuki Tsubota (1989) Microbial biomass in the soils at burned and unburned red pine forests. Bulletin of the Japanese Society of Microbial Ecology 4, 77-87.
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Trabaud L., Michels C. and Grosman J. (1985) Recovery of burnt Pinw hafepensis Mill. forests. II. Pine reconstitution after wildfire. Forest Ecology and Management 13, 167-179.
Vita F., Schiavone M. S. and Forte L. (1993) Mutamenti bioclimatici e vegetazionali indotti da1 fuoco in alcune pinete litoranee dell’arco jonico. Monti e Boschi 2, ll19. Wollum A. G. (1982). Cultural methods for soil microorganisms. In Methods for Soil Analysis (A. L. Page, R. H. Miller and D. R. Keeney, Eds), Vol. 2, pp. 781-801. American Society of Agronomy, Madison.
PII: soo384:17(%)00160-5
Printed
in Great Britain. All rights reserved 0038-0717/96 $15.00 + 0.00
POST-FIRE SOIL MICROBIAL BIOMASS AND NUTRIENT CONTENT OF A PINE FOREST SOIL FROM A DUNAL MEDITERRANEAN ENVIRONMENT S. DUMONTET,’ H. DINEL,Z A. SCOPA,’ A. MAZZATURA’ and A. SARACINO’* ‘Dipartimento di Produzione Vegetale, Universita della Basilicata, 85100 Potenza, Italy and ‘Centre for Land and Biological Resources Research, Agriculture and Agri-Food Canada, Ottawa, Canada (Accepted 23 May 1996)
Summary-A chronosequence of forest fires (from 1981 to 1992) in a homogeneous Mediterranean pine forest ecosystem was investigated. The effect of fire on soil microbial biomass was evaluated in the O5 cm soil layer utilizing the substrate induced respiration (SIR) method (Anderson and Domsch, 1978) and the fumigation extraction (FE) method (Sparling and West, 1988). The C, N and P contents of soil surface layer were greater 1 y after a fire and declined in soils as time since the last fire increased. After 11 y, the soil nutrient content and the size of the microbial biomass were still lower than the neighbouring unburned site suggesting that forest fires have a long-term effect on soil microbiological properties. Copyright 0 1996 Elsevier Science Ltd
MATERIALSAND METHODS
INTRODUCTION
The growing concern about forest wildfires has stimulated an impressive number of scientific papers dealing with fire ecology ~en~u lute. Most of them are devoted to the understanding of the mineral nutrient dynamics in burned soils (DeBano and Conrad, 1978; Christensen, 1987; Kutiel and Naveh, 1987a,b; DeBano, 1991; Klopatek et al., 1991). Little information is available on the modifications imposed by fire on soil microflora, as well as on microbial recolonization of burned soil (Margaris, 1977; Chandler et al., 1983; Takahiro et al., 1989). In addition, no information is available for the Mediterranean region, and more so for the pine forest ecosystems. Most of the papers dealing with the effect of fire on Mediterranean pine forest are devoted to the study of vegetation dynamics (Trabaud et al., 1985; Moravec, 1990; Ne’eman et al., 1992; Vita et al., 1993; Saracino and Leone, 1993). As regards the effects of fire of different intensities on nutrient dynamics, in the Aleppo pine forests of the Mediterranean basin, the work of Kutiel and Inbar (1983), Kutiel and Naveh (1987a,b) and Gillon and Rapp (1989) can be cited. Our aim was to describe nutrient availability and quantitative aspects of microbial biomass over a chronosequence of burned Aleppo pine (Pinus hulepensis Miller) forest soils. The research was carried out in a homogeneous pine forest ecosystem where sites were selected to reflect various post-fire periods. *Author for correspondence.
Study urea The study area was a natural Aleppo pine (P. hulepensis) forest located in a dunal environment facing the Ionian Sea (Southern Italy). Understory vegetation is made up by evergreen sclerophyllous shrub species, mainly Phillyreu ungustifoliu, Rosmurinus oficinulis, Pistuciu lentiscus, and Cistus sulvifolius (Francini, 1953). The forest had not been utilised for logging since 1965. The topography of the area is dominated by fossil dunes running parallel to the coastline and ranging from 1 to 18 m in height. The soil, originated from the recent Quaternary period, is constituted mainly from loose siliceous-calcareous sands. The soil is characterised by a light colour (10YR 6/3 pale brown), according to the Munsell soil color charts (Anonymous, 1990), a high degree of reflectance and a very low waterholding capacity. The Mediterranean climate, semiarid type, is characterized by relatively warm summers (July mean temperature: 24.9”C) and relatively cool winters (January mean temperature: 8.2”C) with an annual mean rainfall of 535 mm, 67% of which falls in the autumn-winter period, 23% in spring and 10% in summer (Fig. 1). Sites selection and sampling Five sites were selected from the Italian Forest Service records and represented five major fire events that occurred from August 1981 to May 1992. All sampling sites were located between 0.5 and 1 km from the sea and along a 10 km transect running roughly parallel to the coastline. The
1467
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50
100 Marina di Ginosa (5 m a.s.1.) (26, 53)
15.8”C
534.7 mm
40
0
- 80
1 J
I
F
I
I
I
I
I
I
I
I
I
(
0
MAMJJASOND Month
Fig. 1. Climate diagram from the meteorological observatory of Marina di Ginosa, located in the site studied. Top: name of observatory (elevation) (numbers of years of observation for temperature and precipitation respectively) (mean annual temperature) (mean annual precipitation). Thick line: mean monthly temperature. Thin line: mean monthly precipitation. Hatched area: relative humid season. Bar area: relative dry season.
control site (no. 1) was an unburned site located on the same transect. All the burned sites had been affected by high intensity fires and the burned areas were between 38 and 110 ha. At sites 2 and 3 the burned stand was removed after fire, while sites 4, 5 and 6 were left undisturbed. A general description of the sites is reported in Table 1. Samples were collected in the dune hollow at each site in June 1992. They were randomly taken in three replicates in the O-5 cm soil layer under the canopy covered zone (or in the zone which was covered by canopy before fire) and in the proximity of tree stumps at sites 5 and 6. The LFH and ash layers were not included in the samples. The three replicates of each sample were pooled and subsequently three sub-samples were obtained and individually processed both for chemical and microbiological analysis.
Analytical methods
Chemical analysis were carried out on air-dried soil samples. Physical and chemical soil characteristics (Table 2) were determined according to MacKeague (1978). Soluble organic C, N and P were measured in filtered solutions obtained after 40 g (air-dried basis) sample was shaken for 1 h in a 0.05 N KzS04 solution. Total organic C analysis was carried out using the potassium dichromate oxidation method, total N using the Kjeldahl method (with selenium as catalyst) (Sparling and West, 1988) and total P using the ammonium molybdate method (MacKeague, 1978).
Microbiological analysis
Microbiological analysis were carried out on soil samples equilibrated for 5 d at 22°C at 60% of their W.H.C. Both physiological and fumigation methods were used to assess fire effects on soil microbial biomass. Microbial biomass C was evaluated using a modified (200 mg glucose kg-’ dry soil) substrate induced respiration (SIR) method (Anderson and Domsch, 1978). Microbial biomass-C and -N were determined using the fumigation extraction (FE) method (Sparling and West, 1988). Counts of nonactinomycete bacteria (B), actinomycete bacteria (A) and fungi (F) were carried out according to Wollum (1982). Statistical
analysis
Simple linear regression was used to evaluate relationships between soil chemical properties and microbial activities. Data were processed utilising the ANOVA procedure and Duncan’s multiple range test of means by the SAS statistical package (SAS Institute (1988)). Principal component analysis was utilised to find the most significant descriptors for the post-fire evolution; this analysis was performed with SASPRINCOMP procedure and interpretation was in accord with the conceptual approach of Jolicoeur and Mosiman (1960) and Gabriel (1971). RESULTS
Climate (Fig. 1) and soil characteristics, such as particle size and pH, confirmed that the different
Post-fire soil microbial biomass and nutrient content
1469
Table 1. General description of sampling sites Sites
Geographical co-ordinates
Fire dates
General description
1
Lat 40”28’44 N Long 16”57’26 E
Unburnt
45 y-old dense pine forest with patchy shrubs understorey chiefly constituted by Phil/yrea angustifolia and Rosmnrinus offieinalis The litter was constituted by a 2 cm undecomposed needles layer.
2
Lat 40”30’42 N Long 17”01’12 E
May 1992
40 y-old pine forest with charred trees. On top soil were present black, white ashes and sparse evergreen sclerophyllous (chiefly P. mgus~i&dia). Experimental plot. The burnt stand was left undisturbed at this station.
3
Lat 40”26’25 N Long 16”53’21E
August 199 1
Old pine forest stand of charred trees with shrub understorey chiefly constituted by P. lentiscus, P. angustifolia, and C. salvifolius The top soil was covered by a 0.5 cm layer of ashesExperimental plot. The burnt stand was left industurbed at this site.
4
Lat 40”30’20 N Long 17”00’08 E
July 1990
Garrigue constituted chiefly by resprouters P. angusfifolia, Pistacia lentiscus, Myrtus commwtis and meseeders C&us salvifolius and Dorycnium hirsurum.ExperimentaI plot. The burnt stand was left undisturbed at this site.
5
Lat 40”28’37 N Long 16”57’29 E
June 1986
As site no. 2
6
Lat 40”27’24 N Long 16’55’07 E
August 198 1
Garrigue constituted chiefly by R. of~cinalis and P. angustifolin. The burnt stand was logged after tire.
sites were pedologically homogeneous (Table 2). The main differences among sites were the date of fire, density of canopy and the degree of shrubs and grass covering in the oldest burned sites. Unburned soil characteristics and microbial biomass
The soil of the unburned site was low in organic matter with an organic C and total N content of 1.1 and 0.07%, respectively (Table 2). The P-Olsen was also low at 12.6 PgP g-‘. The low content of organic matter and mineral nutrients was also reflected in the amount of soluble organic C, soluble total N and soluble P (Table 3). The microbial biomass C for the unburned soil measured by SIR and FE methods was 138 and 176 mg C kg-’ dry soil, respectively, while the N-biomass was 27 mg N kg-’ dry soil, representing approximately 3.8% of the soil N. Burned soils characteristics and microbial biomass
Total organic C, total N and P-Olsen content of the sites sampled are shown in Table 2. It is worth noting that organic C and total N contents are similar for all sites, except for site 3 where the organic C and total N were 5.8 and 5.4 times larger than
those measured in the unburned site. The P-Olsen content varied amongst sites: the largest amount was found in soil 3, the lower in site 1. Soil 2 had a P-Olsen content close to that of soils 6 and 5. The amounts of soluble C, N and P and the total C, total N and P-Olsen of the studied sites are presented in Table 3. Even though site 3 had a solubleC content statistically similar to that of all sites (unburned and burned, except site 2), the percentage of soil C present as soluble-C was 4.6 times lower than the unburned site. The same was observed for the mineral nitrogen (NH: + NO;) content, which was at site 3, 4.5 times higher than the control (which showed the lowest content), but when expressed as percentage of total N it fell to the lowest value (1.2 times lower than the control). The soluble-P content was much more homogeneous and did not show statistically significant differences between sites. In Table 4 biomass-C and -N values obtained by SIR and FE methods are reported. It is interesting to note the consistency of these two methods: the ANOVA test showed that they were not statistically different at P < 0.05. The lowest values in microbial biomass were obtained for sites 6 and 5 (burned in
Table 2. Soil characteristics Site
Sand W)
Silt W)
Clay W)
PH
Organic C’ (g kg-’ dw)
Total N’ (g kg-’ dw)
1 2 3 4 5 6
95.9 94.8 95.0 95.1 96.0 95.9
0.6 0.2 0.7 0.9 0.0 0.7
3.5 5.0 4.3 4.0 4.0 3.4
1.9 7.8 7.3 8.0 1.5 7.9
I l.OC(kO.3)
0.7b 0.6b 3.8a 0.7b 0.7b 0.7b
9.0’ (+0.8) 63.9” (k2.0) 17.gb (kO.5) I I .4’ (kO.4) I I .5c (kO.3)
(kO.08) (kO.08) (H.22) (kO.14) (~0.00) (kO.14)
’ (&standard error). Values in the same column followed by the same letter are not different at P < 0.05 (Duncan’s multiple range test).
P-Olsen’ (mg kg-’ dw) 12.6e (50.49) 18.0k (kO.22) 32.9’ (kO.51) 16.2d (f0.22) 18.9b (kO.22) 17.7c (H.22)
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S. Dumontet et al. Table 3. Soluble C, N and P content Soluble-C’ (mg C kg-’ dw)
Soluble-C (% of soil C)
in burned and unburned
Soluble-N’
Site N-NH;
I
2 3 4 5 6
15.1”~ @l.l3) 12.9’ (kl.95) 17.7” (il.03) 17.5’h (kO.06) 16.6ab (kO.69) 14.5”k (f0.77)
0.14 0.14 0.03 0.10 0.15 0.13
+
3.3e 8..ib 14.v 7.1’ 4.7d 4Y
’ (*standard error) Values in the same column followed
(f0.06) (kO.75) (f0.53) (kO.09) (kO.03) (_+O.lO)
12.5b (kO.23) 10.5’ (+0.23) 11.1’ (iO.21) 14.7” (kO.23) 14.3M(iO.20) 12.5” (irO.29)
by the same letter are not different
soil chemical
properties
and
Soil chemical properties (pH, organic C, soluble C, total N, mineral N, organic-N, P-Olsen and soluble-P) and microbial variables (SIR biomass, BIOSIR), FE biomass (BIOFE), N biomass Table 4. C and N mxrobial Biomass-C’(mg
C kg-’ dw)
Site SIR I 2 3 4 5 6
138’ 172b 290’ 20Sb 79* IOld
(fll.7) (kl2.0) (k8.1) (zt6. I) (zk7.8) (rtl7.4)
Biomass-C’ (mg C kg-’ dw) pooled SIR and FE values
FE 176b 132’ 274’ 197b 75d 7sd
(k15.5) (i3.0) (k21.2) (*6.8) (*9.0) (*9.0)
156’ (kl2.2) 152’ (f12.6) 282a (f10.7) 201 b (k4.4) 77* (f5.4) 8gd (*lo.5)
N-NH; + N-NO: (X of total soil N)
Soluble-P’ (mg P kg-’ dw)
Soluble-P as % of P-Olsen
0.47 1.39 0.39 I .02 0 67 0.64
0.50”b (kO.00) 0.35b (kO.05) 0.75” (kO.15) 0.70” (f0.10) 0.55ab (kO.05) 0.50db (+o.oo)
4.0 2.2 2.4 4.3 2.6 2.8
N-NO; OrgatWN (mg N kg-’ dw)
August 1981 and June 1986, respectively), while the highest one (higher than the unburned site) was measured in site 3 (burned on August 1991) (Table 4). Biomass-C at site 2, which was burned less than 4 weeks before sampling, was similar to those measured in the unburned site. Even the values for sites I and 2 are statistically different for both SIR and FE, the mean values obtained from the two methods did not confirm such a difference. Despite the fact that site 3 had the highest biomass value, the soil had the lowest biomass-C content when expressed as a proportion of total soil organic C. A similar pattern was also observed for biomass-N (Table 4). The biomass immobihsation of N at site 3 was almost twice the value of the unburned soil and the biomass-N value was onethird that of the control when expressed as a percentage of soil total N. On the other hand, the other burned sites showed a decreasing trend of biomass-C and -N as a function of increasing time since the date of the fire. Relationship between microbial activit,v
soils
Biomass-C
at P < 0.05 (Duncan‘s
multiple
range test).
(BION), C-to-N ratio of the biomass (BIOCN), BIOSIR as % of soil organic C (CSIR), BIOFE as % of soil organic C (CFE), N biomass as % of soil total N (NBION) and non-actinomycete bacteriato-actinomycete bacteria + fungi ratio (B/A + F) were tested for their degree of association. The correlation matrix (Table 5) shows that only BIOSIR had a significant positive correlation with organic C and mineral N, while BION was positively and significantly correlated with mineral N. There were other strong correlations among variables which appear to be random in some cases. In contrast, there was no significant correlation amongst total N, P-Olsen and soluble C. In order to determine underlying interrelationships amongst variables, the data were subjected to principal component analysis. The PRINI vs PRINZ explained 79.6% of the total variance (Fig. 2) and revealed that there was a close association between other microbial variables (BIOFE) and soil chemical properties (P-Olsen, total N, organic C and soluble C). The relative position of sites 6 through 4 in Fig. 2 suggests that these sites may have reached a similar state of development after the fire, while positions for sites 3 and 2 indicate that these sites were affected differently by fire and the effect of the fire in site 3 was still present.
biomasses
in burned and unburned
(% of soil C)
SIR
FE
1.3 1.9 0.5 I.2 0.7 0.9
1.6 1.4 0.4 I.1 0.6 0.6
Biomass-N’ (mgd;kK’
27b (k5.0) 37b (k5.0) 508 (i5.4) 30b (k2.9) 13c (kl.2) 14c (*2. I)
’ (*standard error). * The calculation is made taking into account the Biomass-C FE values. 3 Non-actinomycete bacteria/actinomycete bacteria + fungi. Values in the same column followed by the same letter are not different at P < 0.05 (Duncan’s
soils
Biomass-N (% of soil N)
3.8 3.7 1.3 4.3 1.9 2.0
multiple
Biomass C/N2
B/A + F3
6.5 3.6 5.5 6.6 5.8 5.4
9.6 18.6 8.8 9.6 8.6 11.2
range test).
Post-fire soil microbial biomass and nutrient content
1471
Fig. 3 shows the trends of the microbial biomassC and -N (expressed as mg kg’ of dry soil and as % of soil C and N) along the studied chronosequence. It is interesting to note that biomass-C and -N were increasing from site 2 to 3 and decreasing afterwards, while biomass-C and -N were decreasing from site 2 to 3, increasing from site 3 to 4 and decreasing afterwards.
DISCUSSION
Little is known about the effect of fire on soil biomass-C and -N and on the soil nutritional status in Mediterranean environments. Several authors reported an increase of soil mineral nutrients following fires, in different non-Mediterranean ecosys1984; tems (Chandler et al., 1983; Pyne, Christensen, 1987; DeBano, 1991; Klopatek et al., 1991). Such an increase was explained by an enrichment of soil by ash, which would represent one reservoir for mineral N, P, Ca, K and Mg (Christensen, 1987). Klopatek et al. (1991) stated that the soil mineral enrichment by ash, and specifically for N, is strictly related to the fire intensity. From direct observations of the sites investigated and from the records of the Italian Forest Service, it can be assumed that all fires studied here were of high intensity. Klopatek et al. (1991) also stated that one of the greatest initial effects of fire is the reduction of the soil C-to-N ratio resulting from an increase in mineralization processes; however, our data do not support such a statement. The soil Cto-N ratio (Table 2) ranged from 15 to 25 for sites affected by the most recent fires, including the unburned site. The variation of nutritional status of soils along the chronosequence remained unchanged, except for site 3 which was burned 1 y before the sampling. Kutiel and Naveh (1987b) found that the concentration of N-NH: in a brown rendzina soil under a pine forest was more than twice that of an unburned site 8 months after fire, and the concentration of total P increased 3-fold after 2 months following the fire. Furthermore, the characterisation of the soil profiles showed that such a soil enrichment was associated with a decrease of the mineral content of the ash layer. Prieto-Fernandez et al. (1993) also observed 1 month after fire in a HumicCambisol under Pinus pinuster an enrichment in total N and N-NH:, while N-NO; was found to be increased only in the 5510cm soil layer. Saa et al. (1993) reported a meaning 5-fold increase of inorganic soluble P in the O-5 cm layer of a humiccambisol under Pinus pinaster and Ulex europaeus 1 month after wildfire, by comparison with the unburned sites. In our study the substantial unchanged amounts of mineral nutrients in site 6 (Table 2), by comparison with site 1, could be explained by the semi-arid climate of the area stu-
1412
S. Dumontet
et al.
B/A+F Organic N
z”
$ 1
0
.
x
Soluble C
-1
-1
0
1
PRIN 1
Fig. 2. Orthogonal distribution of the principal component (PRIN) analysis for the postfire evolution of the dunal sandy soils. PRIN 1 vs PRIN 2 explained 79.6% of the total variance. BIOSIR and CSIR represent microbial biomass obtained by the substrate induced respiration method and expressed as mg C kg-’ dw and as % of total soil C, respectively. BIOFE and CFE represent microbial biomass obtained by fumigation-extraction method and expressed as mg C kg-’ dw-and as % of soil total C, respectively. BION and NBION represent the microbial biomass expressed as mg N kg-’ dw and % of soil total N respectively. BIOCN represents the C-to-N ratio of soil microbial biomass while B/A + F represents the non-actinomycete bacteria-to-actinomycete bacteria + fungi ratio. The numbered black dots represent the studied sites.
died, characterised by relatively low annual precipitation (535 mm), summer drought and mesic temperature (158°C). After fire events mineral nutrients may temporarily remain in the ash layer, but eventually they may be solubilised, brought downward through the soil profile and utilised by the growing plants and soil microorganisms. Thus, they could be leached by rain from the sandy soil upper layer during the following months. In comparison with other ecosystems in which the fire causes a short term flush of mineral elements (Pyne, 1984; Kutiel and Naveh, 1987a; Klopatek et al., 1991), a relevant soil enrichment was observed only for site 3 which was burned 1 y before sampling (Table 2). This enrichment should not be associated with the condensation of fire-vapourised organic matter downward in the soil profile as proposed by DeBano (1991) due to time elapsed since the fire at this site. If we consider soluble-C, -N and -P content, site 3 behaves in a contrasting manner to the others (Table 3). In fact, although its soluble-C content was comparable with the other sites, it was only 23.3% that of the unburned site when expressed as % of total C. This finding could be explained by the higher values of biomass-C (Table 4), indicating a higher rate of use and immobilisation of the readily available C. In addition, in the O-5 cm layer of site 3 the amount of CHCls-extractable lipids
was 20 and 10 times higher than those found in the unburned soil and in site 2, respectively (Dine], unpublished data). These findings highlight the multiple effects of fire on soil and remaining vegetation, which could also have a direct consequence on some biotic soil characteristics. It was observed at site 3 a relatively well-established herbaceous vegetation (mainly Oryzopsis miliacea), probably due to the soil profile enrichement in mineral N (Table 3) by the rain-solubilized ashes. Kutiel and Naveh (1987b) observed that a rapid compensation of Ndepletion occurred during fire and suggested that it resulted from Na fixation by the free-living diazotrophs. In our study the N content of site 3 was more than 5 times higher than the control (Table 2) but it is unlikely that it can only be explained by the stimulation of bacterial nitrogen fixation. The observed N enrichment may be responsible for the stimulation of microbial biomass growth resulting in an increase of both soil organic matter content and mineralisation. Although sites 3 and 2 showed the lowest soluble-N organic content, site 3 had the higher amount mineral-N (NH: + NOT), which represents only 0.38% of the soil total N (Table 3). These data suggest that a consistent amount of ash mineral-N at this site was immobilised in microbial cells or transformed into another insoluble organic form.
Post-fire soil microbial biomass and nutrient content
z
250
2 if
200
1
150
0 a g
.d $
-O-
Biomass-C (FE)
+
Biomass-N
-..O---
Biomass-Cas%ofsoilC
---A--.
Biomass-N as 46 of soil N
‘& ____.__.__._.___...... -.-..A
: :’
loo
1473
50
OIf
1
0
I
,
I
2
I
4
’
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6 Time afterfire (years)
’
I
8
I
I
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I
i
0
12
Fig. 3. Postfire evolution of biomass and carbon expressed as mg C kg-’ dw (mean f standard error) and as % of soil C and N. The observed behaviours are probably due to the pedological characteristics of the soil which make it unable to limit the leaching of mineral nutrients from the surface ash. Lewis (1974) stated that the importance of nutrient losses due to leaching vary considerably in relation to the absorptive characteristics of soils. The expected low cation exchange capacity of the sandy soil studied may be responsible for the nutrient losses; although leaching was limited in time and intensity by the semi-arid climate, the mineral nutrients could have run off during the years following fire, with a consequent reduction of microbial and plant biomass production. The biomass-C values (Table 4) of sites 1 (unburned) and 2 (burned 4 wk before sampling) are very close to each other considering both SIR and FE values, but they are not statistically different (P < 0.05) if the results of SIR and FE methods are pooled. Biomass-N values of these sites are also not statistically different (P < 0.05). These results are in contrast with those reported by Takahiro et al. (1989), which showed a strong decrease of the soil microbial biomass in a burned Japanese red pine forest 2 and 6 months after fire. Despite similar microbial biomass values for sites 2 and 1 the C-to-N ratio of microbial tissues suggests a fire-stress (Table 4). Biomass C-to-N ratio ranged between 5.4 and 6.6, except for site 2 which had a value of 3.6. These data are confirmed by the non-actinomycete bacteria-to-actinomycete bacteria + fungi ratio which ranged for all soils from 8.8 to 11.2, except for soil 2 which had a value of 18.6. These results are consistent with those of Powlson (1975) who observed in pasteurised greenhouse soils that fungi are more sensitive to heat, fol-
lowed by actinomycete bacteria and nonactinomycete bacteria. Similarly, Margaris (1977) reported that Discomycetes, Pyrenomycetes and Basidiomycetes decreased considerably just after fire and then increased their biomass. Thus, the heat sensitivity of fungi could be responsible for the lowest C-to-N ratio in site 2. The C and N content of the microbial biomass (Fig. 3) showed an increase during the first year after fire and a decrease in subsequent years. The oldest burned sites (site 6 and 5, burned 11 and 6 y before sampling, respectively) still had the lowest microbial biomass values. It was evident that the interference of fire with the microbial commmunities in sandy soil studied lasts for a long period. The lack of information about the size of microbial biomass of pine forests in Mediterranean ecosystems prevents comparison with our data. any Nevertheless, it is worth noting that the size of microbial biomass reported in this work was extremely low as compared to the values reported by other authors who studied the effect of fire on the size of soil microbial biomass. In fact, Takahiro et al. (1989) found values from 737 to 895 mg C-biomass kg-’ dry soil in a O-2 cm layer of an immature clay-loam soil under Pinus densiJora stand, while Margaris (1977) found in the Greek Mediterranean phryganic ecosystem an unlikely microbial biomass value as high as 7150 mg C kg-’ dry soil. CONCLUSIONS
The effects of fire, on the chemical characteristics of the sandy soil studied, Seem to follow a chronosequence which roughly shows the same pattern of
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the already described soil microbial biomass variations (Fig. 3). Organic C, total N, P-Olsen and mineral-N contents are, in fact, higher in site 3 burned 1 y before sampling than in the other sites, including site 2 which was burned a few weeks before sampling. This figure is reversed if soluble-C and mineral-N, expressed as % of soil organic C and total N, are considered. In this case site 3 shows the lowest values. This means that the sandy soil is significantly enriched in C, N and P 1 year after fire, but a small portion of these elements are in a soluble inorganic form. Even though fire seems to affect soil chemical properties hardly at all, except at 1 y after the pine forest was burned (Tables 2 and 3) its effects become evident if microbial variables are considered. Soil microbial biomass values show, in fact, a high degree of relation with the fire date. The most relevant in features in this respect are:
(i) Even 11 y after fire the microbial biomass is far from recovery of the same value of the unburned site; (ii) the increase in microbial biomass recorded 1 y after fire is, under the climatic and pedological characteristics of our study, rather ephemeral; (iii) the increase in microbial biomass observed 1 y after fire (soil 3) is not consistent with the increase of soil organic C. In fact microbial biomass expressed as % of soil organic C and total N at this site accounts for less than half of the control value. These data confirm that fire exerted a strong and long-lasting effect on the microbial communities of our sandy soil. More investigations are needed to fill the gap of information which prevents us from comparing the effects of fire on microbial biomass in similar Mediterranean environments.
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