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20Effects of fires of varying intensity on soil microbial complexes in central siberian scots pine stands
Bogorodskaya A.V. ([email protected]), Sorokin N.D. and Ivanova G.A.
V.N. Sukachev Institute of Forest, Siberian Br., Russian Academy of Sciences,
Krasnoyarsk, Russia
Siberian boreal forests take up half the Earth's surface and have historically played an important biocenotic role in global carbon cycling. Vast forest areas are disturbed by ecological and human factors, with wildfire accounting for 74% of all disturbances. Fires of varying intensity affect 12 to 15 million ha of forests annually [13]. Forest fires disturb natural equilibrium between ecosystem components and often determine forest type and vegetation community dynamics [12]. Soil, as an integral part of forest ecosystems, also suffers from various fire effects. Microbial complexes are are among the soil componets showing the earliest response to fire, which is reflected in changing their structure and functions [1-6].
Study Objects and Methods
The study of how surface fires of varying intensity influence soil biological activity was conducted in Pinis sylvestris-lichen-feather moss stands of the central taiga subzone, Central Siberia (60 38'N, 89 41'E).
The study area is situated in Sym Plain, a well-drained part at the eastern edge of the West Siberian Plain. The local climate is cool and humid, with average annual air temperature ranging -3.2C to -5.7C. The frost-free period lasts for 86-107 days. Annual precipitation totals 450500mm (11). While precipitation occurs primarily in summer, dry periods are frequent and, consequently, forest fire hazard is high during summer months. This is the time when big fires are many [11]. Forests cover 73% of the area, thereof 42.5% are pine stands [11], and 27% of the area in under bogs.
Experimental fires, i.e. contrlled wind-driven fires, were conducted at 200X200m plots in 2000 and 2001 under a joint Russian-American project on modeling fire effects on ecosystems.
The exprimental plots are pure Scots pime-small shrub-lichen-feather moss stands, with trees being 200-250 years old, 30-35cm in diameter, and 22m high on average (site classes IV and V). The regrowth consists only of pine seedlings up to 0.5m high. The total projective area of the grass-small shrub layer varies from 15-20% to 40% among the plots and the layer depth is 20-
35cm. The ground cover is prevailed by small shrubs: Vaccinium vitis-idaea L. in well-draind places, V. myrtillus in mesotrophic places, Ledum palustre in highly moist places, and bog shrubs in wet places.
The soil cover is represented by illuvial-ferrous sandy podzol underlain by alluvial finegrained, carbonate-free sand. The soil is remarkable for high acidity and low humus content (up to 0.5%). Nutrients are present mostly in hardly available forms, which is clear fron the C:N ratio. Due to extremely small content of physical clay, podzol capability of absorbtion is low and soil formation products,thus, move rapidly down the soil profile. Low contents of exchange bases (Ca and Mg) and available phosphorus (P2O5) in the soil profile indicate, along with other parameters, that this is poor soil [14].
The study was carried out on three sample plots subjected to experimental fires of varying intensity: Plot 14 (a high-intensity burn; 2000), Plot 13 (a moderate-intensity burn; 2000), and Plot 19 (a low-intensity burn; 2001) (Table 1). Dr. D. McRae (15) calculated fire line intensities and rates of spread. His data enabled comparison of the impacts of the three fires on soil microbial complexes.
Table 1. Quatitative parameters of experimental fires (based on D. McRae's data)
№ Sample plot Burn date Depth of burn, cm Fire line rate of spread, m/min Fire intensity, kW/m
14 18.07. 2000 6,4 9,0 9018
13 25.07. 2000 4,7 2,0 1067
19 28.07. 2000 3,5 2,9 1016
Soil microflora sampling was done using conventional methodologies [7].
We followed soil microbiology sampling rules, when taking soil samples from soil profiles maximum 80cm deep. In order to obtain the total number of microorganisms and species composition of ecotrophic microbial groups (EMG), we used the following environments: beef-extract agar (BEA) to count ammonificators, wort agar (WA) to count micromycets, starch-ammonia agar (SAA) to determine the number of prototrophic microorganisms that use mineral nitrogen compounds, water agar (WaA) to count oligotrophs, and Eshby's environment for oligonitrophilous microorganisms. Every sowing was repeated three times. Microbial cultures were identified with the help of reference books [8,9]. Catalase, invertase, and urease activity
was found by [10]. Carbonic acid release was obtained by titration using Conway dish [7]. Microsoft Excel 97 program was applied to comlete statistical data processing.
Results and Discussion
Specific hydrothermal characteristics of sandy podzols, fairly high content of hard-to-decompose organic compounds in litter, low soil nutrient content and high acidity - all these factors account for low soil microflora diversity.
As is clear from results of analyses, the total number of microorganisms of the major taxonomic groups (bacteria and fungi) was twice as big under feather moss microgroups as uner lichen microgroups in 2000-2001. This can be due to higher moisture content of organic soil under feather moss. The major ETG's showed low density of cellulose decomposing bacteria and those utilizing organic nitrogen (Table 2). Low density of nitrogen-fixing bateria and the predominance of the oligotropic and oligonirtophlous groups suggest low biological activity of sandy podzol. That mineralization is active in the upper soil layers is attributed to a greater number of mineral nitrogen utilizing bacreria compared to that of ammonificators, which is evident from >1 coefficients of microbial mineralization. The physical and chemical soil proprties promote oligotropic microorganisms that can survive on extremely low contents of mineral nutrients and nitrogen. Low organic matter content resuts in developing oligotrophic and oligonitrophyllous microorganisms feeding on dispersed nutrients (a >2 soil oligotrophy coefficient).
The values of both carbonic acid release rate and soil enzyme activity correspond to the number of microorganisms (their number is up to twice as high in soil under feather moss as under lichen) (Table 3).
Low-to-moderate-intensity fires profuondly change ratios between microorganisms of nitrogen-carbon cycling (Table 2). Ground cover consumption by fire reduces the soil organic matter available for decomposition decreasing, thereby, the number of ammonificators and eliminating vegetative microfungus mycelium. The number of sore-formers and bacteria growing on mineral nitrogen begin to prevail the soil complex following moderate-intensity fire (Plot 13).
Table 2. The number of nitrogen-fixing microorganisms and soil mineralization and oligotrophy coefficients one year following fires of varying intensity (N=28)
№ sample plot and sampling year Sampling depth, cm Bacteria on BEA* Bacteria on SAA* SAA/BEA Fungi on WA* Oligotrophs on WaA* WaA/BEA
Feather moss
14 4-8 32±3 36±3 1,10 5±0,5 37±3 1,20
2000 9-15 39±4 46±5 1,10 44±4 170±12 4,30
14-30 18±2 26±2 1,40 10±2 74±4 4,10
13 4-8 67±4 72±8 1,10 26±4 211±21 3,10
2000 9-15 27±4 46±5 1,70 37±5 250±25 9,20
14-30 14±3 18±1 1,20 - 22±4 1,50
control 2000 nogcT. 0-10 10-20 984±81 477±87 25±3 1406±95 496±74 30±4 1,40 1,20 1,20 1848±96 846±21 34±4 2137±104 721±91 41±10 2,20 1,90 2,80
19 nogcT. 1967±156 699±84 0,36 1322±58 3655±145 1,86
2001 0-5 2899±256 1838±962 0,63 1485±89 1999±452 0,69
5-10 896±57 2410±187 2,69 814±25 31±3 0,04
10-20 393±17 776±45 1,98 353±17 1603±166 4,08
control nogcT. 5475±178 1833±156 0,35 2515±123 10152±2212 1,85
2001 0-5 672±121 1043±117 1,55 238±56 486±122 0,72
5-10 1146±452 2178±196 1,90 93±12 62±8 0,05
10-20 399±16 799±58 2,01 768±56 3185±199 7,98
Lichen
14 4-8 27±4 38±4 1,41 5±0,4 94±10 3,48
2000 9-15 28±3 42±4 1,51 20±3 210±21 7,50
14-30 9±2 10±0,9 1,10 0 20±2 2,22
13 4-8 54±10 74±7 1,41 20±4 180±20 3,33
2000 9-15 21±4 41±4 1,90 16±4 206±22 9,81
14-30 - 6±0,7 - - 15±2 -
control nogcT. 519±70 722±70 1,40 1167±30 1244±116 2,11
2000 0-10 212±41 407±57 1,92 657±89 596±37 2,10
10-20 17±2 24±3 1,42 44±8 31±4 2,65
19 nogcT. 806±32 265±14 0,33 174±10 1825±112 2,26
2001 0-5 778±25 82±6 0,11 134±12 306±45 0,39
5-10 838±51 1022±32 1,22 134±8 337±35 0,41
10-20 162±11 304±21 1,88 61±6 1044±89 6,44
control nogcT. 4025±86 1217±56 0,30 1170±42 4883±115 1,21
2001 0-5 1102±44 1144±45 1,04 375±21 239±44 0,22
5-10 814±26 1638±32 2,02 93±7 309±28 0,38
10-20 101±10 525±12 5,20 30±5 51±8 0,51
* - in colony-forming units (CFU), 1000/1g abs. dry soil.
The number of ammonificators decreases 15-fold and that of cellulose-decomposing mcroorganisms twice. Soil grow more oligotrophic regarding nitrogen (K=WA/BEA>3)
Biochemical soil activity also changes considerably after fire (Table 3). Soil resperation decreases twise and 0-10-cm soil layer enzyme activety decreases 2-4-fold conparing to pre-fire. This indicates a sudden change in oxidation and hydrolitic process activity.
Fire of high intensity (Plot 14) results in that the microbial density and bomass of all ecotrophic groups decreases twice in the upper soil layer comparing to moderate-intensity fire.
Table 3. Fires of vaying intensity and soil biological activity
№ sampl e Soil horizon and sampling depth, sm CO2 release, mg/100g of soil/24hr Enzyme activity NH4, mg/100g of soil NO3, mg/100g of soil
Catalase, cm O2/3min Protease, % Urease, mg NH4/g of soil
Feat ier moss
cont rol nogcT. 0-10 10-20 20-50 47±2,7 19,8±0,9 12,8±0,3 3,4±0,2 46,6±5,2 30,4±3,7 27,1±2,1 5,8±0,7 37±5,6 26±6,1 17±4,1 8±2,1 74,2±4,7 21,4±1,4 14,7±1,8 8,6±1,9 3,50±0,1 2,50±0,18 1,01±0,06 0,10±0,02 0,30±0,03 0,38±0,02 0,15±0,09
14 A1 (4-8) A2 (9-15) B (14-50) 6,1±0,5 8,6±0,6 0,9±0,2 6,4±0,4 21,6±0,6 8,2±0,5 6±0,5 18±0,7 10±0,4 8,7±0,5 18,8±0,6 7,1±0,5 0,96±0,05 0,99±0,05 0,54±0,05 0,09 0,18 0,09
13 Ao (4-8) A2 (9-15) B (15-50) 12,4±1,0 9,8±0,8 3,4±0,8 10,8±0,9 12,1±0,8 4,8±0,7 14±2,0 16±2,0 8±0,8 18,3±1,1 14,6±0,9 8,1±0,7 1,44±0,04 1,67±0,04 0,59±0,03 0,08 0,09 0,04
19 nogcT. 0-10 10-20 8,7±0,4 21,5±1,1 14,2±0,2 15,5±3,9 21,7±4,2 8,2±0,8 - 19,3±0,5 21,5±1,1 11,2±0,5 - -
Lichen
cont rol nogcT. 0-10 10-20 20-50 32,2±2,4 15,5±0,9 12,2±0,5 2,4±0,1 28,6±4,7 14,4±2,2 10,6±0,6 5,8±0,3 22±4,1 18±4,7 12±2,4 4±1,1 28,1±3,1 14,7±2,9 7,4±1,1 3,2±0,7 3,62 1,94 1,07 0,27 0,26 0,11
14 A1 (4-8) A2 (9-14) B (14-30) 5,7±0,5 6,9±0,5 0, 7±0,2 4,6±0,3 18,4±0,7 6,1±0,7 4±0,5 18±1 8±0,5 7,6±0,4 14,8±0,6 5,6±0,4 0,74±0,04 0,98±0,05 0,67±0,04 0,07 0,14 0,09
13 Ao (4-8) A2 (9-14) B (14-30) 10,2±1,4 4,6±0,6 0,9±0,2 10,6±2,1 15,4±3,2 4,7±0,8 9±1,0 14±3,0 4±0,5 14,1±2,1 7,2±1,3 3,4±1,6 1,06±0,04 0,91±0,02 1,04±0,02 0,09 0,14 0,11
19 nogcT. 0-10 10-20 6,2±0,5 14,2±0,6 13,1±0,5 9,5±2,4 22,4±3,2 10,7±1,5 - 8,2±1,2 18,7±1,5 6,4±0,8 - -
(-) - no data.
Soil microflora diversity also decreases following high- and moderate-intensity fires. Microbial complexes are prevailed by bacteria of Pseudomonas genus, such as Ps. desmolyticum and Ps. licuida. Bacillus cerus, Bac. mycoides, and Bac. micelogenosus dominate among spore-
formers. Microfungi are represented mainly by Penicillium, , Mucor, Dematium and Lypomyces yeast.
Besides the above microorganisms, species such as Ps. herbicola, Ps. licuefaciens, as well as Bac. idosus, Bac. filaris, Bac. virgulus spore-formers, and Trichoderma gungus are present in the control plot.
A distinct trend to stabilization of microbial complexes was observed for the upper 10-cm soil layer, below both moss and lichen, in the second year following a moderate-intensity fire as compared to a shrp microorganism density decrease in all ecotrophic groups during the first posfire year (Table 4). Also, microbial soil mineralization was established to increase twice to result in accumulating easy-to-hydrolize nitrogen and its running down to lower soil horizons.
Analysis of microbial groups (communities) 2 years following high-intensity fire (Plot 14) revealed an increase in the number of individuals in all ecotrophic groups, but micromycettes. As soil becomes ash-enriched and forest floor acidity decreases, microfungi decrease in number due to severe competition on the part of bacterial microflora (Table 4). Increasing number of oligotrophs indicates a deficite of readily available nutrients. nutrients.
While the total number of microorganisms increases (Plot 14), organic soil compound mineralization is low (SAA/BEA<1). That soil microorganisms are mainly represented by ammonificators suggests an intensive organic matter supply due to increasing litter (needles and small branches) and ash element content of soil.
Along with the microflora capability to recover and postfire organic soil horizon changes, spatial dynamics of ndividual density of every ecotrophic microbial group depends on the samling year weather. Compared to the year of prescribed burning (2000), for example, the 2002 favorable combination of high air temperatures and enough moisture had resulted in that the numer of microorganisms in all the ecotrophic groups under study increased in both control and experimental plots.
Unlike in previous years, no considerable differece was recorded in microbial complex development between soil below moss and lichen, with soil moisture being sufficiently high in 2002 (Table 4).
In the first year following a low-intensity fire (Plot 19), the number of individuals of all ecotrophic microbial groups decreased 1.5-3 times in the moss layer , while microorganisms of the main physiological groups occurring in lower (0-10cm) soil horizons increased in number.
Table 4. The number of nitrogen-fixing microorganisms and soil mineralization and oligotrophy coefficients two years following fires of varying intensity (N=28)
№ sample
plot and sampling Sampling depth, cm Bacteria on BEA* Bacteria on SAA* SAA/BEA Fungi on WA* Oligotrophs on WaA* WaA/BEA
year
Feather moss
14 nogcT. 0-10 10-20 20-50 87351 ±7973 42020±509 0,48 16935±594 101357±1867 1,16
2002 407±43 228±19 50±10 271±24 0,12 1,18 50±10 378±95 114±19 1955±195 0,28 8,57
115±12 89±16 0,78 62±8 982±23 8,51
13 nogcT. 1496±712 2312±452 1,55 1088±58 7072±108 4,73
2002 0-10 152±41 346±75 2,28 62±11 305±75 2,00
10-20 197±18 395±28 2,00 42±9 466±16 2,37
20-50 42±6 132±19 3,15 0 1432±112 34,01
19 nogcT. 1800±801 4350±223 2,42 2580±573 39300±7390 21,83
2002 0-10 162±29 450±75 2,78 113±36 506±123 3,10
10-20 67±12 940±96 5,31 96±9 1191±458 8,46
20-50 89±17 474±58 5,59 21 ±1 755±155 19,07
control nogcT. 19978±8722 22238±2230 2,23 17038±740 39472±9992 0,40
2002 0-10 1209±564 184±43 0,15 84±45 962±804 0,80
10-20 494±25 800±137 1,62 74±17 518±153 1,05
20-50 346±32 529±35 1,53 52±12 315±23 1,10
Lic ien
14 nogcT. 60424±1578 32204±1211 0,53 35524±1408 65293±1525 1,08
2002 0-10 1046±175 544±56 0,52 78±14 177±36 0,17
10-20 398±42 526±47 1,32 36±5 164±51 0,41
20-50 97±7 202±11 2,08 21±8 195±12 2,01
13 nogcT. 3453±144 2093±123 0,60 1680±121 4760±118 1,38
2002 0-10 201±52 166±46 0,83 97±54 236±44 1,17
10-20 459±26 481±78 1,05 100±23 352±34 0,77
20-50 105±8 1377±112 13,11 28±5 1061±115 10,15
19 nogcT. 3016±154 16859±14446 5,59 13069±1959 57536±7114 19,08
2002 0-10 148±14 262±52 1,77 191±28 749±132 5,06
10-20 203±51 1494±210 7,36 239±51 828±200 4,08
20-50 89±35 573±54 6,44 48±9 389±96 4,37
control 2002 nogcT. 0-10 10-20 20-50 13256±152 175±48 204±88 98±12 21727±146 294±44 246±28 140±16 1,64 1,68 1,5 1,45 16246±1856 56±21 99±26 14±4 30398±4528 224±42 486±23 126±14 2,29 1,28 2,38 1,29
* - in colony-forming units (CFU), 1000/1g abs. dry soil.
This can be attributed to that soil, due to its low heat conductivity, is not heated to a critical temperature lethal for microflora, whereas a very rapidly passing fire promotes microbiological processes and microorganism reproduction. Low-intensity fire has a greater impact on soil below lichen, since lichen is consumed by fire more effectively due to its low moisture content and thus does not protect soil from heating (Table 2).
The second year after a fire of low intensity (Plot 19), microbial complexes bega to recover and microbiological mineralization increased several times (Table 4).
Fire can be generally conluded to stimulate both development of some microbial complexes and soil anzyme activity. Increasing mineralization of organic matter brought in by fire provides, in turn, the nutrient level needed for plant development and improves trophic plant root functioning. So, low-intensity fire contributes to forest regeneration through enriching soil with new elements and incrising its biological activity.
Conclusion
To sum up, our study of influence of fires of varying intensity on soil microflora in central taiga Scots pine stands showed that fire, whatever the intensity, ha a negative impact on structure and functioning of podzolic soil microbial complexes the first year following fire. This is reflected in decreasing number and diversity of microorganisms and their enzyme activity, whereas soil grows more oligotrophic regarding nitrogen. In two years after fire of moderate intensity, a trend occurs for microbial complexes to stabilize themselves. the number of individuals increased in almost all microbial groups after a high-intensity fire and ammonificators began to dominate to suggest a new stage of soil nitrogen transformation process recovery. Low-intensity fire does not influence soil biological activity very much, promotes microorganism functional activity, and improves hydrothermal and trophic soil properties.
Acknowledgement
The aouthors are deeply grateful to the US National Aeronautics and Space Administration (NASA), Land Cover and Land Use Change Program (LCLUC), the US Civil Research and Development Foundation (CRDF), the Forest Service of the US Department of Agriculture, Forestry Canada of Natural Resources Canada, Siberian Branch of the Russian Academy of Science, and the Russian Foundation of Fundamental Research for their financial support of the study.
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