BIOINDICATORS OF FOREST FLOOR DEGRADATION
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A.K. Bhat and J.A.Wani*
Division of Environmental Sciences, Sher-e-Kashmir University of Agricultural Sciences and Technology of Kashmir, Shalimar, Srinagar-191 121
Microbial biomass responds immediately to alterations in soil ecosystem and thus its measurement is a viable tool for understanding and predicting long-term impact of deforestation (Powelson et al., 1987). Plants add energy to the soil system in the form of litter and root exudates which eventually are turned into soil microbial biomass that is a major pool responsible for nutrient cycling and for controlling amounts of nutrients available to plants (Ohtonen et al., 1999). In forest environment, experiments have documented nutrient limitation of tree growth in boreal, temperate and tropical forests (Baker et al., 1994, Heilman et al., 1963) due to deforestation. Ecosystem that receive chronically low inputs of limiting nutrients in the form of litter eventually lead to low degree of nutrient cycling. The importance of biotic regulations of nutrient cycling has been demonstrated for temperate deciduous forests, coniferous forests and grassland (Reichle, et al., 1981). But in the last fifty years deforestation has accelerated in Jammu and Kashmir as a result poor Govt. control, lack of local awareness etc. (Anonymous, 2002). Consequently, the monitoring of environmental degradation might be improved by extensive study of soil biomass dynamics, which may provide early warning signals of ecological disturbances. (Hobbie and Melilo, 1984). An attempt has been made to assess the effect of deforestation on the change in nature of microbial biomass in terms of C and N.
Materials and Methods
Six sites each of forest floor (F) and deforested soils (DF) area from pre -surveyed locations: Shankeracharya (S), Dachigam (D), Ganderbal (G), Kangan (K), Handwara (H) and Tangmarg (T) were selected for soil sampling (Table 1). Soil samples were taken from the humus layer through a depth of 3 cm to 20 cm intervals at 100 m2 quadrant. Soil samples were pooled for each quadrant and material was homogenized, not sieved and stored before analysis. Soil water was determined gravimetrically after drying sub-samples at 105oC for 12 h. For biomass analysis the soils were moistened immediately before analysis to a 250% water content of organic matter, which is reported to be optimal amount for microbial respiration in forest soil (Nordgren et al.1988) fumigated for 24 hrs. With chloroform in vacuum desiccators. After fumigation, samples were incubated with 0.5 g fresh soil, placed in 1 litre airtight glass jar and incubated at 25± 1oC for a period of 60 days. Controls consisting of 20.5 g sub-soil samples were alongside the fumigated samples. Accumulation of CO2 was assessed as absorbed in alkali. Biomass N was determined by extracting incubated samples by 2 M KCL. Biomass-C and N was calculated by equation B= FC/Kc or KN, where B is total biomass of C and N, whereas Kc for biomass C was taken as 0.45 and for KN it was taken as 0.54 as fraction for respective biomass.
Results and Discussion
Soil microbial biomass is an index of overall microbial activities in soil, which is small, but highly labile pool influenced by deforestation. Biomass-C changes were monitored over a period of 60 days. Biomass-C changes over a period of 60 days was observed in the range 100-250, 40-750, 210-877, 100-480, 150-650, 30-520, 175-812, 25-400, 200-910, 76-600, 210-765, 35-502 in the site SF, SDF, DF, DDF, KF, KDF, HF, HDF, TF and TDF respectively (Fig 1). Biomass C in all the
treatments were significantly different. In the present study peak amount of calculated biomass-C values obtained were for 13days only, which was in contrast to the observations made by Jenkinson and Powlson, (1976). Effect of chloroform seems to subside thereafter. Increased rate of biomass-C release was observed in early days of incubation.
In all deforested locations biomass-C has substantially declined between 37% to 50%. The contribution of biomass-C was in the range of 1.38 to 3.12%. Biomass-N level ranged from 31.25 mg kg-1 soil in Shankerachaya forest soil to 93.40 mg kg-1 soil in Dachigam forest soil whereas deforested soils have shown significant decrease. Biomass-N changes were monitored only up to 7 days period. Biomass-N in forest floors ranged from 31.25 to 93.40 mg kg-1 soil.
In all the sites deforested soil has lost biomass C and N substantially. The relation between soil organic carbon content (x) and biomass-C during 13 days period of incubation in 12 soil was as follows Y=0.02x – 2.26 (r-0.85) significant at 5% level. This phenomenon indicates that supply of organic matter from tree is generally larger because of the availability of litter. The small size of biomass-C in deforested soil may be due to the less production of soil microbes per unit of substrate or less longevity of biomass synthesis due to the absence of quantum of plant biomass, which was also observed by Jenkinson and Ladd (1987). Another reason for low biomass-C in deforested soil may be because of decrease in detritus during succession becoming recalcitrant to decomposition.
Ratio of Biomass-C to O.C (Table 2) is significantly higher in forest floors (1.03 – 1.95%) than in deforested soils (0.54-1.78%). Microbial Biomass contributes 4.92 to 8.92% to total N in forest floors whereas deforested soils contribution of Biomass-N is 2.85 to 7.77%. Forest clearing induces lower equilibrium of soil organic matter because of reduced organic input. Martin et al. (1991) has also observed decrease in organic C due to forest clearing which leads to changes in the magnitude of biological and physico-chemical properties of soil. The changes in soil pH between forest floors and deforested soils recorded a difference of 0.10 to 0.80 unit (Table 1). The pH of forest floor is affected by basic cations, which is influenced by litter composition. Binkley and Gardina (1998) have observed that % base saturation is commonly differed by more than 40%, hence affecting pH.
The above findings clearly reveal degradation of soils in future and reminds soils generally improve in suitability for supporting plant growth over a pedogenic time, at least for thousand or tens of thousand years (Van Breeman, 1993). Soil development typically includes accumulation of organic matter and nutrients, development of soil structure and sustained supplies of nutrients through a microbial activity. The matter and energy processed by earlier generation of plants and soil organisms results in a state of negative entropy which can benefit later generations. The long term view may also apply to shorter periods; available evidence show that within a decade trees can substantially alter soils, which is a short enough time to produce feed back effect on the fitness of trees
The rehabilitation and upgrading of the degraded forest soils and afforestation of barren area are colossal tasks (Fotidar, 1989). The environmental variable for humus and mineral soil layer are compared with altitudinal variations (Fig. 2). The thickness of humus layer across altitude did not show any positive correlation but deforestation has lead to significant reduction in humus layer from 54.35% in Dachigam deforested soil to 74.36% in Kangan deforested soils and amount of organic-C decrease observed was 48.57% in Shankerachraya deforested soil to 48.51% in Dachigam deforested soil. The variations in the thickness of humus layer and consequently in the amount of organic-C override most of the variations in biomass-C and biomass – N. Calculation of biomass-C to biomass-N ratios, which reflect the decomposability of litter (Kaye and Hart, 1997) showed the variation of 8 to 10 indicating that C: N ratios stabilize in all the soils despite of resilience. Obviously problems can arise in afforestation because under nutrient deficient condition in pine forests particularly organic N (the main nitrogen pool) is bound in recalcitrant compounds shall be is inaccessible to plants (Nasholm et al. 1998).
In the present study we have not determined the C to N ratio of detritus, which has effect on decomposability. But data suggest that the C: N ratio of detritus must have been higher than critical values for microbes, suggesting N limitation amongst decomposers. Plants and microbes are potential competitators for N (Kaye & Hart, 1997). Table 2 shows the estimate of soil microbial biomass and biomass-C as proportion of total organic carbon of all the soils. To estimate soil microbial biomass-C as proportional of total organic-C, each author has used an individual Kc factor. In the present work, Kc 0.45 was used to calculate amount of soil microbial biomass. It can be seen that the biomass C differ in the same manner as organic C of soil does. Morumoto (1984) had a similar observation.
Table 1 : Site characteristics of forest floors
Site
Forest community
Soil texture
pH
Organic carbon (%)
Total N (ppm)
Humus layer
(cm)
Shankaracharya (F)
Shankaracharya (DF)
Black locust (Rubinia psedoacocia)
Cyprus (Cupressus torulosa)
Cedrus deodara
Silty loam
6.80
7.10
1.80
1.40
545
540
4.5
1.2
Dochigam (F)
Dachigam (DF)
Pinus walliachiana
Fine loamy
6.10
6.90
3.50
1.80
784
784
9.2
4.2
Ganderabal (F)
Ganderbal (DF)
Fir (Abies pindraw)
Silty clay
7.00
7.20
1.97
1.40
737
736
8.46
2.10
Kangan (F)
Kangan (DF)
Pinus wallichiana
Fir (Abies pindoow)
Silty loam
6.90
7.10
2.00
1.50
1400
1400
5.85
1.50
Handwara (F)
Handwara (DF)
Cedrus deodara
Kail (pinus wallichiana)
Loamy
6.20
6.90
3.10
2.10
1374
1300
10.4
2.8
Tangmarg (F)
Tangmarg (DF)
Kail (pinus wallichiana)
Fir (Abies pindoow)
Silty loam
6.30
6.00
2.80
1.60
1478
1450
7.1
1.6
Table 2 : Estimated variable of biomass-C
Biomass-C (mg kg-1 soil)
Treatments
Biomass-C/organic (%)
SF 200
1.03
SDF 75
0.54
DF 600
1.6
DDF 210
1.31
GF 325
178
GDF 175
1.43
KF 403
195
KDF 170
1.13
HF 410
1.39
HDF 300
1.28
TF 300
1.36
TDF 150
1.19
References
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ENVIS Bulletin : Himalayan Ecology 11(2), 2003
G. B. Pant Institute of Himalayan Environment and Development
(An autonomous Institute of Ministry of Environment and Forests, Government of India)
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