CHAPTER 1. INTRODUCTION
CHAPTER 2. REVIEW OF LITERATURE
CHAPTER 3. STUDY SITE: LOCATION, GEOLOGY AND SITE CHARACTERISTICS
CHAPTER 4. VEGETATION AND SOIL
CHAPTER 5. BIOMASS AND NUTRIENT DYNAMICS OF FINE AND COARSE ROOTS
CHAPTER 6. DYNAMICS OF SOIL MICROBIAL POPULATION AND BIOMASS -C, -N AND -P
CHAPTER 7. IN SITU N AND P MINERALIZATION
CHAPTER 8. GENERAL DISCUSSION
LIST OF TABLES
4.1 Phyto-sociological analysis of the undisturbed and disturbed stands
4.2 Importance value index, density and basal area of tree species in the undisturbed and disturbed stands
4.3 Importance value index, density and basal area of shrubs in the undisturbed and disturbed stands
4.4 Importance value index, density and abundance of herbs in the undisturbed and disturbed stands
4.5 Soil physical properties of the undisturbed and disturbed stands
4.6 Soil chemical properties of the undisturbed disturbed stands
4.7 Relationship between woody vegetation characteristics and soil organic C, total N and P
5.1 Seasonal variation in root biomass of different diameter classes in the undisturbed and disturbed stands
5.2 Seasonal and depth wise variation in live and dead mass of fine roots (≤2 mm diameter) in the undisturbed and disturbed stands
5.3 Seasonal and depth wise variation in live and dead mass of coarse roots (2 - ≥5 mm diameter) in the undisturbed and disturbed stands
5.4 Mean annual total root mass in the undisturbed and disturbed stands
5.5 Production and turnover of roots of different diameter classes in the undisturbed and disturbed stands
5.6 Production and turnover of fine and coarse roots in the undisturbed and disturbed stands
5.7 C accumulation in fine and coarse roots and its turnover
5.8 N accumulation in fine and coarse roots and its turnover
5.9 P accumulation in fine and coarse roots and its turnover
5.10 Chemical composition of fine and coarse roots in the undisturbed and disturbed stands
5.11 Decay and mineralization constants of fine and coarse roots in the undisturbed and disturbed stands
5.12 Correlations coefficients showing relationships of root biomass and necromass with soil physico-chemical properties
5.13 Three way ANOVA showing effects of season, soil depth and stand wise variations in biomass and production of roots
5.14 Three way ANOVA showing effects of season, root size and stands on N and P concentrations of roots
5.15 Root decomposition as influenced by climate, vegetation, soil characteristics and root chemistry
6.1 Bacterial and fungal populations in soils of the undisturbed and disturbed stands
6.2 Soil microbial biomass -C, -N and -P and its contribution to total soil nutrient (C, N and P) pool.
6.3 Seasonal changes in percent c ontribution of MBC to total soil organic carbon content in the undisturbed and disturbed stands.
6.4 Seasonal changes in percent contribution of MBN to soil TKN content in the undisturbed and disturbed stands.
6.5 Seasonal changes in percent contribution of MBP to soil phosphorus content in the undisturbed and disturbed stands.
6.6 Three way ANOVA showing effects of season, stand and soil depth on microbial biomass -C, -N and -P.
6.7 Three way ANOVA showing effects of season, stand and soil depth on bacterial and fungal populations.
7.1 Nitrification and N and P mineralization rates in the undisturbed and disturbed stands.
7.2 Three-way ANOVA showing effects of season, stand and soil depth in N and P mineralization rates.
7.3 Correlation coefficients shown relationship between N and P mineralization rate with soil physico-chemical and microbial characteristics and density and basal area of woody vegetation in the undisturbed and disturbed stands.
8.1 Vegetation and soil characteristics of the undisturbed and disturbed stands.
8.2 Biomass and production of fine and coarse roots in the undisturbed and disturbed stands.
8.3 Microbial population and biomass -C, -N and -P in the undisturbed and disturbed stands
8.4 Correlations coefficients showing relationship of total root biomass with certain community parameter, microbial biomass, population and mineralization rate.
8.5 Total root biomass and production in forest ecosystems of the world.
8.6 Microbial biomass carbon, nitrogen and phosphorus in forest ecosystems of the world.
LIST OF FIGURES
3. 1 Location of Jeypore Reserve Forest in Assam, northeastern India.
3.2 Overview of the three study stands: undisturbed, moderately and highly disturbed stands.
3.3 Monthly variation in rainfall and air temperature at the study area.
3.4 Mean monthly variation in relative humidity at the study area.
4.1 Seasonal variation in the climatic variables in the undisturbed and disturbed stands.
4.2 Seasonal variation in soil temperature in the undisturbed and disturbed stands.
4.3 Seasonal variation in soil moisture content in the undisturbed and disturbed stands.
Seasonal and depth wise variation in soil pH in the undisturbed and disturbed stands.
4.5 Seasonal changes in soil organic carbon in the undisturbed and disturbed stands.
4.6 Seasonal variations in total Kjeldahl nitrogen in the undisturbed and disturbed stands.
4.7 Seasonal changes in phosphorus concentration in the undisturbed and disturbed stands.
4.8 Seasonal changes in available NH+4-N in the undisturbed and disturbed stands.
4.9 Seasonal changes in available NO-3-N in the undisturbed and disturbed stands.
4.10 Seasonal changes in available PO-4-P at two soil depths in the undisturbed and disturbed stands.
5.1 Relative proportion of biomass in roots of different diameter classes in the undisturbed and disturbed stands.
5.2 Distribution of fine root mass in the undisturbed and disturbed stands
5.3 Distribution of coarse root mass in the undisturbed and disturbed stands.
5.4 Seasonal variation in live and dead fine root mass in the undisturbed and disturbed stands.
5.5 Seasonal variation in live and dead coarse root mass in the undisturbed and disturbed stands.
5.6 Decay pattern of fine roots in the undisturbed and disturbed stands.
5.7 Decay pattern of coarse roots in the undisturbed and disturbed stands.
5.8 Changes in N concentration in decaying fine and coarse roots in the undisturbed and disturbed stands.
5.9 Changes in P concentration in decaying fine and coarse roots in the undisturbed and disturbed stands.
5.10 N and P release during decomposition of fine roots in the undisturbed and disturbed stands.
5.11 N and P release during decomposition of coarse roots in the undisturbed and disturbed stands.
6.1 Seasonal variation in bacterial population in the undisturbed and disturbed stands.
6.2 Seasonal variation in fungal population in the undisturbed and disturbed stands.
6.3 Seasonal variation in microbial biomass carbon in the undisturbed and disturbed stands.
6.4 Seasonal variation in microbial biomass nitrogen in the undisturbed and disturbed stands.
6.5 Seasonal variation in microbial biomass phosphorus in the undisturbed and disturbed stands.
7.1 Seasonal variation in nitrification rate in the undisturbed and disturbed stands.
7.2 Seasonal variation in nitrogen mineralization rate in the undisturbed and disturbed stands.
7.3 Seasonal variation in phosphorus mineralization rate in the undisturbed and disturbed stands.
8.1 Relationship between microbial biomass -C, -N and -P with total root mass.
The tropical rainforests are dense, evergreen vegetation characterized by high diversity of plant and animal species. They are one of the most fragile and complex terrestrial ecosystems on Earth, presently occupying less than 7% area of Earth’s surface in America, Southeast Asia and Africa (Richards 1952; Whitmore 1998). Within continental Asia, patches of tropical rainforests are found in Indo-China, South China and northeast India (Whitmore 1998). The tropical wet evergreen forest patches also occur in the Western Ghats of India. In northeast India, tropical rainforests are restricted to the far eastern part of the region, particularly in Tirap and Changlang districts of Arunachal Pradesh and Tinsukia and Dibrugarh districts of Assam. Although a major portion of these forests has been brought under protected area management, they are still threatened by anthropogenic activities.
Tropical forests worldwide are exposed to a variety of disturbances ranging from frequent localized events to less frequent, landscape level or multiple disturbance events. Natural disturbances and concomitant recovery are integral aspects of normal ecosystem behaviour (White 1979). Human disturbances, on the other hand, differ in kind, scale, intensity and frequency and sometimes they may be more severe and extensive than the natural disturbances. Shifting cultivation and extraction of timber and NTFP’s species are major causes of disturbance in the humid tropics (Reiners 1980), which have destroyed vast tracts of the humid tropical forest ecosystem. Logging and timber removal or conversion of forest to other land uses has long-term consequences on secondary vegetation, nutrient cycles and water balance (Turner et al. 1997).
Several workers have reported that removal or loss of forest cover alters physico-chemical characteristics of soil (Joergensen and Raubuch 2002) and adversely affects the soil hydrological regime, microclimate, energy balance and enhances soil erodibility (Fenn et al. 1993). Input of organic matter and nutrients to soil through litter and root mass help improve nutrient availability by favourably altering the hydrology and physico-chemical and biological properties of the soil. The periodicity, extent and pattern of litter fall and litter decomposition are important in this respect (Ambasht 1985).
The fine root system of plants play crucial roles in the fluxes of energy and matter in the ecosystem and carry out essential functions of soil resource acquisition (Aerts et al. 1992; Fahey and Hughes 1994). The amount of carbon and nitrogen cycled through fine roots may be as much as or more than that cycled through the above ground litter (Arthur and Fahey 1992). The development of an extensive surface root mat is one of the major mechanisms that enhance nutrient conservation in tropical rain forest, growing particularly on leached and nutrient poor soils (Jordan and Herrera 1981; Cuevas and Medina 1988). The unsuberized roots in the mat are primarily responsible for the retention of nutrients (Edwards and Grubb 1982). In the tropical rainforest fine roots confined to the topsoil layer contribute significantly to soil organic matter and nutrient dynamics in soil by their fast turnover rate (Joslin and Henderson 1987).
The fine roots increase the surface area for growth and multiplication of soil microorganisms, which play an important role in the cycling of mineral nutrients necessary for plant growth (Anderson and Domsch 1980). The soil microbial biomass constitutes a transformation matrix for organic materials in the soil and act as a labile reservoir of plant-available nutrients (Jenkinson and Ladd 1981; Singh et al. 1989). Changes in the microbial population in response to variations in soil conditions (moisture, carbon, nutrients, temperature, pH) have important bearing on nutrient cycling (Diaz-Ravina et al. 1995). Soil microbial biomass serves as an indicator of slower and less easily detectable soil organic matter changes (Johnson and Curl 1972), and plays an active role in nutrient conservation in the tropical soils (Maithani et al. 1998).
The pockets of the tropical rainforest in northeast India are found in the Jeypore Reserve Forest of Assam. These are exposed to varied types of anthropogenic disturbances. Logging and extraction of NTFP’s are two major human activities which are disrupting the structure and functioning of ecosystem.
The present study analyses the role of fine roots and microbial biomass in the nutrient enrichment of the topsoil in a humid tropical forest ecosystem and evaluates the effects of anthropogenic disturbances on their dynamics. The specific objectives of the research were:
(i) To study the seasonal and spatial changes in fine roots (<2 mm diameter) and soil microbial population,
(ii) to study the accumulation of C, N and P in fine roots and microbial biomass and their turnover, and
(iii) to study the N and P mineralization patterns in the undisturbed and disturbed stands.
Review of literature
Forests around the world especially those in the tropics have undergone severe disturbances due to anthropogenic activities. Among these, human settlement in forest areas, clearance of forest and its conversion into agricultural land etc., have in many cases produced harmful long-term effects leading to soil degradation and associated nutrient loss. It is well known that forest clearance for agriculture, a typical situation in the tropics, decreases biodiversity, limits natural vegetation and simplifies the ecosystem structure. The most important consequence of tree felling in the forest ecosystem is degradation of land and soil, both in terms of its structure and quality.
Disturbances which compact the soil, remove the litter layer and top soil, and increase overflow, have significant effects on nutrient cycling processes and forest regrowth. Analysis of physico-chemical properties of soil along with the changes in fine roots biomass and productivity and microbial biomass following disturbance in forest ecosystem are important for evolving appropriate strategies for the reclamation of a degraded ecosystem.
Effects of disturbance on soil
The vegetational diversity of a region is influenced by topography, soil, climate and its geographical location (Tilman 2000). Tropical rain forests are among the most diverse vegetation in the world (Whitmore 1998; Richards 1952), and their species richness and diversity are influenced by a variety of factors including soil nutrient status (Hubbell 1979; Asthon and Hall 1992) and water availability (Walsh 1996). Some studies suggest that forests on soils of high nutrient availability may also have lower species richness than forests on intermediate nutrient availability (Lee et al. 2002). Species diversity has also been correlated with rainfall (Hartshom 1980) and disturbance level (Rao et al. 1990).
Large changes in vegetation cover are taking place in the humid tropics as forests are being converted in to pastures and agricultural fields. An expected result of the removal of tree biomass and changes in land use is decline in soil organic matter, alteration in its physico-chemical characteristics (Boyle 1975; Spaans et al. 1989), soil hydrological regime, energy balance and soil erodibility (Lal 1989). Deterioration of soil quality through the loss of soil organic carbon, nitrogen and other nutrients due to deforestation has been reported by several authors (Srivastava and Singh 1989). Changes in soil microenvironment due to deforestation leads to disruption of decomposition process which ultimately results in to poor carbon and nutrient balance in the soil. Nutrient dynamics in forest soil is also influenced by stand age (Gholtz et al. 1985), tree species composition (Miller 1984) and site conditions.
Fine root dynamics
Fine roots play an important role in the development of soil as a substrate (Berg 1986; Joslin and Henderson 1987), though they represent only a small portion of total plant biomass (Kurz 1989; Vogt et al. 1991). However, the importance of fine roots in nutrient cycling and as a component of forest productivity is not proportional to their contribution to the total biomass.
The high concentration of fine roots in the top few centimeters of soil is an important feature of tropical rain forest (Klinge 1973; Jenik 1978). They are most likely involved in the uptake of nutrients in woody plants (Bohm 1979). The greatest proportion of fine roots in many forests is located in the upper soil horizon (Vogt et al. 1983). They are abundant in the organic horizon accounting for 40-70% of total fine root biomass in the soil profile (Vogt et al. 1996). Dead roots make up 50-80% of the total biomass (Vogt et al. 1986) resulting from the rapid turnover of fine roots (Hendrick and Pregitzer 1993).
Studies have demonstrated large seasonal and yearly variation in fine root biomass (Santantonio and Hermonn 1985; Makkonen and Helmisaari 1998), which is influenced by vegetation and soil characteristics (Arunachalam et al. 1996c). Seasonal fluctuation in fine roots has been reported from tropical and subtropical forests (Silver and Vogt 1993; Singh and Singh 1993; Arunachalam et al. 1996c; Sundarapandian and Swamy 1996). The large structural roots (coarse roots) are more stable and do not exhibit significant seasonal or annual dynamics (Harris et al. 1980; Powell and Day 1991). Harris et al. (1980) suggested that the large roots may experience a cyclic renewal.
Fine root production is an important component of both dry matter production and nutrient cycling in forest ecosystem (Vogt et al. 1986; McClaugherty et al. 1982). Soil temperature, moisture and growth regulating substances, carbohydrate availability, respiration rate, symbiotic and competitive relationships determine root productivity (Persson 1980). The quantity and turnover of roots in forest soils have been determined by Nadelhoffer and Raich (1992) and Hendrick and Pregitzer (1993). Fine root input can be large in many forest ecosystems due to fast turnover and a high percent of total carbon allocated to the belowground compartment (Vogt et al. 1996). Findings of several studies suggest that the fine roots contribute significant amount of detritus to the decomposition system (McClaugherty et al. 1982; Vogt et al. 1982; Bloomfield et al. 1993; Arunachalam et al. 1996b) through fast turnover rate. Root turnover is an important carbon pathway to the soil (Dilustro et al. 2002). Several methods have been employed for estimating fine root turnover in perennial ecosystem and the values obtained by different methods are quite variable (Gill and Jackson 2000). For example, in the northern hardwood forest, the minirhizotron method yielded values ranging from 50 to 90% turnover of fine root biomass annually (Hendrick and Pregitzer 1993; Burton et al. 2000; Tierney and Fahey 2002), while in sequential and ingrowth coring methods 30 to 100% turnover was obtained in an annual cycle (Aber et al. 1985; Fahey and Hughes 1994). N budget method yielded turnover estimates as low as 30% (Aber et al. 1985; Nadelhoffer et al. 1984). The soil core method calls for the sequential collection of replicated organic and mineral soil cores throughout the growing season (Ericsson and Persson 1980). Annual production and turnover rates are based on the observed temporal fluctuations in fine root biomass (Kurz and Kimmins 1987; McClaugherty et al. 1982). This most commonly used method has yielded tremendous amount of information on fine roots biomass in terrestrial ecosystems (Gower et al. 1992). The factors that control root life span are poorly understood. It is generally accepted that in forest systems, root growth and senescence occur simultaneously during the active growing season of the plant. Life span of fine roots in forest system is estimated to vary from several weeks to several years and is often shorter than leaf life span (Hendrick and Pregitzer 1993).
Across a range of ecosystems, net belowground primary production may be greater than the above ground production and nutrient concentrations in fine roots may be higher than those in the foliage (Meier et al. 1985). The amount of carbon and nutrients returned to the soil by fine root turnover may equal or exceed that from leaf litter (Joslin and Henderson 1987; Raich and Nadelhoffer 1989). Minimal retranslocation of nutrients from roots upon senescence also contribute to the importance of fine roots in nutrient cycling (Aerts 1990; Nambiar and Fife 1991). Studies revealed higher nutrient concentrations in <2 mm diameter roots than in those of 2-5 mm diameter roots (Gordon and Jackson 2000).
Nutrient release from decomposing roots is an important pathway of nutrient flux in terrestrial ecosystems (Joslin and Henderson 1987; Fahey et al. 1988). Fogel and Cromack (1977); Persson (1982); Vogt et al. (1983); McClaugherty et al. (1982); Bloomfield et al. (1993); Arunachalam et al. (1996d); Comas et al. (2000); Dilustro et al. (2001) have studied root decomposition in different ecosystems. Temperature, moisture and plant tissue chemistry are considered to have the greatest influence on decomposition rates (Vitousek et al. 1994). Nitrogen, phosphorus and carbon content influence root decomposition (Gorisson et al. 1995). Larger diameter roots decompose at slower rates than smaller diameter roots (Persson 1982; Gholtz et al. 1985; Dilustro et al. 2001).
Microbial biomass C, N and P dynamics
Soil microbial biomass comprises the part of soil organic matter barring the live root fractions and soil organisms larger than 5 x 10-15 m3 (Jenkinson and Ladd 1981). The microbial indices and their relationship with various factors in the temperate (Priha 1999; Leiros et al. 2000; Priha et al. 2001), tropical (Vance et al. 1987; Singh et al. 1989; Sarathchandra et al. 1989; Joergensen et al. 1995; Salamanca et al. 2002; Dinesh et al. 2003) and subtropical (Arunachalam et al. 1996a; 1997; Arunachalam and Arunachalam 2000; Arunachalam and Pandey 2003; Upadhyaya et al. 2004) forests have been extensively studied. Microbial biomass acts as an important ecological indicator and is responsible for decomposition and mineralization of plant and animal residues in soil. The microbial biomass may be a main source of nutrients for the plant and may help in nutrient conservation (Singh et al. 1989). It constitutes a transformation matrix for all the natural organic material in soil and acts as a labile reservoir of plant available nutrients (Jenkinson and Ladd 1981). According to Powlson et al. (1987), soil microbial biomass responds much more rapidly than the total organic matter to any change in organic inputs. Thus microbial biomass can be considered as an ecological marker for analyzing and predicting the long term effects of perturbations in soil subsystem (Smith and Paul 1990).
Due to the rapid turnover rate of microbial biomass, plant available nutrients are released faster; this makes its contribution to nutrient cycling far greater than its size might suggest (Schnurer et al. 1985). Therefore, changes in the size of the microbial biomass affect the cycling of N and P and their availability to plants (Diaz-Ravina et al. 1995).
The role of microorganisms in the turnover of C, N and P compounds in soil has been a focus of several studies of soil productivity (Sikora et al. 1994). The information on variation in microbial biomass and activity in relation to vegetation type, soil factors, climates are limited (Zak et al. 1994). There is even less information available on relationships between microbial biomass and specific microbial activities (eg. N mineralization, soil respiration) in forests (Schimel 1995). These relationships have important implications for evaluation of plant-soil-microbial control on ecosystem processes (Aber et al. 1991). Soil with a relatively high organic matter inputs usually develop a larger microbial biomass (Srivastava 1992). Strong positive correlations between microbial biomass -C and inorganic -P and -N in soil have been reported in dry tropical soils (Srivastava and Singh 1989) and in a range of United Kingdom soils (Brookes et al. 1985). In dry tropical soils of India, Srivastava and Singh (1988) reported that about 96% variability in microbial -P concentration could be explained by organic -P concentration in soil. Primary productivity of forest and grassland ecosystems are generally limited by N availability in soil, which is combined or closely associated with soil organic matter (Schnitzer 1991). Microbial biomass constitutes a significant part of the potentially mineralizable N that is available to plants (Bonde et al. 1988; Singh et al. 1991). In some studies, the amount of microbial biomass was found to be a good indicator of the rate of N mineralization (Paul and Voroney 1984; Azam et al. 1986). Though the soil microbial biomass contributes a relatively small fraction of the total biomass in the terrestrial ecosystems than in plant nutrient cycling. Their growth and activity are influenced by climate, soil moisture content, pH, quality and quantity of substrates and N, P, S concentrations in soil (Orchard and Cook 1983; Anderson and Domsch 1993; Anderson and Joergensen 1997).
Information on the seasonal fluctuation in microbial biomass within an annual cycle is available mostly for agricultural soils. Some workers have reported large annual fluctuations in the microbial biomass (Lynch and Panting 1980; Ross et al. 1981), while others observed only small annual changes (Schnurer et al. 1986; Patra et al. 1990). The tropical forest ecosystems differ from the temperate forests with respect to seasonal fluctuation in microbial biomass. In tropical forest soils, the peak microbial biomass has been recorded during winter (Luizao et al. 1992; Maithani et al. 1996; Arunachalam et al. 1996a; Arunachalam and Arunachalam 2000) when the temperature is low, while the values are low during rainy season when temperature and moisture conditions are plentiful for the microbial activity. In temperate forest peak microbial biomass in soil was recorded during summer or spring by Diaz-Ravina et al. (1993), while von Lutzow et al. (1992) recorded highest biomass N in autumn and lowest during summer.
N and P mineralization
Nutrients returned to the soil through litter is released following its decomposition and mineralization. Rates of litter decomposition and mineralization are influenced by a large number of factors including temperature and soil moisture condition, and by the chemical and physical nature of the litter. An estimation of the release of nitrogen from soil organic matter may be regarded as a prime indicator of soil fertility (Antil et al. 2001). The proportion of total soil organic N that is released annually varies widely from <2% to more than 10% of total N, depending on the soil type and other conditions (Burtholomew and Kirkham 1960). Thus, the accumulation of organic N in the longer term may not be related directly to the ability of the soil to sustain the rate of N released. However, the net amount of N mineralized i. e., the difference between gross mineralization and gross immobilization indicates the ability of the soil to satisfy the immediate requirements of plants.
Of the different fractions of soil organic matter, macro particles comprise a significant component of the ‘light’ fraction of organic matter. Macro-organic matter is generally thought to consist of dead fibrous materials in a state of partial decomposition, including that from roots, but not including living materials (Whitehead et al. 1990). Warren and Whitehead (1988) suggested that the macro-organic matter fractions in grassland soil might contribute substantially to the available N. This conclusion was derived from the observation that plant uptake of N was significantly decreased in soil where macro-organic matter had been removed. The mineral constituents of the soil matrix show close associations with organic materials, which exert important control over the mineralization process (Hassink 1993). A close interaction between the decay products and microbial activity in clay soils has been reported by Gregorich et al. (1991).
The primary microbial processes involved in fresh residue and humus turnover in soils are mineralization and immobilization of soil N. The activities of microorganisms involved in N mineralization are affected by both biotic and abiotic factors (Clarhom and Rosswall 1980; Sarathchandra et al. 1989), and by management practices (Zaman et al. 2002).
Factors which influence the rate of N mineralization include soil texture (Hassink 1992), moisture, pH and temperature (Stanford et al. 1973). The effects of temperature and moisture content on soil N cycling have extensively been studied, often with conflicting results. However, little is known about the influence of understorey management and its interactions with soil temperature and moisture content on N mineralization and nitrification processes (Zaman and Chang 2004). Joergensen et al. (1995) reported that temperature, rather than moisture, appears to be the critical factor affecting microbial biomass and their activities in forest and agricultural soils.
Studies in forests and grasslands have shown that gross nitrogen mineralization is often greater than net nitrogen mineralization and does not differ due to differences in species composition (Stark and Hart 1997; Verchot et al. 2001). The rate of gross mineralization indicates that microbial nitrogen loop strongly dominates nitrogen cycling. This loop consists of decomposition of soil organic matter and return of nitrogen to the soil upon microbial death. Thus plant carbon drives the microbial nitrogen loop and determines net nitrogen mineralization rates (Verchot et al. 2001). Plant available nitrogen is determined by what is left after microbial uptake (Knops et al. 2002). Mature forest stands show a linear relationship between productivity and nitrogen mineralization (Reich et al. 1997), and differences among stands and species are reflected in the soil organic matter pools (Finzi et al. 1998). Soil microbes are often limited by carbon (Jackson and Caldwell 1989). Root exudates and root turnover are important sources of carbon for soil microbes (Grayston et al. 2001). The quality and quantity of this carbon supplied by plants can determine the rate of net nitrogen mineralization (Schmidt et al. 1997), which, in turn, can influence the total amount of NO-3 produced as well as leached from the ecosystem.
Next to nitrogen, phosphorus is a major essential nutrient required by plants, which is absorbed largely as orthophosphate ions (H2PO-4 and HPO4), which are present in the soil solution. In soil, phosphorus exists both in organic and inorganic forms. These are important sources of P for plant and microbial uptake. The organic form exists mostly in humus and other organic materials, while the inorganic form occurs in combination with Al, Fe, Mg, Ca and other elements, most of which are not soluble and therefore not available to plants and microbes. P availability is also controlled by soil, moisture content, aeration and pH which influence microbial transformation of phosphorus.
Soil physico-chemical processes are more important than basal mineralization in releasing plant available inorganic P (Oehl et al. 2004). Duration and intensity of weathering affects forms of soil P. In highly weathered soils, the proportion of organic P is usually greater than in young soils, especially the labile inorganic P. This conceptul model developed by Walker and Syers (1976) from soil sequences in New Zealands has been confirmed for a chronosequence in Hawaii (Crews et al. 1995) and by a literature review of soil P fractions in natural ecosystems (Cross and Schlesinger 1995). In general, P mineralization and immobilization are similar to those of N in that both processes occur simultaneously in soils.
Study site: Location, geology and site characteristics
The study was carried out in and around Jeypore Reserve Forest of Dibrugarh Forest Division of Assam (latitude 27° 05´ to 27° 28´N; longitude 95° 20´ to 95° 38´E; altitude 220 m asl) on the southern bank of the river Brahmaputra (Figure 3.1). Champion and Seth (1968) have classified this forest as Assam Valley Tropical Wet Evergreen Forests (I-IB/CI). The study area is a part of the Joy-Dihing Biosphere Reserve (90-480 m asl) covering an area of about 108 km2. About two-third area in the northern side of Jeypore Reserve Forest is more or less flat and the southern side is hilly. The forest is bounded by Dilli river in the southwest, Namsang river in the east and Buri-Dihing river in the northwest. Apart from these rivers, innumerable seasonal streams and streamlets, mostly rain-fed, drench the reserve forest.
The present study was carried out in and around Jeypore Reserve Forest. Two disturbed and one undisturbed stands were selected for the detailed study. The disturbed stand was divided into two parts on the basis of disturbance index (Rao et al. 1990). Using the ratio of basal area of cut stumps to the total stand basal area as disturbance index, sites were categorized into moderately (MD, disturbance index 54%) and highly-disturbed (HD, disturbance index 88%) stands.
illustration not visible in this excerpt
Figure 3. 1. Location of Jeypore Reserve Forest in Assam, northeast India.
The moderately-disturbed stand (Figure 3.2) was selectively logged (ca. 2 ha) and dominated by Mesua ferrea, Terminalia myriocarpa, Alangium begonifolium, Tetrameles nudiflora, Duabanga grandiflora, Sapium baccatum, etc. The highly disturbed stand (ca. 2.5 ha) was at a distance of about 1 km from the undisturbed stand (Figure 3.2). It was clear-felled 10 years ago for settled cultivation. Here rice, maize, mustard and chilies were grown occasionally. A few individuals of Biscofia javanica, Dillenia indica, Duabanga grandiflora, Bombax ceiba and Albizzia sp. were present at this stand. The undisturbed stand (ca. 2 ha) was in the core area of the Jeypore Reserve Forest. Mature large (>90 cm DBH) trees of Dipterocarpus macrocarpus, Shorea assamica, Mesua ferrea, Tetrameles nudiflora, Castanopsis indica and Vatica lanceaefolia were abundant in this stand.
Geology and soil
The rocks of this region belong to the tertiary and quaternary formation. The oldest tertiary formation consists of a group of gray and black splintery shells with interbeds sandstones and the youngest tertiary sequence consists of pebbles bed alternating with clays and soft sandstones. The quaternary sequence consists of clays, loose coarse sand, gravels and boulder and comprises of a group of old alluvium rocks containing lignified fossil wood. The metamorphic rocks are also found in the area which is composed of quartzes, slates and varieties of schistose rocks (Rahman 2002).
The soil can be classified into two classes, old alluvial and new alluvial. The old alluvial soil or the higher level soil occurring along the Buri-Dihing river is clay or sandy loam. The new alluvial soil or low level soil is of recent origin. It occurs along the banks of Buri-Dihing, Namsang rivers and contains clay, silt, sand and shingles. The texture of the soil varies from sandy loam to clay loam with 1-5% stones.
The area falls within the humid tropical climate with well pronounced wet summer and winter seasons. Mean monthly temperature varies between 7 °C and 36 °C. The hottest months are July and August (35 °C) and the coldest months (8 °C) are December and January. Hail storms are common during March to June end. The annual rainfall ranges between 2500 and 5000 mm, about 85% of which is received during the wet season (Figure 3.3). Relative humidity remains very high through out the year (Figure 3.4).
The vegetation shows the general characteristics of the tropical evergreen forest. The heterogeneous forests found in the area can be broadly classified into the following types as per Champion and Seth’s (1968) classification of forest types of India.
I. IB/CI- Northern Tropical Evergreen Forests- Assam valley tropical wet evergreen (Dipterocarps)
II. 3/ I S2 B- North Indian Tropical Moist Deciduous forests (Eastern Hollock Forests)
III. Miscellaneous Forests.
Type I (IB/ CI) Assam valley tropical wet evergreen forests are easily distinguished by the dominance of Dipterocarpous macrocarpus and Shorea assamica tree species in the canopy layer. Apart from these species, less frequent canopy trees with tall cylindrical boles and comparatively smaller and lighter crowns are also present in the forest. Notable among them are Altingia excelsa. A. excelsa is commonly found on small hill tops and well drained higher sites. Other canopy trees are Artocarpous chaplasha, Cinnamomum glaucesence, Terminalia myriocarpa, T. bellerica, Ailanthus grandis, Canarium strictum, Dysoxylum procerum, Tetrameles nudiflora, etc.
The subcanopy of the forest has tree species like Mesua ferrea, Castonopsis indica, Endospermum chinensis, Taluma hodgsonii, Syzygium cuminii, Duabanga grandiflora, Vatica lanceaefolia, Sapium baccatum, Garcinia paniculata etc. Among these M. ferrea often grows gregariously.
The shade bearing species like Baccaurea sapida, Vatica lanceaefolia, Mallotus roxburghii, Knema longifolia, Sterculia hamiltonii, Dysoxylum binactiferum and Pseudostachyum polymorphum etc constitute an undercanopy layer. The shrubby forest undergrowth has gregarious species like Blastus cocchinensis, Saprosma ternatum, Leea umdraculifera, Capparis multiflora, Saurauia sp., Melastoma sp., Laportea crenulate, Pinanga gracilis etc. The herbaceous flora on the ground cover consists of ferns and Phrynium pubinerve, Bochameria sp., Impatiens sp., Polygonum sp., and Musa sp. Orchids and ferns form the bulk of epiphytes. The common epiphytes are Aexhenanthes sp., Hoya sp., Dischidia sp. and the common lianas include Derris ferruginea, Millettia cunoifolia, Hodgsonia macrocarpa, Entada purseatha, Rhaphidophora descuriva, Ventilage madraspatana, etc. The moist shady forest floor has a thick layer of litter and humus.
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Figure 3.2. Overview of the three study stands: (A) undisturbed (B)
moderately disturbed and (C) highly disturbed stands.
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Figure 3.3. Rainfall, maximum and minimum air temperatures at the study area.
(ٱ) total monthly rainfall, (○) mean monthly maximum and (●) minimum temperatures.
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Figure 3.4. Monthly variation in maximum and minimum relative humidity at the study area.
Vegetation and soil
It is well known that forest clearance for agriculture in the tropics decreases biodiversity, limits natural vegetation and simplifies the ecosystem structure. Besides, it has a detrimental effect on soil properties, since forest clearance modifies the microclimatic conditions at the ground level, and changes the amount and quality of organic input to the soil (Dinesh et al. 2003). Disturbance in the forest also leads to the fragmentation of the community and alteration in tree dynamics. Factors of soil complex such as pH, organic matter and nutrient contents influence plant growth and succession on degraded sites (Pandey and Singh 1985).
Knowledge of the amount, forms and distribution of soil carbon (C), nitrogen (N) and phosphorus (P) are essential for understanding nutrient dynamics in soils, since mechanisms of nutrient dynamics in forest soils vary from one nutrient element to another, stand age (Gholtz et al. 1985), tree species (Miller 1984) and intrinsic properties of the environmental conditions of the sites.
Vegetation structure, microclimatic condition and soil properties of the undisturbed and disturbed forest stands were studied in order to assess the impact of disturbance on these characteristics of the ecosystems.
Vegetation of undisturbed, moderately-disturbed and highly-disturbed stands was studied during 2002-2003. In each stand, density, frequency and basal area of individual plant species were determined in randomly placed quadrats of different sizes. Twenty five quadrats (10m x 10m) for tree species, twenty quadrats (5m x 5m) for shrubs species and fifty quadrats (1m x 1m) were sampled for herbaceous species in each of the three stands. Nomenclature of plant species followed Hooker (1872-1897). Relative frequency, density and basal area were calculated (Philips 1959) for trees and shrubs, and the sum of these represented importance value index (IVI) for the woody species (Curtis 1959). For herbaceous species, IVI was calculated by summing up relative frequency, relative density and relative abundance values.
The Shannon’s species diversity index was computed as, H´=-∑ (ni/N) log (ni/N), where H´=Shannon’s index of general diversity (Shannon and Weiner 1963), ni=importance value index of each species, N=total importance value index.
The microclimate in the three stands was studied by measuring light intensity, relative humidity and air temperature in January, April, July and October during 2002 and 2003 to represent winter, spring, rainy and autumn seasons respectively. All the three parameters were measured randomly at ten places close to the ground surface in each stand. The light intensity was measured using a digital lux meter (TES 1332). The air temperature and relative humidity were measured using a thermo-hygrometer (EXTECH). Soil temperature was measured using a soil thermometer (SYMAX).
Soil sampling: Soil samples were collected in January, April, July and October during 2002 and 2003. In each stand, 20 samples were collected using a steel corer (5.5 cm inner diameter) from two soil depths (0-15 and 15-30 cm) after clearing the litter layer. The replicated samples of a particular depth was thoroughly mixed site-wise to obtain a composite sample. After removing stones, pebbles and large pieces of plant materials, the samples were sieved using 2 mm mesh size sieve. The screened samples were stored in polythene bags for analysis.
Soil analysis: Soil texture was determined by Bouyouncos hydrometer method (Bouyouncos 1962) and bulk density was determined by soil core method (Blake and Hartge 1986). Water holding capacity (WHC) was determined according to Keen’s box method given by Piper (1944), while soil moisture content was measured gravimetrically by incubating 10 g of field moist soil sample in a hot-air oven at 105 ˚C for 24 h. Soil organic carbon (SOC) was determined by dichromate oxidation and titration with ferrous ammonium sulphate (Walkey 1947). Total Kjeldahl nitrogen (TKN) was estimated following semi-micro Kjeldahl procedure by acid-digestion, distillation and titration (Anderson and Ingram 1993). For total P concentration, soil sample was digested using a triacid mixture, followed by colorimetric reaction (molybdenum blue method) with ammonium molybdate and stannous chloride (Jackson 1958). Cation exchange capacity (CEC) was determined after extracting the exchangeable bases from the soil with 1M ammonium acetate (pH 7.0), followed by the replacement of ammonium N with magnesium oxide (Allen et al. 1974). The pH of the soil sample was determined in a soil-water suspension (1:2.5 w/v H2O) using a digital pH meter (Systronics M 335).
For available N and P contents, field moist soil samples (50 g) were shaken with 2M KCL (250 ml) for NH+4-N and NO-3-N and 0.5M NaHCO3 was used for PO-4-P (100 ml, pH 8.5) in a rotatory shaker for 2 h and the suspension was filtered through Whatman filter paper No.1 and/or 44. Nitrate-N (NO-3-N) was measured by phenol disulphonic acid method (Jackson 1958) and ammonium-N (NH+4-N) by phenate method (Wetzel and Lickens 1979). Available phosphorus (PO-4-P) was measured by molybdenum blue method (Jackson 1958). All the analyses were done in triplicates and the final results were expressed on oven-dry weight basis.
Data were statistically analyzed using ANOVA (three-way) to study the effects of stand, season and soil depth on soil physico-chemical variables. Pearson correlation coefficients and regression were worked out according to Zar (1974), wherever necessary.
Floristic composition, density, basal area and diversity index
In total, 201 plant species (88 tree species, 55 shrubs and 58 herbs) belonging to 164 genera and 77 families were recorded in this study. About 57 families were present in the undisturbed stand, 61 in the moderately-disturbed stand and 39 in the highly-disturbed stand. Fifty eight families had single representative species and 8 families had more than 5 species. Members of Euphorbiaceae, Lauraceae, Rubiaceae, Meliaceae and Magnoliaceae were common in the undisturbed stand, while those of Euphorbiaceae, Lauraceae, Rubiaceae, Clusaceae and Meliaceae were common in the moderately-disturbed stand. In the highly-disturbed stand species of Euphorbiaceae, Poaceae and Moraceae families were abundant.
In the undisturbed stand tree species were distributed in four distinct strata (Table 4.2). The emergent trees (height >25 m) included Dipterocarpus macrocarpus, Shorea assamica, Tetrameles nudiflora, Ailanthus grandis, Sapium baccatum, Cinnamomum glanduliferum, Elaeocarpus ganitrus, Talauma phellocarpa, etc. The canopy layer (height 10-25 m) was composed of Mesua ferrea, Castanopsis indica, Canarium bengalense, Terminalia chebula, Talauma hodgsoni, Michelia spp. Litsea salicifolia, and Alstonia scholaris. The subcanopy had Baccaurea sapida, Vatica lanceaefolia, Dysoxylum reticulatum and Diospyros variegata etc. Similar distribution pattern of plants species were also observed in the moderately disturbed stand with lower number of species; whereas in the highly disturbed stand no such stratification was observed. The highly disturbed stand was composed of a few sparsely distributed species like Alangium chinese, Baccurea sapida, Vatica lancefolia and Mesua ferrea.
Species richness varied according to disturbance gradient in different stands. The number of tree species increased from 13 in the highly disturbed stand to 53 and 82 in the moderately disturbed and undisturbed stands respectively. Shrubs species also decreased with the increase in disturbance. However, maximum (35) number of herb species was recorded in the moderately disturbed stand and minimum (20 species) in the undisturbed stand.
Tree density and basal area were negatively related to the intensity of disturbance. In general, the Shannon’s index of diversity for trees and shrub were greater in the moderately-disturbed stand, 1.52 and 1.34 respectively, and lowest (H´=1.04) for tree species in the highly-disturbed stand The diversity index for herbaceous species was lowest (H´=1.09) in the undisturbed stand among the three stands (Table 4.1). The overall species diversity was highest in the moderately disturbed stand, and lowest in the highly disturbed stand; it was at intermediate level in the undisturbed stand.
Table 4.1. Phyto-sociological analysis of the undisturbed and disturbed stands.
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Table 4.2. Importance value index (IVI >5), density (Den., trees ha-1) and basal area (BA, m2 ha-1) of tree species (≥ 30 cm GBH) in the undisturbed and disturbed stands.
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Table 4.3. Importance value index (IVI >10), density (Den. individuals ha-1) and basal area (BA, m2 ha-1) of shrubs in the undisturbed and disturbed stands.
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Table 4.4. Importance value index (IVI>10), density (individuals m-1) and abundance of herbs in the undisturbed and disturbed stands.
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Microclimate exhibited marked seasonal fluctuation with peak temperature and relative humidity during July and trough in winter season. The peak light intensity was observed during spring and lowest during rainy and autumn seasons. The effect of disturbance was more prominent on light intensity, which was several fold higher in the highly disturbed stand than the undisturbed stand. Air temperature also showed an increasing trend with increase in the disturbance level but relative humidity decreased with the increase in disturbance intensity (Figure 4.1).
Physical properties of soil
Soil temperature was high (32.55 ˚C and 33.98 ˚C) during summer and rainy seasons and low (17.73 ˚C and 19.27 ˚C) in winter, and it increased significantly with the increasing degree of disturbance (Figure 4.2). While the proportion of sand particles increased with the disturbance intensity, the proportion of clay decreased significantly (P<0.05) from the undisturbed to highly disturbed stand (Table 4.5). The water holding capacity (WHC) of the soil ranged 38.18- 66.48%; the highest value being in the undisturbed stand (66.48%) and lowest (38.18%) in the highly disturbed stand. Bulk density also varied significantly (P<0.05) between the stands (0.67-1.01g cm-3). Highest bulk density (1.01g cm-3) was recorded in the highly disturbed stand and lowest (0.67 g cm-3) in the undisturbed stand. Soil moisture content varied significantly between seasons and soil depths (F=2.38, P<0.05), though it was invariably higher in the undisturbed stand than the disturbed stands (Figure 4.3).