CHAPTER 8 GENERAL DISCUSSION

Transcription

1 CHAPTER 8 GENERAL DISCUSSION A total of 16 orders belong to three classes under arthropods i.e Arachnida, Insecta and Myriapoda fauna were extracted from all the four study sites during August, 2009 to July, 2011.Among the 16 orders of arthropods extracted, only cryptostigmatid mites and collembola contributed more than 50% of the total microarthropods population in soil and litter in all the study sites. In soil, both cryptostigmatid mites and collembola contributed 58.95%, 64.73%, 60.34%, 73.02% in Dorgakona forest, Dorgakona glassland, Silcoorie forest and Silcoorie grassland, respectively. And in case of litter both contributed 65.88% and 62.45% in Dorgakona forest and Dorgakona grassland, respectively. Among cryptostigmata Galumna flabellifera was found to be dominant in the soil of Dorgakona Forest and Silcoorie Forest sites whereas Scheloribates albialates was recorded in Dorgakona Grassland and Silcoorie Grassland sites. In case of litter Archegozestes magnus and Lanceoppia sp were found to be dominating group in Dorgakona Forest and Dorgakona Grassland, respectively. While for collembola, Lepidocyrtus medius and Isotomina thermophilawere found to be dominant on soil of Dorgakona Forest and Dorgakona Grassland respectively whereas Seira indica was recorded to be dominant group both in Silcoorie Forest and Grassland. The main cause of this might be their body size which unable them to migrate into the pore space of the soil. While for litter, Onychiurus sp in Dorgakona forest and Folsomia sp in Dorgakona grassland were noticed. It may be due to the body size which cannot pass through the 2mm mesh size of litter bags. The total soil microarthropod population from one annual cycle to another cycle has found to be nearly more or less equal in all the four study sites. Among the soil microarthropods extracted from four different sites, collembola and cryptostigmata in particular found to be the predominant arthropod groups in all the study sites. The total soil microarthropods were studied up to 0-10 cm 74

2 layer at all the four sites, among them forest was recorded maximum density as compared to the grassland. Among the forest sites, Dorgakona forest site was found higher population density as compared to Silcoorie forest site. It may be due to presence of denser canopy in Dorgakona forest which block the sunlight and maintain soil moisture high due to precipitation which lead to increase decomposition and organic matters. As regards grassland site, Silcoorie grassland site witnessed more population density than Dorgakona grassland site. This may be due to distributing patterns of soil microarthropods seem to be controlled by the prevailing microclimatic conditions and abiotic factors. Multiple correlation analysis was employed to assess the effect of climatic factors on the population dynamics of different soil and litter dwelling microarthropod groups and the result showed significant and positive relationship with the climatic factors on population dynamics in all the sites. In partial correlation analysis it was found that rainfall, air temperature and relative humidity influenced the majority of the microarthropod groups. As far as community structure is concern, following index i.e. Berger-Parker dominance index, Shannon-Wiener diversity index, Evenness index and Dominance index of soil and litter ranges from in all studied vegetational types of soil and litter. The dominance index in monsoon season (May-October) remains high as compared to the dry season (November- February). Shannon Wiener diversity index and Evenness index were also recoded higher value in wet months as compared to dry months in all the study sites of soil. Dominance index, Shannon-Wiener diversity index and evenness index all are high during wet season may be due to presence of higher rainfall, temperature and relative humidity which favours higher decomposition and nutrient release pattern for the growth of population. The same pattern followed in litter also, as forest recorded higher diversity index compared to grassland. The group diversity of litter dwelling microarthropods in the initial months and monsoon season (May-September) was recorded higher when mass loss was 75

3 higher than dwindled. Both cryptostigmata and collembolan were also followed such type of observation. This result suggested that in the initial months litter mass and occurrence of rainfall was higher which lead to support higher number of species. But in course of time when litter was decomposed the mass of litter also decreased then population may migrate to the soil which leads to decrease in population during the dry season (November February). The index of similarity range was found to be higher in case of soil total microarthropod group. This result may be due to only group level of microarthropod taxas were considered during entire study period. In case of soil cryptostigmata community, high range of similarity was recorded in between the similar system i.e. Dorgakona Forest vs Silcoorie Forest and Dorgakona Grassland vs Silcoorie Grassland. It may be due to occurrence of similar number of cryptostigmatid mite species between the similar system and may be due to similar environmental conditions prevails. The range of similarity was found to be low between the different systems. Same pattern was followed in case of collembola species also where maximum range of similarity was occurred between the similar systems. As far as similarity index of litter is concerned only between two systems i.e. Dorgakona forest and Silcoorie forest was discussed. Like soil dwelling microarthropod groups, in litter also higher values of similarity was recorded for microarthropod group. For cryptostigmatid mites, similarity index was recorded with a range of but maximum number of this range is less than 0.50 which showed less of similarity between these two systems. While collembola species also follows lesser range of similarity may be due to similar nature of climate and soil type prevails. As regards soil physical characteristics is concerned, moisture content was lowest in Dorgakona grassland site and highest was in Silcoorie forest site. This may be due to the thick canopy coverage to prevent the penetration of sunlight and temperature which in turn increase litter decomposition and carbon content. Soil ph also found maximum in forest as compared to the 76

4 grassland. Analysis of soil chemical characteristics revealed that forest had the highest organic carbon, total nitrogen, available phosphorous and extractable potassium as compared to the grassland soil. Soil organic carbon compounds hold basic cations and are a source of energy for decomposers, contributing to increase supply of nutrients, such as N and K in soil. The highest nutrient concentration of NPK were recorded during April to October, it may be due to attributed to release more nutrient during decomposition process because in this period high relative humidity, high temperature and rainfall influence the high rate of litter decomposition. As far as Soil ph is concerned, it did not exhibit any variation among all the vegetational types and its dynamics. Among the soil chemical properties, forest soil had the highest organic carbon, total N, available P and exchangeable K than that of grassland soil. It may be due to close canopy of forest leads to decompose leaf litter faster than grassland. The moisture content of soil exhibited positive and significant influences on soil microarthropods in all the study sites. As moisture depends on the precipitation of soil more moisture content occurred in wet season and least in dry season. Soil ph showed positive but insignificant correlationship with all groups of encountered mcroarthropod populations. Soil became more acidic during winter season showing less influence on the distribution and changes of microarthropod population. Soil organic carbon content showed positive and significant correlationship with the soil microarthropod population in all the study sites except Dorgakona grassland sites. The total nitrogen content of soil was found to be positively and significant relationship with the population in all the study sites (except Dorgakona Forest). It showed that increase concentration of organic carbon and nitrogen direct by influence on the distribution and fluctuation of population densities. Available phosphorous shows strong influence on the soil microarthropod dynamics and it showed positive but insignificant relationship with population of microarthropods in all the studied areas. In case of extractable potassium, it exhibited significant relationship only with the population of Dorgakona forest site while other sites showed insignificant 77

5 relationship. It can be concluded that not as single factor but a combined effect of NPK were responsible to control the population densities of soil microarthropods. As far as litter decomposition is concerned, grassland litter recorded higher value than that of forest. Carbon content found 38.75% and 39.44% in grassland for first and second annual cycles, respectively. In case of forest it was recorded to be 34.04% in first year and 32.75% in second year. In present study, among the two vegetational types (i.e. Dorgakona Forest and Dorgakona Grassland) leaf litter, grassland recorded highest C: N ratio, which indicates low litter quality and decomposability as compared to forest. The result of the present study indicated that within 360 days exposure, the percentage of decomposition of leaf biomass was differ each vegetational type to other from first to second annual cycle. The mass remain after 360 days was higher in grassland and low in forest. The percent of litter weight loss was higher in the initial phase and in during the wet months compared to the dry periods for all the vegetational types. Litter mass loss showed a rapid phase of initial mass loss in which 74.4% and 75.7% of the first and second year litter in forest while 73.9%% and 77.4% in grassland. Decomposition rate constant (k) in forest recorded 2.40 and 2.43 in first and second annual cycle. In case of grassland k value was noticed 1.84 in first and 1.91in second annual cycle. Time required for 50% decomposition in forest were 105 days in1 st year and 103 days in 2 nd year and for grassland it took 136 and 131 days for 1 st and 2 nd annual cycle respectively to decompose 50% of litter. For decay of mass in grassland it took much time may be due to exposure of nature of grassland compared to forest. From the above study it can be conclude that that the micro arthropods population was found to be higher in forest soil than that of grassland soil. The probable reason for this observation could be attributed to the fact that the amount of Nutrients (NPK) in the forest soil was higher than that of Grassland soil. Forest ecosystem being a close canopy, with minimum sunlight reaching the soil provides a favorable condition for rapid decomposition of the 78

6 continuously falling leafs from the tree. This rapid decomposition unlocks the nutrients contained in leaves and returns it back quickly to soils which are available for microarthropods to use. On the other hand grasslands ecosystem soil are more or less continually exposed to the elements of sun thereby making them dry which hinders the rapid decomposition of small biomass of the dead organisms, thus slowing the return of nutrients to soil and limiting its availability to the soil organisms. 79