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2.3 RESULTS AND DISCUSSIONS
2.3.3 Role of microbial biomass and soil enzymes activities in soil fertility and productivity
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2.3.3 Role of microbial biomass and soil enzymes activities in soil fertility and
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wetland sample (S2). The soil CNP ratio in comparison to microbial biomass CNP ratio was quite lower and this may indicate a depletion of carbon pool and active utilization of carbon source for growth and metabolism in flood and precipitation dominated monsoon season. In Kamrup (Amingaon and Umananda), depending on the availability of substrates in river fed floodplain soil, the microbial CNP ratio was higher than soil CNP ratios in soil samples in each season (microbial CNP ratio of 16:3:1 – 28:3:1 in Amingaon and 16:2:1 – 23:3:1 in Umananda in pre-monsoon season; 18:2:1-28:1:1 in Amingaon and 16:1:1 – 32:4:1 in Umananda in monsoon season). Unlike SOM, higher CNP ratios were equally concentrated in disturbed soil site and river bank sediments as well as undisturbed samples. Nutrient mineralization by microorganism was based on MB characterization (including both culturable and unculturable microorganisms).
Correlation studies showed positive and significant association of MB with most of the soil parameters in Majuli as well as Kamrup (Amingaon and Umananda) [Table A 2.7 (A), (B), (C), Appendix 2]. The depthwise discrepancy of microbial biomass was similar to SOM distribution.
2.3.3.2 Soil enzyme activities as a function of soil fertility and productivity
In Majuli soils, there was a continual change in enzyme activities with increasing depth, the decreasing trend of enzymatic activities was uniform and evident in each season [Figure 2.12 (A – H)]. The depth of incidence of high SOM and enzyme activities in most of the sampling sites peaked at 0 – 60 cm and reduced at 80 – 100 cm.
To screen the critical role of microorganisms in soil enzymatic activities, toluene treatment was incorporated in the enzyme assays. Effect of toluene was marked in a
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few enzymes and results complied with the data available in published archives (Pancholy and Rice, 1973; Cole, 1977; Tabatabai and Bremner, 1972; Ladd and Buttler, 1972). Observations indicated a clear demarcation of microbial enzyme activity and extracellular enzyme activity in few enzymes as cellulase, amylase, protease and urease. While in phosphatases, invertase and dehydrogenase, concerned sources of enzyme activities remained similar. The increasing depth conforms to decreasing nutrient content followed by lower enzyme activities. Seasonal variance was minimal in all sampling sites excluding urease [Figure 2.11 (A – H)]. Population of microorganism was higher in monsoon season (from plate count method), hence there is a possibility of more enzymatic activities projected for substrate utilization for maximum growth and metabolism by the microorganisms. This outcome was similar to the reports published by many researchers (Mahalaxmi et al., 2013; Cregger et al., 2012). There was no record of synthetic fertilizer application in Majuli (confirmed by social survey). However application of cow dung in agricultural soils was reported by local inhabitants. Cowdung in the form organic matter may possibly contribute to a small fraction of SOM and enhance soil enzymatic activities. Overall trend of soil enzyme activity was cellulase > amylase > invertase > urease > protease >
dehydrogenase > phosphatases. This trend was equally conspicuous in Kamrup (Amingaon and Umananda) [Figure 2.13 (A – H); 2.14 (A – H)].
In Amingaon and Umananda, soil enzymatic assays depicted fertility and scope of productivity in the soils. Sampling sites in Kamrup were devoid of any kind of agricultural activity or organic amendments other than frequent human intervention.
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Industrial and commercial activities were prominent in Amingaon whereas tourism activities were consistent in Umananda. Soil enzyme activities in Amingaon and Umananda showed that undisturbed soil held a better fertility and productivity potential followed by disturbed sites and bank sediments. In Amingaon and Umananda, there was no record of application of fertilizer or artificial manure from the sampling sites.
The study areas showed organic and inorganic waste deposition activities only.
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Figure 2.12 (A – H) Soil enzyme profile of Majuli River Island in pre-monsoon and monsoon season, at a depth of 0 – 20 cm. Enzyme activities were checked in soil
samples with varied landuse activities and geomorphic origin
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Figure 2.13 (A – H) Soil enzyme profile of Amingaon in pre-monsoon and monsoon season, at a depth of 0 – 20 cm. Enzyme activities were checked in soil samples
from disturbed areas, undisturbed areas and bank sediments
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Figure 2.14 (A – H) Soil enzyme profile of Umananda in pre-monsoon and monsoon season, at a depth of 0 – 20 cm. Enzyme activities were checked in soil
samples from disturbed areas, undisturbed areas and bank sediments
Enzymatic activities inferred soil richness in microbial population as a major factor of macronutrients availability (basically CNP), under influence of secondary factors like pH and trace elements. SOM and soil enzymatic activities are often
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associated with the soil mineralogical nature. Sandy clay loam nature of the soil samples was intended to retain fair amount of SOM and promote enzymatic activities functionalized by soil microorganisms or secondary extracellular sources in Majuli and Kamrup (Amingaon and Umananda). CEC of sandy clay loam soil seem to rely on accessibility of basic cations from SOM that was relatively higher in Kamrup (Amingaon and Umananda). However, in Majuli, Amingaon and Umananda, soil CEC showed no virtual correlations with geochemical parameters proposing the recalcitrant nature of humic substances in Majuli soils and undisturbed soils in Amingaon and Umananda. Additionally, this outcome gave an overview of low humus content in disturbed soils in Amingaon and Umananda soil and bank sediments (Majuli, Amingaon and Umananda).
Enzyme activities and nutrient mineralization in Majuli were positively correlated in both pre-monsoon and monsoon seasons [Table A 2.8 (A), (B), Appendix 2]. The effect of toluene was evident in a few enzymes in the study areas. Results complied with the data published by many researchers (Pancholy et al., 1973; Nelson, 1944; Tabatabai et al., 1972). The by products obtained as a result of enzyme activities added to active nutrient budget or serve as an immediate source of SOM comprising C,N,P and MBC, MBN and MBP. A possible linkage of microorganisms with nutrient mineralization is also supported by the fact that in presence of some important soil enzymes, nutrient mineralization was prominent in both pre-monsoon and monsoon seasons. Factors as pH, sand, silt, clay, CEC and metals in soil were positively linked to a few enzymes (with and without toluene) illustrating that under existing environmental
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conditions, enzymes activities may be accelerated or slowed down [Table A 2.8 (A), (B), Appendix 2]. In Kamrup (Amingaon and Umananda), results were similar, enzyme activities (with and without toluene) showed positive significant correlation with nutrient mineralization, except a few cases like invertase and TOC, dehydrogenase and MBN, etc. [Table A 2.9 (A), (B), Appendix 2] [Table A 2.10 (A), (B), Appendix 2].
CEC showed positive correlations with most of the enzyme activities in Majuli as well Kamrup (Amingaon and Umananda). Overall results indicated that soil enzyme activities in Kamrup (Amingaon and Umananda) were higher than Majuli. Theories apart, one possible reason would be occurrence of relatively greater microbial populations (both bacteria and fungus in Amingaon and Umananda). Significant soil enzyme activities could not be restricted to microbial populations and SOM alone, inherent soil properties characteristics of particular region, have an influence on geochemical parameters known as spatial variability (discussed in Chapter 4).