Nutrient mineralization in soil samples

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2.3 RESULTS AND DISCUSSIONS

2.3.2 Nutrient mineralization in soil samples

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delayed SOM matter degradation activities whereas Group A revealed significant increments of a few soil variables due to residential intervention. Unlike Majuli, Kamrup sampling sites had no specific landuse demarcation, Amingaon was a commercial area with rapid industrial activities whereas Umananda was a pilgrimage site with a few temples located on a hilly terrain. The bank samples were however clearly differentiated. In non-agricultural regions of Amingaon and Umananda, soil functioned as a building material for earth filling, roads and pavements, the physical stability of soil was reflected in the chemical status. In case of disturbed soil, SOM accumulation was irregular, soil mixing released most of the nutrients from soil that leached into ground waters. In undisturbed soils, SOM was functionalized by microorganisms, nutrients were fixed and released gradually in course of time. Under such conditions, the microbial population was anticipated to be higher in Amingaon and Umananda. Results indicated that SOM accumulation was in an ascending order of Group B > Group A > Group C in Amingaon and Umananda.

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Figure 2.6 TOC of soil samples in (A) Majuli, (B) Amingaon and (C) Umananda in pre-monsoon and monsoon season, at a depth of 0 – 20 cm

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Figure 2.7 TN of soil samples in (A) Majuli, (B) Amingaon and (C) Umananda in pre-monsoon and monsoon season, at a depth of 0 – 20 cm

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Figure 2.8 TP of soil samples in (A) Majuli, (B) Amingaon and (C) Umananda in pre-monsoon and monsoon season, at a depth of 0 – 20 cm

Nutrient mineralization in each season showed marginal difference, values of soil variables analysed in pre-monsoon season were slightly higher than values derived in monsoon season. In this context, it was assumed that MC and temperature may have possibly influenced the availability of nutrients in soil by mediating certain mechanisms of ion-soil interactions i.e. release of ions retained by the colloids or facilitating leaching activities. Additionally, Soil MC favoured leaching of elements (basically N and P). A strong of discrepancy of nutrient availability with increasing

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depth in case of Majuli samples was attributed to leaching activities (Figure 2.9; Figure 2.10; Figure 2.11). Soil CNP status in Majuli was positively and negatively correlated to pH, EC, CEC, BCFU, FCFU and grain size distribution respectively [Table A 2.7 (A), Appendix 2]. The correlation studies indicated an inter dependency of the soil variables. However in Amingaon and Umananda, CNP status in Majuli was positively correlated to pH, EC, CEC, BCFU, FCFU and grain size distribution [Table A 2.7 (A), (B), Appendix 2]. The variation in two seasons was significant in Majuli as well as in Amingaon and Umananda.

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Figure 2.9 Depthwise discrepancy soil TOC in Majuli, S1 – S12 divided in three Groups (A, B and C) in pre-monsoon and monsoon season, at depths 0 – 100 cm

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Figure 2.10 Depthwise discrepancy of soil TN in Majuli, S1 – S12 divided in three Groups (A, B and C) in pre-monsoon and monsoon season, at depths 0 – 100 cm

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Figure 2.11 Depthwise discrepancy of soil TP in Majuli, S1 – S12 divided in three Groups (A, B and C) in pre-monsoon and monsoon season, at depths 0 – 100 cm

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In case of bank sediments there was a probability of continuous exchange of nutrients at the water soil interface [Figure 2.6; Figure 2.7; Figure 2.8]. The sediment- water interface is a transition zone between water and underlying soil that affect most of the biochemical processes in flooded soils. OM in river sediments is linked to the type of sediment and its porosity. Moreover, bank sediments comprised of fine sand (visible texture). As aforementioned, adsorption capacity by particles owing to surface area is inversely proportional to the particle size. In bank sediments, higher percentage of fine sand indicated low capabilities of binding OM. In this context, Avnimelech et al (2001) mentioned a structural dependence of sediments on the coating and spacing of inorganic particles by hydrated OM. Thus nutrient mineralization in bank sediments was comparatively lower than the terrestrial sediments. Deposition of river driven OM at the water sediment interface may be linked to slightly higher OC than the terrestrial sediments. However there were a few exceptions in Majuli. There was no clear difference between the TP concentration in terrestrial soils and bank sediments in Majuli and Amingaon, but Umananda bank sediments showed lower TP concentrations than terrestrial soils). The TN concentrations were low in bank sediments in Majuli, Amingaon and Umananda. Availability of TP and TN followed the same concept of OM matter accumulation in flooded sediments.

2.3.2.1 CNP Stoichiometry

Carbon, Nitrogen and Phosphorus tend to follow a stoichiometric ratio in different types of ecosystems known as the Redfield like ratio as in planktonic biomass

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(Cleveland, 2007). A Redfield like ratio in soil and soil MB has been reported to occur in the ratio of 186:13:1 and 60:7:1, respectively based on a worldwide survey. The ratio of CNP mineralization is dependent on substrate utilization rates as well as OM degradation process.

In Majuli, the CNP ratio was in a range of 16:1:1 – 33:6:1 in pre-monsoon season and 12:1:1 – 33:3:1 in monsoon season. CNP ratio was higher in S10 and lower in S12 (both occurring in Group C) in pre-monsoon season whereas in monsoon CNP ratio was higher in S6 (Group A) and lower in S3 (Group B) [Table A 2.6 (A), Appendix 2]. The CNP ratios indicated that substrate utilization (Nitrogen and Phosphorus) was high in Group C or OM degradation was low (evident from CEC), in pre-monsoon season. In monsoon, substrate utilization process by microorganisms was high, possibly due to microbial activity. This outcome is further supported by the fact microbial population was higher in monsoon season than pre-monsoon investigations.

In Amingaon CNP ratio was in a range of 12:3:1 – 20:4:1 in pre-monsoon and 13:3:1 – 24:4:1 in monsoon season respectively. CNP ratio was higher in A6 (Group A) and lower in A3 (Group B) in pre-monsoon season and higher in AB2 (Group C) and lower in A1 and A4 (Group A) in monsoon season [Table A 2.6 (B), Appendix 2]. In Umananda CNP ratio was in a range of 11:4:1 – 22:3:1 in pre-monsoon season and 12:4:1 – 24:5:1 in monsoon season respectively [Table A 2.6 (B), Appendix 2]. CNP ratios in Amingaon and Umananda indicated higher substrate utilization than Majuli samples. The seasonal variance in Amingaon and Umananda was minimal.

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Finally, the experimental data were observed at a global scale by comparing it with SOM in terrestrial soils and alluvial deposits worldwide (Cleveland et al., 2007;

Yang et al., 2013). It was apparent that CNP budget in the test groups in Majuli and Kamrup were rather inclined towards a moderate productive zone. However, Kamrup seemed to be more productive than Majuli (Table 2.5).

Table 2.5 Details of a comparative study of the SOM fractions in Majuli and Kamrup and terrestrial soils worldwide

Elements (g/kg)

Concentration in a wide variety of terrestrial soils (worldwide) (g/kg)

Concentration in Majuli (g/kg)

Concentration in Kamrup

(g/kg)

References

Carbon 13.31 – 469.39* 2.18 – 3.04** 1.93 – 5.53** ISI

Web of Science online database (Cleveland et al., 2007) Nitrogen 0.29 – 18.20* 0.32 – 0.75** 0.29 – 1.14**

Phosphorus upto 0.53*

(alluvial deposits)

0.41 – 0.70** 0.25 – 0.72**

USDA website, Soil and Terrain database for Latin America and the

Caribbean (SOTERLAC) (Yang et al., 2013)

*The unit of actual values have converted to g/kg

**Experimental data

Total Carbon content was reasonable, though less, whereas lower limit of Nitrogen and Phosphorus in the study areas were rational, emphasizing the acute role of terrestrial soils as a nutrient sink. Total phosphorus levels were fairly higher than the amount reported in alluvial deposits worldwide (Yang et al., 2013).

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2.3.3 Role of microbial biomass and soil enzymes activities in soil fertility and