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Soil forms the largest storehouse for terrestrial organic carbon, encompassing
approximately two-thirds of the carbon in the ecosystem. This plays a pivotal role in
reducing atmospheric CO2 and combating global warming and related concerns.
Differences in land use have a considerable effect on soil organic carbon pools and fluxes.
Mineralisation of soil organic carbon enhances CO2 emission into the atmosphere and
salinity of soil can affect the rate of this process. In this context, an investigation was
carried out with the objective to study the effect of salinity on carbon mineralisation under
different land uses in Pokkali ecosystem.
The study was conducted by collecting georeferenced surface soil samples (0-
20 cm) from three land uses of Pokkali ecosystem namely rice-prawn, rice alone and
prawn alone from Kumbalangi, RRS, Vyttila and Kadamakkudy respectively. The
samples were analyzed for different soil properties such as pH, electrical conductivity
(EC), particle size, cation exchange capacity, bulk density and estimated soil organic
carbon pools such as soil organic carbon, labile carbon, water soluble carbon and
microbial biomass carbon. Soil organic carbon stocks and dehydrogenase activity were
also estimated as per standard procedures. Carbon mineralisation was studied by a
laboratory incubation experiment for 74 days where two sets of soil (with and without
1% paddy straw) was amended with different concentrations of CaSO4 and Na2SO4 alone
or both in combinations of 40 and 80 mmol per kg soil in order to get the desired change
in electrical conductivity similar to field conditions.
The mean pH registered for rice-prawn, rice alone and prawn alone land uses were
under neutral range and mean EC were less than 4 dS m-1. Soil texture of rice-prawn and
rice alone belonged to clay and prawn alone in sandy clay loam class. Cation exchange
capacity was highest in rice-prawn and lowest in prawn alone land use. The highest mean
bulk density was observed in prawn alone and the lowest in rice-prawn land uses.
The results revealed that rice-prawn land use recorded the highest organic carbon pools
and prawn alone land use recorded the lowest organic carbon pools. Rice-prawn land use
recorded higher content of soil organic carbon, labile carbon, water soluble carbon,
microbial biomass carbon and total carbon followed by rice alone land use due to
abundance of organic matter in these soils. The maximum soil organic carbon stock was
recorded in rice alone (26.46 Mg ha -1) and minimum in rice-prawn (22.53 Mg ha -1) land
use. The maximum dehydrogenase activity was recorded from rice alone (2959.697 µg
TPF h-1 g-1) and the lowest from prawn alone (2132.491 µg TPF h-1 g-1) land uses.
Carbon mineralisation study revealed that CO2 evolved from soil decreased with
time. The maximum CO2 was evolved on day 11 for rice-prawn and prawn alone land
uses and day 17 for rice alone land use and declined steadily thereafter. The maximum
cumulative CO2 was recorded from the treatments other than control (without paddy straw
and salts). The lowest mean cumulative CO2 in the control was recorded from rice- prawn
(7.2 mg 100 g-1) land use and the highest recorded from prawn alone (26.48 mg 100 g-1)
followed by rice alone (25.15 mg 100 g-1) land uses. A significant positive correlation
was observed between pH, dehydrogenase activity and total organic carbon with
cumulative CO2 in all land uses.
After 74 days of incubation study, the pH of control was reduced and that of other
treatments (with paddy straw and salts) was increased in all land uses. The EC of the
treatments with salts amendments were more than 4 dS m-1 in all land uses before the start
of incubation. After the incubation period, EC decreased in all salt amendments of
different land uses and increased in treatments without salt amendments. Dehydrogenase
activity and total carbon were found higher in the treatments with paddy straw and salts
than in control. Relatively higher dehydrogenase activity was recorded in rice-prawn land
use followed by prawn alone and rice alone land uses. The EC was positively correlated
with dehydrogenase activity which stimulated carbon mineralisation in these soils. In
these land uses, there was no discernible difference between paddy straw and salt
amended treatments with respect to carbon mineralisation. Both the dehydrogenase
activity and the total carbon content contributed to carbon mineralisation process.
In Pokkali ecosystems, rice-prawn land use recorded the highest soil organic
carbon pools and the lowest carbon mineralisation, indicating the existence of an
environment that is conducive for building organic carbon which is crucial for
sequestering more carbon into these soils. As the salinity increased carbon mineralisation
was also increased. The rice cultivation in the low saline phase reduces salinity by
leaching out the accumulated salts, whereas prawn farming does not. Therefore, prawn
alone land use remains saline throughout the year, resulting in increased carbon
mineralisation and decreased carbon sequestration. This study emphasizes the importance
of integrated rice-prawn farming systems in Pokkali lands to maintain soil quality and
slow down the global warming through increased carbon sequestration.