Effect of salinity on carbon mineralisation under different land uses in Pokkali ecosystem

Abstract

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.