PhD

In the current scenario of global climate change, wetland paddy fields are considered as major sources of greenhouse gases (GHG), especially methane (CH 4) and carbon dioxide (CO 2) as they experience both dry and wet situations depending on water availability. On the other hand, wetland characteristics promote the accumulation of organic matter in the soil and sediment, serving as carbon (C) sinks and making them one of the most effective ecosystems for storing soil carbon. In this context, the present study was undertaken with the objective to assess the combined effect of organic and inorganic sources on carbon sequestration and soil health under rice –rice cropping system and to compare the carbon distribution and fluxes with that of adjoining fallow land. The experiment was conducted in the farmer’s field at Varadium, Thrissur with rice as test crop in sandy clay loam soil for two continuous cropping seasons viz., Virippu and Mundakan during April 2015 - January 2016. The treatments consisted of different organic sources like farm yard manure (FYM), jack tree (Artocarpus heterophyllus) leaves, daincha (Sesbania aculeata), rice husk biochar and no organic manure. These were applied in combination with four levels of nitrogen (N) viz., 0, 35, 70 and 105 kg ha -1 represented respectively as N 0, N 35, N 70 and N 105. The soil samples from surface (0-15 cm) and subsurface (15-30 cm) and plant samples were analysed at harvest stage in both seasons to assess the impact of treatments. Gas samples were collected during the second cropping season at three stages viz., active tillering, panicle initiation and near harvest of the crop from fifteen treatments (except N 0 level) so as to evaluate the GHG flux (CO 2 and CH 4) from the cultivated land and it was compared with that of adjoining wetland. Soil characterization of cultivated land (biochar treatment with N 0 level) and fallow wetland (1 m depth) was also carried out after the field experiment.Combined use of both organic and inorganic sources improved the physico-chemical properties of soil over inorganics alone. The impact was more pronounced with biochar + N 0 treatment and its effect on increasing soil pH was also note-worthy. The build- up of soil organic carbon (SOC) as well as total carbon (TC) contents were more in the surface layer compared to the subsurface. Irrespective of treatments, the carbon content decreased after Virippu season as a result of high temperature. The high carbon content noted in the biochar with all levels of N had positive effect even in the second season. The soil carbon storage was also high with this organic source. The distribution of organic C among physical pools of soil organic matter viz., coarse particulate organic carbon (cPOC), fine particulate organic carbon (fPOC), light fraction organic carbon (LFOC), intra light fraction organic carbon (iLFOC), heavy fraction organic carbon (HFOC) and mineral associated organic carbon (MinOC) separated on size and density basis using standard procedures were also studied. Though the organic sources had positive effect on various pools, biochar with all levels of N had a strong impact in the carbon content of cPOC, LFOC and iLFOC. The cPOC concentration decreased over time while the reverse happened with fPOC. However, in the density fractions with biochar treated soils, the LFOC had the highest C concentration followed by iLFOC and the effect was more prominent in the surface layer. Combined application of organic sources and inorganic fertilizers significantly increased the cation exchange capacity and nutrient availability in soil than that of inorganic fertilizers alone. The amount of nutrients decreased with increasing soil depth. Levels of N also had varied effect on these contents. The nutrient content in plant parts like grain, shoot and root of rice crop varied among treatments. Adequate improvement in soil physical, chemical and microbiological parameters on application of green manure contributed to increased grain yield at 35 kg N ha -1 in flooded rice soil while the straw yield was maximum in the treatment with biochar at N 105. Measurement of GHG emission during the rice growth stages showed that the level of CO 2 was high in the active tillering phase and near harvest phase of crop growth, while that of CH 4 was found to be higher in the panicle initiation phase. Application of biochar as well as green manure with 105 kg N ha -1 resulted in greatest emission of CO 2, whereas the FYM + N 105 showed highest emission of CH 4. Soil profile study carried out after the field experiment indicated that the soil profile in the fallow land had a depth of 115 cm and five horizons while a soil profile depth of more than 70 cm with six horizons in the cultivated land. The effect of biochar on distribution of carbon and available nutrients within 30 cm depth was maximum in cultivated land in comparison with fallow wetland. The study revealed that the application of biochar without N had great impact on soil pH and various pools of carbon but its effect at this level on soil available nutrients was reverse. The soil available nutrients were higher with FYM and daincha and their effects increased with higher levels of N which subsequently improved the plant nutrient contents and rice yield. However, the organic sources with higher doses of N had enhanced emission of CO 2 and CH 4. Hence a suitable balance between the organic + inorganic N sources is advocated.