Ph.D

A study entitled “Assessment, mapping and modelling of soil carbon pools and stock in selected agro-ecological units of south Kerala” was carried out to examine the impact of various agricultural land use systems on soil carbon fractions, pools and stock, soil aggregation, glomalin and polysaccharide contents, and to generate soil carbon maps using GIS and predict soil carbon changes in the future climate change scenario using modelling technique. The agro-ecological units (AEUs) of south Kerala namely, southern coastal plain (AEU 1), Onattukara sandy plain (AEU 3), southern laterites (AEU 8), south central laterites (AEU 9) and southern and central foothills (AEU 12) were selected for the study. In each AEU, different agricultural land use categories as described by IPCC for the carbon inventory such as, garden land (coconut), wet land (rice), fallow land (uncultivated) and plantation (rubber) were also selected. Soil profiles upto a depth of one meter was taken from the selected sites and samples were collected from various depth intervals of 0 to 25, 25 to 50, 50 to 75 and 75 to 100 cm.
The surface soil samples (0 – 25 cm) were analyzed for soil properties such as pH, EC, CEC, texture, soil carbon fractions (organic, inorganic, water soluble, permanganate oxidizable, microbial biomass carbon and mineralizable carbon) and organic matter fractions (humic acid, fulvic acid and humin). The depth wise distribution of soil carbon pools (active, passive and slow carbon pools), bulk density and total organic carbon (TOC) were analyzed. Soil organic carbon stock, density, carbon indices (CPI, CLI, CMI, LQI, GWP) and carbon proportion/ turnover were computed. Soil aggregation was evaluated by analyzing water stable aggregates, mean weight diameter, macro-micro aggregate size distribution, aggregate associated organic carbon, glomalin and total polysaccharides in soil. GIS based thematic maps of soil organic carbon, stock, density and land quality were generated in ArcGIS 10.5.1 software. The soil organic carbon changes in the future climate change scenario were predicted using DNDC (Denitrification Decomposition) model.
The results revealed that soil texture was loamy sand to sandy clay loam in AEU 1, sandy loam to sandy clay loam in AEU 3 and 9, sandy loam to sandy clay in AEU 8 and sandy clay loam to sandy clay in AEU 12. The sand fraction (45.32-80.34 %) was found to be more than silt (9.60 – 21.14%) and clay (8.87-39.15 %). The pH and EC of soils ranged 254
from 4.77 to 5.73 and 0.06 to 0.44 dS m⁻1 respectively. The CEC of soil varied between 2.60 and 6.69 c mol(p+) kg-1 with the highest value in rubber land use and the lowest in uncultivated land.
Soil organic carbon (SOC) ranged from 0.34 to 0.73, 0.44 to 0.99, 0.40 to 1.09, 0.25 to 0.97 and 0.26 to 1.26 per cent in AEU 1, 3, 8, 9 and 12 respectively. Among the land uses, rubber land use recorded the highest SOC (1.03 %) followed by coconut (0.75 %) and rice (0.60 %) and the lowest SOC was observed in uncultivated land (0.34 %). The water soluble, permanganate oxidizable and the particulate organic carbon varied from 34.01 to 162.81 mg kg-1, 1.21 to 7.37 mg g-1 and 0.37 to 2.75 per cent respectively. The different AEUs and land uses followed the order AEU 12 > AEU 9 >AEU 8 > AEU 3>AEU 1 and rubber > coconut > rice >uncultivated land respectively. Similar trend was observed for microbial biomass carbon which ranged from 121.2 to 424.4 mg kg-1 with the highest value in AEU 12 and rubber land use. Percentage contribution of water soluble, particulate and permanganate oxidizable C to TOC was 0.31 to 0.43, 40.12 to 69.72 and 12.62 to 19.73 per cent respectively.
The organic matter fractions viz. humic acid, fulvic acid and humin varied from 0.57 to 2.06, 0.73 to 2.33, 0.62 to 1.59 per cent respectively in different AEUs. The rubber land use showed significantly higher humic acid (1.72 %), fulvic acid (2.01 %) and humin (1.44 %) than coconut, rice and uncultivated land. Percentage contribution of humic acid, fulvic acid and humin to total organic matter ranged from 29.40 to 32.51, 32.30 to 36.42 and 26 to 29.25 per cent respectively.
The active, slow and passive pools of carbon in soil ranged between 0.09 and 1.03, 0.17 and 0.78, 0.13 and 1.26 per cent respectively with the highest value in AEU 12 and the lowest in AEU 1. In different land uses it followed the order rubber >coconut> rice> uncultivated land. The soil carbon pool showed a gradual decline from 0 - 25 cm to 75 - 100 cm depth with a decrease of 0.82 to 0.14, 0.63 to 0.18 and 0.97 to 0.24 per cent for active, slow and passive carbon pools respectively. The passive pool of carbon (31.95 – 38.08 %) contributed more towards total organic carbon than active (23.64-37.66 %) and slow (22.73-32.12 %) pools. 255
With depth (0-25 cm to 75-100 cm) bulk density of soil increased from 1.38 to 1.66, 1.33 to 1.68, 1.35 to 1.64, 1.33 to 1.60 and 1.29 to 1.55 Mg m-3 in AEU 1, 3, 8, 9 and 12 respectively. Higher bulk density was observed in uncultivated land than rice, coconut and rubber land uses. The TOC in soil ranged from 0.72 to 2.80, 0.69 to 2.89, 0.79 to 3.46, 0.88 to 3.18 and 0.75 to 3.56 per cent in AEU 1, 3, 8, 9 and 12 respectively. In all the AEUs the highest TOC was registered from rubber land use followed by coconut, rice and the lowest from uncultivated land. The TOC content decreased with depth (0 - 25 cm to 75 - 100 cm) and the values were 1.99 to 0.95, 2.24 to 0.92, 2.45 to 1.05, 2.53 to 1.05 and 2.38 to 1.01 per cent for AEU 1, 3, 8, 9 and 12 respectively. Total soil organic carbon stock varied from 162.71 to 293.22, 175.80 to 275.79, 188.63 to 322.36, 199.73 to 331.48, 144.54 to 355.63 Mg ha-1 in AEU 1, 3, 8, 9 and 12 respectively with the highest stock in AEU 12. Among the land uses it followed the order rubber>coconut>rice>uncultivated land. SOC stock decreased from 0 to 25 cm to 75 to 100 cm depths and the values were 76.28 to 36.59, 79.97 to 35.88, 86.70 to 39.61, 88.10 to 39.48 and 79.90 to 35.80 Mg C ha-1 for AEU 1,3, 8, 9 and 12 respectively. The soil organic carbon density of the surface (0-25 cm) soil varied from 3.07 to 6.01, 3.68 to 5.76, 3.84 to 7.81, 4.19 to 7.19 and 3.27 to 6.96 kg m-2 in AEU 1, 3, 8, 9 and 12 respectively with the highest density observed for AEU 12. With respect to different land uses it followed the order rubber (6.96 kg m⁻2 )> coconut (5.86 kg m⁻2) > rice (5.10 kg m⁻2) > uncultivated land (3.61 kg m⁻2).
The carbon indices such as lability, pool and management index ranged from 0.30 to 0.93, 0.19 to 0.51 and 5.55 to 38.42 respectively in different AEUs and were found to be the highest in AEU 12 and rubber land use. The land quality index based on SOC stock in kg m-2 was rated as medium (6 - 9 kg m-2 ) in all AEUs.
The mineralizable C content in soil varied from 1.40 to 3.45, 1.18 to 3.41, 1.04 to 3.04, 1.23 to 3.35 and 1.01 to 3.02 mg g⁻¹ in AEU 1, 3, 8, 9 and 12 respectively. The highest value was observed from AEU 1 and rice land use. Similar trend was obtained for global warming potential of soils based on CO2 evolution which varied from 31.82 to 78.41, 26.89 to 79.02, 22.89 to 69.02, 28.03 to 76.06 and 22.96 to 68.64 in AEU 1, 3, 8, 9 and 12 respectively. The C proportion and turnover rates were in the range of 0.25 to 0.77 256
and 0.04 to 0.17 respectively. The C proportion was the highest in AEU 12 and rubber land use whereas the C turnover was the highest in AEU 1 and rice land use.
With respect to aggregate distribution, the larger size fractions (5-8, 2-5, 1-2 & 0.5-1 mm) were found to be higher in rubber land use and the smaller size fractions (0.25-0.5 & 0.1-0.25 mm) were higher in rice land use. The macro aggregate fraction (> 250 μm) ranged from 22.60 to 75.70 per cent with the highest value in rubber land use while the micro aggregate fraction (53 – 250 μm) ranged from 10.78 to 56.46 per cent with the highest value in rice land use. The water stable aggregates and mean weight diameter were ranged from 22.60 to 34.31 per cent and 0.56 to 2.90 mm respectively and were the highest in rubber land use.
The aggregate associated organic carbon was the highest in the size fraction of 1 to 2 and 0.5 to 1 mm (1.03-1.27 g kg-1) compared to other fractions. The organic carbon associated with macro (> 250 μm) and micro (53-250 μm) aggregates are in the range of 2.16 to 5.70 and 0.30 to 0.95 g kg-1 respectively with the highest value in rubber land use. The glomalin related soil protein ranged from 1.22 to 6.35 mg g-1 and polysaccharides from 2.12 to 9.24 mg g-1 . Irrespective of the AEUs the highest glomalin and polysaccharides were observed in rubber land use.
The DNDC model predicted organic carbon changes in coconut land use systems in Thuravoor (AEU 1), Bharanikkavu (AEU 3), Ookkod (AEU 8), Karavaram (AEU 9) and Vellarada (AEU 12) under RCP 4.5 and RCP 8.5 future climate change scenario. The DNDC model predicted an increase in soil organic carbon status from 2020 to 2050 under RCP 4.5 and RCP 8.5 scenario but the rate of increase in soil carbon were more pronounced under 4.5 scenario where relatively lower CO2 emission was observed. The predicted SOC for 2050 under RCP 4.5 scenario were 1.26, 1.04, 1.44, 1.23 and 1.27 for Thuravoor (AEU 1), Bharanikkavu (AEU 3), Ookkod (AEU 8), Karavaram (AEU 9) and Vellarada (AEU 12) respectively.
The organic carbon fractions, pools and stock were the highest in agro-ecological unit 12. Among the land uses, rubber contributed more to the SOC stock and pools indicating the prevalence of conducive environment for the buildup of carbon. The macro 257
and micro aggregates, aggregate associated carbon, glomalin, polysaccharides and C proportion were also higher in rubber land use indicating it as a potential carbon sink. Among the carbon fractions particulate organic carbon contributed more to total organic carbon. Among the carbon pools passive pool contributed more towards the total organic carbon. The carbon associated with larger fractions (1-2 mm and 0.5-1 mm) and macro aggregates (> 250 μm) were high in rubber land use indicating physical protection and sequestration in soil. The DNDC model predicted an increase in the SOC status from 2020 to 2050 under RCP 4.5 and RCP 8.5 scenario of future climate change.