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Digital mapping of soil organic and inorganic carbon status in India
The annual flux of C passing through the soil and the magnitude of the soil C pool are two of the reasons that soil organic carbon plays a significant role in the global C cycle. Soil C is also a crucial factor to soil fertility and the environment because of its vast potential of sequestration in soil and the biota. The amount of organic C stored in various soil pools at a specific time is the balance between the rate of input of biomass-C and that of its mineralization in each of the organic C pools. Change in the size of the soil C pool, therefore, can significantly affect atmospheric CO2 concentration. In this context, Soil and Land Resources assessment division, NRSC is working since 2007 to digitally map organic and inorganic carbon density as well as to estimate the size of these pools for India using intensive field data and remote sensing data under ISRO Geosphere Biosphere Programme through National Carbon Project (NCP).
In this study an attempt was made to estimate soil organic and inorganic carbon stock through Random forest based digital soil mapping technique. The spatially explicit mapping of SOC and SIC at 250 m resolution and an estimate of their pool size in India was undertaken using a large number of remote sensing derived data layers and data mining approach. The SOC and SIC densities up to 100 cm depth or paralithic contact (whichever is shallower) were estimated for 1198 soil samples located across India using a stratified random sampling that integrated land use, soil, topography and agro-ecological regions. Using Random forests (RF) based spatial prediction procedure with climatic, land cover, rock type, soil type, multi-year NDVI, irrigation status as independent input variables, models for predicting carbon density at 250 m spatial resolution were developed. For modelling with RF algorithm, about 898 soil profile observations were used, while the rest of 300 were used for validation. The SOC, SIC and total soil carbon pool size of India has been estimated at 22.72 ± 0.93 Pg,12.83 ± 1.35 Pg and 35.55 ± 1.87 Pg, respectively, which are comparable to previous studies while providing first spatially explicit 250m map of their distribution. The spatial distribution indicates that majority of the carbon stock resides in the northern part of India.
The soil carbon stock of eastern India has contribution from organic carbon, while the western portion has contribution mainly from inorganic carbon. This sort of geospatial modelling approach would help in monitoring soil carbon by detecting changes in SOC and SIC stocks as a function of change in climate, land use and can be linked to global models to understand the global carbon dynamics in a better way. Besides, the spatial distribution of carbon densities coupled with process based models should help in developing necessary management interventions so as to reduce CO2 emissions from soils. The mean total organic carbon density and mean total inorganic carbon density generated on 5x5 km grid for entire India using spatial analysis and modelling technologies is available in www.nrsc.gov.in.
Sreenivas K, Dadhwal V K, Suresh Kumar, Sri Harsha G, Tarik Mitran, Sujatha G, Janaki Rama Suresh G, Fyzee M A, Ravisankar T, (2016). Digital Organic and Inorganic carbon mapping of India. Geoderma, 269, 160–173.