Dr. Vivak M. Arya & Dr. Charu Sharma
Soil organic matter that is an essential reservoir of carbon, nutrients and energy in the cycle of life plays multipurpose role, the relative importance of which differ with soil type, climate and land use. Though soil organic matter constitutes only a small fraction of the total mass of mineral soils, it exerts a profound influence on physical, chemical and biological function of the soils. Soil carbon determines ecosystem and agro ecosystem function, influencing soil fertility, water holding capacity and many other soil parameters. It is of global importance because of its role in the global carbon cycle and therefore, the part it plays in the mitigation of atmospheric levels of green house gases (GHGS) with special reference to CO2.The atmospheric concentration of CO2 has increased by 35% from about 280 ppmv (parts per million volume) at the beginning of the industrial revolution (ca1850) to about 380 ppmv today. Fossil fuel burning, land use changes and tropical deforestation are the major sources of CO2 emission, which respectively contribute 6.4, 1.1, and 1.6 pg C annually.
Global Carbon Pool Size
The main pools of actively cycling carbon are atmospheric CO2, biota (soil vegetation), soil organic matter and the ocean. Ocean contains the largest reserves of C-about 39,000 Pg C- though most of it is in deep ocean layers and not in active circulation. Atmosphere contains about 785 Pg C as CO2 that is equal to about 15 t C above ha of the earth’s surface. Biota stock about 400-600 Pg C, 75 percent of which occurs in forests. The global pool of SOM is estimated to contain about 1500 pg C to a depth of 1 m. To reduce the emission of CO2, carbon capture and storage (CCS) has been found to be an important option and they are: capturing CO2 at large and stationary point sources, and injecting the CO2 in suited geological reservoir or sinks.
Processes and Practices Influencing SOC
There are several factors and processes that affect SOC pool and its depletion. Decline in soil quality, soil’s productivity and environment-moderating capacity exacerbates SOC depletion. Emission of CO2 from soil to the atmosphere is influenced by the mineralization of C in SOM through microbial processes that use it as a source of energy, combine C with O2 leading to release of CO2 and H2O. The CO2 emission from soil is accentuated by ploughing and mixing crop residue and other biomass in the soil surface.
Soil as C Sink
Carbon stored in soils worldwide represents the 3rd largest sink in existence, after oceans and geologic sinks. There is 2-4 times as much C stored in soils as there is in the atmosphere and approximately 4 times the C stored in vegetative material (i.e. plants). Soil C is found as either inorganic (i.e. mineral) or organic materials. Inorganic soil C is generally found as carbonates of calcium (CaCO3, or limestone) and magnesium (MgCO3). The organic forms of C in soil are a very diverse group of materials that can be defined as ‘everything in or on the soil that is of biological origin, whether it’s alive or dead. It therefore includes live plant roots and litter (not shoots), humus, charcoal and other recalcitrant residues of organic matter decomposition. It also includes the organisms that live in the soil that are collectively called the soil biota (e.g., fungi, bacteria, mites, earthworms, ants and centipedes).
Soil Carbon Measurement
The commercially available techniques for measuring and monitoring soil C, or more importantly components of the soil C pools, are currently limited. Total soil C measurement, either by combustion or by Heanes wet oxidation, provide realistic measures of total C status. Most routine soil tests report soil organic C measured using the Walkley-Black wet oxidation technique, but this only measures 70-90% of the total soil organic C, depending on soil type. Both wet oxidation techniques will only measure the organic C, which is a clear advantage they have over the combustion methods.
Soil Carbon Decline
Soil organic C represents an equilibrium condition reflecting the balance between C inputs (as residues, roots etc) and C mineralization and loss as CO2 – for a given climate and soil type. We often hear about declining soil organic matter and C stores in cropland, because the change from a native pasture or woodland to cropping has meant a relative reduction in C inputs and an increase in carbon removed in harvested products and by gaseous loss. The result is a slow shift towards a new (lower soil C) equilibrium position. This changed position will reflect a new balance between inputs and losses, and particularly under rainfed (kandi) cropping conditions (even under the best conservation tillage techniques) will be significantly lower than under native vegetation or grassland. This is logical, as while native vegetation or pastures use every available drop of moisture to grow biomass and fix atmospheric C all year round, there are periods (in recent years some of those periods have been quite long!) where crops aren’t growing but microbes are decomposing soil organic matter.
How Can Soil C be restored?
If we wish to change soil organic C status, presumably in order to improve soil health and the productivity/profitability of the enterprise, there has to be a shift in the balance between inputs and losses. In other words, we have to increase the inputs while minimizing losses. Increasing C inputs can generally be achieved by increasing productivity (more biomass grown generally more residues returned. However, it is worth considering the size of the C pool to get some perspective on the use of either strategy, but particularly that of organic amendments. A black cracking clay with 1% organic C will contain approx. 10 t C/ha in the top 10 cm layer, while a 5 t/ha grain sorghum crop will return approx 10 t/ha organic matter as roots and surface residues (about 4 t C/ha). Contrast that with applying 5 t/ha of manure (say 30% C) once every 5 years.Good examples from cropping include retaining crop residues in the soil by eliminating burning and reducing tillage. Again, combinations of economic pressures and climatic sequences will have a big impact on the practicality and effectiveness of these strategies. An interesting approach gaining publicity at present lies in the conversion of organic wastes and crop residues into relatively inert C compounds by pyrolysis to create biochar, and then adding this to soil. While economics will ultimately decide the feasibility of this strategy, it is worth remembering that inert materials like biochar have long residence times in soil because they are relatively recalcitrant to soil microbial decomposition.
Soil Carbon sequestration in India
The prevalent low levels of SOC concentration in India are attributed to soil mining practices of excessive tillage, imbalanced fertilizer use, little or no crop residue, crop residue burning and severe soil erosion. Benbi reported that total organic carbon pool in soils of India is estimated at 21 Pg to 30 cm depth and 63 Pg to 150 cm depth, which represents 2.2% of the world pool for 1 m depth. The has been decrease of 30 to 60% in Soc concentration of cultivated soils even by 1960.Soil organic carbon(SIC) pool is generally high in calcareous soils of arid and semiarid regions. Calcareous soils are widely distributed covering almost 54% of the geographical area of our country. The potential of carbon sequestration in India ranges from 39.3 to 52.0 Tg yr-1, which comprises of restoration of degraded soils (7.2-9.4 Tgyr-1),SIC sequestration(21.8-25.6 Tg yr-1).
Practical Use and Conclusion
Organic carbon and organic matter are the keys to healthy soils that are able to support productive and sustainable land uses – both in agricultural and natural ecosystems. Technological options that have been found to be efficient for soil C sequestration in India include green manuring, mulch farming/conservation, zero or no tillage, aforestation/agro forestry, grazing management especially in Kandi area, integrated nutrient management, manuring and choice of cropping sequence. Soil C sequestration is strategy to achieve food security through improvement in soil quality. Soil organic carbon is an extremely valuable national resource. Irrespective of the climate debate, the SOC tock must be restored, enhanced, and improved. A soil carbon management policy at national and state levels that include regulation-based trading soil carbon must be developed.
(The authors are from SKUAST-J)
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