Nalina Gnanavelrajah
Crops and trees are natural carbon sinks. Both plants as well as soils rich in organic matter can greatly help in absorbing the atmospheric carbon. Farming practices that enhance plant diversity and organic matter in soils leads to sustainable agriculture and also environment.
Carbon sequestration in terrestrial ecosystems has beneficial effects at both global and local levels. Any action to improve situations at global level will have impact locally too. But actions need to be beneficial to the local communities, to be implemented. Carbon sequestration includes increasing Carbon pools of both biomass and soil carbon. The idea is to capture and store the carbon emissions produced by human activities from reaching the atmosphere.
Improving soil Carbon has desirable effects on increased productivity of lands, reduced land degradation and increased biodiversity. Soil Carbon pools increase productivity of soils by lessening the constraints of nutrient availability, water availability, toxicity and erosion. Despite the well recognition of forest lands as best carbon sequestration agents, it is impossible to have forests at the expense of food security.
Improving carbon stocks of agricultural land-uses could be effected by increasing biomass Carbon, soil Carbon or both. Biomass Carbon could be significantly increased by introducing tree crops in the field. Identifying appropriate agricultural land uses to increase carbon sequestration is a better option to solve the problems of environmental degradation and food insecurity.
Soil carbon could be increased by incorporating crop/plant residues into the soil and organic manure application. Soil organic matter is the main determinant of biological activity. The amount, diversity and activity of soil fauna and microorganisms are directly related to the organic matter. Organic matter, and the biological activity that it generates, have a major influence on the physical and chemical properties of soils. Nutrient availability is increased through decomposition of organic matter and increased cation exchange capacity of soils. Water availability is increased by improved soil structure and water holding capacity by organic matter itself. Improved soil structure by application of organic matter also enhances root penetration, aeration and drainage. In addition to reducing toxicity, it also helps as a buffer against acidity or alkalinity. The reaction of soil organic compounds with phytotoxic chemicals is an important component of beneficial effect of organic matter. Decomposition and transformation of applied pesticides by soil microorganisms prevent the accumulation of soil toxicities. The reduction of soil organic matter and microbial activities reduce the capacity of the soil system to adjust to the toxic effect of natural and applied phytotoxic compounds and residues.
Study on Carbon stock
A study was conducted in the Khlong Yai sub-watershed of Thailand to assess the agricultural land-uses in terms of carbon sequestration and to get information about farmer perception on environment. Soil carbon in coming ten years was forecasted by Roth C model for which existing soil data and information about organic matter management in field were required. A questionnaire survey was conducted to get information regarding crop residue management, organic manure incorporation and farmer perception on environment.
Use of organic manures
Along with fertilizer use, use of organic manure is also popular in the study area. About 64% of the farmer respondents were using organic manure. All the farmers of monocrop and intercrop land-uses (pineapple, sugarcane, coconut-cassava and sugarcane-cassava) use organic manures. On the other hand, none of the farmers growing eucalyptus or paddy use organic manures. Chicken manure is the main organic manure used by 98% of those applying organic manure. Other organic manure used was cattle manure. The average amount of organic manure used by different land users ranged between 1.96 – 11 tons/ha, the minimum usage being to coconut and maximum usage to sugarcane crop.
About half of the farmers surveyed felt that over the last ten years, there was a decrease in the organic matter in the field. Majority of the remaining (36%) felt that there was no change, while very few farmers felt that the organic matter had increased in the last ten years. Almost all the farmers perceived organic matter as important for improving land quality. However, many of them felt that the amount they applied was not enough owing to high cost and lack of livestock. Due to insufficient quantities, most often, farmers did not apply manure to long standing plantations.
About 87% of farmers used inorganic fertilizers. Average amount of inorganic fertilizer use ranged from 0.31 – 1.63 tons/ha in different land uses. The inorganic fertilizers mainly included nitrogen, phosphorus and potassium nutrients. Farmers of coconut, mixed orchard and eucalyptus land uses did not use inorganic fertilizers.
Measuring carbon content
Field measurements were carried out to estimate biomass of each type of land-use. The agricultural land-uses studied were mono-cropped land use such as cassava, pineapple, sugarcane, para rubber, eucalyptus and paddy, inter-cropped land use – coconut-cassava, mixed cropped land use – mixed orchards and mono-cropped rotations namely pineapple-cassava and sugarcane-cassava.
The total carbon stock from agricultural land uses in the sub-watershed was 20.5 million tons of which 41.49% was biomass carbon and 58.51% was soil carbon. Of the total carbon stock, 39.59% was contributed from Para rubber which covers 23.3% of agricultural land uses. Mixed orchard land use contributed 19.79% of total carbon stock, covering 24.49% of land uses.
The total soil carbon in all types of land uses amounted to 12 million tons, of which land use of para rubber, mixed orchard, pineapple, cassava and sugarcane–cassava covers 28%, 18%, 17%, 13% and 11%, respectively. Results show that there will be a total of 0.21 million ton of soil carbon accumulation in the study area in ten years. In ten years, land uses like cassava, sugarcane – cassava rotation, and pineapple-cassava rotation will accumulate 97, 77 and 60 thousand tons of carbon, respectively. Similarly, coconut-cassava, mixed orchard, sugarcane, eucalyptus and pineapple land uses will be contributing 9, 7, 6.6, 2 and 1 thousand tons respectively. In case of Para rubber and coconut land uses, about 43 and 3 thousand tons of soil carbon, respectively are estimated to be reduced in ten years. Contrary to the general belief that soil carbon is depleted in agricultural land uses, the modelled Soil Organic Carbon (SOC) values in this study indicated the increase in SOC. This is mainly due to the management practices of organic manure and crop residue incorporation by farmers in the study area (Table 1).
Majority of the farmers (79%) believed that trees had a positive impact on the environment. However monocrop and intercrop land use types did not include trees. More than half of the farmer respondents felt that, it is difficult to manage the trees in the crop fields. Other reasons for not including trees in crop fields are lack of knowledge on managing trees with crops and fear of losing some land for crop cultivation.
It is worth noting that biomass carbon is higher in tree crop land uses compared to shrub crop land uses while the opposite is true in case of soil carbon accumulation. However, intercropping of tree-shrub (coconut-cassava) had higher records in both biomass and soil carbon accumulation. This finding opens a window to future land use planning and research in terms of carbon stock management in agricultural land uses.
Lessons learned
Soil carbon accumulation was more in land-uses having short-term crops such as cassava, sugarcane and pineapple. Intercropping with perennial tree crops and short-term crops such as coconut-cassava increased both biomass carbon and soil carbon. Research on farming systems of mixed/inter cropping tree crops with field crops with addition of organic manure and residue management would help to improve further carbon sequestration in agricultural land-uses.
Farmers’ perception about organic matter management and tree planting could help to increase carbon sequestration and soil quality. Farmers in the Khlong Yai sub-watershed are successful in increasing soil organic matter content in their fields by incorporating organic manures and crop residues to their soils. In addition, by selecting tree crops they could increase biomass carbon in their fields.
Dr. (Mrs.) Nalina Gnanavelrajah
Senior Lecturer, Department of Agricultural Chemistry, Faculty of Agriculture, University of Jaffna P.O.Box 57, Thirunelvely, Jaffna, Sri Lanka, email: nalina12@yahoo.com
