Global Theme on Agroecosystems
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Global Theme on Agroecosystems
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Potential for Carbon Sequestration in SAT Soils:
Global climate change due to the Greenhouse Effect and its impact on plant productivity, and therefore world food supply, is a major issue of concern to the scientific community. Soil organic carbon (SOC) is a key part of the carbon cycle. When manipulated through soil management, SOC makes soils a significant sink. Of the 173.3 million tones of sediments discharged into the sea by soil erosion, the Asia region contributes half. Erosion acts on the soil surface, the most fertile layer of the soil profile, containing 30-60% of the total SOC. In addition to the carbon, the sediments which are eroded are rich in plant nutrients. The whole process not only impoverishes the soil, but the mineralization of a part of the dissolved and particulate SOC becomes a land-degradation-induced source of C emission. Among terrestrial C sinks, agricultural systems with intensified cropping play a dual role by augmenting the amount of C entering the soil, and suppressing decomposition rates of SOC by reducing evaporative losses due to lowered soil temperatures. Soils in the semi-arid tropics (SAT) are highly prone to degradation; have low stocks of SOC; and are continuously under pressure to produce more food and feed to fulfill the ever-increasing demand for growing human and animal populations. This region occupies an area of about 11.1 million km 2 and is densely populated with about 850 million people of whom 300 million are considered food insecure. Several recent studies have assessed the potential for C sequestration on agricultural soils, primarily in industrialized countries. There are many obstacles to enhance the productivity of SAT soils as water availability being the primary constraint. The ability of SAT soils as C sinks is also being debated. The Approach Since 1976, operational scale Vertisol watersheds have been used at ICRISAT to optimize efficiency of rainwater, and to sustain productivity of Vertisols using an integrated watershed approach. Data from these long-term watershed experiments are used in this study to validate the hypothesis that improved management of Vertisols through integrated watershed management in the SAT not only increases the productivity but also promotes SOC sequestration and thereby enhances soil quality. In this study, an improved management system with soil, water, nutrient, pests and cropping systems was evaluated. A conventional system with farmer-adapted practices, mimicking the traditional low-input, low-output system, was maintained to serve as a comparison and control. In the improved system a 2-year rotation with improved high-yielding varieties of sorghum (Sorghum bicolor) intercropped with pigeonpea (Cajanus cajan) (2:1 proportion) (Figure 1) and maize during the rainy season followed by chickpea (Cicer arietinum) in the postrainy season were grown on a broad-bed and furrow (BBF) landform from 1976 to 1988 with application of 60:20:0 kg N:P:K ha -1 yr -1 . From 1989 to 1998, the field was divided into two plots, and a 2-year rotation with sorghum (cv CSH 9 and ICSV 745) and pigeonpea (cv ICP 1-6) as intercrops in one year and in next year green gram (Vigna radiata) was sown during the rainy season and sorghum in the postrainy season; these two plots were grown as mirror images. In the conventional system from 1976 to 1988, a local variety of sorghum (cv M 35-1) was grown on flat land in the postrainy season and in the rainy season, the field was maintained as cultivated fallow. These plots received 10 t ha -1 of farmyard manure (FYM) once in 2 years. From 1989, the field was divided into two plots, with a crop rotation of fallow-sorghum in one year and fallow-chickpea (cv Annegiri) in the next year; the two plots were treated as mirror images. Organic C, total N, mineral N, microbial biomass C and N were estimated by standard methods.
In our experiment on an average we observed 34% more organic C (g C kg -1 ) up to a soil depth of 120 cm in soils under the improved system, compared to traditional system. In case of surface layer (0-15 cm), the increase in organic C content was 67 % more in the improved system (4.35 g C kg -1 soil) than that of the traditional system (2.6 g C kg -1 soil). In traditional as well as in the case of the improved system, organic C content of soil decreased with the increase in soil depth (Table 1). However, even up to 90 cm depth improved management of Vertisols resulted in increased organic C contents by 21% over Vertisols managed with the traditional system (2.5 g C vs 2.05 g C kg -1 soil). On the whole, by taking bulk density into consideration, we observed that in 22 years in the improved system, carbon sequestered was 7.4 t C ha -1 more than in the conventional system, resulting in a gain of 335 kg C ha -1 y -1 .
Table 1. Organic carbon content (g C kg -1 soil) of semi arid tropical Vertisol under improved and conventional systems in a watershed, ICRISAT, Patancheru, 1998.
Values in parentheses indicate the percent increase in organic C content in the improved system compared to the conventional system. Carbon inputs were found to increase with continuous cropping, particularly where fertilizers were applied and when legumes were included in the system. When there is a fallow for one season, the decomposition is enhanced due to higher soil moisture and temperatures. Fertilization also enhanced crop production, with many experiments showing a positive effect of N additions on the soil C balance. Our findings show that the improved system not only increased productivity (Figure 2) but also increased organic C content in the soils, along with an increase in total N, available N, and Olsen's P. With an increase in biomass C, there was an 83% increase in mineral N, 105% increase in microbial biomass N, and about 18% increase in total N in the improved system compared to the traditional system. The relationship between the microbial biomass and mineral N was found to be positive in the improved system, there was a continuous release of plant-available N from the microbial biomass. Cropping system involving legumes and other land, soil and water management practices are effective with respect to increase in microbial biomass, available N and P leading to higher productivity of crops. Biomass N comprised about 2.6% of total soil N in the improved system, whereas in the traditional system, the biomass N constituted only 1.6% of total N. Microbial biomass C is about 10.3% of total soil organic C in the improved system compared to only 6.4% in the traditional system. Soil respiration in terms of both soil C and microbial biomass C was observed to be more in the improved system than in the traditional system. Biomass C as a proportion of total soil C serves as a surrogate for soil quality. In the present study improved management practices of Vertisols resulted in higher values (10.3% vs 6.4%) of biomass C as a proportion of soil organic C up to 120 cm depth indicating that with improved management practices in these Vertisols would reach a new stage of equilibrium and these results are supported by continued increase of 81 kg ha -1 yr -1 over the last 22 years with improved management, and such improved soil properties as reported earlier also support the increased yields. Increased grain yields of 26 kg ha -1 y -1 observed in the traditional system suggest that, even the traditional system is also sustainable at lower production levels (1000 kg ha -1 ) which can support 4 persons ha -1 for 1 year and whereas an improved system can support about 18 persons ha -1 in integrated watershed system for rainfed Vertisols. More respiration and more microbial activity resulted in more net N mineralization. Increased soil respiration in the improved system, compared to the conventional system indicates increased biological activity, and this increase in biological activity resulted in increased nutrient availability. An increase in biomass C resulted in more labile N pool for mineralization. At lower depths, microbial biomass was more than that in the traditional system. However, biomass N was slightly lower indicating less availability of N for microbial growth, and there is an increase in microbial biomass C as there is an increased C substrate availability. Microbial biomass at deeper depths may be metabolically less active as compared to the biomass in top layers because of reduced substrate availability of O2 due to poor aeration under field conditions.
The SAT of the world occupies 11.1 million km 2 (15.6% of global ice-free land) containing 1.49 million km 2 of Vertisols. As all these Vertisols occur in developing countries, they are not fully utilized and their potential for food production is not fully realized. The present study has shown that it is possible to sequester 335 kg C ha -1 yr -1 with improved soil management. Over a 10-year period, if such kind of soil quality enhancement can be accomplished, it is possible to sequester about 0.5 Gigatonnes of atmospheric C. In an eventual carbon market, this would cost between US$ 10-15 billion, funds that could be used to address food security and environmental quality. This process can help to alleviate the C emission and accompanying global warming process. However, in addition, if the resource-poor farmers of the SAT can be helped to implement such technology, it will also assist in poverty alleviation through increased productivity and stabilization of productivity resulting from the improved land quality. Actively promoting C sequestration through good soil management has the potential to alter the bleak picture of poverty, land degradation, and food security, particularly in Asia and Africa.
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