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Laboratory. Some of the research findings with these facilities are indicated in this section. Enhanced carbon dioxide concentration effects the carbon dioxide assimilation and their partitioning within the source leaf and transport to the sink in mung-bean and wheat. Carbon dioxide elevation partially compensates for the negative effect of moisture stress in Brassica plants and may possibly help to grow in the drier habitat than they are currently grown. Brassica spp. responded differently to elevated carbon dioxide levels. All the yield components in rice viz. panicle number (effective tillers), filled grains per panicle and grain weight responded positively to enhanced carbon dioxide levels. Increased photo-assimilate supply possibly increased the maturity percentage of the seeds. The studies with rice, wheat and mustard are currently being used to refine the existing crop growth models. Presently, the FACE facility has been used to study crop response to CO2 on growth and yield. The efforts are being made to include also the rising temperature effects (Uprety 1998). This kind of study needs to be also extended to some of the traditional crops of the country, like chickpea, pigeonpea, groundnut and potato.

Gadgil (1995) and Gadgil et al. (1999a, b) used PNUTGRO model to determine the sowing window for rainfed groundnut. Variation in the model yield with sowing date showed that broad sowing window of 22nd June-17th August is the optimum for minimizing the risk of failure. It was also shown that incidence of locally triggered pests/diseases viz. leaf miner and late leaf spot (tikka) is low when sowing is postponed to after mid-July and thus does not involve much risk. It was also seen that pod filling stage was critical for moisture availability.

Lal et al. (1999) projected 50% increased yield for soybean for a doubling of CO2 in Central India. However, a 3°C rise in surface air temperature almost cancels out the positive effects of doubling of carbon dioxide concentration. A decline in daily rainfall amount by 10% restricts the grain yield to about 32%.

Hundal and Kaur (1996) examined the climate change impact on productivity of wheat, rice, maize and groundnut crop in Punjab. If all other climate variables were to remain constant, temperature increase of 1,2 and 3°C from present day condition, would reduce the grain yield of wheat by 8.1, 18.7 and 25.7%, rice by 5.4, 7.4 and 25.1%, maize by 10.4, 14.6 and 21.4% and seed yield in groundnut by 8.7,23.2 and 36.2%, respectively.

Lal et al. (1998) examined the vulnerability of wheat and rice crops in northwest India to climate change through sensitivity experiments with CERES model and found that under elevated CO2 levels, yields of rice and wheat increased significantly (15 and 28% for a doubling of CO2). However, a 3°C (2°C) rise in temperature cancelled out the positive effect of elevated CO2 on wheat (rice). The combined effect of enhanced CO2 and imposed thermal stress on the wheat (rice) crop is 21% (4%) increase in yield for the irrigation schedule presently practiced in the region. While the adverse impact of likely water shortage on wheat crops would be minimized to a certain extent under elevated CO2 levels, it would largely be maintained for the rice crops resulting in net decline in the rice yields.

Mandal (1998), Chatterjee (1998) and Sahoo (1999) calibrated and validated the CERES-maize, CERES-sorghum and WOFOST models for the Indian environment and subsequently used them to study the impact of climate change (CO2 levels: 350

and 700 ppm and temperature rise from 1 to 4°C with 1 °C increment) on phenology, growth and yield of different cultivars. Chatterjee (1998) observed that an increase in temperature consistently decreased maize and sorghum yields from the present day conditions. Increase in temperature by 1 and 2°C, the sorghum potential yields decreased by 7-12%, on an average. An increase in 50 ppm CO2 increases yields by only 0.5%. The beneficial effect of 700 ppm CO2 was nullified by an increase of only 0.9°C in temperature.

However, Mandal (1998) observed that an increase in temperature up to 2°C did not influence potential and irrigated yields of chickpea as well as above ground biomass significantly. Pre-anthesis and total crop duration got reduced with the temperature rise. Nitrogen uptake and total water use (as evapo-transpiration) were not significantly different upto 2°C rise. The elevated CO2 increased grain yield under potential, irrigated and rainfed conditions. There was a linear increase in grain yield, as the CO2 concentration increased from 350 to 700 ppm. Potential grain yield of pigeonpea decreased over the control when the temperature was increased by 1 °C (using WOFOST).

Sahoo (1999) carried out simulation studies of maize for climate change under irrigated and rainfed conditions. Rise in temperature decreased the yield under both the conditions. At CO2 level of 350 ppm, grain yield decreased continuously with temperature rise till 4°C. This was possibly due to reduction in days to 50% silking and physiological maturity. At CO2 level of 700 ppm, grain yield increased by about 9%. The temperature rise effect in reduction of yield was noted in several maize cultivars. Effect of elevated carbon dioxide concentration on growth and yield of maize was established, but less pronounced when compared with crops, like wheat, chickpea and mustard crops. The beneficial effect of 700 ppm CO2 was nullified by an increase of only 0.6°C in temperature. Further increase in temperature always resulted in lower yields than control.

The sensitivity experiments of the CERES-rice model to CO2 concentration changes, as conducted by (Saseendran et al. 1999), indicated that over the Kerala State, an increase in CO2 concentration led to yield increase due to its fertilization effect and also enhanced the water use efficiency. The temperature sensitivity experiments have shown that for a positive change in temperature up to 5°C, there is a continuous decline in the yield. For every one degree increment, the decline in the yield is about 6%. Also, in another experiment, it was noticed that the physiological effect of ambient CO2 at 2°C in temperature was compensated for the yield losses at 425 ppm CO2 concentration.

Estimates of impact of climate change on crop production could be biased depending upon the uncertainties in climate change scenarios, region of study, crop models used for impact assessment and the level of management. Aggarwal and Mall (2002) studied the impact of climate change on grain yields of irrigated rice with two popular crop simulation models - Ceres-Rice and ORYZA1N at different levels of N management. The climate change scenarios used were 0.1 °C increase in temperature and 416 ppm CO2 (2010 scenario) and 0.4°C temperature and 755 CO2 (2070 scenario) as the optimistic scenario, whereas increase of 0.3°C temperature and 397 ppm CO2 (2010 scenario) and 2.0°C temperature and 605 ppm CO2 (2070

scenario) as the pessimistic scenarios of climate change, as adopted from studies of Watson et al. (1998). The results showed that the direct effect of climate change on rice crops in different agro-climatic regions in India would always be positive irrespective of the various uncertainties. Depending upon the scenario, rice yields increased between 1.0 and 16.8% in pessimistic scenarios of climate change depending upon the level of management and model used. These increases were between 3.5 and 33.8% in optimistic scenarios. These conclusions are highly dependent on the specific thresholds of phenology and photosynthesis to change in temperature used in the models. Caution is needed in using the impact assessment results made with the average simulated grain yields and mean changes in climatic parameters.

Screening of cultivars for tolerance to sterility under enhanced temperatures during post anthesis phase for the major crops needs to be evaluated in the phytotron (control chambers), for choosing the appropriate cultivars for sustained productivity under climate change. Quality aspects for important crops, like wheat (aestivum and durum), basmati rice and mustard under the climate change, need to be addressed. There is also a need to develop a selection criterion for screening of the cultivars for adaptation to drought and temperature stresses.

Adaptation of crops to gradual change in the climatic conditions needs to be included in the existing crop growth models, as it is not well understood. Moreover, the suitable agronomic and resource management options may nullify the ill effects of climate change on growth and yield of crops.

2.3.2 Soil Productivity

The most important process is the accelerated decomposition of organic matter, which releases the nutrients in short run, but may reduce the fertility in the long run. Soil temperature influences the rates at which organic matter decomposes, nutrients are released and taken up, and plant metabolic processes proceed. Chemical reactions, that affect soil minerals and organic matter, are strongly influenced by higher soil and water temperature. Soil productivity and nutrient cycling are, therefore, influenced by the amount and activity of soil microorganisms. Soil microorganisms fulfill two major functions, i.e. they act as agents of nutrient element transportation as well as store carbon and mineral nutrients (mainly N, P and S) in their own living biomass, acting as a liable reservoir for plant available nutrients with a fast turnover. The doubling of CO2 increases plant biomass production, soil water use efficiency by the plants, and C/N ratios of plants. The changes in the C/N ratios of plant residues returned to the soil, have impact on soil microbial processes and affect the production of trace gases NOx and N2O.

Results of the All India Co-ordinated Long-term Fertility Trials indicate that regions, having higher organic carbon content (>0.6%) in the beginning, showed a declining trend, whereas the regions with lower organic carbon content remained more or less static or slight increase in the organic carbon content was noticed in around 25 years. In general, Indian agricultural soils are low in organic carbon content, and for achieving higher agricultural production, we have to depend upon the fertilizers. The hypothesis of increased organic carbon degradation with temperature rise has to be linked with the crop intensity factor, which is significantly higher for India, where proportion of the small and marginal land holdings is increasing due to rapid growth in population with time.

The interaction of nitrogen, irrigation and seasonal climatic variability, particularly at low input of irrigation, has several implications. Under adequate moisture supply situation, like for Punjab and Haryana, the yield benefits are obtained up to higher nitrogen application, whereas in the regions of limited to moderate water supply situations, the increasing trends in yield are noted up to relatively lower values of nitrogen. At low levels of water availability, it is difficult to decide optimal levels of N fertilizer for maximizing yield returns in view of uncertainty of N response, which is strongly related to a good post monsoon rainfall received during crop growing period (Kalra and Aggarwal 1996).

Das and Kalra (1995) evaluated the fertilizer and resource management for enhancing crop productivity under inter-annual variations in weather conditions. The results revealed sensitivity of crop yields to climatic variability and the need of inputs management in relation to climatic variability. Simulation models forjudging the soil nutrient availability and subsequently relating to growth and yield of crops are available, but needs to be refined and thoroughly tested for the climate change event.

Analysis of the food grains production data for the last few decades reveals a tremendous increase in yield due to technological advancement, but it appears that impact of vagaries of monsoon has been large throughout the period. The annual food production showed an increasing trend, and the deviations around the technology trend line were significantly related to seasonal rainfall. But no definite trend is noticed in case of rabi season food production with the winter season's rainfall, as majority of the food production in this season comes from the irrigated areas.

Changes in rainfall due to global climate change may affect the surface moisture availability, which becomes important for germination and crop stand establishment in the rainfed areas. Modifications in the surface and ground water availabilities with the rainfall change, are difficult to be observed when the land use and land cover are so rapidly changing.

Farmers have several agronomic management options to face the situation of water scarcity, through choice of crops, cultivars, adoption of suitable irrigation, nutrient and pesticides application schedules.

Water production functions, which relate to water availability and its use with crop yields, help in identifying critical growth stages at which the limited amount of water can be applied to get the maximum benefits (Kalra and Aggarwal 1994).

Soils dominate the cycling of many atmospheric trace gases because of the highest abundance and diversity of microbes in them. Earlier, equilibrium used to exist between the sources and sinks of GHGs, but a shift in this equilibrium has started becoming evident as a consequence of human induced activities. In order to comprehend the shift of source - sink equilibrium, one needs to understand the processes involved in generating the net flux (a function of production processes, consumption processes and gas transport) at the soil atmosphere interface.

Microbes have emerged as the major contributor as well as consumer of GHGs as they are the main intermediaries of C turnover in soil. They are also considered as sole agents for soil humus formation, cycling of nutrients, soil tilth and structure and also perform myriad of other functions. What will happen to the soil fertility in the event of global climate change needs to be addressed through soil organic matter (SOM)? The assessment of soil health/quality/fertility through changes in SOM is difficult, and therefore, other soil parameters are being used as proxy indicators. For example, soil microbial biomass (the living part of organic matter) due to dynamic character has been shown to quickly respond to changes and perturbations, often before the measurable changes occur in organic C and N, thus acting as an indicator of long term changes in SOM content (Powlson and Brookes 1987). However, the measurement of microbial biomass (Cmic) alone will not serve the purpose, because they are generally influenced by climatic variables. Hence, for real measurement of the impact of soil processes, one needs to consider proportion of total organic C or N within the microbial biomass i.e. microbial quotient. Under the equilibrium conditions, Corg of agricultural soils contains 2.3-4% Cmic. Soils exhibiting a C^ to Corg ratio higher or lower than these values appearing in the equilibrium line would be either accumulating or losing C, respectively (Anderson and Domsch 1986). Different climatic conditions, in particular precipitation/evaporation, influence the equilibrium Cmic to Corg ratio (Insam 1990) and a very high correlation was found in which 73% variation could be explained with the quadratic function, and thereby one can predict the soil fertility in terms of accumulation or losses of C.

Under the changed scenario of atmospheric composition due to global warming, the tropical region, such as India, with small organic C reserves, will show net efflux of CO2, because rates of soil respiration increase exponentially with temperature. Thus, CO2 effluxes from tropical system should increase markedly with small change in temperature without any increase in inputs from the above ground communities, thereby leading to rapid losses over a short period of a few decades and later on, it will sustain the balance because of the shortage of substrate for decomposition as well as adaptation of microbial communities towards the climatic change. The alterations in microbial community structure and their physiology can be interpreted in terms of differences in phospho-lipid fatty acids (PLFA) - fingerprinting. In general, PLFA profiles had decreasing unsaturation, greater chain length and larger number of cyclopropyl fatty acids at higher temperatures.

It has also been suggested that climate change could increase rates of soil erosion, further hampering food production. Increases in rainfall will accelerate the rates of soil loss, reducing farm productivity even more. A further negative consequence of accelerated erosion will be increased sedimentation in streams and reservoirs. This will shorten the life span of dams, which helps to prevent floods and provide both electricity and water for irrigation. Another way, in which erosion could accelerate, is through a decrease in rainfall, which could lead to dry spells and increased risk of wind erosion (Parry et al. 1999). If erosion rates go unchecked, continued soil impoverishment would eventually force farmers to abandon their lands. Thus, erosion is among the major threats to food production in a warmer climate. But, these qualitative assessments have not been studied in depth, where the rapid changes in land use patterns may totally reverse our thinking.

Other land degradation problems, such as water logging, soil salinity and sodicity development, are emerging due to rapid land use pattern and land cover changes. The impact of climate change on these aspects needs to be looked into for sustaining the agricultural production.

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