EFFECTS OF ALTERNATE WET AND DRY IRRIGATION ON CROP YIELD AND SOIL NUTRIENT DYNAMICS

Abstract

Rice is a staple food for more than half of the world’s population. In India, the most common method of rice cultivation is the cultivation under continuous flooding irrigation (CFI) which uses a large amount of water. The prevailing water scarcity has led to the practice of the water saving method of irrigation i.e by practicing alternate wet and dry irrigation (AWDI) where the field is left alternately flooded and dried. The effects of AWDI on crop yield are different in different studies. This study was conducted to address the contradictory results of the effects of AWDI on crop yield. The crops used in the study were rice and wheat but only rice was included in the AWDI versus CFI scenario, and wheat was only irrigated using CFI because wheat does not need standing water to grow. The effects of CFI and AWDI on crop yield and soil nutrient dynamics were simulated for 97 years (2004-2100) using the DeNitrification DeComposition (DNDC) model. The model was calibrated from 2004-2011 and validated from 2012-2017. The model performed fairly well in simulating crop yield and soil nutrient dynamics. The crop yield in AWDI was higher than the yield in CFI by about 4-6 %. Crop yield has a direct relationship with SOM levels of the soil if the crop residue is returned to the field, therefore, the SOC and SON levels were also higher in AWDI than CFI. To check the effects of crop residue on SOM levels of the soil, three crop residue return scenarios of 0%, 5%, and 25% were performed. The SOM levels were highest in the scenario where 25% crop residue was returned and lowest in the scenario where 0% crop residue was returned. Crop yield was not significantly affected by the amount of crop residue returned to the field because the field was fertilized. To eliminate the effects of fertilizer on crop yield, scenarios of zero fertilization were also performed. In these scenarios, the crop yield was 88-92% lower than the fertilization scenarios and the crop yield was highest in the scenario where 25% of crop residue was returned. The novelty of the study is the analysis of SOC and SON dynamics based on AWDI because, in the majority of the previous studies, the SOC and SON dynamics were analyzed based on fertilization or crop rotation but not concerning AWDI. AWDI has higher SOC and SON levels than CFI. In CFI, the lower SOC and SON levels can be attributed to the higher leaching and runoff losses that took place due to the huge amount of water applied to the field. The CO2 and CH4 losses were also higher in the case of CFI due to the favorable condition created for methanogenesis in CFI. In AWDI, the loss of nitrogen through denitrification was higher than the losses in CFI, but the SOC and SON levels were higher in AWDI than CFI because N losses through leaching and runoff were lower in AWDI. Carbon and nitrogen mineralization was higher in AWDI because of the availability of more oxygen due to the intermittent wetting and drying of the field. When more crop residue was returned, the SOC and SON levels also increased. In the fertilization scenarios, the carbon and nitrogen balance was positive in all the scenarios where crop residue was returned but it was negative in the scenarios where no crop residue was returned. In the zero fertilization scenarios, the carbon and nitrogen balances were negative in all the scenarios irrespective of the percentage of residue that was returned to the field. This is due to the extremely reduced crop yield because of the unavailability of nutrients required for the proper growth and development of the crops. From the results of our study, it was observed that the crop yield and nutrient dynamics were all better in AWDI than CFI. When AWDI was combined with crop residue return, the results were even better. It is recommended that more crop residue (at least more than 25%) should be returned to the soil to improve soil health in the long run as well as creating a sustainable environment. From this study, farmers can get an idea of how much water is sufficient for growing rice crops and the impact of crop residue return on the crop yield as well as soil health by improving the SOC and SON dynamics of the soil.