Bholanath Saha

Microbial Transformation of Sulphur: An Approach to Combat the Sulphur Deficiencies in Agricultural Soils

Sulphur, an essential component for plant as well as animals, is present in soils in both organic and inorganic forms, with organic form particularly sulphate esters and carbon-bonded sulphur contributing ~75–90% of the total. The major sources of sulphur in soils are sulphur-containing minerals, plant and elemental residue and external addition including atmospheric deposition. Sulphur deficiency in plants results in poor nitrogen metabolism thus protein biosynthesis, chlorosis, low oil percentage and ultimately low yield. The conversion of organic sulphur in organic matter to inorganic form and vice versa is dominantly a microbiological process. In well-aerated soil, organic sulphur is mineralized to sulphate and taken up by plants. Concurrently inorganic sulphur is immobilized to organic form and incorporated in microbial tissue. The rate of these processes obviously depends on soil reaction, temperature, moisture and addition of crop residue and many other factors that ultimately affect the activity of microorganism. Several enzymes in soil, viz. arylsulphatase, play a major role in sulphur mineralization process though very little information is available till now towards the pathway of decomposition. In addition to this process, inorganic sulphur in soil undergoes various oxidation and reduction process, modulated by microorganisms. Various reduced inorganic sulphur compounds are oxidized by a group of bacteria in suitable condition and utilize the energy. The wide range of stable redox states and their interconversion affect sulphur cycle, fate of applied fertilizer and ultimately its availability to plants and microbes. In this chapter we reviewed the sulphur cycle and its transformation by various microbial processes.

Integrated nutrient management (INM) on yield trends and sustainability, nutrient balance and soil fertility in a long-term (30 years) rice-wheat system in the Indo-Gangetic plains of India

Abstract We analyzed data from a long-term rice–wheat cropping sequence to evaluate the effects of integrated nutrient management (INM) on yield trends and sustainability, nutrient balance and soil fertility of the system. After 30th cycle, grain yield of both the crops significantly declined under control and highest rice and wheat grain yields were obtained when 50% N supplied through green manure and farm yard manure, respectively. The magnitude of yield slope under INM was found considerably higher than 100% recommended fertilizer (RDF). Sustainable yield index (SYI) for both the crops were found lower in control but considerably higher with the 100% RDF and under INM. The soils under all the treatments suffered an apparent loss of K and N (except where organics replacing 50% N). Correlation study also reveals K is the sole factor for the yield sustainability as apparent K balance was negatively correlated with SYI and yield slope.

Biological Nitrogen Fixation for Sustainable Agriculture

Worldwide the increasing use of inorganic nitrogenous fertilizer becomes a threat to the environment and consequently to the mankind. It is high time to think about the alternate nitrogenous source for assuring sustainable agriculture. Biological nitrogen fixation (BNF), involving beneficial microorganism, is a process of converting the free atmospheric nitrogen into plant available form. This biological process harmonizes the ecosystem and offers an economic and environment-friendly approach for reducing the external inputs and improving internal sources. In nature a symbiotic relationship exists between most of the agriculturally important leguminous plants and beneficial microorganism, where bacteria can fix atmospheric free N2 and provide it to the plant in available form in exchange of nutrition and shelter. These rhizobia (bacteria) dwell in the nodule present in the roots of leguminous plants. This type of symbiosis also exists between free-living microorganisms, viz., Azolla and Anabaena, and with rice plants in anaerobic condition. Therefore, BNF is considered as an important biological process for harnessing soil health as well as for assuring economic, environmental, and agronomic benefit. However, the necessity of inclusion of leguminous crop in the cropping system due to BNF concern is not getting popularize among the farming community particularly to the marginal and low-income group farmer due to lack of availability of specific rhizobia strain for specific crop, as well as socioecomic constraints. The present book chapter is focusing on the importance of BNF in agricultural system and the effectiveness of various legume species and their beneficial microsymbiont. The genetics, biochemistry behind the BNF, and the probable strategy for improving the N2 fixation process are also getting concern for understanding these important biological phenomena.