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Excessive nitrogen (N) fertilization may impair soil health through negative effects on the soil biological community and soil reaction. Given the limited and conflicting reports of N fertilizers in relation to soil health, there is a need to look into positive and negative effects of using fertilizer N in cropping systems in India. It has been endeavored in this chapter to analyze research data from India to answer some specific questions: Does high and continuous N input threaten soil quality? Do Indian farmers overuse N? What is the soil health issue vis-a-vis crop productivity? What happens to soil quality when N is used only as mineral fertilizer, or manures/sewage, or a combination of the two? How to use N for a better soil health? The chapter concludes that: (1) mineral fertilizers (and even N) use can enhance soil health in many soils (mainly in the alkaline and neutral pH range) in India through increasing soil organic matter as a result of the greater root growth associated with improved crop yields and often accompanied by enhanced microbial activity; (2) in acid soils, long-term N use has deleterious impacts on soil quality and crop productivity; and (3) the combined use of mineral fertilizers and organic nutrient sources is the best way forward.

The effect of presubmergence and green manuring on various processes involved in [ 15 N]-urea transformations were studied in a growth chamber after [ 15 N]-urea application to floodwater. Presubmergence for 14 days increased urea hydrolysis rates and floodwater pH, resulting in higher NH 3 volatilization as compared to without presubmergence. Presubmergence also increased nitrification and subsequent denitrification but lower N assimilation by floodwater algae caused higher gaseous losses. Addition of green manure maintained higher NH 4 +-N concentration in floodwater mainly because of lower nitrification rates but resulted in highest NH 3 volatilization losses. Although green manure did not affect the KCI extractable NH 4 +-N from applied fertilizer, it maintained higher NH 4 +-N content due to its decomposition and increased mineralization of organic N. After 32 days about 36.9 % (T 1 ), 23.9 % (T 2 ), and 36.4 % (T 3 ) of the applied urea N was incorporated in the pool of soil organic N in treatments. It was evident that the presubmergence has effected the recovery of applied urea N.

Algal N labelled with 15N added to a flooded soil in laboratory columns without plants was studied to determine the changes over time in the fate of N assimilated by algae and to study how its fate is affected by (a) exclusion of light simulating complete closure of the rice canopy, and (b) addition of fertilizer-NH4*. In the light but with no added fertilizer-N there was little net mineralization of the added algal N during the first 4 weeks, but after 8 weeks 42% had been mineralized, of which 95% was denitrified. Exclusion of light caused net mineralization to proceed more rapidly in the first 4 weeks due to the death of algal cells and lowered reassimilation. After 8 weeks 51% had been mineralized, of which 54% was denitrified, 16% volatilized and 30% was present as KCl exchangeable NH4+-N. Application of fertilizer-NH4+ apparently caused mineralization of 25% of the algal N within one week but the results were probably affected by pool substitution in which labelled N mineralized to NH4+-N was diluted with fertilizer – NH+4 and then immobilized leaving more labelled NH4–N in the mineral pool. After 8 weeks, 42% of algal N had been mineralized, of which 69% was estimated to have been denitrified, 19% lost through NH3 volatilization and 12% remained as extracted NH4++NO-3. Uptake of N by a rice crop would reduce the gaseous losses. Algal N was mineralized quickly enough to be available during the growing season of a rice crop and, depending on field conditions, algae may have a role in assimilating N and protecting it from loss as well as being a major driving force for NH3 volatilization through diurnal increases in pH.