Long-term Effect of Manure, Fertilizer and Lime Application on Active and Passive Pools of Soil Organic Carbon under Maize-Wheat Cropping System in an Alfisol

Permanent manurial trial (PMT) which has been in progress since 1956 under maize-wheat cropping system at BAU, Ranchi was monitored for the impact of continuous application of amendments on active and passive pools of soil organic matter (SOM). Treatments include control, 100% N, 100% NP, 100% NPK, 100% NPK+ lime, FYM alone and ¹/2[(N+FYM)+P(A-X/2)+K(B-Y/2)] to each compound crop. Continuous application of organic manure alone or in combination with inorganic fertilizer significantly influenced the total soil organic carbon, CO₂ evolution, microbial biomass, water soluble carbon, acid hydrolysable carbohydrates, humic fraction and E₄/E₆ ratio over 100% NPK. The active fraction of soil organic carbon (SOC) was significantly higher in a plot receiving 100% NP over the one receiving only 100% N or control. Continuous application of organic manure alone or in combination with inorganic fertilizer enriched the soil with total organic carbon (10. 9 g kg¹), humic fractions (2. 43 g kg¹) and raised optical density of humic acid (4. 40) and fulvic acid (7. 70). However, sole application of fertilizers caused gradual deterioration in soil fertility with lower organic carbon content. On the other hand, liming in presence of fertilizers hastened the process of oxidation of soil organic matter. Total and active fractions of SOC ultimately support the productivity and their losses reduce the nutrient supplying capacity of soil in the long run. Carbon content in humic and fulvic acid fractions was generally higher under 100% NPK+FYM as compared to other treatments. However, fulvic acid carbon was higher than humic acid in all the treatments except in 100% NPK+lime. The E₄/E₆ ratio of fulvic acid in permanent manurial trial (PMT) soil under different treatments was in general wider (5. 0–8. 0) than that of humic acid (3. 0–4. 0). Integrated use of FYM with chemical fertilizers or use of FYM alone exerted significant effect on the active and passive pools of soil carbon.