Understanding the effects of long-term use of fertilizers on soil carbon and nitrogen pools and their activities is essential for sustaining soil productivity. Our objectives were to quantify longterm changes in soil organic carbon (SOC), soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN) and mineralizable C in maize-wheat cropping sequence in fertilized and unfertilized plots (control, N, NP, NPK, and NPK + FYM). Continuous application of fertilizers increased SOC over its initial content. Active fractions of SOC, i.e., water-soluble carbon, hydrolysable carbohydrates, SMBC, SMBN and dehydrogenase activity, improved significantly with an application of NPK and NPK + FYM. A general increase in carbon mineralization with time period was observed throughout the experiment and was maximum in 100% NPK + FYM treated plots. The estimated annual C input value in NPK + FYM treatment was 1.05 MgC ha -1 year -1 . The overall net change in organic carbon was maximum in treatment receiving FYM along with inorganic fertilizers. Therefore, these results suggest that the integrated use of NPK and FYM is an important nutrient management option for sustaining maize-wheat cropping system.

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An understanding of the adsorptiondesorption behaviour of phosphate (P) in soils after three decades of long-term fertilization would be an invaluable supplement to our knowledge of the chemistry of P in soils and would assist in developing P application strategies for successive crops. With this objective and within the framework of a long-term experiment, we collected surface soil samples (0-0.15 m) from agricultural crop land on which a rotation of maize-wheat-cowpea (fodder) crops had been grown for 32 years. The soil samples were investigated for adsorption and desorption behaviour of P, and the buffering capacities of soil samples were computed from the adsorption data. Soils differed widely in their P supply characteristics. Phosphate adsorption increased while percentage P adsorbed decreased gradually with increasing levels of added P. Adsorption maxima and the extent of P adsorption was at its maximum in unfertilized soil, and P adsorption decreased with increased P applications. The incorporation of farmyard manure with optimal P levels decreased P adsorption even more than a super-optimal application of P fertilizers. Bonding energy and Freundlich constant 'n' also decreased with increased P application and with the incorporation of farmyard manure. The adsorption and desorption of applied P were inversely related, and the soils that adsorbed P the most readily released it the least into the soil solution, and vice-versa. Computation of maximum buffering capacities (MBC) indicated that increased levels of P application decreased MBC and increased P supply in soil. The beneficial effects of an annual incorporation of farmyard manure with optimal P levels were clearly pronounced, and P availability in these soils was found to be even higher than the soil receiving the superoptimal application of P fertilizers. Various adsorption-desorption parameters were significantly related with the P uptake of different summer (rice, maize, soybean) and winter (wheat, berseem and raya) crops. Adsorption maxima, desorption maxima and maximum buffering capacity are the major parameters governing P availability in soils.