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Field experiments were conducted from 1985 through 2001 on medium deep vertisol to determine the effect of the continuous application of manure and fertilizers on a two-year cotton-sorghum rotation. The results indicate that the response to N was greatest during the initial years, while after five to six rotation cycles the yield levels declined to the zero level in the N and NK plots. The application of P along with N prevented the decline in seed cotton and sorghum grain yields. The effect was more pronounced at the higher level. K application did not result in any yield increase. Balanced fertilizer at the higher level (N90P19K37) resulted in a significant yield increase over the recommended dose (N60P13K25); however, the percentage increase declined with duration, indicating a decline in factor productivity. Seed cotton yields were the highest when part of the fertilizer N was applied from an organic source (farmyard manure: FYM). Of the eight years, a significant response was observed in four years at the lower level (N30P13K25 + 5 t FYM ha-1) and six years at the higher level (N45P19K37 + 7.5 t FYM ha-1), while in sorghum a response was only observed in two years. The cotton crop should, therefore, be preferred to sorghum for the application of organic manure. In areas where no organic manure is available, N60P13K25 is sufficient for cotton, while sorghum needs to be fertilized at 1.5 times the recommended dose (N90P19K37).

Soil organic carbon (SOC) and rainfall are generally positively related, whereas a negative relationship between soil inorganic carbon (SIC) and rainfall with some exception is observed. Land use pattern in black soil region (BSR) of the semi-arid tropical (SAT) India, consists of 80% under agriculture, followed by forest, horticulture, wasteland and permanent fallow. For sustainable agriculture on these soils, there is a concern about their low OC status, which warrants fresh initiatives to enhance their OC status by suitable management interventions. In the BSR region, cotton, soybean and cereal-based systems dominate but it is not yet clear as to which cropping system in the SAT black soils is most suitable for higher OC sequestration. Many short-term experiments on cotton or cereal-based systems clearly suggest that cotton or cereal-based cropping systems including leguminous crops perform better in terms of SOC sequestration whereas soybean-legume combination do not add any substantial amount of OC. In sub-humid bioclimatic zones (1053-1209 mm mean annual rainfall), soybean is grown successfully with wheat or fallowing, and SOC concentration is maintained at 0.75% in the 0.30 m soil layer under integrated nutrient management. In view of enhancement and maintenance of OC in many shortterm experiments conducted in various agro-climate zones of SAT, it is realized that OC accumulation in soils of the semi-arid ecosystem with suitable cropping and management practices could be substantial especially in cotton-pigeon pea rotation, and thus the discussed crop rotations in each major bio-climatic zone stand for wide acceptance by the SAT farmers.