A sustainable agro-ecosystem requires the conservation or enhancement of the soil resource, and it is imperative that the organic matter content of the soils be sustained. A decrease in organic matter content is an indication of lower soil quality in most soils. Analysis made by Rozanov ( 1990 ) showed that 16 per cent of original soil carbon stock might have disappeared since agriculture began 10,000 years ago. Also an overall loss of humus, estimated at the rate of 25.3 million tons / year in the past 300 years.

The Indian soils, generally low in organic matter, under intensive cropping with imbalanced fertiliser use, declined in soil organic matter ( SOM ), as evidenced from the long term fertility experiments ( Swarup et al.,1998 ). This paper analyses the dynamics of soil organic carbon (SOC), under three different experimental systems and cropping patterns over a period of time conducted at Tamil Nadu Agricultural University, Coimbatore, India. These three experiments are being conducted with an aim of monitoring soil fertility due to continuous intensive cultivation under different farming systems.

The Old Permanent Manurial Experiment (OPME ) was started in 1909, originally as an irrigated experiment and that was later (1937) converted into a rainfed experiment and at this time the cropping system was converted from two crops per year into a single crop per year. The New Permanent Experiment ( NPME ) was started in 1925, and is conducted in an irrigated system with one set of treatments receiving 2t ha-1 of cattle manure, since its inception. These two experiments are conducted with different nutrient input management systems of single inorganic nutrients and their combinations.

Along with these, the changes in the OC status of another irrigated long term fertilisation trial is being conducted since 1972, under intensive cultivation ( 3 crops per year) is also discussed. All the three experiments concludes that there is a build up in the organic carbon status of the soil for a balanced application of N, P, and K along with an application of organic manure. Among the inorganic fertiliser treatments, combined application of all the three major nutrients resulted in a higher organic carbon status, than to the application of single nutrients, owing to a better growth and higher amount of root residues added after the harvest of each crop, over the years. But, both in OPME and NPME, there was a decline in the OC content after seven years and a reversal in the last nine years has happened, proving an establishment of an equilibrium. Under intensive cropping system of cultivation, the soil organic carbon build up resulted in all the treatments, including unmanured control, which is maximum for the combined application of inorganics and organics ( 100% NPK + FYM ).

When compared with the initial soil carbon status in 1972 at the inception of the experiment ( 0.30% ), the intensive cropping since then had resulted in the enhanced OC content, regardless of the nutrient input system including control. Continuous adoption of all the treatments to the high intensive cropping system tended to build up soil organic matter ( Brar et al., 1998 ). The build up is maximum in 100% NPK+FYM applied plots (133% increase) followed by 150% NPK ( 123% increase) with reference to initial level of 0.3%. The increase was recorded in control (40% increase) and N alone (67% increase)

The increase in OC is attributed to added root biomass over a period of 30 years. Appreciable improvement in soil OC with respect to the initial level was observed at optimal to super optimal (100-150% ) NPK doses on almost all the soils (Nambiar, 1994). Decline in SOM with continuous use of nitrogenous fertilizers has been reported especially on laterite and black cotton soils by Goswami and Rattan (2000). Such ill effects can be set right when balanced application of FYM was practiced.

Long term sustenance of soil fertility and higher productivity has been accomplished, provided optimum NPK are supplemented with FYM, as it is due to the manifold beneficial effects, where FYM acts as a store house of nutrients, improver of soil health, synergiser of the microbial activity, soil biomass accumulator and preserver of bio- diversity. Hence it is concluded, continuous cultivation (even intensive) with optimum fertilization and organic manuring, would buildup soil organic carbon status.