Soil microbial biomass (SMB) was determined in a long tern experiment including six crop rotations with different agricultural management. The soil, of volcanic origin (Typic Melanoxerand), was located in the Central Valley of the South-center region in Chile. Soil biomass was performed by using fumigation extraction (FE) technique, ninhydrin nitrogen (N) reaction and ultraviolet absorbance at 280nm (UV280). Ninhydrin-N reaction gave a good comparison of microbial N between treatments, verifying its usefulness in measuring soil biomass; at the same time UV280 technique presented some limitation (small ?UV280); but it was equally effective in separating effects between treatments. SMB was a sensitive parameter to changes in agricultural management and therefore reflected changes in the labile fraction of soil organic matter. Its use is suggested as biological indicator in order to determine early changes in soil quality and fertility.

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