Organic carbon balance in Chilean volcanic soils after human intrusion and under different management practices

Abstract

It is well known that humified soil organic matter ( SOM) plays a central role in soil fertility due to its chemical and physical properties, especially by sustaining soil biological and related ecological equilibrium. Soil organic matter quantity and quality needs to be preserved in order to maintain soil productivity. In this study total organic carbon contents and their distribution were determined in several volcanic ash derived soils of Chile. Carbon balances are expressed in terms of a labile fraction (alkali-extractable less than 10.000 Dalton and polysaccharide compounds) and a stable fraction ( fulvic, humic and humine compounds). The effects of different agronomic management on soil organic matter ( SOM) quality and quantity are estimated and compared between agricultural and virgin forest soil sites. The results indicate that soils under native forest show a C level higher than agricultural soils. After human intrusion the stable C ( m.w. > 10,000 Daltons), viewed as a measurement of the humification grade, shows a mean value of 70% over the total C and follows the order: ultisols ('Rojo arcillosos') > andisols-placandepts ('Nadis') > andisols-distrandepts ('Trumaos'). C balance demonstrates that in agricultural soils the soluble C, or 'labile C' is increased and accounts for 12-30% of the total C; these organic compounds contribute largely to the solubilization, mobility and availability of plant nutrients but also enhance losses of organic carbon by lixiviation. By using the carbon balance methodology discussed in this work, changes of quality and quantity of SOM can be followed up. The SOM of Chilean volcanic soils is being degraded after human intrusion; this needs to be considered in cultivation of these soils in order to preserve their fundamental properties.