ChileanJAR

Measurement of organic matter turnover is usefull to understand changes in the distribution of organic matter in intensive management systems. Particulate organic carbon (CMOP), humic and fulvic acid carbon (COH and COF, respectively) in several treatments of a cultivated and virgin Kandihumult soil from the Misiones province, Argentine (27° 39' S Lat. and 55° 26' WLong.) were assessed in arder to study the dynamics of organic matter. The agroecosystems were: a) Suhtropical native forest; b) 50 year-old plantation of "yerba mate" (Ilex paraguariensis Saint Hil) crop with mechanical weed control between crop lines; c) The same as b) but with no mechanical weed control between crop lines; and d) The same as b) but interplanted during the last six years with elephant grass (Pennisetum purpureum Schumach) between the crop lines to be used as green manure. Mineralassociated organic matter was isolated through the dispersion of aggregates by mechanical shaking in water, using 5 mm glass beads and removing CMOP by passing the dispersed aggregates through a 100 J.lm sieve. Native forest subtropical soil and the 50 year-old "yerba mate" crop with mechanical weed control had the highest and lawest soil organic carbon and CMOP content, respectively. The COH under agricultural management practices decreased in the 0.05-0.15 m layer whereas the COF increased in both layers. Six years ofelephant grass cover crop reversed that trend to approach those values found in the virgin soil studied.