Organic carbon fractions in a yerba mate plantation on a subtropical Kandihumult of Argentina

An investigation was conducted on the amount and rate of soil organic matter (SOM) turnover, as influenced by agricultural operations in ultisols in Argentina. The decrease of particulate organic carbon (POC) and decomposition or relative stability of its free (POCf) and occluded (POCo) fraction were determined at sites under a forest, yerba mate (Ilex paraguariensis St. Hill), and in a yerba mate plantation using elephant grass as cover crop. Aggregate stability was measured by the dry sieving method for 5 min, using four sieves of different sizes and a Retsch stirrer. Wet sieving was performed to separate the mineral-associated organic matter (MOC) from the POC. The effect of 50 years of yerba mate cultivation resulted in a significant decrease of total POC. The highest POC were noted in the microaggregate (0.1–0.25 mm) fractions in the surface layers of the three ecosystems. The POCo concentration was higher than the POCf content in the soil within the plant rows and between the rows. This suggested that POCo was more resistant than POCf to decomposition, due to protection within the microaggregates. The few years under elephant grass has resulted in a significant increase in POC, probably due to the relatively slow rate of decomposition of fresh residues, characterized by high dry matter input and C/N ratios. Elephant grass-derived POC is believed to play an important role in the nutrient cycling of the degraded yerba mate soils.