CARBON AND NITROGEN IN THE PARTICLE SIZE FRACTIONS OF SOIL ORGANIC MATTER IN CROP SYSTEMS UNDER NO-TILLAGE

Soil quality in no-till systems is related to the crop systems and may be evaluated through particulate organic matter (>53 mu m) stock due to the functionality this property provides to the soil and its sensitivity to management practices. Aiming to assess soil quality no-till crop systems, this study was carried out through a long-term experiment (21 years) in a Latossolo Vermelho distrofico tipico (typical Oxisol) in Ponta Grossa, PR, Brazil. Six crop systems were assessed. Wheat-TR (Triticum aestivum L.), soybean-SO (Glycine max L.), corn-MI (Zea mays L.), oat-AV (Avena strigosa Schreb.) as a cover crop, vetch-ER (Vicia villosa Roth) as a cover crop, ryegrass-AZ (Lolium multiflorum Lam) for hay, or alfalfa-AL (Medicago sativa L.) for hay comprised the following systems: TR-SO (reference), ER-MI-AV-SO-TR-SO, ER-MI-TR-SO, AV-MI-TR-SO, AZ-MI-AZ-SO and AL-MI (corn every three years). The stocks of total organic carbon (COT) and total N (NT) and C and N in particulate (>53 mu m) and mineral-associated organic matter (<53 mu m) were determined at 0-5, 5-10 and 10-20 cm. The semiperennial AL-MI system had the greatest COT and NT stocks in the 0-20 cm layer (63.6 Mg ha(-1) COT and 4.6 Mg ha(-1) NT), and increased annual sequestration rates in relation to the TR-SO baseline system (0.23 Mg ha(-1) year(-1) COT and 0.03 Mg ha(-1) year(-1) NT). The AL-MI system also had the highest C and N stocks in the particulate organic matter in the same layer (12.5 and 0.91 Mg ha(-1), respectively) due to its higher C addition from root plant matter and the physical protection of organic crop litter. The ER-MI-TR-SO system had the lowest C and N stocks in the 0-20 cm layer with values of totals stocks of 57.8 and 4.03 Mg ha(-1), respectively, and without an increased annual sequestration rate because the stocks were similar to the TR-SO baseline system; C and N stocks in particulate organic matter were 10.4 and 0.67 Mg ha(-1), respectively. This tendency was repeated for individual layers with a significant difference between systems in the 0-5 cm layer and no significant difference in the 5-10 and 10-20 cm layers. The particulate organic matter contributed around 30 % to total C stocks in the 0-5 cm layer on average in the systems. Crop rotations with species that have active root systems for a longer time, such as the semi-perennial AL-MI system, could increase total C and N stocks, especially the particulate organic matter, which promote soil functionality and, consequently, greater soil quality.