Influence of summer crop residues on 15N present in organic matter fractions under two lowland soils

The state of Rio Grande do Sul has about 20% of the total area as lowland soils, suitable for flooded rice (Oryza sativa). In order to mitigate damage caused by rice monoculture, new crops such as sorghum (Sorghum bicolor) and soybean (Glycine max) have been cultivated in these areas. With different qualities of crop residues, it is expected a change in soil organic matter (SOM) dynamics and consequently, nitrogen (N) availability. The objective of this study was to evaluate the influence of rice, soybean and sorghum crop residues on the N present in physical fractions of SOM of two lowland soils, using labeled N-15 technique, under incubation for 180 days in aerobic condition and more 180 days in anaerobic condition. At 30, 180 and 360 days of incubation the remaining N of the plant residues and N destination from the residues in both soils were quantified in the physical fractions of SOM >250 mu m, 250-53 mu m and <53 mu m. The soil with higher amount of clay-Fsilt received a larger quantity of N-15 from residues, while flooding of the soil after 180 days caused a loss of N added to the soil by the soybean and sorghum residues. In general, larger amounts of N-15 were reported in the fraction <53 mu m, associated with clay minerals, throughout the incubation period. These N losses should be considered in N fertilization for the following crops in rotation with flooded rice.