Soil carbon and nitrogen fractions and wheat yield in the south-east of Buenos Aires, Argentina.

Continued soil management, tillage system and summer crop sequences affect the distribution and stocks of soil organic carbon (OC), total nitrogen (TN) and their fractions, with different lability degrees. A longterm experiment was conducted in the Barrow Integrated Experimental Farm (INTA), where three crop sequences were evaluated, alternating legumes, oilseeds and grasses, N fertilization and conventional tillage. The aims of this study were: 1) to determine the effect of crop rotations and fertilization on the organic C and N fractions; 2) to evaluate the changes in the organic fractions content with different degrees of recalcitrance (mineral organic carbon, MOC, and recalcitrant organic carbon, ROC) and the recalcitrance index, RI; 3) to evaluate C lability through the relationships between the organic carbon fractions; 4) to study the potentially mineralizable N (Nan) related to N soil contribution. Labile and recalcitrant fractions were separated with acid hydrolysis and the RI was calculated. The crops provided different amounts of residues, which made each system behave differently even with similar OC levels. Grass-legume sequences had higher TOC and coarse particulate organic carbon fraction, with 18% lability. The sequence with soybean, the residues of which had a lower C:N ratio, could be more susceptible to chemical degradation. The potentially mineralizable N was not associated with OC fractions and was more abundant in the sequence with soy. Oilseeds were found to be better predecessors than summer grasses for wheat yield.