No-tillage and ryegrass grazing effects on stocks, stratification and lability of carbon and nitrogen in a subtropical Umbric Ferralsol

No-tillage farming combined with temporary cattle grazing is becoming a common practice in parts of South America. We quantified the effects of no-tillage and winter grazing of annual ryegrass (Lolium multiflorum Lam.) on soil carbon and nitrogen stock, stratification ratio (concentration in 0-5 to 10-20-cm layer) and lability (based on particulate organic matter, POM), relative to conventional tillage or ungrazed ryegrass. A 9-year-old experiment was conducted in a southern Brazilian Ferralsol. Soil under no-tillage accumulated 1.11 Mg C ha(-1) year(-1) and 0.10 Mg N ha(-1) year(-1) to 100-cm depth relative to conventional tillage when ryegrass was not grazed, and also increased the stratification ratio of carbon (1.48 vs. 1.11) and nitrogen (1.66 vs. 1.17). The carbon and nitrogen lability was also greater in no-tillage soil, as carbon and nitrogen stocks increased proportionally more in sand-POM than in silt or clay size fractions (carbon lability index = 1.57 vs. 1.00; nitrogen lability index = 1.57 vs. 1.00). Semiquinone concentrations in physical fractions were 4-28% smaller in no-tillage, suggesting less organic matter aromaticity in this treatment. Grazing, however, caused no further improvement of stocks of carbon and nitrogen in the 0-100-cm layer of no-tillage soil (e.g., 209 vs. 212 Mg C ha(-1), ns), or enhancement of stratification or lability of those elements, relative to ungrazed ryegrass. Overall, no-tillage effectively promoted soil carbon and nitrogen accumulation and lability in this subtropical Ferralsol, whereas grazing did not compromise the gains of no-tillage. Highlights Capacity of no-tillage and grazing to enhance stocks, stratification and lability of soil C and N. C sequestration with no-tillage is being debated, and little is known when combined with grazing. No-tillage accumulated 1.11 Mg C ha(-1) year(-1) but grazing promoted no further C accumulation. No-tillage effectively accumulated C in soil and grazing did not compromise such gain of no-tillage.