Soil carbon and nitrogen stocks and fractions in a long-term integrated crop–livestock system under no-tillage in southern Brazil

• Different animal and plant residue additions impacted soil C and N stocks.
• Moderate, low, and no grazing treatments resulted in similar stocks of soil C and N.
• Most intensive grazing led to C and N losses, with soil organic matter degradation.
• High grazing intensity indicated soil quality loss compared with native area.

Managing grazing stocks in integrated crop–livestock (ICL) systems under no-tillage is a key variable for reaching equilibrium in soil C and N budgets. Understanding how different plant and animal residues affect soil C and N stocks in these systems goes beyond soil dynamics since these elements are crucial for the functioning of the soil–plant–atmosphere system. The objective of this research was to determine soil C and N fractions, stocks, budgets and the carbon management index as affected by nine years of ICL with grazing intensities under no-tillage conditions. The experiment established in May 2001 in a Rhodic Hapludult (Oxisol) of southern Brazil was composed of black oat (Avena sativa) plus ryegrass (Lolium multiflorum) pasture in winter and soybean (Glycine max) crop in summer. Treatments were regulated by grazing pressures to maintain forage at 10, 20, 30 and 40 cm high (G10, G20, G30 and G40, respectively). Non-grazed (NG) treatment was the control. Changes in soil C and N stocks and fractions (particulate and mineral-associated) were assessed in the ninth year of the experiment. Moderate and light grazing intensities (G20, G30 and G40) resulted in similar increases in total organic C, particulate organic C, total N, and particulate organic N compared with NG treatment. Soil C additions ranged from 0.54 to 8.68 Mg ha−1 from NG to the other grazing treatments. The G10 led to a soil N loss of 1.17 Mg ha−1 due to soil organic matter degradation. The carbon management index (CMI) values, compared with native forest (NF) as a reference, indicated soil quality loss and degradation under high grazing intensity (G10). For a positive contribution to the soil system, ICL must be managed with moderate grazing intensities and adjustment of N additions through N fixation or fertilization.