Soil C and N stocks as affected by cropping systems and nitrogen fertilisation in a southern Brazil Acrisol managed under no-tillage for 17 years

Soil organic matter is an ecosystem component with agronomic and environmental functions and is affected by soil management. The objectives of this study were to evaluate (i) soil organic C and N losses during a period of conventional cultivation (1969-1983) that followed on native grassland and (ii) the potential of four long-term (17 years) no-till cereal- and legume-based cropping systems (bare soil, oat (Avena strigosa Schreb.)/maize (Zea mays L.), lablab (Lablab purpureum L. Sweet) + maize and pigeon pea (Cajanus cajan L. Millsp.)+maize) with different N fertilisation levels (0 and 180 kg N ha−1 year−1) to increase the C and N stocks of a southern Brazilian Acrisol. Samples were taken from 0 to 107.5 cm depth, in 10 soil layers. The C content in the 0-17.5 cm layer of grassland decreased by 22% (8.6 Mg C ha−1) during the period of conventional cultivation. Meanwhile, N decreased by 14% (0.44 Mg N ha−1). Additional C and N losses occurred after the establishment of bare soil and oat/maize (no N). With N fertilisation, the C and N stocks of oat/maize were steady with time. Legume-based cropping systems (lablab + maize and pigeon pea + maize) increased C and N stocks due to the higher residue input. Although the major soil management effects were found in the 0-17.5 cm layer, up to 24% of the overall C losses and up to 63% of the gains of the whole 0-107.5 cm layer occurred below the 17.5 cm depth, reinforcing the importance of subsoil as a C source or sink. The average C sequestration rate of legume-based cropping systems (with N) was 0.83 Mg C ha−1 year−1 in the top 0-17.5 cm layer and 1.42 Mg C ha−1 year−1 in the whole 0-107.5 cm layer, indicating the remarkable potential of legume cover crops and N fertilisation under no-tillage to improve SOM stocks and thus, soil and environmental quality in humid subtropical regions.