Effects of tillage on contents of organic carbon, nitrogen, water-stable aggregates and light fraction for four different long-term trials

Soil management may affect C and N dynamics in soils, but the underlying processes are not well understood. Our objective was to quantify the impact of different tillage treatments on the amount and distribution of free and occluded light fractions (fLF and oLF, respectively), on the water-stable macro-aggregate (> 250 μm) contents, and on organic carbon (Corg) storage. Four long-term tillage trials were carried out on loess soils in Germany with sugar beet followed by two years of winter wheat as crop rotations. The different tillage treatments trialled were regular conventional tillage (CT, to 30 cm), mulch tillage (MT, to 10 cm) and no-tillage (NT). Soils were sampled in 0–5 cm, 5–25 cm and 25–40 cm depth after 18–25 years of the different tillage treatments. These four long-term tillage trials on plots differing in soil texture and climatic conditions revealed consistent results between them. Average crop yields of sugar beet and winter wheat from 2004 to 2010 were higher under CT and MT than under NT. The NT and MT treatments produced significantly higher Corg contents than the CT treatment in 0–5 cm soil depth. The Corg stocks in the sampled profile, based on the equivalent soil mass approach (CT: 0–40, MT: 0–38, NT: 0–36 cm), were significantly higher for the MT treatment than for the CT and NT treatments. The fLF, oLF, and macro-aggregate contents were significantly higher for the NT and MT treatments than for the CT treatment in the top 5 cm, whereas in 5–25 cm depth, the oLF contents were significantly higher for the CT and MT treatments. The correlation of the macro-aggregate content against the fLF and oLF contents suggested that the macro-aggregate content is not directly influenced by the different tillage treatments but by the contents of available biomass, presumably due to the higher biomass input via higher crop yields under CT and MT and the vertical distribution of the residue input by mulching and plowing. Stepwise multiple linear regression analysis suggested that the Corg content was the most important factor influencing the macro-aggregate content in the soils of the four long-term trials, whereas the contents of fLF and silt were negatively related to the macro-aggregate content.

► Four study sites showed the same trend in the C storage for 3 tillage treatments.
► The C stocks (0–40 cm) were highest in the minimum tillage treatment.
► The C content was the most important factor influencing the macro-aggregate content.
► The free light fraction content was negatively related to the macro-aggregate content.
► The silt content was negatively related to the macro-aggregate content.