Temporal dynamics of soil organic matter composition and aggregate distribution in permanent grassland after a single tillage event in a temperate climate

The dynamics of soil organic matter (SOM) composition in grassland after a single tillage event is not completely understood. Our objective was to study the long-term effects of sporadic tillage on SOM stocks, aggregate distribution and density fractions in grassland soils. We hypothesized that tillage decreased organic carbon (Corg) stocks, concentration of water-stable macroaggregates, and SOM light fractions. In April 2010 soil samples were taken at three depths two and five years after either one or two tillage operation(s). The trial site has loamy sandy soils (Cambisols, Eutric Luvisols, Stagnosols, Anthrosols) and is situated north of Kiel in Germany, with a mean annual temperature of 8.9 °C and a precipitation of 768 mm. Treatments consisted of permanent grassland (control), tillage of grassland followed by a re-establishment of grassland (G–G) and tillage of grassland followed by a re-establishment of grassland with one season of winter wheat in between (G–W–G). Two years after tillage, the Corg stocks, corrected for differences in soil bulk density, were significantly reduced (p ≤ 0.05) in the top 10 cm of the soil profile in both tilled treatments (25.4, 16.8 and 18.6 Mg Corg ha−1 in 1250 Mg soil in the control, G–G and G–W–G, respectively). However, in the entire soil profile (0–40 cm) the Corg stocks in the G–G treatment were only 5% lower and in the G–W–G treatment 4% higher than in the control and no longer significant. The concentration of water-stable macroaggregates (>250 μm) in the top 10 cm was decreased by 10 g and 9 g in 100 g−1 soil in the G–G and G–W–G treatments, respectively, compared with the control. The water-stable aggregate-size classes >2000 μm and 250–1000 μm were most affected by the tillage treatments. The concentration of the SOM light fraction decreased by 31% and 41% in the surface soil of the G–G and G–W–G treatments, respectively. Five years after tillage, the effects of tillage treatments on Corg stocks, water-stable aggregates, and SOM composition were still detectable; however the differences were much smaller and no longer significant. Our results suggest that sporadic tillage of grassland may not result in marked effects on Corg stocks, aggregate stability or SOM composition.