Dynamics of soil organic carbon fractions one year after the re-conversion of poplar and willow plantations to arable use and perennial grassland

• We re-converted tree-plantations back to maize cropping and perennial grassland.
• C stocks of incorporated harvest residues distinctly decreased within one year.
• No loss of bulk SOC was detected within one year.
• But first losses in labile soil fractions were detectable.
• Perennial grassland provided a better stabilization of macroaggregates in loamy soils.

Due to the intensive soil tillage during the re-conversion of fast growing tree plantations back to agricultural use, high losses of accumulated soil organic matter are expected. A field trial was conducted at two former fast growing poplar plantations and one willow plantation in northern Germany to study the effects of different tillage depths and land use systems during such a re-conversion of the plantations on soil organic carbon (SOC) dynamics. Re-conversion was performed at tillage depths of 5, 15 and 30 cm and rye-grass and maize cropping, were established. Directly after re-conversion and again one year after re-conversion, bulk soil C, distribution of C within various soil fractions (microbial biomass, water-stable aggregates, free and occluded light fraction organic matter) and C amounts added with coarse harvest residues were determined at 0–30 cm soil depths. After re-conversion, the amount of C stored in the harvest residues was 17–39 t C ha−1. One year after re-conversion, it had declined distinctly but rarely significantly, due to the high spatial variability of the harvest residues in the field. Nevertheless, C of the bulk soil did not change, but a decrease of microbial biomass C, macroaggregate (250–2000 μm) C and free light fraction organic matter indicate a loss of important fractions of soil organic C. More C was found in macroaggregates under rye-grass than under maize one year after re-conversion in loamy soils. Overall, one year after re-conversion yet no clear effects of tillage depth or land-use on SOC dynamics could be detected.