2. Materials and methods2.1. Site description and soil samplingSoil samples were taken in April 2013 from sites in Friemar (west of Erfurt) and Zschortau (north of Leipzig) in eastern Germany. The sites are part of long-term field experiments installed to analyze the impact of tillage intensity on soil properties and productivity in 1992/93 and 1997/98, respectively (Koch et al., 2009). The site in Friemar is a Haplic Phaeozem (3% sand, 68% silt, 29% clay) with a mean annual precipitation of 517 mm and a mean annual temperature of 7.8 C, while the site in Zschortau is a Gleyic Luvisol (33% sand, 53% silt, 14% clay) with a mean annual precipitation of 512mm and a mean annual temperature of 8.8 C (Koch et al., 2009). The two contrasting soils were chosen as field replicates. Samples were taken from the conventionally tilled plots, managed with annual moldboard ploughing to the depth of 30cm. More detailed site information are given in Koch et al. (2009). A composite soil sample was taken from each plot consisting of twelve individual core samples taken from 0 to 30 cm depth. The soil samples were sieved <5 mm.2.2. Experimental setup: incubation experimentThe dry soil was carefully ground to pass a 250mm sieve (Andruschkewitsch et al., 2014; Helfrich et al., 2008) to ensure the complete dispersion of macro-aggregates. The amount of the remaining material on top of the 250 mm sieve was negligible and discarded in case of the clayey soil but was collected and mixed in equivalent parts into each of the <250 mm subsamples (see below) in case of the sandy soil.The organic soil additives analyzed in this study were biochar, slurry and manure. We used commercially available biochar (Verora GmbH, Switzerland) manufactured from woodchips by combustion at 575 C, containing 77.4% C and 1% N (C/N = 77.4), and produced according to the European biochar certificate (Schim- melpfennig and Glaser, 2012). The biochar was dried at 40 C and ground to pass a 53mm sieve. The analyzed slurry and manure were obtained from an organic cattle farm in Schwäbisch Hall (Southern Germany). The manure was cut into smaller pieces in order to allow a more homogenous application.The sample size for the incubation was 150 g dry soil material, which was split up in two 75 g subsamples filled into 120 ml pots because of spatial restrictions. The biochar was added at a rate of 30 g per kg of dry soil (equivalent to 30 t ha 1, 0–10 cm, assuming a dry bulk density of 1 t m-3), which is in the range of commonly used rates in previous studies (e.g. Bruun et al., 2014; Sun and Lu, 2014; Lu et al., 2014). Afterwards, the samples were shaken over night to assure a homogenous mixture of the biochar with the soil. For slurry and manure, the amounts added were adapted to the common field application rate of 150 kg N ha 1 corresponding to 0.3 g N per kg dry soil. Due to different C/N ratios, the slurry (C/ N = 10.9) treatments received 3.28 g C kg 1 soil and the manure (C/ N = 19.7) treatments 5.91 g C kg 1 soil. The dry soil or the soil/ biochar mixture was thoroughly mixed with the slurry or the
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