Changes in P fractions after long-term application of biogas slurry to soils under organic farming.

In the present field study, the effects of applying biogas slurry for 25 years on soil microbiological and soil chemical properties were investigated in comparison with the application of raw cattle slurry. An on-farm research approach was used with pairwise comparison of neighboring fields, taking samples at a high spatial resolution from two biodynamic farms down to 10 cm. Texture, soil organic C (SOC), total N and P, microbial biomass C, N, and P as well as P fractions were determined by different extraction procedures. Application of biogas slurry had no significant effects on SOC, total N, or microbial biomass C, N, and P. Clay and pH effects outweighed site-specific differences for most properties. Biogas slurry increased the fractions of plant-available inorganic phosphate (+66 % more Olson-P) and of Ca-associated phosphates (+40 % 1 M HCl-P) at the expense of hardly soluble Fe and Al phosphates (-25 % 6 M HCl-P). This is in line with the observation that biogas slurry contains significantly more HO-P and 1 M HCl-P than raw slurry. The long-term application of biogas slurry had no negative impacts on any of the soil properties analyzed. [ABSTRACT FROM AUTHOR]