Funding: DFG (German Research Foundation); 2007 - 2010

Responsible researchers: H.-W. Scherer (INRES – Plant Nutrition, Uni Bonn), G. Welp, Saskia Foerster

A sulphur fertilization which is orientated at the need of the crops presupposes above all knowledge of the medium- and long-term S supply by mineralization of organic S forms. Organic S forms can constitute up to 95 % of total S. They are mainly composed of carbon bonded sulfur (C-S; e.g. S-containing amino acids) and not directly carbon bonded sulfur (C-O-S; e.g. ester sulfates). The latter probably play the more important role for S mineralization. Yet, statements are inconsistent regarding the contribution of various organic S forms. This might be to due to a different "stability" in relation to the attack of  Arylsulfatase enzymes. We hypothezise that ester sulfates are not available for the enzymes when they are protected within soil aggregates.

In this project, we want to check out, whether there is a causal relation between aggregate parameters (size, stability) and a biochemical S-mineralization of the ester sulfates. For this purpose, we use soil samples from a randomized long-term experiment with a differentiated, 45 year organic fertilization and a mineral fertilization variant as control. Besides, we will spike soil samples with  35S-sulfate and follow its fate over time (entrapment within ester sulfates, remobilization etc.).