Sulphur availability to crops in organic and conventional farming –
predicting the net sulphur mineralization

Sulphur (S) deficiency in crops has become common during the last decade, which has increased the need to better understand S dynamics. Since normally >90% of the soil S is in organic form, it is important to understand how organic S becomes available to plants and how soil processes or properties are involved. Improved predictions of plant availability should result in more accurate fertilizer recommendations. Knowledge of organic S behaviour is particularly important for organic and low-input farmers.
The main objective of this PhD-project is to relate S mineralization and plant availability
of S in agricultural soils to inherent soil properties. The soil is collected from 6 locations in South and Central Sweden. All the locations are part of a long-term field experiment, where factorial combinations of farmyard manure and mineral fertilizer (N, S and other nutrients) are tested in different crop rotations. Using soil from this experiment provides opportunities to study the effect of different fertilizer treatments on different soils in a longer time perspective. Incubation experiments are used to quantify the potential S mineralization in the soils, while a pot experiment will provide indications of plant availability. The incubations include estimates of net S mineralization and more elaborate studies of S transformations, using labelling with carrier free ³⁵SO₄^2-.
Most of the soil properties are analyzed by standard methods. However, the chemical S speciation is determined by XANES (X-ray Absorption Near Edge Structure)spectroscopy, rather than the destructive traditional wet chemistry methods. XANES
spectroscopy has recently been successfully used for S speciation in soil and sediment samples and has the advantage of being a non-destructive solid state analysis which gives a qualitative, and sometimes also quantitative, fingerprint of the S oxidation states and speciation. As a complement to the general characterization of soil properties, the physically protected (less available) organic S fraction is isolated, through acetylacetoneextraction combined with ultrasonic dispersion, and analyzed by XANES spectroscopy. Covariation between soil properties and experimental results is analyzed by multivariate models.