Sterols in soil organic matter in relation to nitrogen mineralization in sandy arable soils

Unusually low net N mineralization in soils relatively rich in total organic C and N was repeatedly reported for sandy arable soils in NW Europe. In order to adequately account for it in simulation models, it is necessary to know the involved substances and processes. Therefore, 9 arable top soils (< 6% clay) with a wide range of total organic C (1.1%-5.2%) and C : N ratios (12-35) were studied. The soils varied strongly in the mineralizability of soil organic N which was determined via long-term laboratory incubations (> 200 d). It was hypothesized that mineralization was controlled by antioxidants, and the Trolox equivalent antioxidant capacity (TEAC) of the soils was measured. In addition, pyrolysis-field ionization mass spectrometry (Py-FIMS) was applied to investigate the influence of the molecular-chemical composition of soil organic matter. In these soils, the compound class of sterols from Py-FIMS analysis was most closely, negatively correlated with the mineralizability of soil organic N (r(2) = 0.75, p = 0.003). This was probably not an antioxidative effect, because the TEAC values did not correlate sufficiently with the mineralizability and the sterol intensities. However, the negative relation with sterols could be causal, since the correlation was about as close with other components of the compound class of sterols and even closer with the main plant sterol beta-sitosterol (r(2) = 0.84, p = 0.001). In addition, the variability among samples was strongly governed by the proportions of sterols, and sterols also had a high discriminating power in discriminant analysis. Furthermore, the proportions of sterols were extraordinary in those arable podzol soils that developed under previous heath-or woodland (up to 10.2% of total ion intensity from Py-FIMS). In conclusion, the inhibitory effect of these compounds needs to be investigated in more detail in order to optimize parameterization of N as well as C simulation models especially for podzolized, sandy arable soils with former heath-or woodland vegetation.