Sorption and desorption controls on alanine bioavailability in volcanic soils

Recent studies have shown that mineral-associated organic N is an important source of bioavailable N, and organic N sorption to/desorption from clay minerals may be a key factor of N dynamics in soils. This study aims to elucidate the importance of sorption/desorption to the mineralization of amino acids in volcanic soils. We hypothesized that in volcanic soils, sorption of amino acid to minerals reduces its mineralization and that desorption of amino acid differs reflecting soil properties.

Soils sampled from the O, A, and B horizons of three volcanic soils were used. Incubation experiments using the tracer method were carried out to assess the mineralization of alanine, which was used as a representative amino acid in the soil. Soils were placed in glass jars and were amended with ¹³C labeled alanine at a rate of 1% of total N to investigate the effects of alanine sorption on its mineralization. Similarly, separate soils were amended with ¹³C labeled alanine sorbed to iron oxides to examine the desorption of alanine. The percentage of mineralization of the added alanine over 7 days of incubation was determined. Acid oxalate extractable Al and Fe (Al_o and Fe_o) of soil were measured as representative clay mineral components contributing to sorption. Sorption isotherm experiments were carried out to understand each soil’s sorption characteristics, and the results were fitted to Freundlich’s sorption equation.

The percentage of mineralization of free alanine was the highest in O horizon soils at 40% and showed a decreasing trend going down the soil profile. The mineralization of alanine sorbed to iron oxides was approximately 56% of that of free alanine, regardless of the soil properties. Our results suggest that approximately 44% of the sorbed alanine was strongly sorbed to the iron oxides, but the remaining alanine was easily desorbed and mineralized similarly to free alanine. Freundlich-k constants were correlated with both Al_o and Fe_o content and mineralization of alanine (r = 0. 70, P < 0.05, r = −0.63, P < 0.05, respectively). Furthermore, based on the fitted Freundlich’s equation, the amount of alanine that was sorbed of the added alanine in the incubation experiment was calculated, and the results indicated that alanine mineralization was strongly correlated with the ratio of sorbed to added alanine (r = −0.95, P < 0.001).

We conclude that the desorption of sorbed alanine was constant regardless of soil properties and that sorption reduced alanine mineralization rather than delaying it.