Amino acid and n mineralization dynamics in heathland soil after long-term warming and repetitive drought

Monomeric organic nitrogen (N) compounds such as free amino acids (FAAs) are an important resource for both plants and soil microorganisms and a source of ammonium (NH ) via microbial FAA mineralization. We compared gross FAA dynamics with gross N mineralization in a Dutch heathland soil using a N tracing technique. A special focus was made on the effects of climate change factors warming and drought, followed by rewetting. Our aims were to (1) compare FAA mineralization (NH production from FAAs) with gross N mineralization, (2) assess gross FAA production rate (depolymerization) and turnover time relative to gross N mineralization rate, and (3) assess the effects of a 14 years of warming and drought treatment on these rates. The turnover of FAA in the soil was ca. 3 h, which is almost 2 orders of magnitude faster than that of NH (i.e. ca. 4 days). This suggests that FAA is an extensively used resource by soil microorganisms. In control soil (i.e. no climatic treatment), the gross N mineralization rate (10 ± 2.9 µg N g day) was 8 times smaller than the total gross FAA production rate of five AAs (alanine, valine, leucine, isoleucine, proline: 127.4 to 25.0 µg N g day). Gross FAA mineralization (3.4 ± 0.2 µg N g day) contributed 34 % to the gross N mineralization rate and is therefore an important component of N mineralization. In the drought treatment, a 6–29 % reduction in annual precipitation caused a decrease of gross FAA production by 65 % and of gross FAA mineralization by 41 % compared to control. On the other hand, gross N mineralization was unaffected by drought, indicating an increased mineralization of other soil organic nitrogen (SON) components. A 0.5–1.5 C warming did not significantly affect N transformations, even though gross FAA production declined. Overall our results suggest that in heathland soil exposed to droughts a different type of SON pool is min-eralized. Furthermore, compared to agricultural soils, FAA mineralization was relatively less important in the investigated heathland. This indicates more complex mineralization dynamics in semi-natural ecosystems.