Soil organic nitrogen cycling increases with temperature and precipitation along a boreal forest latitudinal transect

The biodegradation of soil organic nitrogen (SON) into forms available for plant uptake is an important control on nitrogen availability, plant growth, and carbon (C) sequestration in terrestrial ecosystems. Changes in biochemical composition during SON diagenesis were investigated by analyzing hydrolysable amino acids in the organic (L, F, and H) horizons and the surface mineral (B) horizon of soil profiles from the Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect (NL-BELT). The diagenetic continuum in the soil profiles provides insights about the mechanisms of SON transformations, and comparison of regions along the latitudinal transect provides insight on the effects of climate. The fraction of N accounted for as amino acids decreased with depth in the profile, indicating a preferential loss of amino acids compared to bulk N during diagenesis. The amino acid composition also changed consistently with depth, and was used to construct a degradation index for assessing the extent of SON diagenesis. The compositional changes were similar to those observed in marine and lake sediments, indicating leaching and biodegradation have similar effects on SON composition in all three systems. This was supported by the use of the amino acids hydroxproline and diaminopimelic acid as tracers of plant and bacterial remains, respectively. The contribution of microbial remains increased with depth to > 34 % of amino acids and > 19 % of total N in the B horizon, consistent with increasing microbial decomposition. The extent of SON diagenesis was similar in the three regions despite a younger mean age of the organic matter at the southernmost site, indicating a higher rate of N turnover with higher temperature and precipitation. The resulting increase in N availability likely contributes to higher productivity and thus the higher litter inputs observed in the southern region, maintaining similar C stocks despite more rapid decomposition.