Isotopic Elucidation of Microbial Nitrogen Transformations in Forest Soils

Abstract Soil nitrogen (N) transformations between labile N forms (extractable organic N [EON], ammonium [NH 4 + ], and nitrate [NO 3 − ]) regulate soil N availability. However, it has long been difficult to quantify the transformations of total soil organic and labile N forms in soils, which has left large uncertainties in evaluating atmospheric N deposition effects on soil N dynamics. Based on concentrations and natural abundances of N isotopes of soil organic N, EON, NH 4 + , and NO 3 − across 11 forests with variant N deposition levels, we established a quantitative isotopic framework to estimate the fractions of soil N depolymerization ( f D ), mineralization ( f M ), nitrification ( f N ), and of NO 3 − losses ( f L ) via denitrification and leaching. Based on the fractions, the gross production and storage of corresponding soil labile N were estimated for forests of China and Japan. We found that f D , f M , and f N increased, while f L decreased with the increase of N deposition among the study forests. And the contribution of denitrification (relative to the NO 3 − leaching) to total NO 3 − losses also increased with increasing N deposition. Our method provides new and straightforward insights into the present soil N transformations and allows to evaluate the soil N status. These findings are useful for modeling forest N cycles under different N deposition regimes.