Proteins comprise the largest soil N reservoir but cannot be taken up directly by microorganisms and plants due to size constraints and stabilization of proteins in organo-mineral associations. Therefore the cleavage of this high molecular weight organic N to smaller soluble compounds as amino acids is a key step in the terrestrial N cycle. In the last years two isotope pool dilution approaches have been successfully established to measure gross rates of protein depolymerization and microbial amino acid uptake in soils. However, both require laborious sample preparation and analyses, which limits sample throughput. Therefore, we here present a novel isotope pool dilution approach based on the addition of ¹⁵N-labeled amino acids to soils and subsequent concentration and ¹⁵N analysis by the oxidation of α-amino groups to NO₂⁻ and further reduction to N₂O, followed by purge-and-trap isotope ratio mass spectrometry (PT-IRMS). We applied this method in mesocosm experiments with forest and meadow soils as well as with a cropland soil amended with either organic C (cellulose) or organic N (bovine serum albumin). To measure direct organic N mineralization to NH₄⁺, the latter was captured in acid traps and analyzed by an elemental analyzer coupled to an isotope ratio mass spectrometer (EA-IRMS). Our results demonstrate that the proposed method provides fast and precise measurements of at%¹⁵N even at low amino acid concentrations, allows high sample throughput and enables parallel estimations of instantaneous organic N mineralization rates.

蛋白质是最大的土壤氮库，但由于有机矿物组合中蛋白质的大小限制和稳定性，不能直接被微生物和植物吸收。因此，将这种高分子量有机氮裂解为较小的可溶性化合物（如氨基酸）是陆地氮循环的关键步骤。在过去的几年中，已经成功建立了两种同位素池稀释方法来测量土壤中蛋白质解聚和微生物氨基酸吸收的总速率。然而，两者都需要费力的样品制备和分析，这限制了样品通量。因此，我们在此提出一种新的同位素池稀释方法，该方法基于向土壤中添加¹⁵ N 标记氨基酸，然后通过将 α-氨基氧化为 NO ₂ ⁻并进一步还原为 N ₂ O进行浓缩和¹⁵ N 分析，然后进行吹扫捕集同位素比质谱 (PT-IRMS)。我们将该方法应用于森林和草甸土壤以及用有机 C（纤维素）或有机 N（牛血清白蛋白）改良的农田土壤的中生态实验。为了测量有机氮直接矿化为NH ₄ ⁺，将后者捕获在酸阱中并通过与同位素比质谱仪（EA-IRMS）联用的元素分析仪进行分析。我们的结果表明，即使在低氨基酸浓度下，所提出的方法也能快速、精确地测量 % ^{15 N，允许高样品通量，并能够并行估计瞬时有机氮矿化速率。}