Despite the enormous size of the organic nitrogen (N) pool contained in mineral subsoils, rates of N cycling and soil exoenzyme activities are rarely measured in soils below 10 or 20 cm depth. Furthermore, assumed relationships between N mineralization rates and the activities of various decomposition exoenzymes are poorly characterized. We measured rates of gross and net N mineralization and nitrification as well as the potential activities of hydrolytic and oxidative enzymes at five soil depths (forest floor to 50 cm) in Spodosols at three hardwood forests of varying age (45 and 100 years post-harvest and old growth) at and near the Hubbard Brook Experimental Forest in New Hampshire, USA. As expected, rates of N cycling and potential enzyme activities per unit soil mass correlated strongly with soil carbon (C) concentration, and these parameters declined exponentially with increasing soil depth. After normalization per unit soil organic matter, N cycling rates and specific enzyme activities generally decreased little with depth within the mineral soil. Gross N mineralization rates correlated with specific activities of those enzymes that hydrolyze cellulose (β-glucosidase, cellobiohydrolase) and N-rich glucosamine polymers (N-acetylglucosaminidase), but not those that degrade protein or more complex C compounds. Hence, gross N cycling appear associated with the N released during microbial N recycling, rather than from decomposition of soil organic matter. Across the three stands, the youngest had a larger ratio of N- to-phosphorus-acquiring enzyme activities, indicating a greater N demand in younger than older forests. For all three stands, mineral soil below 10 cm contributed 30–53% of total gross and net N cycling per unit area to 50 cm depth. Overall, even though microbial N cycling and enzyme activities per unit soil mass decreased with depth, microbial processes in subsoils contributed substantially to ecosystem-scale gross N fluxes because of the sustained microbial activity per unit soil organic matter at depth and the large size of the organic matter pool in the mineral soil. These results support the inclusion of often-ignored mineral subsoils and microbial N recycling in both ecosystem N budgets and in model simulations, due to their contribution to soil N fluxes and the importance of microbial N dynamics in forest stands.

尽管矿物底土中含有巨大的有机氮（N）库，但在深度小于10或20 cm的土壤中，很少测量N循环速率和土壤外酶活性。此外，N矿化速率与各种分解外切酶活性之间的假定关系很难被表征。我们测量了三个不同年龄（收获后45年和100年）的硬木林在五种土壤深度（林地至50厘米）中的五种土壤深度中总和净氮矿化和硝化的速率以及水解和氧化酶的潜在活性和旧的增长）在美国新罕布什尔州的哈伯德布鲁克实验林及其附近。正如预期的那样，每单位土壤质量的氮循环速率和潜在的酶活性与土壤碳（C）浓度密切相关，这些参数随着土壤深度的增加呈指数下降。在将每单位土壤有机物归一化后，氮循环速率和比酶活性通常随矿质土壤中的深度而降低很少。总的N矿化速率与那些水解纤维素（β-葡萄糖苷酶，纤维二糖水解酶）和富含N的葡萄糖胺聚合物（N-乙酰氨基葡糖苷酶）的酶的比活相关，但与降解蛋白质或更复杂的C化合物的酶的比活没有关系。因此，总氮循环似乎与微生物氮循环过程中释放的氮有关，而不是与土壤有机质的分解有关。在这三个林分中，最年轻的具有较高的吸收氮磷的酶活性，这表明较年轻的森林对年轻人的氮需求更大。对于所有三个展位，10厘米以下的矿质土壤对50厘米深度的土壤贡献了总单位净N循环的30–53％。总体而言，尽管每单位土壤质量的微生物氮循环和酶活性随深度而降低，但深层土壤中的微生物过程仍对生态系统规模的总氮通量有很大贡献，这是因为每单位土壤深度的微生物活动持续，且土壤中微生物的大小很大。矿物土壤中的有机物池。这些结果支持在生态系统氮预算和模型模拟中包括经常被忽略的矿物地下土壤和微生物氮循环，因为它们对土壤氮通量的贡献以及林分中微生物氮动态的重要性。尽管每单位土壤质量的微生物氮循环和酶活性随深度降低，但由于每单位深度土壤有机物的持续微生物活性和有机物的较大尺寸，深层土壤中的微生物过程对生态系统规模的总氮通量有很大贡献。在矿物土壤中汇聚。这些结果支持在生态系统氮预算和模型模拟中包括经常被忽略的矿物地下土壤和微生物氮循环，因为它们对土壤氮通量的贡献以及林分中微生物氮动态的重要性。尽管每单位土壤质量的微生物氮循环和酶活性随深度降低，但由于每单位深度土壤有机物的持续微生物活性和有机物的较大尺寸，深层土壤中的微生物过程对生态系统规模的总氮通量有很大贡献。在矿物土壤中汇聚。这些结果支持在生态系统氮预算和模型模拟中包括经常被忽略的矿物地下土壤和微生物氮循环，因为它们对土壤氮通量的贡献以及林分中微生物氮动态的重要性。