Organic carbon and total nitrogen stocks in soils of the Lena River Delta

The Lena River Delta, which is the largest delta in the Arctic, extends over an area of 32 000 km**2 and likely holds more than half of the entire soil organic carbon (SOC) mass stored in the seven major deltas in the northern permafrost regions. The geomorphic units of the Lena River Delta which were formed by true deltaic sedimentation processes are a Holocene river terrace and the active floodplains. Their mean SOC stocks for the upper 1 m of soils were estimated at 29 kg/m**2 ± 10 kg/m**2 and at 14 kg/m**2 ± 7 kg/m**2, respectively. For the depth of 1 m, the total SOC pool of the Holocene river terrace was estimated at 121 Tg ± 43 Tg, and the SOC pool of the active floodplains was estimated at 120 Tg ± 66 Tg. The mass of SOC stored within the observed seasonally thawed active layer was estimated at about 127 Tg assuming an average maximum active layer depth of 50 cm. The SOC mass which is stored in the perennially frozen ground at the increment 50-100 cm soil depth, which is currently excluded from intense biogeochemical exchange with the atmosphere, was estimated at 113 Tg. The mean nitrogen (N) stocks for the upper 1 m of soils were estimated at 1.2 kg/m**2 ± 0.4 kg/m**2 for the Holocene river terrace and at 0.9 kg/m**2 ± 0.4 kg/m**2 for the active floodplain levels, respectively. For the depth of 1 m, the total N pool of the river terrace was estimated at 4.8 Tg ± 1.5 Tg, and the total N pool of the floodplains was estimated at 7.7 Tg ± 3.6 Tg. Considering the projections for deepening of the seasonally thawed active layer up to 120 cm in the Lena River Delta region within the 21st century, these large carbon and nitrogen stocks could become increasingly available for decomposition and mineralization processes.作为北极地区规模最大的三角洲，勒拿河三角洲（Lena River Delta）总面积达32000平方千米，其储存的土壤有机碳（soil organic carbon, SOC）总量或占北半球永久冻土区七大主要三角洲总土壤有机碳储量的一半以上。勒拿河三角洲由典型三角洲沉积作用形成的地貌单元包括全新世河阶地与活动泛滥平原，二者0-1米表层土壤的平均SOC储量分别为29 kg/m² ±10 kg/m²和14 kg/m² ±7 kg/m²。按1米深度计算，全新世河阶地的总SOC库估算为121 Tg ±43 Tg，活动泛滥平原的总SOC库则为120 Tg ±66 Tg。假设该区域平均最大活动层厚度为50厘米，观测到的季节解冻活动层内储存的SOC总量约为127 Tg；当前未参与强烈大气生物地球化学交换、埋藏于50-100厘米深度多年冻土中的SOC储量估算为113 Tg。全新世河阶地0-1米表层土壤的平均氮（N）储量为1.2 kg/m² ±0.4 kg/m²，活动泛滥平原的对应值为0.9 kg/m² ±0.4 kg/m²；按1米深度计算，河阶地的总氮库估算为4.8 Tg ±1.5 Tg，泛滥平原的总氮库则为7.7 Tg ±3.6 Tg。考虑到21世纪内勒拿河三角洲地区季节解冻活动层厚度将加深至120厘米的预测，这些巨量的碳、氮储量将越来越易于被分解与矿化作用利用。