Assessing stabilisation and destabilisation processes - Tracing young and old carbon in subsoils

地下土壤储存了全球50%以上的土壤碳，但对地下土壤的稳定和不稳定过程知之甚少。地下有机碳（OC）主要来源于以下三种来源：作为溶解有机碳（DOC）淋滤到地下土壤中的OC，来自根凋落物和根渗出物（两种生物源OC）的OC，以及随沉积基岩形成而沉积的OC，称为地源OC或化石OC。关于这些年轻和年老OC源对地下OC库贡献的研究很少。SUBSOM的这个子项目的目的是追踪地下土壤有机碳的来源，区分地球成因和生物成因的有机碳，并评估地下土壤中年轻的DOC衍生的地下土壤有机碳和古老的地球成因有机碳的稳定性。在三个工作包（WPs）中，将现场试验与室内分析和培养试验相结合，研究不同土壤类型中不同OC组分的脆弱性。同位素标记（13C）将用于追踪年轻的DOC衍生的OC组分，14C将用于追踪非常古老的地球成因OC组分。在SUBSOM观测站，将继续监测土壤二氧化碳通量（WP1），并将使用Switch off联合实验验证基于二氧化碳梯度的模型。基于对这一大型观测站数据集的多因素分析（机器学习算法），将利用根系周转量等新获得的数据来约束底土中二氧化碳通量的驱动因素。在一个联合实验中（DO13C注入，由我们的子项目领导的现场活动），通过在三个地点的两个不同的土壤深度（WP2）应用标记的DOC溶液，评估底土中DOC的命运和稳定性。同样，在观察站，将探索20个月前施用于土壤表面的标记凋落物的命运（关闭实验，由我们的子项目领导的采样活动）。添加营养物质和易获得的OC基质的培养实验将揭示这种年轻DOC衍生OC输入的稳定程度和机制。我们将在四个不同沉积基岩（黄土、第三系砂、砂石、石灰石）的地点（WP3）进行联合实验（由子项目牵头），探讨地球成因OC在底土中的作用。从深岩心（10米）基岩样品中获得几乎100%的地球成因OC，以量化和表征地球成因OC分数。将开发分馏方法来分离地球成因和生物成因的有机碳。地源性有机碳进入地下土壤（与营养物和易获得的有机碳相遇）的稳定性将在培养实验中进行评估。在SUBSOM开发的数值模型中，DOC在不同土壤类型下的翻转和原位运移数据以及地质OC对地下OC的贡献将直接作为输入变量。

Subsoils store more than 50% of the global soil carbon, but little is known on stabilisation and destabilisation processes in subsoils. Subsoil organic carbon (OC) is derived from three major sources: OC leached into subsoils as dissolved organic carbon (DOC), OC derived from root litter and root exudates (together the two biogenic OC sources) and OC that was deposited with the formation of sedimentary bedrocks, so called geogenic OC or fossil OC. There are few studies on the contribution of these young and old OC sources to subsoil OC stocks. The aim of this subproject within SUBSOM is to trace the sources of subsoil OC, to differentiate between geogenic and biogenic OC and to assess the stability of young DOC derived subsoil OC and old geogenic OC in subsoils. In three workpackages (WPs) field experiments will be combined with laboratory analysis and incubation experiments on the vulnerability of different OC fractions in different soil types. Isotopic labelling (13C) will be used to trace the young DOC derived OC fraction and 14C will be used to trace the very old, geogenic OC fraction.At the SUBSOM observatories monitoring of soil CO2 fluxes will be continued (WP1) and the CO2 gradient based model will be validated using the Switch off joint experiment. Newly available data such as root turnover will be used to constrain the drivers of CO2 fluxes in subsoils based in a multifactorial analysis (machine learning algorithms) of this large observatory data set.The fate and stability of DOC in subsoil will be assessed in a joint experiment (DO13C inject, field campaigns led by our subproject) by applying a labelled DOC solution at three sites to two differed soil depths (WP2). Similarly, at the observatories the fate of the labeled litter applied onto the soil surface 20 months ago will be explored (Switch off experiment, sampling campaigns led by our subproject). Incubation experiments with addition of nutrients and easily available OC substrate will reveal the degree and mechanisms of stabilisation of such young DOC derived OC input. The role of geogenic OC in subsoils will be explored in a joint experiment (led by our subproject) at four sites with different sedimentary bedrock (loess, tertiary sand, sand stone, lime stone)(WP3). From deep cores (10 m) bedrock samples with almost 100% geogenic OC will be derived to quantify and characterised the geogenic OC fraction. Fractionation methods will be developed to separate geogenic from biogenic OC. The stability of geogenic OC entering subsoils (encounter with nutrients and easily available OC) will be assessed in incubation experiments.Data on the turnover and in-situ transport of DOC in different soil types and the contribution of geogenic OC to subsoil OC will be directly used as input variables in the numerical model developed within SUBSOM.