FOR 1806: The Forgotten Part of Carbon Cycling: Organic Matter Storage and Turnover in Subsoils (SUBSOM)

这个项目的重点是研究地下土壤中有机质的储存和周转。全球总碳库的40- 60%固定在30厘米至2米深的较深土壤中。对较深层土壤层的有机碳进行的分析显示，有机碳的14 - c年龄高达几千年。这表明OM在很长一段时间内积累在底土中，因此看起来非常稳定。因此，认为地下OM与全球碳循环无关。然而，由于溶解的有机质和生物扰动，新鲜的碳输入到达底土，从而导致土壤有机质（SOM）分解。为了加强对周转机制的了解，研究单位提出了以下假设：(1)底土有机质主要由顽固性化合物组成，这些化合物是许多代谢和生化过程中最难降解的最终产物，因此在新鲜生物质输入的降解过程中积累起来。(2)新鲜有机质的年输入量很低，其在地下的封存对池大小变化和14C活动没有显著贡献。(3)地下土壤中的旧SOM与成土矿物的结合更强，因此更有效地稳定了生物降解。这是因为限制碳含量的底土环境比表层土壤提供了更大的吸附位点密度，并且在吸附相互作用过程中有机质组分之间的竞争效应更小。(4)大块底土有机质的高放射性碳年龄是由一小部分化石碳或地质碳造成的，因此有机质在底土中的平均停留时间被严重高估。(5)下层土壤微生物密度低，活动受环境因素（如低氧分压和低温）的限制，因此SOM以极低的速率矿化。(6)微生物群落和活动仅限于鲜有机质供应、根系、渗出物或穴居动物的极少数热点地区。大块土壤的大部分是如此稀少，即使是潜在的可降解的OM也可以持续存在。在十个子项目中，假设将在现场和实验室实验中进行研究，这些实验遵循一个共同的实验设计，以相互参考提升的数据。总的来说，研究股的目的是发展一个概念模型，以加强对土壤物理、化学和生物参数的土壤OM储存和周转过程的认识。

This project focusses on the storage and turnover of organic matter (OM) in subsoils. 40-60 percent of the globally total carbon pool is fixed in the deeper soil from 30 cm up to 2 m depth. The organic carbon analysed in deeper soil horizons showed a high 14C-age of several thousand years. This suggests that OM was accumulated in subsoils over a very long time and, therefore, appears to be very stable. Thus subsoil OM was not considered to be relevant for global C-cycle. Nevertheless, fresh C-inputs due to dissolved OM and bioturbation reach the subsoil and consequently lead to soil organic matter (SOM) decomposition. To enhance the understanding of the turnover mechanisms the Research Unit raised the following hypotheses: (1) Subsoil OM largely consists of recalcitrant compounds that are the least degradable end-products of numerous metabolic and biochemical processes and which, thus, accumulate during the degradation of fresh biomass inputs. (2) The annual inputs of fresh OM into the subsoil are so low that their sequestration in the subsoil does not significantly contribute to pool size changes or 14C activity over centuries. (3) Old SOM in subsoils is more strongly bound to pedogenic minerals and, thus, is more effectively stabilised against biodegradation. This is because C-limited subsoil environments provide a larger sorption site density than topsoils with less competition effects among organic matter components during sorptive interactions. (4) The high radiocarbon age of bulk subsoil OM is caused by a small fraction of fossil or geogenic C. The mean residence time of OM in subsoil is thus grossly overestimated. (5) Microbial densities in subsoils are low and activities are limited by environmental factors like low partial O2 pressures and low temperatures so that SOM is mineralised at extremely low rates. (6) Microbial communities and activities are limited to very few hot spots where fresh OM is supplied with SOM, roots, exudates or burrowing animals. Large parts of the bulk soil are so sparsely populated that even potentially degradable OM can persist. Within ten subprojects the hypotheses will be studied in the field and in laboratory experiments, which follow a common experimental design to refer the elevated data to each other. Overall, the Research Unit aims to develop a conceptual model to enhance the knowledge of OM storage and turnover processes in the subsoil considering soil physical, chemical and biological parameters.