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

该项目主要研究有机质在底土中的储存和周转。全球总碳库的40%-60%固定在从30厘米到2米深的更深的土壤中。对深层土壤中有机碳的分析表明，14C年龄高达几千年。这表明有机质在底土中积累了很长一段时间，因此显得非常稳定。因此，底土有机质不被认为与全球碳循环有关。然而，由于溶解的OM和生物扰动导致的新鲜C-输入到达底土，从而导致土壤有机质(SOM)的分解。为了加强对周转机制的理解，研究组提出了以下假设：(1)底土有机质主要由难降解的化合物组成，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，因此在新鲜生物质投入的降解过程中积累。(2)新鲜有机质在底土中的年输入量很低，以至于它们在底土中的固定不会在几个世纪内对池塘大小的变化或14C的活性有显著的贡献。(3)底土中的老有机质与成壤矿物结合更强，从而更有效地稳定了土壤，防止生物降解。这是因为碳限制的下层土壤环境提供了比表层土壤更大的吸附位置密度，而在吸附相互作用过程中，有机质组分之间的竞争效应较小。(4)大量底土有机质的高放射性碳年龄是由少量化石或地源C引起的，因此，有机质在底土中的平均停留时间被严重高估。(5)底土微生物密度低，活性受到低氧分压和低温等环境因素的限制，土壤有机质矿化速率极低。(6)微生物群落和活性仅限于极少数提供有机质、根、分泌物或穴居动物的热区。大部分散装土壤的人口稀少，即使是潜在的可降解有机质也可以持续存在。在十个子项目中，这些假设将在现场和实验室实验中进行研究，这些实验遵循共同的实验设计，将提升的数据相互参照。总体而言，研究股的目标是开发一个概念模型，以加强对考虑土壤物理、化学和生物参数的底土有机质储存和周转过程的了解。

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.