Centre for Environmental and Climate Science, Lund University, 223 62 Lund, Sweden.
Department of Biology, Microbial Ecology Group, Lund University, 223 62 Lund, Sweden.
Sci Total Environ. 2022 Jul 20;831:154876. doi: 10.1016/j.scitotenv.2022.154876. Epub 2022 Mar 28.
Boreal forests have a large impact on the global greenhouse gas balance and their soils constitute an important carbon (C) reservoir. Mature boreal forests are typically a net CO sink, but there are also examples of boreal forests that are persistent CO sources. The reasons remain often unknown, presumably due to a lack of understanding of how biotic and abiotic drivers interact to determine the microbial respiration of soil organic matter (SOM). This study aimed at identifying the main drivers of microbial SOM respiration and CO and CH soil chamber-fluxes within dry and wet sampling areas at the mature boreal forest of Norunda, Sweden, a persistent net CO source. The spatial heterogeneity of the drivers was assessed with a geostatistical approach combined with stepwise multiple regression. We found that heterotrophic soil respiration increased with SOM content and nitrogen (N) availability, while the SOM reactivity, i.e., SOM specific respiration, was determined by soil moisture and N availability. The latter suggests that microbial activity was N rather than C limited and that microbial N mining might be driving old-SOM decomposition, which was observed through a positive correlation between soil respiration and its δC values. SOM specific heterotrophic respiration was lower in wet than in dry areas, while no such dependencies were found for chamber-based soil CO fluxes, implying that oxygen depletion resulted in lower SOM reactivity. The chamber-based soil CH flux differed significantly between the wet and dry areas. In the wet area, we observed net CH emission that was positively related to soil moisture and NH-N content. Taken together, our findings suggest that N availability has a strong regulatory effect on soil CO and CH emissions at Norunda, and that microbial decomposition of old-SOM to release bioavailable N might be partly responsible for the net CO emission at the site.
北方森林对全球温室气体平衡有很大影响，其土壤构成了一个重要的碳（C）储存库。成熟的北方森林通常是 CO 的净汇，但也有一些北方森林是持续的 CO 源。原因通常未知，可能是由于缺乏了解生物和非生物驱动因素如何相互作用来确定土壤有机物质（SOM）的微生物呼吸。本研究旨在确定主要驱动因素，以确定瑞典诺伦达成熟北方森林干、湿采样区的微生物 SOM 呼吸以及 CO 和 CH 土壤室通量。利用地质统计学方法和逐步多元回归评估了驱动因素的空间异质性。我们发现，异养土壤呼吸随着 SOM 含量和氮（N）可利用性的增加而增加，而 SOM 反应性，即 SOM 特定呼吸，则由土壤水分和 N 可利用性决定。后者表明微生物活性受 N 而不是 C 的限制，微生物 N 开采可能正在驱动旧-SOM 的分解，这通过土壤呼吸与其 δC 值之间的正相关得到证实。湿区的 SOM 特异性异养呼吸低于干区，而基于腔室的土壤 CO 通量则没有这种依赖性，这表明氧气耗尽导致 SOM 反应性降低。基于腔室的土壤 CH 通量在湿区和干区之间有显著差异。在湿区，我们观察到净 CH 排放与土壤水分和 NH-N 含量呈正相关。总的来说，我们的研究结果表明，在诺伦达，N 的可利用性对土壤 CO 和 CH 排放具有很强的调节作用，而微生物分解旧-SOM 以释放生物可利用的 N 可能是该地点净 CO 排放的部分原因。
