The origin of soil organic matter controls its composition and bioreactivity across a mesic boreal forest latitudinal gradient

Abstract

Warmer climates have been associated with reduced bioreactivity of soil organic matter (SOM) typically attributed to increased diagenesis; the combined biological and physiochemical transformation of SOM. In addition, cross-site studies have indicated that ecosystem regime shifts, associated with long-term climate warming, can affect SOM properties through changes in vegetation and plant litter production thereby altering the composition of soil inputs. The relative importance of these two controls, diagenesis and inputs, on SOM properties as ecosystems experience climate warming, however, remains poorly understood. To address this issue we characterized the elemental, chemical (nuclear magnetic resonance spectroscopy and total hydrolysable amino acids analysis), and isotopic composition of plant litter and SOM across a well-constrained mesic boreal forest latitudinal transect in Atlantic Canada. Results across forest sites within each of three climate regions indicated that (1) climate history and diagenesis affect distinct parameters of SOM chemistry, (2) increases in SOM bioreactivity with latitude were associated with elevated proportions of carbohydrates relative to plant waxes and lignin, and (3) despite the common forest type across regions, differences in SOM chemistry by climate region were associated with chemically distinct litter inputs and not different degrees of diagenesis. The observed climate effects on vascular plant litter chemistry, however, explained only part of the regional differences in SOM chemistry, most notably the higher protein content of SOM from warmer regions. Greater proportions of lignin and aliphatic compounds and smaller proportions of carbohydrates in warmer sites' soils were explained by the higher proportion of vascular plant relative to moss litter in the warmer relative to cooler forests. These results indicate that climate change induced decreases in the proportion of moss inputs not only impacts SOM chemistry but also increases the resistance of SOM to decomposition, thus significantly altering SOM cycling in these boreal forest soils.

Temperature sensitivity of biomass‐specific microbial exo‐enzyme activities and CO2 efflux is resistant to change across short‐ and long‐term timescales

“…Knowing this, we employed organic-rich, mesic soils to help minimize any indirect effects of environmental changes co-varying with temperature (e.g., substrate and water availability) on microbial responses. Previous studies demonstrated that the mesic soils along the latitudinal transect in this study harbor similar, abundant C stocks and moisture regimes regardless of long-term differences in mean annual temperature (MAT; Kohl et al, 2017;Ziegler et al, 2017). An additional study along the same transect reports that microbial CO 2 efflux rates did not decrease over 96 days of incubation (Podrebarac, Laganière, Billings, Edwards, & Ziegler, 2016), demonstrating that substrate limitation is unlikely, at least on monthly timescales, in soils from these sites.…”

“…Microbial decomposition of soil organic matter and CO 2 efflux in situ is influenced by plant and rhizosphere activities (Kuzyakov, ; Kuzyakov & Xu, ; Talbot, Allison, & Treseder, ), phenomena excluded from our study. Along the transect employed in the current study, increasing mean annual litter input in spite of the similar soil C content (Ziegler et al, ) and decreasing relative inputs of moss‐derived organic matter (Kohl et al, ) with decreasing latitude represent ecosystem level factors that may play a role in the in situ soil CO 2 effluxes at study sites. Therefore, although the current study suggests that microbial modules in Earth‐system models may consider temperature sensitivity of multiple microbial processes to be constant across time, incorporation of complex feedbacks among plants, microbes, and climate is necessary to scale up microbial process rates to the ecosystem level.…”

“…Microbial decomposition of soil organic matter and CO 2 efflux in situ is influenced by plant and rhizosphere activities (Kuzyakov, 2010;Kuzyakov & Xu, 2013;Talbot, Allison, & Treseder, 2008), phenomena excluded from our study. Along the transect employed in the current study, increasing mean annual litter input in spite of the similar soil C content and decreasing relative inputs of moss-derived organic matter (Kohl et al, 2017) with decreasing latitude represent ecosystem level factors that may play a role in the in situ soil CO 2 effluxes at study sites.…”

Biochemical and structural controls on the decomposition dynamics of boreal upland forest moss tissues

“…The low decay rates observed are unlikely to be an artifact of the laboratory approach, as Bengtsson et al (2016) compared fieldbased litter bag and laboratory incubation approaches using a common set of Sphagnum mosses and found that the laboratory approach generally resulted in greater mass loss. The northern forest mosses, but not the southern forest mosses, exhibited higher Q 10 than the bulk L horizon soil (Laganière et al, 2015;Podrebarac et al, 2016) and previous findings for vascular plant tissue decomposition (e.g., Fierer et al, 2005) based on the decay rate of the labile C fraction. However, the higher temperature only slightly increased the total C degraded after 959 days.…”

Biochemical and structural controls on the decomposition dynamics of boreal upland forest moss tissues

“…This idea is supported by the 13 C NMR data, which indicate that the moss OM is rich in carbohydrates with little aromatic or alkyl C. The proportions of O-alkyl C in mosses were higher than vascular 5 plant litter collected from the same study sites (55.3% vs. 37.0%; Kohl et al 2018). Further, the OM composition did not change significantly following incubation, unlike the decomposition of vascular plant tissues and SOM in which O-alkyl C is often preferentially degraded and alkyl C increases in relative abundance (Baldock et al, 1997;Kögel-Knabner, 1997;Preston et al, 2009).…”