Plant Secondary Metabolites—Missing Pieces in the Soil Organic Matter Puzzle of Boreal Forests

Abstract

Processes underlying soil organic matter (SOM) transformations are meeting growing interest as SOM contains more carbon (C) than global vegetation and the atmosphere combined. Therefore, SOM is a crucial element of the C cycle, especially in ecosystems rich in organic matter, such as boreal forests. However, climate change may shift the fate of this SOM from C sink into C source, accelerating global warming. These processes require a better understanding of the involved mechanisms driving both the C cycle and the interlinked nitrogen (N) cycle. SOM transformations are balanced by a network of interactions between biological, chemical and physical factors. In this review, we discuss the findings of the most recent studies to the current state of knowledge about the main drivers in SOM transformations. We focus on plant-derived secondary metabolites, as their biochemical traits, especially interactions with soil microbial communities, organic N compounds and enzymes make them potential regulators of SOM decomposition. However, these regulatory abilities of plant-derived compounds are not fully explored.

Surplus Carbon Drives Allocation and Plant–Soil Interactions

“…Likewise, Glycine max (soybean) roots release more metabolites including amino acids and organic acids when grown with limited P [73]. Roots may also release a large spectrum of plant secondary metabolites [74,75], particularly when plants are nutrient limited [76,77]. Increased concentrations of phenolic compounds [68], flavanoids [78], and organic acids [79] have been reported in root exudates from nutrient-deficient plants.…”

Mechanisms of Carbon Sequestration in Highly Organic Ecosystems – Importance of Chemical Ecology

“…Their potential effect on decomposition and stabilization processes in the soil can be significant, because PSMs can comprise up to 30 % of dry weight of plants and their concentration depends on species, age, organ, as well as environmental conditions including soil nutrient status . A quantitatively dominating group of PSMs, tannins (Figure ), affect C and N mineralization as well as microbial community structure and biomass depending on concentration and chemical structure. The underlying mechanism of inhibition by tannins seems to include the formation of recalcitrant complexes with proteins (including enzymes), chelating metal ions, and/or their direct toxicity towards microbes .…”

“…Evidence from previous studies suggests that the complexation of proteins by tannins conserves N derived from litter with consequences for plant N acquisition via resulting in organic over mineral N dominated pathways . Overall, scientists are only beginning to consider PSMs in these processes, and an in‐depth understanding of litter and subsequently SOM degradation as influenced by PSMs on the basis of N and C pools and the resulting consequences for plant N acquisition is also crucial for improving litter decomposition models in climate change scenarios …”

Volatile organic compound fluxes from northern forest soils

“…Roots produce BVOCs to interact with other soil organisms and to strengthen their resilience to pathogens and herbivores, while root exudates also stimulate microbial production or the uptake of VOCs (Rasmann et al, 2005;Kai et al, 2007;Peñuelas et al, 2014). Plant-derived BVOCs may also affect soil organic matter decomposition (Adamczyk et al, 2018). VOCs are released by decomposers as metabolic side products in aerobic carbon metabolism, fermentation, amino acid degradation, isoprenoid biosynthesis and sulphur reduction (Peñuelas et al, 2014) or are metabolized to transmit signals from ectomycorrhizal fungi to plant root (Ditengou et al, 2015).…”