Tannins, an abundant group of plant secondary compounds, raise interest in different fields of science, owing to their unique chemical characteristics. In chemical ecology, tannins play a crucial role in plant defense against pathogens, herbivores, and changing environmental conditions. In the food industry and in medicine, tannins are important because of their proven positive effect on human health and disease treatment. Such wide interests fueled studies on tannin chemistry, especially on their flagship ability to precipitate proteins. In this Review, we expand the basic knowledge on tannin chemistry to the newest insights from the field. We focus especially on tannin reactions with different non‐protein organic N compounds, as well as the complex interactions of tannins with enzymes, resulting in either an increase or decrease in enzyme activity.

Two major groups of plant secondary compounds, phenolic compounds and terpenes, may according to current evidence mediate changes in soil C and N cycling, but their exact role and importance in boreal forest soils are largely unknown. In this review we discuss the occurrence of these compounds in forest plants and soils, the great challenges faced when their concentrations are measured, their possible effects in regulating soil C and N transformations and finally, we attempt to evaluate their role in connection with certain forest management practices. In laboratory experiments, volatile monoterpenes, in the concentrations found in the coniferous soil atmosphere, have been shown to inhibit net nitrogen mineralization and nitrification; they probably provide a C source to part of the soil microbial population but are toxic to another part. However, there is a large gap in our knowledge of the effects of higher terpenes on soil processes. According to results from laboratory experiments, an important group of phenolic compounds, condensed tannins, may also affect microbial processes related to soil C and N cycling; one mechanism is binding of proteins and certain other organic N-containing compounds. Field studies revealed interesting correlations between the occurrence of terpenes or phenolic compounds and C or net N mineralization in forest soils; in some cases these correlations point in the same direction as do the results from laboratory experiments, but not always. Different forest management practices may result in changes in both the quantity and quality of terpenes and phenolic compounds entering the soil. Possible effects of tree species composition, clear-cutting and removal of logging residue for bioenergy on plant secondary compound composition in soil are discussed in relation to changes observed in soil N transformations.

Boreal forests are ecosystems with low nitrogen (N) availability that store globally significant amounts of carbon (C), mainly in plant biomass and soil organic matter (SOM). Although crucial for future climate change predictions, the mechanisms controlling boreal C and N pools are not well understood. Here, using a three-year field experiment, we compare SOM decomposition and stabilization in the presence of roots, with exclusion of roots but presence of fungal hyphae and with exclusion of both roots and fungal hyphae. Roots accelerate SOM decomposition compared to the root exclusion treatments, but also promote a different soil N economy with higher concentrations of organic soil N compared to inorganic soil N accompanied with the build-up of stable SOM-N. In contrast, root exclusion leads to an inorganic soil N economy (i.e., high level of inorganic N) with reduced stable SOM-N build-up. Based on our findings, we provide a framework on how plant roots affect SOM decomposition and stabilization.

See also the Commentary on this article by Hättenschwiler et al., 223: 5–7.