Plant roots increase both decomposition and stable organic matter formation in boreal forest soil

Abstract

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.

Fig. 1

Effect of the treatments and time on chemical and biological soil properties. Partial constrained correspondence analysis (pCCA) illustrates the effect of the three treatments and experimental time (year) on the chemical and biological compositions of the mesh bags. The CCA1 axis explains 28.4% (P ≤ 0.001, F = 31.36) of the data variability, and the CCA2 axis explains 6.8% (P ≤ 0.001, F = 7.59) of the data. The different mesh treatments are represented with a green triangle (1000 µm mesh), magenta circle (50 µm mesh) and black square (1 µm mesh). The variables responsible for the separation of the different samples in the analysis have been marked on the diagram with the following abbreviations: total AAs total free amino acids, CT condensed tannins, stableN stable SOM-nitrogen (N) pool, SOM soil organic matter, NH₄ ammonium, PLFA_B bacterial phospholipid fatty acids (PLFA), PLFA_F fungal PLFA

Fig. 2

Soil decomposition and microbiology: a Soil organic matter (SOM) loss, b N loss, c PLFA fungal biomarkers, d PLFA bacterial biomarkers. The different mesh sizes are represented with different colours (from left to right): 1000 µm (green), 50 µm (magenta), and 1 µm (black). The given values are the means of 24 replicates. Significant differences (P < 0.05) between treatments within 1 year are indicated by different letters, and the differences between different years for the same treatment are indicated by capital letters. The error bars represent ±s.e.m. Source data are provided as a Source Data file

Fig. 3

Soil chemistry and correlations with condensed tannins: a stable SOM-N pool, b condensed tannins, c Fourier-transform infrared (FTIR) polyphenolics (sums of 1512, 1421 and 1388 infrared (IR) bands), d correlation between stable SOM-N vs CT plotted for the whole set of data, e FTIR polypeptides (sums of 1668 and 1543 IR bands) (see Supplementary Figs. 5 and 6), e chitin concentration, f chitin concentration, g dissolved organic N/inorganic N (DON/IN) ratio (logarithmic values). The different mesh sizes are represented with different colours (from left to right): 1000 µm (green), 50 µm (magenta) and 1 µm (black). The given values are the means of the 24 replicates (except 9 for FTIR and chitin). More results characterizing the soil are given in Supplementary Figs. 3–6. Significant differences (P < 0.05) between treatments within 1 year are indicated by different letters, and the differences between different years for the same treatment are indicated by capital letters. The error bars represent ±s.e.m. Source data are provided as a Source Data file

Fig. 4

Framework of SOM-N transformations: organic N economy under natural conditions (1000 µm), inorganic N economy for exclusion of roots (50 µm), as well as roots and fungal hyphae (1 µm). Organic N economy (i.e. the domination of organic N) includes also build-up of stable SOM due to stabilization of organic N compounds with plant-derived tannins. Exclusion treatments (50 and 1 µm) led to an inorganic N economy (i.e. increased concentration of inorganic N forms) with reduced SOM build-up. The arrow thickness is proportional to the magnitude of the net flux. CT condensed tannins (shown as a monomer), SOM soil organic matter, DON dissolved organic N, IN inorganic N. The sizes of DON and IN are proportional to their concentrations