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Soil organic matter formation and loss are mediated by root exudates in a temperate forest

Abstract

The amount and composition of root exudates—low-molecular-weight carbon compounds released from living plant roots into soil—are expected to shift under global change, and a growing body of work indicates that root exudates have important impacts on stable soil organic matter dynamics. However, most research on exudate effects on soil organic matter uses highly homogenized or artificial soil, leaving major uncertainties in how exudates will influence carbon dynamics in natural, intact soil systems. We used 13C-labelled artificial root exudates to examine the effects of exudation rate and type on stable soil organic matter formation and loss in intact forest soil cores collected over a heterogeneous gradient in a temperate hardwood forest. We observed effects of different exudate treatments on stable soil carbon dynamics that overrode native soil heterogeneity, and higher exudation rates enhanced mineral-associated organic matter turnover but not accumulation. Organic and amino acid exudates led to net mineral-associated organic matter accumulation, with amino acids having particularly strong positive effects on microbial biomass. Simple sugars increased mineral-associated organic matter turnover (both formation and loss) but did not alter the size of this pool. Our results suggest that predicted increases in root exudation rates and compositional shifts towards simple sugars under global change may reduce soils’ C storage capacity.

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Fig. 1: Effects of exudation rate on MAOM dynamics.
Fig. 2: Effects of exudate compound type on MAOM dynamics.
Fig. 3: Interactive effects of exudate compound type and rate on soil variables.
Fig. 4: Conceptual framework explaining possible roles of exudate compound type and rate in regulating MAOM dynamics.

Data availability

Data underlying this study can be found at https://doi.org/10.6084/m9.figshare.21221288 and metadata at https://doi.org/10.6084/m9.figshare.21221294.

Code availability

Code for statistics and figure generation can be found at https://doi.org/10.6084/m9.figshare.21221300.

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Acknowledgements

We thank C. Berlingeri and A. Aguilar for help collecting and processing samples, S. Frey and M. Knorr for facilitating our work at the field site and C. Heslop for statistics advice. Thanks to A. Becker (B & B Fabrications) for the help on designing exudate delivery pumps and B. Erkkila (YASIC) for analytical advice. We also thank the Harvard Forest LTER and specifically A. Barker Plotkin and J. Thompson for their ongoing support with this project. N.R.C. was supported by the Harvard Forest LTER Graduate Student Research Award. The land Harvard Forest occupies is the unceded home territory of the Nipmuc People. Centuries of Indigenous stewardship and knowledge helped shape the ecosystems we study today, and we honour the vital role of Nipmuc community members in shaping these ecosystems into the future. This honouring is an active process that includes co-developing questions and knowledges, promoting Indigenous community self-determination and continuing to build a reciprocal relationship with the Nipmuc that ensures that this land and its life-giving benefits are mutually accessible, affirming and sustaining.

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N.R.C. and B.N.T. conceived and designed the project. N.R.C. conducted the experiments and analysed the data. N.R.C. and B.N.T. wrote the manuscript.

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Correspondence to Nikhil R. Chari or Benton N. Taylor.

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Nature Geoscience thanks Ashley Keiser and Matthew Craig for their contribution to the peer review of this work. Primary Handling Editor: Xujia Jiang, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Variation in MAOM % C of replicate cores across the sampling transect.

Each point represents a single replicate core, which were analyzed immediately (1 day) after sampling and did not receive any exudate manipulations. Thus, these data represent natural variation in MAOM % C across our sampling transect. We note there is fourfold variation in MAOM % C in samples along this gradient. Here, we maintain this heterogeneity in contrast to previous ARE experiments in homogenized soils.

Extended Data Fig. 2 Change in POM C between experimental and replicate samples (ΔPOM) by treatment.

All treatments had mean negative ΔPOM, indicating POM loss over the course of the experiment. There were no treatment differences from the control under interactive or individual effects of type and rate (ANOVA; n = 10, n = 19 for control), which may be due to greater heterogeneity in POM than MAOM pools. Center lines for each box represent the median and the upper and lower limits of the box represent the interquartile range. Whiskers represent lower and upper range limits (excluding outliers), and outliers are indicated with bold points.

Extended Data Fig. 3 Treatment effects on pore-water metals.

Interactive effects of exudate compound type and rate (two-way ANOVA; n = 10, n = 19 for control) on a) pore-water Al (pct-Glc high = 0.05) and b) pore- water Fe. Asterisks (*) indicate treatments which are significantly different from the control. Center lines for each box represent the median and the upper and lower limits of the box represent the interquartile range. Whiskers represent lower and upper range limits (excluding outliers), and outliers are indicated with bold points.

Extended Data Fig. 4 Change in MAOM N (ΔMAOM N) between experimental and replicate cores.

Asterisks (*) indicate treatments which are significantly different from the control (two-way ANOVA; n = 10, n = 19 for control; pct-AA high = 0.005). When ΔMAOM N was modelled as a function of exudate type alone, AA treatments had significantly higher ΔMAOM N than control (p ≤ 0.001). Center lines for each box represent the median and the upper and lower limits of the box represent the interquartile range. Whiskers represent lower and upper range limits (excluding outliers), and outliers are indicated with bold points.

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Chari, N.R., Taylor, B.N. Soil organic matter formation and loss are mediated by root exudates in a temperate forest. Nat. Geosci. 15, 1011–1016 (2022). https://doi.org/10.1038/s41561-022-01079-x

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