Increase in fine root biomass enhances root exudation by long-term soil warming in a temperate forest

Abstract

Trees can invest up to one-third of the carbon (C) fixed by photosynthesis into belowground allocation, including fine root exudation into the rhizosphere. It is still unclear how climate and soil warming affect tree root C exudation, in particular quantifying longer-term warming effects remains a challenge. In this study, using a C-free cuvette incubation method, in situ C exudation rates from tree fine roots of a mature spruce dominated temperate forest were measured in regular intervals during the 14th and 15th year of experimental soil warming (+ 4°C). In addition, a short-term temperature sensitivity experiment (up to + 10°C warming within 4 days) was conducted to determine the inherent temperature sensitivity of root exudation. Root exudation rates in the long-term warmed soil (17.9 μg C g–1 root biomass h–1) did not differ from those in untreated soil (16.2 μg C g–1 root biomass h–1). However, a clear increase (Q10 ∼5.0) during the short-term temperature sensitivity experiment suggested that fine root exudation can be affected by short-term changes in soil temperature. The absence of response in long-term warmed soils suggests a downregulation of C exudation from the individual fine roots in the warmed soils. The lack of any relationship between exudation rates and the seasonal temperature course, further suggests that plant phenology and plant C allocation dynamics have more influence on seasonal changes in fine root C exudation. Although exudation rates per g dry mass of fine roots were only marginally higher in the warmed soil, total fine root C exudation per m2 soil surface area increased by ∼30% from 0.33 to 0.43 Mg C ha–1 yr–1 because long-term soil warming has led to an increase in total fine root biomass. Mineralization of additional fine root exudates could have added to the sustained increase in soil CO2 efflux from the warmed forest soil at the experimental site.

Long‐term soil warming decreases soil microbial necromass carbon by adversely affecting its production and decomposition

“…For example, the turnover rate of the amino sugars was found to be faster under elevated CO 2 (Glaser et al, 2006). Secondly, our previous studies have confirmed enhanced plant root C input in warming plots, due to increased root growth/turnover and root exudation (Heinzle et al, 2023;Kengdo et al, 2022Kengdo et al, , 2023, indicating a shift in the balance of plant-versus microbial-derived SOM under climate warming.…”

“…(2022) and 2 Heinzle et al. (2023). Similarly, SOC content data is from studies conducted at the same site: 3 Tian, Schindlbacher, et al.…”

“…Thus, whether lower pH levels in the warming plots (Tian, Schindlbacher, et al., 2023) can potentially enhance the desorption of mineral‐associated MNC remains to be demonstrated. In addition, increased root biomass triggered by soil warming may enhance MNC decomposition through greater root activity and a 30% rise in root exudation, especially organic acids that can release carbon from mineral‐bound MNC (Heinzle et al., 2023; Keiluweit et al., 2015; Kengdo et al., 2022, 2023). Organic acid exudation potentially accelerates MNC breakdown by stimulating rhizosphere priming processes (Buckeridge, Creamer, & Whitaker, 2022; Kästner et al., 2021; Keiluweit et al., 2015).…”

“…Our specific objectives were (1) to investigate how soil warming impacts the content of soil MNC and (2) to identify the key factors and processes that underlie the observed changes in MNC content. Our previous work found that long‐term soil warming increased belowground plant C inputs (Heinzle et al., 2023; Kengdo et al., 2023) but decreased SOC content through increased EEA (Tian, Schindlbacher, et al., 2023) and decreased the soil P pools (Tian, Shi, et al., 2023). Therefore, we hypothesized that long‐term warming reduced MNC as a matter of decreased soil P and soil microbial biomass and that warming promotes the decomposition of MNC by increased soil EEA.…”

Canopy and understory nitrogen additions differently affect soil microbial residual carbon in a temperate forest

“…Elevated N deposition alters soil N availability for plants and microbes (Stevens et al., 2018; Figure 1). Because N is the primary limiting factor of plant productivity in terrestrial ecosystems, N promotion of plant growth likely enhances organic C for microorganisms that also transform more plant‐derived C into microbial residues (Ataka et al., 2020; Heinzle et al., 2023; Wang et al., 2022). Furthermore, N input can modify the composition of the soil microbial community, altering soil organic matter decomposition (Morrison et al., 2016).…”