Tao Wang

Large amounts of dissolved organic carbon (DOC) are transported laterally from uplands and wetlands to headwater streams; however, the global magnitude of this flux and its role in the carbon (C) budget remain unclear. By compiling 20,403 DOC concentration observations and applying machine learning models, we estimated the annual DOC flux from forested headwater streams north of 30°S to be 116.2 Tg C yr ⁻¹ (90% CI: 86.5–145.2). Higher yields were observed in tropical regions (3.8 g C m ⁻² yr ⁻¹ ), where vegetation productivity is high, and in boreal regions (3.1 g C m ⁻² yr ⁻¹ ), where peatland cover and soil organic C stocks are substantial. When comparing our results with global terrestrial C flux estimates upscaled from eddy covariance data, we found that the fraction of net ecosystem production (NEP) lost annually as DOC ranged from negligible (<0.1%) to 20.7%, with discharge explaining 34% of the variation. Our study suggests that neglecting the lateral export of DOC could lead to an overestimation of NEP in forested headwater catchments, a bias that is further amplified by increased discharge.

Plant roots and their mycorrhizal symbionts regulate soil organic carbon (SOC) dynamics in tropical forests, where succession‐driven shifts in mycorrhizal associations alter carbon allocation patterns. However, the relative contributions of roots versus extraradical mycorrhizal hyphae to SOC accumulation and decomposition across forest succession remain unclear. Using a 2‐year isotopic ingrowth‐core field experiment along a tropical forest chronosequence (70–400 years), we quantified the roles of roots and extraradical hyphae in new SOC accumulation and priming effects. After 2 years, roots contributed nearly four times more to SOC accumulation than hyphae across tropical forest succession. Roots enhanced new SOC accumulation while suppressing the positive priming effect on native SOC, leading to a net gain of 1.6 mg C g ⁻¹ soil yr. ⁻¹ , predominantly as particulate organic carbon. In contrast, extraradical hyphae had a marginal effect, contributing only 0.4 mg C g ⁻¹ soil yr. ⁻¹ . Root‐driven SOC input peaked in early succession, whereas hyphal contributions remained consistent. Our findings highlight the dominant role of roots—rather than mycorrhizal hyphae—in SOC accumulation and stabilization during tropical forest succession, emphasizing the need to incorporate root traits into management strategies to enhance SOC sequestration, particularly in tropical forests under increasing land‐use pressures.

Effects of global warming on soil organic carbon (C) can be investigated by comparing sites experiencing different temperatures. However, observations can be affected by covariance of temperature with other environmental properties. Here, we studied a thermal gradient in forest soils derived from volcanic materials on Mount Taranaki (New Zealand) to disentangle the effects of temperature and reactive minerals on soil organic C quantity and composition. We collected soils at four depths and four elevations with mean annual temperatures ranging from 7.3 to 10.5 °C. Soil C stocks were not significantly different across sites (average 162 MgC ha⁻¹ to 85 cm depth, P > .05). Neither aluminium (Al)-complexed C, nor mineral-associated C changed significantly (P > .05) with temperature. The molecular characterisation of soil organic matter showed that plant-derived C declined with increasing temperature, while microbial-processed C increased. Accompanying these changes, soil short-range order (SRO) constituents (including allophane) generally increased with temperature. Results from structural equation modelling revealed that, although a warmer temperature tended to accelerate soil organic C decomposition as inferred from molecular fingerprints, it also exerted a positive effect on soil total C presumably by enhancing plant C input. Despite a close linkage between mineral-associated C and soil organic C, the increased abundance of reactive minerals at 30–85 cm depth with temperature did not increase soil organic C concentration at that depth. We therefore propose that fresh C inputs, rather than reactive minerals, mediate soil C responses to temperature across the thermal gradient of volcanic soils under humid-temperate climatic conditions.

A major uncertainty in the estimation of soils acting as net carbon (C) sinks or sources stem from the effects of anthropogenic nitrogen (N) input on the balance between plant C input and soil C loss. In contrast to the generally observed increasing pattern of plant C input, the response of soil C loss to increased N deposition remains elusive, largely due to its large temporal variation. Here simultaneous measurements of two major soil C loss pathways, including dissolved organic carbon (DOC) leaching and soil respiration, were conducted for 5 and 3 yr, respectively, to assess the effects of N addition on soil C loss in an N‐limited montane forest. The effects were seasonal, depth and N level dependent and the two pathways responded asynchronously to N addition. Significant decreases in DOC concentrations and fluxes in leachates from the organic layer were observed during autumn/winter under a high N addition rate (40 kg N/ha/yr). No significant impact of N addition on DOC concentrations or fluxes was observed for leachates from the mineral soil horizon. Biodegradability was low for DOC from both soil layers and was not consistently influenced by N addition. Soil respiration was significantly decreased under high N addition. Annual soil C loss (estimated by summing DOC leaching from the mineral horizon and soil respiration) showed that N addition reduced soil C loss consistently over years, implying that the forest soil is likely a C sink under excess N deposition, which should be confirmed with longer term monitoring.

Summary: A large fraction of nitrogen (N) in forest soils is present in mineral‐associated proteinaceous compounds. The strong association between proteins and minerals limits microbial accessibility to this source, which is a relatively stable reservoir of soil N. We have shown that the ectomycorrhizal (ECM) fungus Paxillus involutus can acquire N from iron oxide‐associated proteins. Using tightly controlled isotopic, spectroscopic and chromatographic experiments, we demonstrated that the capacity to access N from iron oxide‐associated bovine serum albumin (BSA) is shared with the ECM fungi Hebeloma cylindrosporum and Piloderma olivaceum . Despite differences in evolutionary history, growth rates, exploration types and the decomposition mechanisms of organic matter, their N acquisition mechanisms were similar to those described for P. involutus . The fungi released N from mineral‐associated BSA by direct action of extracellular aspartic proteases on the mineral‐associated BSA, without initial desorption of the protein. Hydrolysis was suppressed by the adsorption of proteases to minerals, but this adverse effect was counteracted by the secretion of compounds that conditioned the mineral surface. These data suggest that the enzymatic exudate‐driven mechanism to access N from mineral‐associated proteins is found in ECM fungi of multiple lineages and exploration types.

The mechanisms by which lime and/or phosphate addition impacts the preservation of soil organic matter (OM) are poorly understood. We explored the changes in quantity and chemistry of water-extractable organic matter (WEOM) in the bulk soil and its heavy density fraction (>1.6 g/cm³) of an unmanaged C-rich volcanic soil caused by lime and/or phosphate application. The addition of lime or phosphate caused (i) a significant increase in the WEOM, along with a decrease in its C/N ratio and an increase in its aromaticity, and (ii) changes in the WEOM chemical composition, measured with pyrolysis-gas chromatography/mass spectrometry, this being most impacted by lime application. The combined effect of lime and phosphate addition on the quantity and chemistry of WEOM was larger than the effects of separate lime and phosphate additions. By comparing the response of the bulk soil and the heavy fraction, we infer that phosphate has a greater contribution to the destabilization of vulnerable particulate OM, while lime causes a comparable disruption in the particulate OM and that in the heavy fraction. These findings provide a mechanistic insight into the decreased OM stability after liming and/or P fertilizing Andosols. They have implications for designing climate-smart management practices for these soils.