Tingting Ren

Hydrochar application to soil inevitably releases hydrochar-derived dissolved organic matter (HDOM), yet its specific impact on soil microbial communities, independent of the hydrochar solid matrix, remains poorly understood. This study investigated, for the first time, the dose-dependent effects of HDOM on bacterial communities in three distinct soil types (red, yellow-brown, and black soils). A concentration gradient, including undiluted stock solution and 10-, 100-, and 1000-fold dilutions with ultrapure water, was established to test for hormesis-like responses. High-throughput 16S rRNA gene sequencing revealed that HDOM induced profound, soil-specific shifts in bacterial community structure. The application of HDOM induced the emergence of numerous specific bacterial taxa, with unique ASVs reaching up to 15,372. However, no significant changes were observed in microbial community richness or evenness (alpha diversity). Drastic shifts in beta diversity were evident only in red soil and yellow-brown soil, and exclusively under the undiluted HDOM treatment. At the phylum level, HDOM application did not alter the dominant bacterial types (top 10 phyla); however, their relative abundances were jointly regulated by both HDOM dose and soil type. Significant HDOM-induced changes in key bacterial biomarkers were primarily detected in red soil (e.g., phylum Elusimicrobia, class Fimbriimonadia, and family Alicyclobacillaceae) and yellow-brown soil (e.g., phylum Proteobacteria, class Alphaproteobacteria, and family Rhizobiaceae), while in black soil, such changes were observed only under the undiluted HDOM treatment (e.g., species Streptomyces rochei). Predictive functional profiling suggested limited impact on major metabolic pathways, with soil type remaining the primary determinant. These findings demonstrate that HDOM exerts a direct, dose-dependent, and soil-specific influence on bacterial communities, providing key insights into the environmental behavior of hydrochar and guiding its safe application.

Earthworms play a dual role in the global carbon cycle: they accelerate organic matter decomposition yet are often associated with greater soil organic carbon (SOC) storage. However, uncertainty regarding the mechanisms and magnitudes through which earthworms concurrently influence SOC mineralization and stabilization has limited the integration of soil fauna into carbon models. Here, we synthesize 696 paired observations from 122 studies worldwide to resolve this uncertainty. On average, earthworms increase SOC by 5.4% (95% CI: 2.2%–9.1%), with effects strengthening over time under sustained plant‐derived carbon inputs. Earthworms enhance mineral‐associated organic carbon (MAOC) by 21.2%, while particulate organic carbon (POC) remains unchanged. These patterns suggest that earthworm activity promotes a transition from short‐term carbon mineralization to long‐term stabilization, likely mediated by the coupling of microbial processing and physical protection. Specifically, epigeic earthworms boost microbial biomass carbon, whereas endogeic species enhance macroaggregate formation, facilitating the incorporation of microbial necromass into MAOC. The magnitude and direction of these effects depend on sustained carbon inputs and earthworm functional type. Collectively, these results reconcile decades of conflicting evidence and provide the first quantitative global synthesis showing that earthworms increased soil carbon over time under sustained plant carbon inputs. This microbial–mineral formation pathway has direct implications for climate‐smart land management, soil biodiversity conservation, and the representation of earthworm bioturbation in global carbon models.

Long-term nitrogen (N) addition induces N saturation in forests, but its impacts on nutrient allocation across plant organs and life forms remain unclear. Utilizing a 12-year N addition experiment in a coastal poplar plantation in southeastern China, which included five treatments (0 [CK], 50 [N₁], 100 [N₂], 150 [N₃], and 300 [N₄] kg N ha⁻¹ yr⁻¹), we examined N, phosphorus (P), and potassium (K) concentrations in leaves and roots of overstory poplar trees and understory herbs in 2024. Results showed understory leaf N increased by 14.9–31.4% in vigorous growth, while overstory leaf N rose only 7.0% under the highest N treatment. By late growth, N₃ treatment understory herbs had 17.2% and 21.9% higher leaf and root N, respectively. In contrast, overstory root N decreased by 15.7% under N₁, but root P increased by 55.4% under N₂. These findings reveal that understory herbs preferentially allocate nutrients to leaves, whereas overstory poplar trees shift allocation to roots under elevated N. Such divergent strategies may reshape interlayer nutrient competition, thereby affecting forest structure and function and providing theoretical and practical guidance for predicting N addition responses and plantation management.

As key detritivores and fungal grazers, terrestrial isopods (Isopoda: Oniscidea) play crucial roles in mediating ecosystem processes. Although nitrogen enrichment represents a major global change driver known to modify soil food webs, its long‐term effects on the abundance of these keystone detritivores remain largely unknown. For this study, we conducted a 10‐year nitrogen enrichment experiment to monitor the active density and biomass of terrestrial isopods across eight seasons over 2 years during 2020 to 2022 in 13‐ and 17‐year‐old poplar plantations, respectively. Our results revealed that nitrogen enrichment increased the abundance and biomass of terrestrial isopods. For nitrogen enrichment levels of 5, 10, 15 and 30 g N m ⁻² year ⁻¹ , the corresponding increases in isopods active density were estimated to be 11.6%, 24.5%, 38.9% and 92.9% higher, respectively, relative to the ambient N level. Furthermore, nitrogen enrichment did not alter the carbon to nitrogen ratio (C:N) of the body of isopods, but reduced the C:N of poplar leaf litter, understorey plants and detritus. Nitrogen enrichment also reduced the understorey plant diversity and altered understorey plant species composition. The structural equation models revealed that the responses of isopod abundance to nitrogen enrichment were always inversely related to the C:N of the understorey community, while showing no correlation with poplar leaf litter quality. This study reveals a new mechanism by which nitrogen enrichment promotes soil invertebrates by affecting the quality of the understorey rather than the overstory. Our results suggest that long‐term nitrogen enrichment affects soil detritivores through seasonally dependent pathways mediated by the understorey plant community. This reveals a previously unrecognized mechanism through which nitrogen deposition influences soil food web structure. Read the free Plain Language Summary for this article on the Journal blog.

Management practices are expected to influence the capacity of forests to mitigate climate change. However, the long‐term effects of afforestation on soil carbon accumulation in response to contrasting management regimes remain poorly understood. Here, we combined organic matter fractionation with a nine‐year‐long organic fertilization experiment to investigate the influences of largely accepted practices such as biochar (BC) and biogas‐slurry (BS) inputs on the accumulation of soil particulate organic carbon (POC) and mineral‐associated organic carbon (MAOC) in three soil horizon depths (0–25, 25–50, and 50–75 cm, respectively). Our results suggested that both BS and BC significantly enhanced the POC and total soil organic carbon (SOC) content but overall did not significantly influence MAOC. Moreover, the POC and MAOC was more responsive to BS than BC. Further, our analyses revealed that the effects of BC on POC and MAOC were indirectly regulated by changes in the SOC: total nitrogen (TN) ratio, while BS influenced POC and MAOC by regulating TN. However, the responses of MAOC to the infiltration of organic fertilizers into the mineral soil should not be ignored, especially under high BC levels. Our work revealed that management practices are critical for supporting the long‐term capacity of new forests to accumulate soil carbon, thereby facilitating the provision of nature‐based solutions in response to climate change.