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Abstract Soil extractable nitrate, ammonium, and organic nitrogen (N) are essential N sources supporting primary productivity and regulating species composition of terrestrial plants. However, it remains unclear how plants utilize these N sources and how surface-earth environments regulate plant N utilization. Here, we establish a framework to analyze observational data of natural N isotopes in plants and soils globally, we quantify fractional contributions of soil nitrate (f NO3-), ammonium (f NH4+), and organic N (f EON) to plant-used N in soils. We find that mean annual temperature (MAT), not mean annual precipitation or atmospheric N deposition, regulates global variations of f NO3-, f NH4+, and f EON. The f NO3- increases with MAT, reaching 46% at 28.5 °C. The f NH4+ also increases with MAT, achieving a maximum of 46% at 14.4 °C, showing a decline as temperatures further increase. Meanwhile, the f EON gradually decreases with MAT, stabilizing at about 20% when the MAT exceeds 15 °C. These results clarify global plant N-use patterns and reveal temperature rather than human N loading as a key regulator, which should be considered in evaluating influences of global changes on terrestrial ecosystems.

Abstract Understanding the mechanisms controlling forest carbon accumulation is crucial for predicting and mitigating future climate change. Yet, it remains unclear whether the dominance of ectomycorrhizal (EcM) trees influences the carbon accumulation of entire forests. In this study, we analyzed forest inventory data from over 4000 forest plots across Northeast China. We find that EcM tree dominance consistently exerts a positive effect on tree, soil, and forest carbon stocks. Moreover, we observe that these positive effects are more pronounced during unfavorable climate conditions, at lower tree species richness, and during early successional stages. This underscores the potential of increasing the dominance of native EcM tree species not only to enhance carbon stocks but also to bolster resilience against climate change in high-latitude forests. Here we show that forest managers can make informed decisions to optimize carbon accumulation by considering various factors such as mycorrhizal types, climate, successional stages, and species richness.

The resorption of nutrients by plants before litter fall and the mineralization of nutrients from plant litter by soil processes are both important pathways supporting primary productivity. While the positive relationship between plant biodiversity and primary productivity is widely accepted for natural ecosystems, the roles of nutrient resorption and mineralization in mediating that relationship remains largely unknown. Here, we quantified the relative importance of nitrogen (N) resorption and N mineralization in driving plant community N investment and the correlation between species diversity and community productivity along an N-limited successional chronosequence of the mixed broadleaved–Korean pine (Pinus koraiensis) forest in northeastern China. Leaf N resorption efficiency (NRE) at the community level increased significantly along the successional chronosequence, whereas litter N mineralization rate decreased significantly. Leaf NRE was more important than litter N mineralization rate in driving the diversity–productivity relationship. However, higher leaf NRE led to less N mineralization as succession progressed along the chronosequence. Our results highlight the importance of the N resorption pathway rather than the N mineralization pathway for forest N acquisition with community succession, and they provide mechanistic insights into the positive effects of biodiversity on ecosystem functioning. In future forest management practices, we recommend appropriate application of N fertilizer to mitigate the adverse effects of N-poor soil on seedling regeneration during late succession and thus maintain the sustainable development of temperate forest ecosystems.

Identifying the thresholds for the positive responses of total net primary productivity (NPP) to nitrogen (N) enrichment is an essential prerequisite for predicting the benefits of N deposition on ecosystem carbon sequestration. However, the responses of below-ground NPP (BNPP) to N enrichment are unknown in many ecosystems, which limits our ability to understand the carbon cycling under the scenario of increasing N availability. We examined the changes in above-ground NPP (ANPP), BNPP, and NPP of a temperate meadow steppe across a wide-ranging N addition gradient (0, 2, 5, 10, 20, and 50 g N m-2 year-1) during 5 years. Both ANPP and NPP increased nonlinearly with N addition rates. The N saturation threshold for ANPP (TA) and NPP (TN) was at the rate of 13.11 and 6.70 g N m-2 year-1, respectively. BNPP decreased with increasing N addition when N addition rates 5 g N m-2 year-1, resulting in much lower TN than TA. Soil N enrichment played a key role in driving the negative impacts of high N addition rates on BNPP, and consequently on the earlier occurrence of N saturation threshold for NPP. Our results highlight the negative effects of soil N enrichment on NPP in natural grasslands super-saturated with N. Furthermore, by considering ANPP and BNPP simultaneously, our results indicate that previous findings from above-ground might have over-estimated the positive effects of N deposition on primary productivity. [ABSTRACT FROM AUTHOR]

Cold spray (CS) has emerged as a representative of solid-state additive manufacturing (AM) via supersonic impact. It enables a high deposition rate of solid-state microparticles. Delamination, however, tends to occur when depositing too thick; this remains to be conquered. Here, a CS-like process, warm spray (WS), was presented. Interestingly, it was found that the appropriate increase in particle temperature can effectively reduce the residual stress amplitude, relieving the concentrated tensile stress and safeguarding the additively manufactured components from interfacial delamination even when depositing too thick. The key role of temperature on delamination was identified in solid-state AM via supersonic impact.

Abstract A dense NiAl coating with low oxide content was deposited on the GH4202 superalloy substrate by atmospheric plasma spraying (APS) using carbon-containing Ni/Al composite powders. Compared with traditional APS NiAl coatings with high oxide content, this coating exhibits better long-term oxidation resistance at 900 °C in air. For the NiAl coating with low oxide content, it was found that the improved inter-splat bonding, due to limited oxide inclusions, promotes the formation of a continuous protective Al2O3 layer on the coating surface during oxidation. During long-term oxidation, the dispersion of oxide scales within the coating and element diffusion across splats result in the in-situ healing of unbonded inter-splat interfaces.

Large degradation in thermal insulation and strain tolerance is a main headache and a primary cause of the failure for plasma-sprayed thermal barrier coatings (TBCs) during service. One mechanism behind such degradation is the healing of interlamellar pores formed by multiple connections between edges of a pore, which significantly speeds up healing during thermal exposure. The objective of this study is to obtain sintering-resistant TBCs by tailoring the width of interlamellar pores to avoid multiple connections. Firstly, the mechanism responsible for the multiple connections was revealed. The splat surfaces before and after thermal treatments were characterized via an atomic force microscope (AFM). The roughening of the pore surface occurs during thermal exposure, along with the grain growth inside the splats. Consequently, the local surface height increases, which causes multiple connections and healing of the interlamellar pores. Secondly, critical widths of the interlamellar pores for avoiding the multiple connections during thermal exposure are established by correlating the extent of surface roughening with the growth of individual grains. The height increase of the splat surface and the growth of the grain size (D) were found to increase with the exposure temperature and duration. A relationship linking the height increase and the growth of the grain size induced by thermal exposure in plasma-sprayed ceramic splats was obtained. Finally, composite TBCs were prepared to form wide interlamellar pores in the coatings. Using this design, the increases in the thermal conductivity (λ) and the elastic modulus (E) can be prevented to a large extent. Thus, sintering-resistant TBCs that maintain high thermal insulation and strain tolerance, even after long thermal exposure, can be created.

Cold spray (CS) is a highly potential solid-state additive manufacturing (AM) technique. In situ shot-peening-assisted CSAM was proposed to additively manufacture fully dense deposits using cost-effective and renewable nitrogen gas. The role of in situ shot-peening particles is critical but remains unclear. Here, the process was quantitatively modeled to visualize the dynamic deformation, energy conversion, as well as cell/sub-grain size and microhardness evolutions, compared to those during the conventional CSAM process, identifying the key role of in situ shot-peening particles in the AA6061 extreme deformation and microstructure characteristics during in situ shot-peening-assisted CSAM. High-fidelity modeling was verified fully by comparing the experimental and model-reproduced deformation profiles, cell/sub-grain size distributions, and increases in microhardness. The results show that the kinetic energy of in situ shot-peening particles was 470 times higher and dissipated mainly through AA6061 plastic deformation (86.36% of total energy), leading to significant enhancement of microhardness and tensile strength. Moreover, the mixing ratio of large-size SS410 particles required to create a fully dense deposit was evaluated from an energy perspective, in good agreement with the experiment. This study elucidates the microforging principle during in situ shot-peening-assisted CSAM, providing scientific guidelines for high-quality and low-cost CSAM of high-strength aluminum alloys.

Atmospheric plasma sprayed metallic coatings always contain oxides because of the entrainment of ambient air into plasma jet, which significantly limits their performances and applications. This paper presents an approach to deposit highly dense Ni-based alloy coatings with oxide-free particles achieved by introducing boron as deoxidizer element through plasma spraying to generate ultrahigh temperature Ni20Cr4B droplets. During spraying, boron is preferentially oxidized and the formed B2O3 is completely removed due to rapid evaporation. Thus, the in-flight oxidation of metal can be suppressed to achieve oxide-free particles. The oxygen content of plasma sprayed Ni20Cr4B coating is lower than 0.18 wt.% and presents little change over a wide spray distance range from 60 mm to 140 mm. With such a low oxygen content, the Ni20Cr4B coating presents a highly dense microstructure (porosity of 0.21 ± 0.11%) and well-bonded inter-splat interfaces. The hot-corrosion test at 600 °C for 200 h shows that the highly dense Ni20Cr4B coating can effectively block the penetration of the corrosion media and the corrosion only takes place at the coating surface, while severe inner corrosion was observed for the traditional Ni20Cr coating with poor inter-splat bonding.