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Nitrogen (N) availability limits plant growth and soil carbon (C) sequestration in N‐limited ecosystems, however, plant and soil C feedback on the free‐living nitrogen fixation (FLNF) process is poorly understood. Moreover, whether this feedback is influenced by initial N availability in leguminous and non‐leguminous forest soils has not been clarified. Here, we found that the addition of plant‐ and microbial‐derived C significantly enhanced soil nitrogenase activity (13∼28%) and that microbial‐derived C had a more positive impact. These positive effects were attributed to the direct C‐energy supply (0.49∼0.84) rather than variations in soil microbial activity (−0.01∼0.21) and substrate resources (−0.45∼0.27). Long‐term N addition did not inhibit FLNF. C addition promoted FLNF in soils of the two forests, but the response rate was higher in the leguminous forest soils. Our study reveals that increased soil C availability can drive FLNF in tropical forests, enhancing our understanding of the soil C‐N coupling mechanism. Plain Language Summary: Explicit consideration of controlling factors for free‐living nitrogen fixation (an important process of nitrogen biogeochemistry), is critical to understanding the response of ecosystems to environmental change. Our study reveals a previously overlooked but important process in which increased soil carbon availability exerts positive feedback on soil nitrogen inputs via free‐living nitrogen fixation. This insight enhances our understanding of soil carbon‐nitrogen coupling mechanisms and highlights an important nitrogen source in tropical forests based on increased vegetation and soil carbon sink conditions under global environmental change scenario. Key Points: Carbon (C) input affected free‐living nitrogen fixation (FLNF) more than nitrogen (N) application in tropical forest soilsBoth plant‐ and microbial‐derived C enhanced FLNF, but microbial‐derived C had a more positive impactC addition promoted FLNF in soils of the two forests, but the response rate was higher in the leguminous forest soils [ABSTRACT FROM AUTHOR]

Highly weathered lowland (sub)tropical forests are widely recognized as nitrogen (N)‐rich and phosphorus (P)‐poor, and the input of N and P affects soil carbon (C) cycling and storage in these ecosystems. Microbial residual C (MRC) plays a crucial role in regulating soil organic C (SOC) stability in forest soils. However, the effects of long‐term N and P addition on soil MRC across different soil layers remain unclear. This study conducted a 12‐year N and P addition experiment in two typical subtropical plantation forests dominated by Acacia auriculiformis and Eucalyptus urophylla trees, respectively. We measured plant C input (fine root biomass, fine root C, and litter C), microbial community structure, enzyme activity (C/N/P‐cycling enzymes), mineral properties, and MRC. Our results showed that continuous P addition reduced MRC in the subsoil (20–40 cm) of both plantations (A. auriculiformis: 28.44% and E. urophylla: 28.29%), whereas no significant changes occurred in the topsoil (0–20 cm). N addition decreased MRC in the subsoil of E. urophylla (25.44%), but had no significant effects on A. auriculiformis. Combined N and P addition reduced MRC (34.63%) in the subsoil of A. auriculiformis but not in that of E. urophylla. The factors regulating MRC varied across soil layers. In the topsoil (0–10 cm), plant C input (the relative contributions to the total variance was 20%, hereafter) and mineral protection (47.2%) were dominant factors. In the soil layer of 10–20 cm, both microbial characteristics (41.3%) and mineral protection (32.3%) had substantial effects, whereas the deeper layer (20–40 cm) was predominantly regulated by microbial characteristics (37.9%) and mineral protection (18.8%). Understanding differential drivers of MRC across soil depth, particularly in deeper soil layers, is crucial for accurately predicting the stability and storage of SOC and its responses to chronic N enrichment and/or increased P limitation in (sub)tropical forests. [ABSTRACT FROM AUTHOR]

To investigate the influence of PAC mixture type and parameters on noise reduction performance and their variation patterns. This study investigated: the mixes' gradation characteristics parameters, the coefficient of uniformity (CU) and coefficient of curvature (CC); the nominal maximum size (DT), the material's composition parameter; the void ratio (VV) and thickness (H), the mixes' structural composition parameters, and finally, the surface density (BD). PAC specimens' sound absorption coefficient was measured indoors by the standing wave tube method, and the variation law were observed under different PAC parameters. PAC parameters' degree of influence on noise reduction performance was investigated through grey correlation theory and correlation analysis. The results indicated that increasing the void ratio and thickness improved the peak absorption coefficient and enhanced PAC's noise reduction performance effectively. Increasing the nominal maximum particle size weakened the noise reduction performance. In addition, the mix type and thickness had a greater influence on the basic trend in the absorption coefficient spectrogram. PAC's noise reduction effect was reflected primarily in the middle and high frequency band, and moved to the high frequency band gradually as the thickness increased. Overall, VV, H, and DT had a relatively large effect on PAC's noise reduction performance. [ABSTRACT FROM AUTHOR]