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Optically levitated oscillators in high vacuum have excellent environmental isolation and low mass compared with conventional solid-state sensors, which makes them suitable for ultrasensitive force detection. The force resolution usually scales with the measurement bandwidth, which represents the ultimate detection capability of the system under ideal conditions if sufficient time is provided for measurement. However, considering the stability of a real system, a method based on the Allan variance is more reliable to evaluate the actual force detection performance. In this study, a levitated optomechanical system with a force detection sensitivity of 6.33 ± 1.62 zN/Hz1/2 was demonstrated. And for the first time, the Allan variance was introduced to evaluate the system stability due to the force sensitivity fluctuations. The force detection resolution of 166.40 ± 55.48 yN was reached at the optimal measurement time of 2751 s. The system demonstrated in this work has the best force detection performance in both sensitivity and resolution that have been reported so far for optically levitated particles. The reported high-sensitivity force detection system is an excellent candidate for the exploration of new physics such as fifth force searching, high-frequency gravitational waves detection, dark matter research and so on.

【Objective】The differences in leaf nutrient content and nutrient resorption efficiency of different canopies of A. montana forest were analyzed, and the relationship between A. montana leaf nutrient resorption efficiency and nutrient contents in leaves and soil was studied.【Method】The Aleurites montana forest, planted after the coniferous forests with low production being cut down and cultivated as ecological forest conforming to the nature, was taken as test materials. The contents of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) in leaves of the upper and lower canopies of A. montana were determined in the peak vegetative period (July) and the defoliating period (October).【Result】The macronutrients N, P, K, Ca and Mg in senescent leaves were significantly lower than those in mature leaves and there was no nutrient enrichment in senescent leaves. The N contents of mature and senescent leaves in the upper canopy were significantly higher than those in the lower canopy. The K content in mature leaves of upper layer was significantly higher than that in the lower canopy. Ca resorption efficiency was significantly lower than that of other nutrients, with only 14.935 (±3.191) %. In addition, the resorption efficiency of Mg in the lower canopy was significantly higher than that in the upper canopy. There was no significant difference in the resorption efficiencies of other nutrients between upper and lower canopies. There was a significant negative correlation between K resorption efficiency and K content in mature and senescent leaves, and a significant positive correlation between Ca resorption efficiency and Ca content in mature leaves. There was a significant positive correlation between N resorption efficiency and soil hydrolyzed nitrogen. The K resorption efficiency was negatively correlated with soil total K content, but positively correlated with soil available K content.【Conclusion】The nutrient contents in senescent leaves of A. montana were significant lower than those in mature leaves, and there was a certain correlation between nutrient resorption efficiency and nutrients in leaves and soil. The study results could provide a scientific basis for further exploring the nutrient cycling mechanism of ecological public welfare forests.

The dynamic frictional behaviors of natural discontinuities (joints, fractures, faults) play an important role in geohazards assessment; however, the mechanisms of the dynamic fault weakening/strengthening are still unclear. In this paper, a dynamic shear box was used to perform direct shear tests on saw-cut (planar) and natural (rough) granite fractures, with different normal load oscillation amplitudes. Based on the recorded shear forces and normal displacements, the shear forces, apparent friction coefficients and normal displacements are found to change periodically with oscillated normal loads and are characterized by a series of time shifts. The observed changing patterns are similar for the rough and planar fractures. Compared with the test data under constant normal load (CNL), small/large normal load oscillation amplitude enhances/reduces the peak shear strength, with a critical point. The magnitude of critical normal load oscillation for the rough fractures is smaller than the planer fractures. The results imply that dynamic fault weakening/strengthening can be achieved by both normal load oscillation amplitudes and slip surface topography. The rough fractures with larger normal oscillation amplitude can easily cause frictional weakening under stress disturbance.