Effects of cotton field management practices on soil CO2 emission and C balance in an arid region of Northwest China

Changes in both soil organic C storage and soil respiration in farmland ecosystems may affect atmospheric CO2 concentration and global C cycle. The objective of this field experiment was to study the effects of three crop field management practices on soil CO2 emission and C balance in a cotton field in an arid region of Northwest China. The three management practices were irrigation methods（drip and flood）, stubble managements（stubble-incorporated and stubble-removed） and fertilizer amendments（no fertilizer（CK）, chicken manure（OM）, inorganic N, P and K fertilizer（NPK）, and inorganic fertilizer plus chicken manure（NPK＋OM））. The results showed that within the C pool range, soil CO2 emission during the whole growing season was higher in the drip irrigation treatment than in the corresponding flood irrigation treatment, while soil organic C concentration was larger in the flood irrigation treatment than in the corresponding drip irrigation treatment. Furthermore, soil CO2 emission and organic C concentration were all higher in the stubble-incorporated treatment than in the corresponding stubble-removed treatment, and larger in the NPK＋OM treatment than in the other three fertilizer amendments within the C pool range. The combination of flood irrigation, stubble incorporation and application of either NPK＋OM or OM increased soil organic C concentration in the 0-60 cm soil depth. Calculation of net ecosystem productivity（NEP） under different management practices indicated that the combination of drip irrigation, stubble incorporation and NPK＋OM increased the size of the C pool most, followed by the combination of drip irrigation, stubble incorporation and NPK. In conclusion, management practices have significant impacts on soil CO2 emission, organic C concentration and C balance in cotton fields. Consequently, appropriate management practices, such as the combination of drip irrigation, stubble incorporation, and either NPK＋OM or NPK could increase soil C storage in cotton fields of Northwest China.

Integrated contra‑directionally coupled chirped Bragg grating waveguide with a linear group delay spectrum

Due to the advantages of low propagation loss,wide operation bandwidth,continuous delay tuning,fast tuning speed,and compact footprints,chirped Bragg grating waveguide has great application potential in wideband phased array beamforming systems.However,the disadvantage of large group delay error hinders their practical applications.The nonlinear group delay spectrum is one of the main factors causing large group delay errors.To solve this problem,waveguides with nonlinear gradient widths are adopted in this study to compensate for the nonlinear efect of the grating apodization on the mode efective index.As a result,a linear group delay spectrum is obtained in the experiment,and the group delay error is halved.

Predicting community traits along an alpine grassland transect using field imaging spectroscopy

Assessing plant community traits is important for understanding how terrestrial ecosystems respond and adapt to global climate change.Field hyperspectral remote sensing is effective for quantitatively estimating vegetation properties in most terrestrial ecosystems,although it remains to be tested in areas with dwarf and sparse vegetation,such as the Tibetan Plateau.We measured canopy reflectance in the Tibetan Plateau using a handheld imaging spectrometer and conducted plant community investigations along an alpine grassland transect.We estimated community structural and functional traits,as well as community function based on a field survey and laboratory analysis using 14 spectral vegetation indices(VIs)derived from hyperspectral images.We quantified the contributions of environmental drivers,VIs,and community traits to community function by structural equation modelling(SEM).Univariate linear regression analysis showed that plant community traits are best predicted by the normalized difference vegetation index,enhanced vegetation index,and simple ratio.Structural equation modelling showed that VIs and community traits positively affected community function,whereas environmental drivers and specific leaf area had the opposite effect.Additionally,VIs integrated with environmental drivers were indirectly linked to community function by characterizing the variations in community structural and functional traits.This study demonstrates that community-level spectral reflectance will help scale plant trait information measured at the leaf level to larger-scale ecological processes.Field imaging spectroscopy represents a promising tool to predict the responses of alpine grassland communities to climate change.

A broadband integrated microwave photonic mixer based on balanced photodetection

An integrated microwave photonic mixer based on silicon photonic platforms is proposed,which consist of a dual-drive Mach–Zehnder modulator and a balanced photodetector.The modulated optical signals from microwave photonic links can be directly demodulated and down-converted to intermediate frequency(IF)signals by the photonic mixer.The converted signal is obtained by conducting of-chip subtraction of the outputs from the balanced photodetector,and subsequent fltering of the high frequency items by an electrical low-pass flter.Benefting from balanced detection,the conversion gain of the IF signal is improved by 6 dB,and radio frequency leakage and common-mode noise are suppressed signifcantly.System-level simulations show that the frequency mixing system has a spurious-free dynamic range of 89 dB·Hz^(2/3),even with deteriorated linearity caused by the two cascaded modulators.The spur suppression ratio of the photonic mixer remains higher than 40 dB when the IF varies from 0.5 to 4 GHz.The electrical-electrical 3 dB bandwidth of frequency conversion is 11 GHz.The integrated frequency mixing approach is quite simple,requiring no extra optical flters or electrical 90°hybrid coupler,which makes the system more stable and with broader bandwidth so that it can meet the potential demand in practical applications.

Statistical characteristics of forest litterfall in China

Dear Editor, Litterfall represents a major pathway of carbon and mineral transfer from vegetation to soils, and thus is an important eco- logical process in carbon and nutrient cycling. Annual litter- fall production also influences the amount of fuel accumu- lated in the forests, which serves as a part of an indicator for risk from forest fire. China covers a broad climatic gradi- ent and contains almost all major forest types of the north- ern hemisphere from boreal to tropical forests. However, few synthesis studies on the production of litterfall at a national scale in Chinese forests have been reported so far. This study attempted to characterize the integrated forest litterfall dataset of China （N=904, 1970-2014, Jia et al., 2016） to provide es- sential information required by regional biogeochemical cy- cle and fire risk models.