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This paper describes the L-band airborne Synthetic Aperture Radar (SAR) system and an experimental study of repeat-pass SAR interferometry. The discussions mainly focus on the internal calibration method, the imaging method based on Differential Global Positioning System/Inertial Measurement Unit (DGPS/IMU) data, Residual Motion Error (RME) estimation and compensation, and the Digital Elevation Model (DEM) inversion method with Interferometric SAR (InSAR). Two typical test areas were chosen from the acquired SAR data for detailed analyses. The DEMs in these two areas were inversed with repeat-pass InSAR, and comparisons with the measured ground points show errors in these two areas with a standard deviation 3.40 m and 2.85 m, respectively. This demonstrates the good performance of this L-band repeat-pass interferometric SAR system. These results have important significance for the design of airborne repeat-pass InSAR systems and typical terrain mapping. When the flight tracks are under good control, this L-band SAR system is applicable in the fields of differential InSAR, polarimetric InSAR, and tomographic SAR.

Developing modern circular agriculture is one of the important ways to promote agricultural sustainable development, facilitating the achievement of the Sustainable Development Goals adopted by the United Nations. However, when agricultural production operators constructed particular systems, they often lacked accurate data support and parameter matching. Consequently, problems such as unknown input and output, low circulation efficiency, and lack of comprehensive evaluation occurred, increasing challenges for efficient circulation of the systems. Therefore, based on sufficient data collection and field investigation, this research integrated emergy analysis and life cycle assessment to conduct sustainability evaluation on the modern Straw–Sheep–Cropland agro-pastoral circular system. Then the system was optimized by means of coupling parameter adjustment and key technology regulation. The results showed that the whole system required lower total emergy input after optimization. And the total weighted value of potential environmental impacts of the optimized system was 47.12% of that of the original system. Meanwhile, annual environmental service emergy in air, water, and soil was reduced significantly compared with the original one. In general, the optimized system had good performance in reduction, reuse, and controllability, so its sustainability was also high. This research formed a systematic method suitable for evaluating and optimizing the modern agro-pastoral circular system, which provided accurate guidance for the scientific construction and sustainable development of circular agriculture systems. [ABSTRACT FROM AUTHOR]

Biochar combined with fertilizer as a soil amendment benefits to improving soil fertility, especially soil organic carbon and crop yield. However, the effect of biochar on the improvement of soil properties and crop yield was varied from soil properties and limited for medium–low‐yield farmland in the North China. During the completely randomized field experiment, SIX treatments (biochar applied as 0, 15 and 30 t·ha‐1, under 240 and 300 kg N ha‐1 nitrogen fertilizer) were applied in wheat season and examined to reveal changes in the SOC and other properties of 0‐ to 10‐cm and 10‐ to 20‐cm soil layers. The results showed that two years after the application of biochar, a significant increase in the SOC was observed, ranging from 19.52% to 97.50% (p < 0.05) in the 0‐ to 20‐cm soil layer. Wheat yield and SOC content increased with increasing amount of biochar applied under the same amount of nitrogen fertilizer. The content of soil available potassium increased significantly under 30 t·ha‐1 biochar application (p < 0.05). Both biochar and nitrogen fertilizer application could increase wheat yield, and the effect of biochar application for increasing wheat yield was better than that of nitrogen fertilizer. Wheat yield and SOC content increased with increasing nitrogen fertilizer at the same amount of biochar application. The principal component analysis results showed that biochar input, SOC, available potassium and total nitrogen were the key factors affecting wheat yield. Biochar application is a fast and effective measure to improve SOC and wheat yield in medium‐ and low‐yield farmlands. [ABSTRACT FROM AUTHOR]

Aquatic plants play important roles in the productivity of intensive Chinese mitten crab (CMC) rearing ponds. This study aimed to evaluate the lower limit of aquatic plant coverage (APC) in CMC ponds. A field investigation was conducted and unmanned aerial vehicle was used to estimate APCs. A follow‐on single factor experiment was performed to verify the results and evaluated the efficacy of APC values with 15%, 45%, 65% and 85% APC treatments. Average total nitrogen (0.81 mg/L), total phosphorus (0.06 mg/L), ammonium‐N (0.12 mg/L) and nitrate‐N (0.10 mg/L) met the grade II–IV standards for surface water quality in China in the investigations. Further, the use of an exponential decay model for water qualities and APCs indicated that an effective APC threshold was around 30%, and crab yields (1,536 kg ha–1 year–1) significantly decreased by 19.2% if the pond APCs could not reach this threshold. Crab yields were also correlated with crab feed characteristics, and especially the level of fish meat that was used. The single factor experiment for APC again confirmed our APC value threshold via exponential model. Meanwhile, the water quality measurements of the 15% APC treatment were significantly larger than the other APC treatments, but the crab yield, individual weight and carapace length indeed decreased in this single factor experiment for APC. We therefore conclude that the empirical APC level of 60% is preferred, but that smaller APCs could be acceptable in intensive CMC ponds if an APC of at least 30% is maintained. [ABSTRACT FROM AUTHOR]

Considering the influence of the ionosphere, troposphere, and other systematic errors on double-differenced ambiguity resolution (AR), we present an optimal triple-frequency code-phase combination determination method driven by both the model and the real data. The new method makes full use of triple-frequency code measurements (especially the low-noise of the code on the B3 signal) to minimize the total noise level and achieve the largest AR success rate (model-driven) under different ionosphere residual situations (data-driven), thus speeding up the AR by directly rounding. With the triple-frequency Beidou Navigation Satellite System (BDS) data collected at five stations from a continuously-operating reference station network in Guangdong Province of China, different testing scenarios are defined (a medium baseline, whose distance is between 20 km and 50 km; a medium-long baseline, whose distance is between 50 km and 100 km; and a long baseline, whose distance is larger than 100 km). The efficiency of the optimal code-phase combination on the AR success rate was compared with that of the geometry-free and ionosphere-free (GIF) combination and the Hatch-Melbourne-Wübbena (HMW) combination. Results show that the optimal combinations can always achieve better results than the HMW combination with B2 and B3 signals, especially when the satellite elevation angle is larger than 45°. For the wide-lane AR which aims to obtain decimeter-level kinematic positioning service, the standard deviation (STD) of ambiguity residuals for the suboptimal combination are only about 0.2 cycles, and the AR success rate by directly rounding can be up to 99%. Compared with the HMW combinations using B1 and B2 signals and using B1 and B3 signals, the suboptimal combination achieves the best results in all baselines, with an overall improvement of about 40% and 20%, respectively. Additionally, the STD difference between the optimal and the GIF code-phase combinations decreases as the baseline length increases. This indicates that the GIF combination is more suitable for long baselines. The proposed optimal code-phase combination determination method can be applied to other multi-frequency global navigation satellite systems, such as new-generation BDS, Galileo, and modernized GPS. [ABSTRACT FROM AUTHOR]

The European Global Navigation Satellite System Galileo is gradually deploying its constellation. In order to provide reliable navigation and position services, the effectiveness and reliability of ambiguity resolution between reference stations is necessary in network real-time kinematic (NRTK). The multifrequency signal of Galileo could much enhance the ambiguity resolution (AR) reliability and robustness. In this study, to exploit full advantage of this, the geometry-free (GF) TCAR and ionospheric-free (IF) triple-carrier ambiguity resolution (TCAR) methods were utilized in solving the ambiguity in the Hong Kong area, which is an ionosphere disturbance active area, and compared with each other. The IF TCAR method was then used to combine multi-systems to improve Galileo E1 AR performance, which is named as the combined IF (CIF) TCAR method. Three experiments were carried out in the Hong Kong area and the results showed that the Galileo-only system could fix ambiguities on all satellite pairs correctly and reliably by the IF TCAR method, while the GF TCAR method showed a weaker performance. The wide-lane (WL) convergence time of the IF TCAR method is improved by about 37.6%. The IF TCAR method with respect to the GF TCAR method could improve the WL accuracy by 21.6% and the E1 accuracy by 72.7%, respectively. Compared with GPS-only TCAR or Galileo-only TCAR, the ambiguity accuracy and the convergence time of the CIF TCAR method, which combines GPS and Galileo, could be improved by about 25.7% and 47.1%, respectively. [ABSTRACT FROM AUTHOR]

The network real-time differential positioning technique is a good choice for meter and sub-meter level's navigation. More attention has been paid to the Global Positioning System (GPS) and GPS + GLONASS (GLObal NAvigation Satellite System) network real-time differential positioning, but less on the GPS + BDS (BeiDou Navigation Satellite System) combination. This paper focuses on the GPS + BDS network real-time differential positioning. Since the noise of pseudorange observation is large, carrier-phase-smoothed pseudorange is usually used in the network real-time differential positioning to improve the positioning accuracy, while it will be interrupted once the satellite signal is lost or a cycle slip occurs. An improved algorithm in the position domain based on position variation information is proposed. The improved method is immune to the smoothing window and only depends on the number of available satellites. The performance of the network real-time differential positioning using the improved method is evaluated. The performance of GPS + BDS combination is compared with GPS-only solution as well. The results show that the positioning accuracy can be increased by around 10%–40% using the improved method compared with the traditional one. The improved method is less affected by the satellite constellation. The positioning accuracy of GPS + BDS solution is better than that of GPS-only solution, and can reach up to 0.217 m, 0.159 m and 0.330 m in the north, east and up components for the static user station, and 0.122 m, 0.133 m and 0.432 m for the dynamic user station. The positioning accuracy variation does not only depend on whether the user is inside or outside the network, but also on the position relation between the user and network. [ABSTRACT FROM AUTHOR]

Straw returning is a vital agronomic practice for soil organic carbon (SOC) sequestration and stabilization by changing the storage and fraction composition. Iron and aluminum (Fe/Al) oxides have a protective effect on SOC through adsorption and co-precipitation, which depends on the fraction composition of SOC itself, especially originated from externally added organic materials. However, the changes and interactions of SOC components and Fe/Al oxides in straw returning soil are not clearly understood. To evaluate the effect of straw returning on SOC composition and Fe/Al oxides, we have conducted an in situ field experiment for 10 yrs., which including five treatments: no rice and wheat straw returning (NRW, as control), total amount wheat straw returning (W), total amount rice straw returning (R), half amount rice and wheat straw returning (HRW), and total amount rice and wheat straw returning (TRW). The results showed that SOC and heavy fraction organic carbon (HFOC) increased under all five treatments, especially for R, HRW, and TRW treatments (P < 0.05). Compared with wheat straw returning, rice straw returning was more conducive to increasing the contents of SOC and its fractions, free Fe/Al oxides, complex aluminum oxides and the carbon utilization efficiency. Collectively, these properties were strongly affected by both the type and amount of straw returning. Soil organic carbon was significantly correlated with HFOC, water-soluble organic carbon, alkali-hydrolyzed nitrogen, available phosphorous, available potassium, and total nitrogen (P < 0.05). Of these, HFOC had the most significant effect on SOC and was vitally important for improving SOC sequestration and stability (P < 0.001). The increase of labile carbon fractions was more conducive to the absorption and utilization of nutrients by crops. Straw returning promoted the interaction between increasing organic matter and complex iron oxide (Fe c). This served to strengthen the protective effect of SOC, which increased the content of Fe c and stability of SOC, resulting in decreased soil pH. Taken together, these results highlight the importance of straw returning in promoting the storage and stability of SOC pool. Critically, rice straw returning can reduce soil pH and Fe c content, therefore could be considered as an ideal model for a rice-wheat rotation system. • Long-term straw returning increased the sequestration and stability of soil organic carbon by increasing heavy fractions. • Soil organic carbon fraction composition and Fe/Al oxides changed simultaneously with straw type and amount. • The effect of labile organic carbon of straw returning on Al oxides was greater than that of Fe oxides. • Rice straw returning has greater potential in improving soil organic carbon and soil properties. [ABSTRACT FROM AUTHOR]

China is a key global region vulnerable to climate change; however, limited studies have focused on the combined impacts of atmospheric CO 2 enrichment and warming on crop production in arable land, especially in rice paddies in China. To address this issue, a 4 year open-air field experiment during 2010–2014 was conducted to simulate the impact of climate change on crop production in a rice paddy in southeast of China. Four treatments including the ambient condition (CK), CO 2 enrichment (500 ppmv, CE), warming of canopy air (2 °C above the ambient, WA), and the combined CO 2 enrichment and warming (CW) were used to investigate the responses of total biomass, crop yield and harvest index. In general, different treatments significantly affected wheat and rice production. Compared to CK, CE significantly increased grain yield of rice by 8%. In contrast, the decreases of 26.2% and 10% in wheat and rice yield were observed under WA. However, there was no significant difference of wheat production between CW and CK, while rice yield and biomass were slightly decreased by a mean of 4.8% and 5.3% over 4 years, indicating the positive effect of CO 2 enrichment was unable to compensate for the negative impact of warming. The interannual variations of the responses were also observed in this study. The variation of wheat yields during 4 years was much higher than that of rice yields; however, significant changes in the stability of rice biomass and harvest index were observed under CE and WA. The results indicated both stabilizing and increasing grain yield under climate change are major challenges for agriculture in developing countries. [ABSTRACT FROM AUTHOR]

Significant efforts have been made to assess the impact of tillage regimes on crop yields and/or greenhouse gas (GHG) emissions across single crop growing season. However, few studies have quantified the impact across a whole rotation cycle in multiple cropping systems. Utilizing on a long-term tillage experiment with the rice–wheat rotation system in East China, we examined the GHG emissions under different tillage practices with or without crop straw incorporation. Results showed that compared to the no-straw control, straw incorporation increased wheat yield by 28.3% ( P < 0.05), irrespective of tillage practices, but had no significant effect on rice yield. Although straw incorporation did not significantly affect CH 4 emissions during the wheat season and N 2 O emissions during the whole rice–wheat cycle, it significantly stimulated CH 4 emissions by 98.8% ( P < 0.01) during the rice season. Also, there were no significant differences in CH 4 and N 2 O emissions between tillage practices during the wheat season. Compared to plowing, rotary tillage increased CH 4 emissions significantly by an average of 38.8% ( P < 0.01) but had no significant impacts on N 2 O emissions during the rice season. Across the rotation cycle, annual yield-scaled global warming potential of CH 4 and N 2 O emissions under no-tillage plus rotary tillage was 26.8% ( P < 0.01) greater than that of rotary tillage plus plowing with or without straw incorporation. Significant interactions between soil tillage and straw management practices were found on annual GHG emissions, but not on crop yields. Together, these results indicate that plowing in the rice season plus rotary tillage in the wheat season may reduce GHG emissions while increasing crop yield in rice–wheat cropping areas. [ABSTRACT FROM AUTHOR]

• 4R strategy for controlling AGNPS pollution was applied in Yangtze River basin. • N and P fertilizer rate reduced by 20 % and 27 % without any yield loss. • More than 40 % of N & P in the drainage was removed by eco-ditches & eco-ponds. • TN in the final field drainage was lower than 2 mg/L during 3 out of 19 times. • TN, NH 4 +-N, TP in the river decreased by 28.9 %, 30.4 %, 21.9 % after eco-restoration. Non-point pollution is becoming one of the main causes of the degradation of rural environment in China. A systematic strategy encompassing source reduction, process retention, nutrient reuse, and water restoration for controlling agricultural non-point source pollution was applied to a 1.5 km2 intensive rice-wheat rotation area at Yaoqiao county, Zhenjiang city, Jiangsu Province, in the downstream reach of the Yangtze River, China, from 2016 to 2018. The results showed that the use of organic and controlled-release fertilizers reduced the total nitrogen (TN) and total phosphorus (TP) input for rice and wheat by 20 % and 27 %, respectively, without any yield loss. The N and P concentration in the ecological ditches declined by 8%–50.4% for TN and 12%–69.7% for TP in comparison with that in the control ditches, and then further declined after the purification of eco-ponds. The removal efficiency was positive related with the N and P centration of inflow. TN concentration in the drainage water finally flow into the Shangshe River was lower than 2 mg/L during three out of 19 times. The river water quality was distinctly improved, wherein the TN, NH 4 +-N, TP, and chemical oxygen demand (COD Cr) concentrations were lower by 28.9 %, 30.4 %, 21.9 %, and 35.5 %, respectively following implementation of ecological water restoration technologies. The water quality improvement was mainly attributed to reduced pollutant load from agricultural fields and the aquatic plant uptake. This study found that the systematic "4R" approach is a promising method for decreasing the N and P loads from agricultural fields and improving the downstream water eutrophication problem, and can be extended to other agricultural intensive regions. [ABSTRACT FROM AUTHOR]

Elodea nuttallii is widely used in Chinese mitten crab (CMC) rearing practice, but it is not a native aquatic plant and cannot endure high temperature. Thus, large E. nuttallii mortality and water deterioration events could occur during high-temperature seasons. The aim of this study was to identify the use of local macrophytes in CMC rearing practice, including Ipomoea aquatic and Oryza sativa . A completely randomized field experiment was conducted to investigate the crab yield, water quality, bacterioplankton community and functions in the three different systems ( E. nuttallii , I. aquatic , and O. sativa ). Average crab yields in the different macrophyte systems did not differ significantly. The I. aquatic and O. sativa systems significantly decreased the total nitrogen and nitrate-N quantities in the outflow waters during the rearing period compared to the E. nuttallii system, and the I. aquatic and O. sativa plants assimilated more nitrogen than the E. nuttallii plant. Moreover, the significant changes of bacterioplankton abundances and biodiversity in the three systems implied that cleanliness of rearing waters was concomitantly attributed to the differential microbial community and functions. In addition, principle component analysis successfully differentiated the bacterioplankton communities of the three macrophytes systems. Environmental factor fitting and the co-occurrence network analyses indicated that pH was the driver of bacterioplankton community structure. Functional predictions using PICRUSt (v.1.1.3) software based on evolutionary modeling indicated a higher potential for microbial denitrification in the I. aquatic and O. sativa systems. Notably, the O. sativa plants stopped growing in the middle of the rearing period. Thus, the I. aquatic system rather than the O. sativa system could be a feasible and environmental-friendly alternative to the E. nuttallii system in CMC rearing practice., Competing Interests: The authors declare that they have no competing interests.

Straw incorporation is a common long-term practice to improve soil fertility in croplands worldwide. However, straw amendments often increase methane (CH 4 ) emissions from rice paddies, one of the main sources of anthropogenic CH 4 . Intergovernmental Panel on Climate Change (IPCC) methodologies to estimate CH 4 emissions from rice agriculture assume that the effect of straw addition remains constant over time. Here, we show through a series of experiments and meta-analysis that these CH 4 emissions acclimate. Effects of long-term (>5 years) straw application on CH 4 emissions were, on average, 48% lower than IPCC estimates. Long-term straw incorporation increased soil methanotrophic abundance and rice root size, suggesting an increase in CH 4 oxidation rates through improved O 2 transport into the rhizosphere. Our results suggest that recent model projections may have overestimated CH 4 emissions from rice agriculture and that CH 4 emission estimates can be improved by considering the duration of straw incorporation and other management practices.

Effects of fertilisation and other management techniques on a weed community were evaluated during wheat growth in a rice-wheat cropping system. Fertiliser treatments were C0 (C means chemical, C0 means zero chemical fertiliser.), CN (N fertiliser), CNK (N plus K fertiliser), CNPK (N plus P and K fertiliser), CNP (N plus P fertiliser), and CPK (P plus K fertiliser). Weed density, biomass, and bio-diversity were determined. Redundancy analysis (RDA) was used to investigate the relationship between fertiliser management, weed species, and weed density. The overall weed densities in the C0 and CPK treatments were the greatest during wheat seeding and ripening periods and were significantly greater than densities in the other treatments. N, P and organic matter in soil were highly correlated with weed species and density, whereas K in soil was not significantly correlated with weed species and weed density. N fertiliser significantly reduced weed density. Balanced fertilisation maintained weed species richness and resulting in a high yield of wheat. CNPK application reduced weed damage and improved the productivity and stability of the farmland ecosystem.