Potential of UAV-Based Active Sensing for Monitoring Rice Leaf Nitrogen Status

Unmanned aerial vehicle (UAV) based active canopy sensors can serve as a promising sensing solution for the estimation of crop nitrogen (N) status with great applicability and flexibility. This study was endeavored to determine the feasibility of UAV-based active sensing to monitor the leaf N status of rice (Oryza sativa L.) and to examine the transferability of handheld-based predictive models to UAV-based active sensing. In this 3-year multi-locational study, varied N-rates (0–405 kg N ha−1) field experiments were conducted using five rice varieties. Plant samples and sensing data were collected at critical growth stages for growth analysis and monitoring. The portable active canopy sensor RapidSCAN CS-45 with red, red edge, and near infrared wavebands was used in handheld mode and aerial mode on a gimbal under a multi-rotor UAV. The results showed the great potential of UAV-based active sensing for monitoring rice leaf N status. The vegetation index-based regression models were built and evaluated based on Akaike information criterion and independent validation to predict rice leaf dry matter, leaf area index, and leaf N accumulation. Vegetation indices composed of near-infrared and red edge bands (NDRE or RERVI) acquired at a 1.5 m aviation height had a good performance for the practical application. Future studies are needed on the proper operation mode and means for precision N management with this system

A New Curve of Critical Nitrogen Concentration Based on Spike Dry Matter for Winter Wheat in Eastern China.

Diagnosing the status of crop nitrogen (N) helps to optimize crop yield, improve N use efficiency, and reduce the risk of environmental pollution. The objectives of the present study were to develop a critical N (Nc) dilution curve for winter wheat (based on spike dry matter [SDM] during the reproductive growth period), to compare this curve with the existing Nc dilution curve (based on plant dry matter [DM] of winter wheat), and to explore its ability to reliably estimate the N status of winter wheat. Four field experiments, using varied N fertilizer rates (0-375 kg ha-1) and six cultivars (Yangmai16, Ningmai13, Ningmai9, Aikang58, Yangmai12, Huaimai 17), were conducted in the Jiangsu province of eastern China. Twenty plants from each plot were sampled to determine the SDM and spike N concentration (SNC) during the reproductive growth period. The spike Nc curve was described by Nc = 2.85×SDM-0.17, with SDM ranging from 0.752 to 7.233 t ha-1. The newly developed curve was lower than the Nc curve based on plant DM. The N nutrition index (NNI) for spike dry matter ranged from 0.62 to 1.1 during the reproductive growth period across the seasons. Relative yield (RY) increased with increasing NNI; however, when NNI was greater than 0.96, RY plateaued and remained stable. The spike Nc dilution curve can be used to correctly identify the N nutrition status of winter wheat to support N management during the reproductive growth period for winter wheat in eastern China

Evaluation of Variable Application Rate of Fertilizers Based on Site-Specific Management Zones for Winter Wheat in Small-Scale Farming

China is currently experiencing a severe issue of excessive fertilization. Variable rate fertilization (VRF) technology is key to solving this issue in precision agriculture, and one way to implement VRF is through management zone (MZ) delineation. This study is aimed at evaluating the feasibility and potential benefits of VRF based on site-specific MZs in smallholder farm fields. This study determined the amounts of basal and top-dressing fertilizers in different spatial units, based on soil nutrient MZs and crop growth MZs, respectively. The potential agronomic, economic, and environmental advantages of spatial variable rate fertilization were further assessed by comparing the farmer’s treatment, the expert’s treatment, and the variable rate fertilization treatment based on management zones (VR-MZ). The results showed that VR-MZ reduced the use of nitrogen (N), phosphorus (P), and potassium (K) fertilizers by 22.90–43.95%, 59.11–100%, and 8.21–100%, respectively, and it also increased the use efficiency of N, P, and K by 12.27–28.71, 89.64–176.85, and 5.48–266.89 kg/kg, respectively, without yield loss. The net incomes of VR-MZ were 15.5–449.61 USD ha−1 higher than that of traditional spatially uniform rate fertilization. Meanwhile, less nitrous oxide emission (23.50–45.81%), ammonia volatilization (19.38–51.60%), and nitrate ion leaching amounts (28.77–53.98%) were found in VR-MZ compared to those in uniform fertilization. The results suggest that the VR-MZ has great potential for saving fertilizers, significantly increasing farmers’ net income, reducing environmental pollution, and promoting the sustainable use of resources. This study provides a theoretical basis and technical support for exploring a VRF suitable for village-scale farming

Critical N data points used to develop spike N_c curves.

<p>a: 2009–2011 NM13, b: 2009–2011 YM16. The symbols (◇) and (○) represent the data points obtained from the N-limiting treatments for YM16 and NM13, and (×) and (+) represent the data points obtained from the non-N-limiting treatments for YM16 and NM13. The symbols (●) represent the calculated N_c points for each sampling date. The solid line represents the spike N_c curve that describes the relationship between spike N_c concentration and SDM of winter wheat.</p

Changes in winter wheat spike N concentration with time under different N application rates in the 2009 and 2011 seasons.

<p>a: 2009–2010 NM13, b: 2010–2011 NM13, c: 2009–2010 YM16, d: 2010–2011 YM16. Data are presented as days after sowing. The vertical bars represent standard error of the mean value on each sampling date. The vertical dotted bars without line end represent LSD values (<i>P</i><0.05) on each sampling date.</p

The relationship between RY and NNI for winter wheat in 2009–2011.

<p>The NNI value was averaged over all sampling dates.</p

Changes of spike dry matter (t ha^-1) for winter wheat with time (days after sowing) under different N application rates in experiments conducted during 2009–2011 seasons.

<p>Changes of spike dry matter (t ha^-1) for winter wheat with time (days after sowing) under different N application rates in experiments conducted during 2009–2011 seasons.</p

Comprehensive validation of the spike N_c curve using the independent data set from the experiment conducted during 2007–2008.

<p>Data points (□) and (△) represent N-limiting and non-N-limiting treatments, respectively. The solid line in the middle represents the critical N curve (N_c = 2.85×SDM^-0.17) that describes the relationship between the critical N concentration and spike dry matter of rice. The data points (●) and (○) that were not used for establishing the parameters of the allometric function (2009–2011) were used to develop two limit curves (--------): minimum limit curve (N_min = 1.86×SDM^-0.41) and maximum limit curve (N_max = 3.57×SDM^-0.12).</p

The basic information about the four experiments conducted in present study.

<p>The basic information about the four experiments conducted in present study.</p

Changes in the N nutrition index for YM16 and NM13 during the reproductive growth period.

<p>a: 2009–2010 NM13, b: 2010–2011 NM13, c: 2009–2010 YM16, d: 2010–2011 YM16. The vertical bars represent standard error of the mean value on each sampling date. The vertical dotted bars without line end represent LSD values (<i>P</i><0.05) on each sampling date.</p