Abstract
The nitrogen (N) nutrition index (NNI) is a reliable indicator of crop N status and there is an urgent need to develop efficient technologies for non-destructive estimation of NNI to support the practical applications of precision N management strategies. The objectives of this study were to: (i) validate a newly established critical N dilution curve for spring maize in Northeast China; (ii) determine the potential of using the GreenSeeker active optical sensor to non-destructively estimate NNI; and (iii) evaluate the performance of different N status diagnostic approaches based on estimated NNI via the GreenSeeker sensor measurements. Four field experiments involving six N rates (0, 60, 120,180, 240, and 300 kg·ha−1) were conducted in 2014 and 2015 in Lishu County, Jilin Province in Northeast China. The results indicated that the newly established critical N dilution curve was suitable for spring maize N status diagnosis in the study region. Across site-years and growth stages (V5–V10), GreenSeeker sensor-based vegetation indices (VIs) explained 87%–90%, 87%–89% and 83%–84% variability of leaf area index (LAI), aboveground biomass (AGB) and plant N uptake (PNU), respectively. However, normalized difference vegetation index (NDVI) became saturated when LAI > 2 m2·m−2, AGB > 3 t·ha−1 or PNU > 80 kg·ha−1. The GreenSeeker-based VIs performed better for estimating LAI, AGB and PNU at V5–V6 and V7–V8 than the V9–V10 growth stages, but were very weakly related to plant N concentration. The response index calculated with GreenSeeker NDVI (RI–NDVI) and ratio vegetation index (R2 = 0.56–0.68) performed consistently better than the original VIs (R2 = 0.33–0.55) for estimating NNI. The N status diagnosis accuracy rate using RI–NDVI was 81% and 71% at V7–V8 and V9–V10 growth stages, respectively. We conclude that the response indices calculated with the GreenSeeker-based vegetation indices can be used to estimate spring maize NNI non-destructively and for in-season N status diagnosis between V7 and V10 growth stages under experimental conditions with variable N supplies. More studies are needed to further evaluate different approaches under diverse on-farm conditions and develop side-dressing N recommendation algorithms.
Highlights
Maize (Zea mays L.) is the second most important staple food crop in China and accounts for about 33% of total Chinese cereal production [1]
This study evaluated the potential of using GreenSeeker active optical sensor for estimating
Nstatus indicators of spring maize in Northeast China and evaluated different N status diagnostic approaches based on estimated nutrition index (NNI) via GreenSeeker sensor measurements
Summary
Maize (Zea mays L.) is the second most important staple food crop in China and accounts for about 33% of total Chinese cereal production [1]. N management (PNM) strategies that match N supply with maize N demand in both space and time are urgently needed to increase NUE and mitigate negative environment impacts [4,5,6]. For such strategies to be successful and practical, non-destructive methods for real-time diagnosis of maize N status before making side-dressing N application recommendations need to be developed. It has been observed that plant N concentration (PNC) decreases with increasing plant aboveground biomass (AGB), which can be described with a critical N dilution curve [7,8]. As the ratio of actual PNC over critical PNC, the NNI can be used to diagnose crop
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