Estimating winter wheat biomass and nitrogen status using an active crop sensor

Plant biomass and nitrogen status are important factors to consider when making in-season crop management decisions. Traditional sampling and analysis are time-consuming, labor-intensive and costly. It is desirable to estimate these parameters nondestructively using remote sensing technology. The objective of this study is to evaluate the potential of using an active crop canopy sensor, GreenSeeker, for estimating winter wheat biomass, nitrogen concentration and uptake in North China Plain. A total of 13 field experiments involving different N rates, varieties and sites were conducted from 2004 to 2007 in Shandong Province, China. In addition, data from 69 farmer's fields were also collected to further evaluate the sensor's potential application under on-farm conditions. The results indicated that across sites, years, experiments and growth stages, normalized difference vegetation index became saturated when biomass reached 3736 kg ha-1, or when plant nitrogen uptake reached 131 kg ha-1. Ratio vegetation index was linearly related with winter wheat biomass and plant nitrogen uptake and did not show obvious saturation effect. However, none of the two vegetation indices performed well for nitrogen concentration estimation. We conclude that RVI should be selected when using the Green Seeker crop sensor to estimate winter wheat biomass or N uptake across sites, years and growth stages. The NDVI index can also be used before plant biomass and N uptake reach threshold values. More research is needed to further evaluate the results under more diverse conditions, and develop strategies of using the Green Seeker active sensor for diagnosing crop growth and N status and making in-season management decisions, especially under high-yielding conditions.