MODELING RICE PLANT NITROGEN EFFECT ON CANOPY REFLECTANCE WITH PARTIAL LEAST SQUARE REGRESSION (PLSR)

Abstract

In Arkansas and other major rice growing states in the U.S., nitrogen fertilizer is commonly applied in a splitapplication. The split application consists of a pre-flood ground application and one or two mid-season applications.Currently there are no fast and accurate methods for estimating mid-season nitrogen requirements of a rice crop. The goalof this research was to test spectral characteristics of a rice canopy to model and predict nitrogen status of a rice crop at thebeginning of internodal elongation (BIE). The early part of internodal elongation is a critical period for mid-season nitrogenapplication due to the increased demand for nitrogen in the reproductive stage. Plot experiments were conducted with twopopular rice varieties, Cocodrie and Wells, and six nitrogen levels of 0, 34, 67, 101, 134, 168 kg N/ha (0, 30, 60, 90, 120,and 150 lbs/acre) in a completely randomized block design with four replications. Data on canopy reflectance, biomass, andtissue nitrogen were measured three times at 7-day intervals starting at BIE. A t-test comparison showed no significantdifference between the two cultivars in plant nitrogen, tissue nitrogen, or biomass at BIE. Both tissue nitrogen and plantnitrogen showed a parabolic relationship with yield, with yield maximized at 13 g/m2 plant nitrogen and 3% tissue nitrogen.A partial least square regression (PLSR) model on canopy reflectance could explain 47%, 63%, and 71% of the variation inplant nitrogen for weeks 0, 1, and 2, respectively, from BIE. The cultivar did not affect the sensitivity of the PLSR models.With these models, it is possible to identify rice nitrogen requirements during the internodal elongation with moderate to highaccuracy. A nitrogen management regime can be developed based on rice canopy reflectance using the PLSR models. Theamount of nitrogen required by the field can be calculated as the difference between the predicted plant nitrogen status andthe optimum level of 13 g/m2, at which the rice yield was maximized.