Suitable Locations for Reference Plots Based on the Nitrogen Suffiency Index (NSI)

Abstract

Nitrogen (N) is critical to the quantity and quality of agricultural yields. Excess N fertilization is costly, both economically and environmentally (nitrate leaching, eutrophication, greenhouse gas release, soil degradation). This research identifies zones that could substitute the field-long N-rich strips by using spatial analysis of the nitrogen sufficiency index (NSI) and the relation with Apparent Electrical Conductivity (ECa), Elevation, Slope and Soil. NSI calculated from ECa grouped into three classes was capable of minimizing the effects on NDVI. Correlation coefficients (R) between three-class NSI and NSI calculated from the nearest ECA values were very high for all the fields with values between 0.82< R <0.94, with the highest coefficients associated with fields in 2005 and 2007. Meanwhile, three-class NSI coefficients were consistently significant in relation to the NSI reference, with an average of R=0.79 for all the fields. The highest coefficient was detected for 2007, with R=0.89, whereas the lowest values were associated with 2006 (R=0.67). In the case of elevation grouped into four classes, the correlation results were not statistically significant, with overall average values of R<0.70. The maps elaborated from the NSI for ECa grouped into three classes show a high level of accuracy compared to the NSI reference map. The new N-rich zones not only can contribute to mitigating the environmental impact of agricultural practices (reducing 77% of N inputs) but also be an accurate source of data for the analysis of NSI and within-field N variability.