Estimation of leaf nitrogen content from nondestructive methods in Eucalyptus tereticornis and Eucalyptus saligna plantations

The determination of leaf nitrogen content (LNC) by indirect methods is essential for silvicultural management of forest crops. The application of photography or rapid measurement equipment, such as chlorophyll index (soil-plant analysis development-SPAD), is increasingly used due to its low-cost, ease of estimation and accuracy. Therefore, the aim of this study was to estimate foliar nitrogen content from nondestructive methods in plantations of Eucalyptus tereticornis and Eucalyptus saligna using three urea treatments (120 kg N ha-1, 240 kg N ha-1 and a control treatment without urea). For each treatment, 10 trees were selected, including four for the validation of the equations. The LNC was directly evaluated for color with the CIEL*a*b* model, photographic measurement with the RGB model, SPAD measurement and destructive estimation of nitrogen in leaves. The results showed negative relationships with the L* (luminosity) and b* (trend from yellow to green) indices, while the a* (red to green trend) index was discarded, with SPAD positive relationships were found with LNC and RGB space. In the R and B indices, the greatest negative relationships were found. It was determined that the multivariate equation Y=a+b1x1+b2x2+…+bnxn can be used for this type of study. It was also determined that the LNC=0.389+0.026SPAD model was the optimum for E. tereticornis and the LNC=3.826-0.001R-0.10B equation was the optimum for E. saligna.