Soil and plant indices for predicting eucalypt response to nitrogen in Uruguay

Eucalypt plantations have been increasingly fertilized with N applied postplanting in Uruguay, but information on the benefits of the practice is scarce. The objective of this study was to identify the N availability indices (NAI) able to differentiate responsive from unresponsive sites when N was applied to eucalypts 6 or 12 mo after planting (MAP). The NAI were based on soil and plant analyses. Volume yield response to N was evaluated in 20 experiments conducted in plantations of Eucalyptus globulus Labille and E. grandis Hill ex Maiden. The NAI were related to volume response expressions by using linear and quadratic-plateau models, and the model with the highest [R.sup.2] was selected. Leaf N concentration (LNC) was the NAI most strongly related to N response in both E. grandis fertilized at 6 MAP (seven sites) and E. glabulus fertilized at 12 MAP (six sites) and the estimated critical levels were 34.6 and 20.9 g [kg.sup.-1], respectively. None of the NAI could be selected in the E. globulus sites fertilized at 6 MAP, but soil mineralizable N (NMIN) was clearly related to N response when all E. glabulus sites were pooled (12 sites), resulting in a critical level of 109 mg N [kg.sup.-l]. Although the [R.sup.2] of the models describing relationships between the selected NAI and N response varied from 0.52 to 0.94, most NAI separated responsive from unresponsive sites. The results suggest that LNC and NMIN can be used as tools to improve prediction of the early eucalypt volume response to N in Uruguay. Abbreviations: AVI, absolute volume increase; CONTV, control volume; LNC, leaf nitrogen concentration; MAP, months after planting; MAXV, maximum volume; NM, nitrogen availability indices; NMIN, soil mineralizable nitrogen; RV, relative volume; SOM, soil organic matter.