Field-scale variability in site conditions explain phenotypic plasticity in response to nitrogen source in Pinus radiata D. Don

Aims Productivity of forest ecosystems is constrained by site resource availability and utilisation at an individual tree level. A better understanding of nitrogen (N) nutrition addition to forest ecosystems is critical for maintaining optimal plantation productivity, given the influence of an environment gradient, genetics, and their interactions. Methods We studied the aboveground growth response in a plantation setting of ten commercial P. radiata genotypes to N-fertilisation using three different N sources, and also assessed the effect of on-site environmental factors on this response. We compared, on equimolar basis, the effect of N-fertilisation with inorganic N (NH4NO3), organic N (L-arginine), and the two N sources combined (L-arginine:NO3-) to that of unfertilised trees on tree height, diameter, descriptors of microsite variability, and climate and seasonal information. After 2.5 years of fertilisation, genotype-specific variation in aboveground growth response to N sources were measured, and these were significantly influenced by field-scale heterogeneity. Results Across P. radiata genotypes, trees treated with inorganic N forms showed suppressed growth compared to unfertilised trees, while trees fertilised with organic N (either alone or in combination with inorganic N) were not significantly different than the untreated controls. We provide evidence of significant interactions between N source and genotype, N source and cover as well as genotype and microsite variability affecting temporal trends in tree volume. Conclusions We conclude that the comprehension of field-scale variability in soil properties and associated environmental variables is essential for understanding genotype performance as they are crucial determinants of intraspecific variation in response to N-fertilisation.