Research paperNet primary productivity and allocation to fine-root production in field-grown sweet cherry trees under different soil nitrogen regimes

- Sweet cherry trees on a dwarfing rootstock were grown with two N rates since planting.
- N application hastened the turnover rates and increased the biomass of fine-roots.
- Trees receiving N allocated a higher proportion of NPP to fine-roots.
- Fruit and fine-root biomass production were positively correlated.

Fine-roots have a high replacement rate or turnover during growing season, constituting an important sink for C allocation and a substantial metabolic cost for trees. However, studies in fruit-bearing trees are scarce. Here we report the annual net primary production (NPP) and relative allocation to seasonal fine-root production in field-grown sweet cherry trees (Prunus avium L. cv 'Bing' on Gisela®6) fertilized with two N rates (0 and 60 kg ha−1). Seasonal fine-root production and total tree NPP were estimated combining destructive and non-destructive methods. Vegetative growth variables, fruit yield, and air/soil variables were also monitored. N fertilization hastened the turnover rates of fine-roots, resulting in an increased absolute amount of fine-root biomass production, on average 70% higher than in control trees. Total NPP was unchanged, but the relative fraction of annual NPP accounted by fine-roots was higher in N-fertilized trees (24% versus 17%), which did not affect the fruit yield and quality. However, the relative C allocation to leaves and shoots was diminished, suggesting that N-fertilized trees preferentially allocated the fixed C into fruits and fine-roots at the expense of vegetative canopy growth. Inputs from fine-roots to soil organic cycle were 22 kg N ha−1 and 690 kg C ha−1 in control trees; while, 42 kg N ha−1 and 1170 kg C ha−1 in N-fertilized trees. Our results demonstrate that fine-root growth represents a major C cost for young sweet cherry trees, as well as an important contribution of C and nutrients to soil cycling.