Abstract:
The effects of ¹⁵N-labeled fertilizer applied to mature summer-bearing red
raspberry (Rubus idaeus L.) plants were measured over two years. Four nitrogen (N)
treatments were applied singularly at 0, 40, or 80 kg·ha⁻¹ of N in early spring
(budbreak), or split with 40 kg·ha⁻¹ of N (unlabeled) applied at budbreak and
40 kg·ha⁻¹ of N (¹⁵N-labeled) applied 8 weeks later. Plants were sampled six times per
year to determine N and ¹⁵N content in the plant components throughout the growing
season. Soil was also sampled seven times per year to determine inorganic N
concentrations within the four treatments as well as a bare soil plot. There was a trend
for the unfertilized treatment to have the lowest and for the split-N treatment to have
the highest yield in both years. N application had no significant effect on plant dry
weight or total N content in either year. Dry weight accumulation was 5.5 t·ha⁻¹ and
total N accumulation was 88-96 kg·ha⁻¹ for aboveground biomass in fertilized plots in
2001. Of the total N present averaged over two years, 17% was removed in prunings,
12% was lost through primocane leaf senescence, 13% was removed through fruit
harvest, 30% remained in the over-wintering plant, and 28% was considered lost or
transported to the roots. Peak fertilizer N uptake occurred by July for the single N applications and by September for the last application in the split-N treatment. This
uptake accounted for 36-37% (single applications) and 24% (split application) of the
¹⁵N applied. Plants receiving the highest single rate of fertilizer took up more fertilizer
N while plants receiving the lower rate took up more N from the soil and from storage
tissues. By mid-harvest, fertilizer N was found primarily in the fruit, fruiting laterals,
and primocanes (94%) for all fertilized treatments; however, in the split application,
the majority of this fertilizer N was located in the primocanes (60%). Stored fertilizer
N distribution was similar in all fertilized treatments. By the end of the second year,
5-12% of the fertilizer acquired in 2001 remained m the fertilized plants. Soil nitrate
concentrations increased after fertilization to 10.6 g·sampled volume⁻¹, but declined to
an average of 4.8 g·sampled volume⁻¹ by fruit harvest. Seasonal soil N decline was
partially attributed to plant uptake; however, leaching and immobilization into the
organic fraction also contributed to the decline.
