Experimental approach to assess fertilizer nitrogen use, distribution, and loss in pear fruit trees.

Enhanced use efficiency of fertilizer nitrogen (N) is paramount for sustainable pear fruit production. Sufficient fruit N content is a major factor for pear fruit storage potential, but fertilizer N use efficiency in pear fruit trees is generally low. The objective of this work was to test a methodology to quantify N uptake, partitioning and leaching loss as influenced by different calcium nitrate (Ca(NO3)2) fertilizer timings. To this end, 10-year-old 'Conference' pear trees (Pyrus communis L.) were transplanted and grown in soil-filled pallet boxes subjected to different timing of fertilizer addition. A fraction of the calcium nitrate was labelled with 15N and applied one month before full bloom, during summer and post-harvest. By sampling newly formed biomass (i.e., leaves, fruit and flower buds), temporal dynamics in the N fraction derived from calcium nitrate fertilization were determined. Simultaneously NO3- leaching derived from calcium nitrate fertilization was quantified. Our data suggest that spring application of N was most efficiently partitioned to leaves, fruits and buds at the time of harvest with a mean percentage of calcium nitrate derived N (Ndff) of 9.2%, 20.4% and 18.6%, respectively. Summer application of calcium nitrate was far less efficient at the time of harvest with Ndff of 0.56%, 0.89% and 1.4%, respectively, and substantially higher NO3- leaching was observed compared to spring fertilization. Post-harvest application showed even higher levels of leaching. Applying a split fertilization scheme with 60 kg N ha-1 evenly spread over spring, summer, and post-harvest, about 9, 15 and 30%, respectively, of the fertilizer N had leached out as NO3- at the end of the growing season. This experimental approach may offer potential for detailed N budget studies for various fruit tree studies.

Inoculation of pear flowers with Metschnikowia reukaufii and Acinetobacter nectaris enhances attraction of honeybees and hoverflies, but does not increase fruit and seed set.

Currently, one of the most important challenges is to provide sufficient and affordable food and energy for a fast-growing world population, alongside preserving natural habitats and maintaining biodiversity. About 35% of the global food production depends on animals for pollination. In recent years, an alarming worldwide decline in pollinators has been reported, putting our food production under additional pressure. Therefore, there is an urgent need to find sustainable ways to ensure this crucial ecosystem service. Recent studies have shown that floral nectar is generally colonized by microorganisms, specifically yeasts and bacteria, which may alter nectar chemistry and enhance attraction of pollinators. In this study, we investigated changes in pollinator foraging behavior and pollination success in European pear (Pyrus communis L.) cultivars 'Regal Red' and 'Sweet Sensation' (red sports of 'Doyenné de Comice') after flower inoculation with the typical nectar-inhabiting microorganisms Metschnikowia reukaufii and Acinetobacter nectaris, and a combination of both. Pollination success was monitored by measuring the number of flower visits, fruit set and seed set in two consecutive years, 2019 and 2020. Results revealed that application of a mixture of M. reukaufii and A. nectaris resulted in significantly higher visitation rates of honeybees and hoverflies. By contrast, no effects on flower visits were found when yeasts and bacteria were applied separately. Fruit set and seed set were not significantly affected by any of the inoculation treatments. The only factors affecting fruit set were initial number of flower clusters on the trees and the year. The absence of treatment effects can most likely be attributed to the fact that pollination was not a limiting factor for fruit set in our experiments. Altogether, our results show that inoculation of flowers with nectar microbes can modify pollinator foraging patterns, but did not lead to increased pollination success under the conditions tested.