Date Published: April 12, 2019
Publisher: Public Library of Science
Author(s): Charles Marty, Josée-Anne Lévesque, Robert L. Bradley, Jean Lafond, Maxime C. Paré, Sergio Saia.
Numerous studies have speculated that lowbush blueberry (Vaccinium angustifolium) is less efficient than weed species at taking up inorganic nitrogen (N) derived from fertilizers, thus raising questions as to the effectiveness of N fertilization in commercial fields. However, competition for acquiring N as well as specific interactions between blueberry and companion weeds characterized by contrasted functional traits remain poorly documented. Here, we assessed fertilizer-derived N acquisition efficiency and biomass production in lowbush blueberry and two common weed species that have different functional traits—sweet fern (Comptonia peregrina), a N2-fixing shrub, and poverty oat grass (Danthonia spicata), a perennial grass—in a commercial blueberry field in Québec, Canada. In 2015, 15N-labelled ammonium sulfate was applied at a rate of 45 kg ha-1 to 1 m2 field plots containing lowbush blueberry and one of the two weeds present at several different density levels (0 to 25 plants m-2). In 2016, each plot was harvested to determine vegetative biomass and the percentage of fertilizer-derived N recovered (PFNR) in each species. The PFNR was higher in blueberry (24.4 ± 9.3%) than in sweet fern (13.4 ± 2.6%) and poverty oat grass (3.3 ± 2.9%). However, lowbush blueberry required about four times more root biomass than sweet fern and poverty oat grass to uptake an equivalent amount of N from ammonium sulfate. The PFNR in poverty oat grass increased with plant density (from 0.8% to 6.4% at 2–3 and >6 plants m-2, respectively), which resulted in a decrease in blueberry’s PFNR (from 26.0 ± 1.4% to 8.6 ± 1.8%) and aboveground vegetative biomass production (from 152 ± 58 to 80 ± 28 g m-2). The increase in biomass production and N content in sweet fern with increasing plant density was not accompanied by an increase in PFNR (29.7 ± 8.4%), suggesting an increasing contribution of atmospherically-derived N. This mechanism (i.e., N sparing) likely explained blueberry’s higher biomass production and N concentration in association with sweet fern than with poverty oat grass. Overall, our study confirms lowbush blueberry low efficiency (on a mass basis) at taking up N derived from the fertilizer as compared to weeds and reveals contrasted and complex interactions between blueberry and both weed species. Our results also suggest that the use of herbicides may not be necessary when poverty oat grass is present at a low density (<15 plants of poverty oat grass m-2) and that adding inorganic N fertilizer is counterproductive when this species is present at a high density as it takes up as much fertilizer as lowbush blueberry.
Lowbush blueberry (Vaccinium angustifolium) is an ericaceous shrub native to eastern North America that grows on disturbed sites [1,2]. It is an economically significant crop in Québec, Canada’s Maritimes Provinces and Maine , where it is managed for its fruits from naturally occurring plant populations . Commercial lowbush blueberry fields are often developed on forest clear cuts where plants regrow from their large rhizomatic network and clones can spread naturally [2,5,6]. Lowbush blueberry fruits are produced generally through a 2-year crop cycle; the shrubs are pruned to near ground level in the first year (vegetative year) to stimulate new shoot production, and the plants bloom and produce fruits in the second year [6,7]. Lowbush blueberry lives in symbiosis with ericoid mycorrhizal fungi, which gives the plant access to otherwise inaccessible nutrients, especially from recalcitrant soil organic matter [8–10]. Although lowbush blueberry is well adapted to low nutrient availability and has low N requirements, repeated use of inorganic or organic fertilizers has been shown to increase leaf nutrient concentrations and growth [7,11–14] as well as fruit yields [11,14–16]. Weed control is, however, often necessary for fertilizers to be efficient [4,6,11,13,17,18].
Our data show that lowbush blueberry is not as efficient as weed species at acquiring inorganic N from the fertilizer. In our experiment, lowbush blueberry captured most of the added fertilizer only because individual plants had a well-developed root system. Fertilization in young lowbush blueberry stands having an undeveloped root system may not stimulate lowbush blueberry growth, especially when poverty oat grass density is >10 plants m-2, in which case fertilization may benefit the weed species more than the targeted plant. Our results show that poverty oat grass relies strongly on the fertilizer to fulfill its N demand relative to the other studied species. At plant densities >25 plants m-2, poverty oat grass captured about the same amount of fertilizer as blueberry despite a much smaller belowground biomass, resulting in a significant negative impact on blueberry’s vegetative biomass. In contrast, sweet fern uses atmospherically-derived N to support its N demand, especially when present at high density. This likely resulted in a “N sparing” phenomenon that alleviated the competition for acquiring N and explained the absence of detrimental effect of sweet fern density on lowbush blueberry fertilizer-derived N uptake. Based on our results, we recommend poverty oat grass suppression before applying N fertilizer in areas where poverty oat grass density is >10 plants m-2. In contrast, fewer precautions are required for applying fertilizer in plots having sweet fern, as this species does not compete with lowbush blueberry for acquiring N. Further research is required to assess the impacts of both weeds on lowbush blueberry fruit production.