Contrasting impacts of two weed species on lowbush blueberry fertilizer nitrogen uptake in a commercial field

Abstract

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 N 2 -fixing shrub, and poverty oat grass ( Danthonia spicata ), a perennial grass—in a commercial blueberry field in Québec, Canada. In 2015, 15 N-labelled ammonium sulfate was applied at a rate of 45 kg ha -1 to 1 m 2 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 fruit yield and growth response to inorganic and organic N-fertilization when competing with two common weed species

“…A previous 15 N-labelling study has shown that D . spicata specifically relies heavily on inorganic N fertilizer for producing its aboveground biomass [7]. The absence of an effect for the organic N fertilizer on the three plant species may have resulted from the specific organic fertilizer used.…”

“…This commercial field is located near Saint-Eugène d’Argentenay in the Saguenay–Lac-Saint-Jean region, Quebec, Canada (48°59′N, 72°18′W), which contains 82% of Quebec’s commercial lowbush blueberry surface area [2]. A study was previously conducted on this field to assess the ability of the three aforementioned species to acquire 15 N from an inorganic fertilizer [7]. Soils in the lowbush blueberry stands are podzols characterized by fine sands that originate from fluvioglacial deposits.…”

“…This fertilizer is usually applied in the early spring of the vegetative year [4], and its application is often accompanied by weed control treatments [5]. The presence of weeds tends to diminish yields and the efficacy of inorganic N-fertilization [6] because some weed species are more efficient than blueberries in the taking up of inorganic N [7]. On the other hand, boreal ericaceous shrubs are more efficient in taking up organic N-forms from humus than other boreal forest plants [8,9] due to their association with ericoid mycorrhizal fungi [10,11].…”

“…peregrina ). These two species are characterized by distinct functional traits which drive the direction and the magnitude of interactions with lowbush blueberry [7]. D .…”

“…spicata is a strong competitor for inorganic N fertilizer acquisition due to its shallow and dense root system, whereas C . peregrina has a beneficial impact on blueberry growth due to its N 2 -fixation ability [7]. However, the latter may be a stronger competitor when N-fertilization occurs as organic N due to its cluster roots.…”