Soil mineral nitrogen dynamics in cover-cropped irrigated vineyards with contrasting soil textures

Abstract

Context and purpose of the study. Cover cropping in vineyards supports grape yield, quality, and soil health. This study examined alleyway and undervine cover crops’ effects on soil mineral nitrogen (N) at 0-30 cm over three years in two organic vineyards with different soil textures in the Okanagan Valley, British Columbia (BC), Canada.

Material and methods. Superior cover crop species were selected through prior screening. Two organic vineyards in Oliver (loamy sand soil; S1) and West Kelowna (sandy loam soil; S2), BC, seeded to three cover crop mixtures in alleyways and undervine areas in 2021, with annual species reseeded in 2022. Undervine crops included Ladino white clover, winter lentil, and ryegrass + birdsfoot trefoil, while alley treatments included ryegrass + tillage radish + white clover, fescue + white clover, and fall rye + hairy vetch. Soil samples (0-15 cm and 15-30 cm) were collected every fall (2020-2022) and analysed for KCl extractable NH₄⁺-N and NO₃⁻-N using a LECO 628 CHN analyser, with data processed using repeated measures in JMP v.18.

Results. NH₄⁺-N and NO₃⁻-N concentrations were significantly higher in fine- than coarse-textured soils. End-of-season NH₄⁺-N ranged from 0.08–8.96 mg kg⁻¹ (S1) and 0.30–13.1 mg kg⁻¹ (S2), while NO₃⁻-N ranged from 1.73–9.82 mg kg⁻¹ (S1) and 2.30–19.04 mg kg⁻¹ (S2). In S1, NH₄⁺-N levels in undervine areas at both depths were significantly influenced by interactions between undervine and alley cover crops. At 0–15 cm, winter lentil with ryegrass + tillage radish + white clover produced the highest NH₄⁺-N, while at 15–30 cm, Ladino white clover with fescue + white clover had the highest levels. NH₄⁺-N peaked in 2020, with similar levels in 2021 and 2022. In S1, NO₃⁻-N in undervine areas showed significant interactions between year, undervine cover crops, and alley cover crops, with 2022 levels exceeding those in 2021 and 2020. Treatments with Ladino white clover produced the highest NO₃⁻-N, while winter lentil resulted in the lowest. In S2, cover crops had no significant impact on NH₄⁺-N or NO₃⁻-N, except in 2022, when ryegrass + tillage radish + white clover increased NO₃⁻-N at 15–30 cm in alleys. Overall, finer soils had higher mineral N concentrations, with Ladino white clover enhancing NO₃⁻-N for vine nutrition and winter lentil reducing NO₃⁻-N to manage vine vigour.