Mitigation of salinity stress in 'Chandler' walnuts through the application of Ascophyllum nodosum extracts to soil: effects on growth, yield, and its impact on physical and biological soil properties

In Chile most of the walnut orchards are located in semiarid areas prone to saline conditions. Therefore, mitigating salinity stress is a relevant issue for farmers, especially for 'Chandler', the most sensitive planted variety. The aim of this study was to evaluate the effect of an extract of Ascophyllum nodosum (ANE; Acadian Organic (R)), applied through drip irrigation, on the vegetative and productive response of 'Chandler' walnut orchard affected by salinity (soil EC 1.8 dS m(-1); irrigation water 2.0 dS m(-1)), effects on physical and biological soil properties. A dose of 4 L ha(-1) ANE was applied monthly during two seasons (2021-2022 and 2022-2023) from the beginning of shoot growth until one month before harvest (24 L ha(-1) per season), compared to unamended control. ANE applications promoted good conditions for tree development, greater shoot thickness, higher SPAD index measurements and better vegetative expression, represented by the NDVI spectral index. Consequently, a greater proportion of kernel filling and sizes was found in the ANE treatment (kernel size >32 mm, first season: 54% ANE vs 45% control; second season: 26% ANE vs 6% control). The stress mitigation of ANE promoted a positive cumulative effect after 1 season, with early vegetative development in the second season and significantly greater fruit size and yield efficiency. In addition, ANE application significantly improved physical and biological soil properties, possibly increased soil microbial activity, stimulated organic matter accumulation, improved the soil aggregate stability, and increased soil macroporosity at greater depths. The latter is relevant for salt management since a more stable soil porous system favors adequate water distribution in the soil profile under irrigation cycles, improving the displacement of salts and, consequently, reducing salinity stress.