Effects of Shallow Groundwater Salinity on the Field Performance of Rainfed Soybean (Glicine max), maize (Zea mays) and wheat (Triticum aestivum) in the Argentine Pampas

Abstract

While shallow groundwater can buffer rainfed crops against drought, its usefulness depends critically on water quality, as groundwater salinity can limit this contribution. In the Chaco–Pampas plain, widespread shallow water tables are increasingly regarded as a water source by farmers, yet field-based evidence on how groundwater salinity affects crop performance under real production conditions remains limited. Most of the existing knowledge comes from controlled experiments using ‘top-down’ saline irrigation, which do not adequately represent the ‘bottom-up’ capillary contribution of saline groundwater typical of rainfed field settings. Understanding these responses and their interaction with rainfall variability is essential to guide adaptive management in these agroecosystems. We took advantage of a natural gradient of groundwater salinity (≈2–11 dS m⁻¹) with uniformly shallow depths (< 2 m) under real field conditions in the western Pampas (temperate sub-humid climate, coarse-textured soils) to evaluate its effects on rainfed soybean, maize, and wheat. Manual yield sampling and yield-component analysis (grain number m⁻², thousand-grain weight) were complemented with ecophysiological measurements (leaf temperature and stomatal conductance) and time series of the fraction of absorbed photosynthetically active radiation (fAPAR) derived from Sentinel-2 NDVI. Relative yields were modelled using a curvilinear, exponential-type function to estimate the salinity level causing a 50% yield reduction (C₅₀) and the Salinity Tolerance Index (ST-index). Groundwater salinity significantly reduced the yields of the three crops, following a clear tolerance ranking: wheat (C₅₀ = 8.4 dS m⁻¹) > maize (C₅₀ = 5.0 dS m⁻¹) > soybean (average C₅₀ = 4.8 dS m⁻¹). Observations in multiple growing seasons for soybean revealed some inter-annual variability, with tolerance (C₅₀) tending to increase under wetter conditions (R² = 0.98, n = 4, p = 0.009). Yield reductions were primarily associated with decreases in grain number (56%–98% variability explanation), while grain weight was mainly affected in maize and in soybean during dry seasons (36%–43% variability explanation). Higher groundwater salinity also impaired plant water status (lower stomatal conductance, higher leaf temperature) and reduced fAPAR, more markedly in maize and soybean under dry conditions. Crop response thresholds to groundwater salinity should not be regarded as a fixed physiological attribute but rather as a dynamic ecohydrological property modulated by rainfall inputs. Depending on its salinity and the prevailing climatic conditions, shallow groundwater can act either as a resource or as a stress factor for crops. These findings highlight the need for adaptive management strategies that integrate groundwater salinity monitoring with seasonal climate forecasts, enabling risk zoning and the prioritization of more tolerant crops (such as wheat) in high-risk areas or years.