Soil composition and plant nutrients at the interface between crops and saline wetlands in arid environments in NE Spain

In arid and semiarid environments, saline wetlands are both high-value habitats threatened by landscape transformations, and a constraint to agricultural production. There is a lack of knowledge about nutrients distribution in intermittently flooded environments subjected to soil and water salinity. The aim of this work was to characterize soil and plant nutrient status along soil transects at the interface between agricultural land and saline wetlands. These aspects were studied in Guallar and Gallocanta, two Ramsar saline wetlands in NE Spain that represent arid and semiarid conditions, respectively. Soils were characterized for nitrogen (N) and phosphorous (P) status; crop and natural vegetation nutrient status was measured with P Nutrition Index (PNI) and Nitrogen Nutrition Index (NNI). Soil salinity (EC1:5) increases towards the halophyte areas, reaching 5.4 dS m⁻¹ in Guallar and 3.7 dS m⁻¹ in Gallocanta. On average, the soils are loams or sandy loams. Carbonate-rich soils are common (39% mean calcium carbonate equivalent) and some soils in Guallar have a high gypsum content (up to 72%). The soils have a low to moderate cation exchange capacity regardless of soil salinity, with a maximum of 14 cmol⁺ kg⁻¹ in the soil with the highest organic matter content (3.8%). Overall, cereal soils show total P and N contents within the normal range for agricultural soils, although the available P is above the requirements for cereals, up to 67 mg kg⁻¹. The difference in soil nutrients between cropped and halophyte land is more noticeable when it comes to total and available P content than for N levels. The relatively high nutrient content in some halophyte soils and the total and available P gradients along the soil transects, reflect downslope nutrient movement towards the lake. Halophyte soils can accumulate between 11% and 71% of the available P found in cropped soils. No relationship was found between the soil nutrient content and that of plants, nor between these two parameters and soil salinity. NNI and PNI indicate non-limiting nutrient status for cereals grown in Gallocanta and nutrient limitation in Guallar, probably related to the high gypsum content, and/or soil salinity. Studying the plant available nutrient content in arid and saline soils, at the interface between halophytes and crops, is crucial for improving land management strategies and reconciling production and conservation in protected saline habitats.