Calcium supplementation ameliorates salinity stress in apricot trees irrigated with reclaimed wastewater: a case study at the fragile terraces ecosystem of Al Jabal Al Akhdar, northern Oman Fower

Abstract

The agricultural terraces of Al Jabal Al Akhdar in northern Oman date back millennia. However, this unique farming system is collapsing due to critical water stress brought about by climate change and rapid urban development. Irrigation with reclaimed wastewater (RWW) can be a solution to preserve and mitigate freshwater demand, but with an attendant risk of salinity. This study examined the mitigating effects of calcium (Ca2+) on Na+ toxicity and nutrient translocation of apricot (Prunus armeniaca ‘San Castrese’) grown in RWW solution along the soil-shoot-fruit continuum. Significant changes were observed in the soil, vegetative growth, and fruit quality attributes of the RWW-irrigated trees vs. the well water WW-irrigated trees. Although some of these changes may improve soil health (e.g., rise in total N, Ca2+, K+ and Mg2+ nutrients), other attributes have an adverse impact on soil health (e.g., total Na+, Cl-, electrical conductivity (EC), and sodium absorption ratio (SAR)). The apricot fruit per se was evidently not a sink for Na+ as it maintained low concentrations with increasing salinity. This Na+ ion selectivity at the root level results in limited translocation to photosynthesizing leaf tissues and fruit. The fruit uptake pattern of nutrients followed the order of Ca2+ > K+ > P > Mg2+ > Na+ > Fe > Mn2+. Ca2+ was negatively correlated with fruit K+ concentration, implying antagonistic interactions. Tree growth, trunk cross-sectional area TCSA, yield components; crop load efficiency (yield cm‑2), fruit shelf life, and physiological reaction (chlorophyll Fv/Fm) all demonstrated a consistent declining trend, indicating a photoinhibitory response to saline RWW. Overall, Ca2+ supply ameliorated Na+ toxicity through decreased Na+ in the apricot leaf tissues and soil SAR ratio.