Salinity intrusion reduces grain yield in coastal paddy fields: case study in two estuaries in the Red River Delta, Vietnam

We characterized 63 fields sampled based on farmers’ perception of production risks within each commune (referred as either safe or at-risk fields), from the 3 communes in the 2 contrasting estuaries with different farming transformation in Red River Delta, Vietnam (Giao Huong and Giao Thien communes in Ba Lat estuary, Nam Dien communes in Day estuary), for 6 cropping seasons from 2015 to 2017. Rice yield was lower in Nam Dien (472 g/m2) than in Giao Huong (619 g/m2) and Giao Thien (662 g/m2), because of higher salinity levels in Day estuary than in Ba Lat estuary (8.0 ± 4.7 vs 2.9 ± 5.5‰), which led to higher salinity in standing water in the paddies (1.1 ± 0.3 vs 0.8 ± 0.3‰ in spring rice, 0.6 ± 0.3 vs 0.2 ± 0.2‰ in summer rice), and higher [Na+] in the surface soil (2.13 vs 1.7 meq/100 g). At-risk fields had higher water salinity and soil Na+, higher sand proportion, deeper standing water and lower yield than safe fields (558 vs 649 g/m2). Short duration inbred variety was most sensitive to increasing salinity, and farmers planted more hybrid varieties at at-risk fields in spring. Tall local glutinous cultivars with lower yield potential were planted at at-risk fields in summer with less N fertilizer. N fertilizer application rate was high (ca. 200 kg/ha/season) with low use efficiency. This study highlighted small-scale environmental variation and adaptation and yield reduction due to salinity in the two contrasting estuaries in the Red River Delta.