Effects of water and NPK fertigation on watermelon yield, quality, irrigation-water, and nutrient use efficiency under alternate partial root-zone drip irrigation

Alternate partial root-zone drip irrigation (APRD) is efficient in improving irrigation water use efficiency (IWUE). However, little is known about the coupling effects of irrigation and NPK fertilisers on watermelon performance and resource use efficiency under APRD. This study investigated the effects of drip irrigation modes (conventional drip irrigation [CDI], APRD) coupled with NPK fertilisation rates on watermelon growth, IWUE, nutrient use efficiency, yield, and fruit quality. There were five treatments coded as CSH, CDH, CDL, ADH and ADL. The capital letters in the codes indicate drip irrigation modes as CDI (C) or APRD (A), irrigation amount as sufficient (S) or deficit (D, 60% of the total amount of S), and NPK fertilisation rates as high (H) or low (L, 60% of the amount of H). The results showed that compared with CDI at the same fertigation level, APRD increased the photosynthetic rate by 10.1%, plant growth by 25.5%, and NPK absorption by 29.1% across all growth stages, respectively. The promotion effect of APRD was more prominent at early growth stages. APRD increased watermelon yield by 10.4%, thus increased IWUE and fertiliser partial productivity at the same extent, while decreased the NPK use efficiency by 7.0% compared with the CDI counterparts. Compared with CSH, APRD increased vitamin C, total soluble solids (TSS), and the ratio of TSS to total organic acids by 63.2%, 6.2% and 17.2%, respectively. Additionally, the yield didn’t decrease in ADL whereas it increased in ADH, leading to an increase in IWUE of 67.8–78.7% under APRD treatments compared with CSH. Conclusively, APRD could save 40% of total irrigation water and NPK fertilisers without yield reduction while improving fruit quality. Thus, it is suggested as a sustainable practice for watermelon production.