Plasma-Activated Water as a Potential Low-Carbon Complement to Synthetic Nitrogen Fertilizers: A Comparative Review

Conventional nitrogen fertilizers are essential to food production but impose substantial energy, greenhouse-gas, and reactive-nitrogen losses. This review compares Haber–Bosch-derived urea, ammonium nitrate, calcium nitrate, green ammonia, and fertigation with plasma-activated water (PAW), in which non-thermal plasma fixes atmospheric nitrogen directly into water as NO3−/NO2− and, in some systems, NH4+. A PRISMA-adapted Scopus screening retrieved 765 records. Automated screening excluded 312 records; all 453 provisionally retained records were then manually audited, removing 88 additional false positives and yielding 365 plasma nitrogen-fixation studies, including 157 PAW/plasma-in-liquid records. The comparison uses explicit system boundaries for energy, carbon intensity, nitrogen-use efficiency, and technology readiness. The lowest verified directly measured in-water system reports 1.14 MJ mol−1 N for total soluble nitrogen, whereas lower values near 0.4–0.5 MJ mol−1 N refer mainly to gas-phase or modeled plasma fixation and are not directly interchangeable with PAW. Controlled-environment studies report improved germination or vegetative growth in several crops and, in one full-cycle controlled horticultural study with a nitrate-equivalent control, fruit performance comparable with conventional nitrate fertilization. Nevertheless, PAW is not a general replacement for synthetic fertilizer. Its most credible near-term role is as a decentralized complement in fertigation, protected cultivation, hydroponics, and remote or supply-constrained systems supplied by low-carbon electricity. Major barriers are dilute and variable nitrogen concentration, reactor durability, storage stability, incomplete techno-economic accounting, and the absence of replicated multi-season field validation. Minimum reporting requirements and research priorities are proposed.