Aims: Biochar (BC) and humic acid product (HAP) soil amendments may improve plant performance under water-limited conditions. Our aim was to investigate if BC and HAP amendments, alone or in combination, will have positive and synergistic effects. Methods: A three-factorial fully randomized study was carried out in the greenhouse for 66 days, including the factors 'BC', 'HAP' and 'water regime'. Maize ( Zea mays var. 'Amadeo' DKC-3399) was grown in pots (6 kg sandy soil pot -1) amended with/without BC (0, 1.5 and 3%; w/ w) and with/without HAP (0 or an equivalent of 8 kg ha -1). Two water regimes, limited and frequent (H 2O limit , H 2O frequ ), were applied after day 28 following seedling establishment at 60% water holding capacity (WHC). In the H 2O limit treatment, the soil water content was allowed to drop until wilting symptoms became visible (25-30% WHC) while in H 2O frequ the WHC was brought to 60% of the maximum on a daily basis Results BC but not HAP, added alone or in combination with BC, significantly increased the biomass yield and the water and N use efficiency of plants at both water regimes. The BC-mediated relative increase in the yield was equal with both watering regimes, refuting initial hypotheses. BC had generally a stimulating effect on water relations and photosynthesis, it increased the relative water content and the leaf osmotic potential, decreased the stomatal resistance and stimulated the leaf gas exchange (transpiration). Both, BC and pure HAP addition, stimulated photosynthesis by increasing the electron transport rate (ETR) of photosystem II (PSII) and of the ratio between effective photochemical quantum yield to non-photochemical quenching (Y(II)/Y(NPQ)), revealing reduced heat dissipation. Conclusions: Biochar use in poor sandy soils can improve plant growth by improving soil-plant water relations and photosynthesis under both H 2O frequ and H 2O limit conditions. HAP loading, however, did not improve the effect of biochar or vice versa.
