Improving tomato nitrogen use efficiency with lignite-derived humic substances

Improving plant nitrogen use efficiency (NUE) is critical in sustainable cropping systems. Humic substances (HS), a type of plant biostimulant, have the potential to increase plant nitrogen (N) uptake and assimilation, while having the ability to change soil environment using their stable and solid products as soil amendments. In this two-year field study, the effects of lignite-derived HS soil amendments applied to topsoil at a rate of 1.5 t/ha was evaluated at two levels of N inputs (90 and 180 kg/ha) on tomato (Solanum lycopersicum cv. 'Camaro') growth performance, nitrogen use status, and soil property changes. HS increased plant canopy growth (leaf area index, LAI), shoot and fruit biomass, total N and NUE across different growth stages. During early growth, increased NUE caused by HS application was mainly due to increased nitrogen uptake efficiency (NUpE) under high N input management, while during the mid-late growth period, HS reduced the detrimental effects of N stress caused by low N input, as evidenced by improved biomass accumulation and NUE resulting from increased nitrogen utilization efficiency (NUtE). Overall, HS application improved the yield of tomatoes grown under low N input by 35.2% and 31.1%, in the first and second years of the experiment, respectively. Low N input reduced tomato plant normalized difference vegetation index (NDVI), LAI, biomass accumulation and total N, but improved plant NUE, NUpE and NutE compared with the high N input treatment. At the end-season, HS treated soil had lower soil NO3-N retention, which was positively correlated with increased plant total N. Soil organic matter content was improved after incorporating HS for two years. These results show that lignite-derived solid HS can be used as a soil amendment to sustainably improve the productivity and resource use efficiency of tomatoes and improve their soil environment.