Co-addition of humic substances and humic acids with urea enhances foliar nitrogen use efficiency in sugarcane (Saccharum officinarum L.)

Humic substances (HS) and humic acids (HA) are proven to enhance nutrient uptake and growth in plants. Foliar application of urea combined with HS and HA offers an alternative strategy to increase nitrogen use efficiency (NUE). The objective of this study was to understand the effects of foliar application of HA and HS along with urea on NUE and response of different biometric, biochemical and physiological traits of sugarcane with respect to cultivar, mode of foliar application, geographic location and intervals of foliar application. To study this, two different independent Experiments were conducted in green house facilities at two different agro-climatic zones (USA and Brazil) using two different predominant varieties, modes and intervals of foliar applications. The three different foliar applications used in this study were (1) urea (U), (2) mixture of urea and HS (U+HS) and (3) HA (U+HA). In both Experiments, (15)N (nitrogen isotope) recovery or NUE was higher in U+HS followed by U+HA. However, magnitude of NUE changed according to the differences in two Experiments. Results showed that foliar application of U+HS and U+HA was rapidly absorbed and stored in the form of protein and starch. Also induced changes in photosynthesis, intrinsic water use efficiency, protein, total soluble sugars and starch signifying a synergistic effect of U+HS and U+HA on carbon and nitrogen metabolism. These results showed promising use of HS and HA with urea to improve NUE in sugarcane compared to using the urea alone. Simultaneously, mode, quantity, and interval of foliar application should be standardized based on the geographic locations and varieties to optimize the NUE.