Interaction among fire and sward height increases leaf nitrogen per unit area of annual ryegrass in subtropical grasslands from southern Brazil

Fire is one of the main drivers of vegetation structure from terrestrial ecosystems and its effects on plants allocation resources and biomass are mediated by leaf functional traits. These attributes can provide useful information for future management using fire as pre-grazing treatment to increase forage quality. In May 2023, we established an experiment to assess combined pre-grazing effects of two vegetation treatments (fire; control) and two sward heights (15; 25 cm) on annual ryegrass leaf functional traits (specific leaf area [SLA], leaf dry matter content [LDMC]), nutrients concentration [nitrogen, phosphorus], and biomass. Fire killed all plants from sowed seeds but there was annual ryegrass regrowth from recruitment of soil seed bank. Differences in LDMC and biomass were found only regarding sward height, wherein at 25 cm LDMC and biomass were, respectively, 26.1% and 4.5 times higher than 15 cm. An interactive effect of pre-grazing treatment and sward height was found for leaf nitrogen concentration per unit of area, which was 52.2% higher in fire treatment at 15 cm in comparison to other treatments. There were no differences for leaf phosphorus concentration per area and SLA among treatments. Our findings suggested that, in areas that have seed of annual ryegrass in soil seed bank, interaction of fire and sward height of 15 cm increased forage quality by higher amounts of leaf nitrogen per area. Given the low-cost technique, fire can be a useful vegetation management tool to remove dead biomass while enhancing the performance of annual ryegrass from soil seed bank.