Effects of a high-severity wildfire and post-fire straw mulching on gross nitrogen dynamics in Mediterranean shrubland soil.

摘要

Little is known about the combined impacts of fire and straw mulching, a widely used post‐fire emergency\udmeasure, on the soil nitrogen (N) cycle. Unburnt (US) and severely‐burnt soils without (BS) and with straw\udmulching (BSM) were preincubated (3 and 6 months) in the laboratory before fire and mulching effects on gross\udN transformations were investigated with a paired 15N‐labelling experiment. The ammonium‐to‐nitrate\ud(NH4\ud+/NO3\ud‐) ratio of burnt soils decreased with preincubation time from 21 to 1.3, consistent with a shift of the\udN cycle towards net nitrification. After 3 months of preincubation, gross mineralisation (MSON) and gross NH4\ud+\udimmobilisation (INH4) in BS more than doubled compared to US, in the latter being MSON 4.82 mg N kg‐1 day‐1 and\udINH4 3.01 mg N kg‐1 day‐1. Mulching partly mitigated this stimulation in the mineralisation‐immobilisation\udturnover (MIT). After 6 months, MIT differences among treatments disappeared and gross rates approached those\udin US after 3 months. After three months, autotrophic nitrification (NH4\ud+ oxidation) in all treatments was 0.41‐0.52\udN kg‐1 day‐1, while after 6 months it remained similar in US but increased 8‐fold in burnt soils. Heterotrophic\udnitrification of organic N only occurred in burnt soils, and its importance was similar to autotrophic nitrification\udafter 3 months, but around 4‐fold lower after 6 months. To conclude, burning opened up the N cycle and NO3\ud‐\udaccumulated, increasing the potential for ecosystem N losses. In the short term, straw mulching slightly mitigates\udthe effects of fire on the N cycle.