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

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