Soil gross N transformation rates after a wildfire and straw mulch application for burned soil emergency stabilisation

Abstract

Effects of fire and post-fire mulching for soil protection on N transformations were evaluated in the laboratory 4, 8 and 12 months after a wildfire in the 0-2-cm layer of a burned soil without (BS) or with straw mulch (BSM) and an unburned soil (US). The relationships between the gross N rates calculated with the FLUAZ model (Mary et al. 1998) and 19 soil characteristics were also explored. The gross N mineralisation (m) and NH 4 + immobilisation rates (ia) were similar and significantly correlated (p<0.001) and decreased during the incubation. Both m and ia were higher in burned than in unburned soils and, despite a reduction with time, differences were still 3-fold at t=12 months. Treatment explained half the variation of gross nitrification (kn), which was always low in US and nil in burned soils. Nitrate immobilisation (in) was the only gross rate that was (a) higher in unburned than in burned soils, (b) higher after 12 months than in previous samplings and (c) negatively affected by mulching. Both m and ia were positively correlated with soil pH, initial inorganic-N pools and extractable-Mn (p<0.05 to p<0.001) and negatively correlated with soil water-holding capacity and contents of total C, total N, extractable Cu and extractable Zn (p<0.05 to p<0.001). Both kn and in were negatively correlated with soil characteristics. Despite the strong correlations of pH and total C and N with the gross N transformations, the former variables were never included in the best multiple linear models for the latter. Besides the initial NH 4 + -N or NO 3 − -N contents, all but one best multiple regressions for m and ia included one or two available nutrients, in particular Cu, K, P and Mn. Multiple regressions of kn were only found for US and they included two nutrients (Ca and K, or K, Zn and electrical conductivity). Considering all samples and incubation intervals together, two thirds of in variance was explained by a model with the initial NO 3 − -N, the available Cu and P content. Mulching of burned soil gave a shortlived stimulation of m and ia and a sustained reduction of in.

Fire severity and post‐fire mulching effects on N transformation rates of temperate soils during the first critical winter–spring period

“…The immobilisation of NO 3 − ( I NO3 ) was infrequently modelled in the studied soils, except in those burnt with intermediate severity where it can be, occasionally, the most important NO 3 − consuming process. This result agrees with the fact that, in general, microorganisms immobilise NH 4 + rather than NO 3 − (see Booth et al (2005) and references therein), as also found in other studies with burnt soils (Fernández‐Fernández et al, 2017; Gómez‐Rey & González‐Prieto, 2015). Moreover, NO 3 − immobilisation seems to be suppressed when NH 4 + concentrations are elevated and it tends to be more frequent in microsites where NH 4 + has been depleted (Chen & Stark, 2000).…”

“…Gross N mineralization is usually stimulated by mulching in unburnt soils between 1 month and 1 year after mulch application (Huang et al, 2008; Murphy et al, 2003; Shindo & Nishio, 2005). In burnt soils, Gómez‐Rey and González‐Prieto (2015) also found an increase of gross N mineralization, but short‐lived: it was observed 4 months after the fire, but not after 8 months. Conversely, Fernández‐Fernández et al (2017) reported a decrease in gross N mineralization in mulched burnt soils both after 3 and 6 months of incubation under humidity and temperature conditions optimal for microbial activity.…”

“…Our results are partially coincident with previous studies. Gómez‐Rey and González‐Prieto (2015) found an increase in the gross NH 4 + immobilisation that persisted at least for 1 year in a soil burnt with medium‐to‐high severity, while Fernández‐Fernández et al (2017) reported a short‐lived increase of $$$ {I}_{{\mathrm{NH}}_4\mathrm{lab}} $$$ in a severely burnt soil. As also noticed by these authors, $$$ {I}_{{\mathrm{NH}}_4\mathrm{lab}} $$$ was the main NH 4 + consuming process in most of the studied soils.…”

Aged biochar affects gross nitrogen mineralization and recovery: a 15N study in two contrasting soils

“…The GNM rate in our study ranged between 1.85 and 4.67 mg N kg −1 soil day −1 , which is consistent with several other studies (1–10 mg N kg −1 soil day −1 ; Cheng et al ., ; Gómez‐Rey & González‐Prieto, ), but it is greater than the rate (0.73 mg N kg −1 soil day −1 ) observed by Nelissen et al . ().…”

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

“…In , 2014 straw application after fire effectively reduced erosion but did not affect the soil nutrient concentrations. Mulching has been shown to affect gross N fluxes in unburnt soils (Cheng et al, 2012;Huang et al, 2008), but we are aware of only one experiment testing the effect of straw application on gross N rates in burnt soils (Gómez-Rey and González-Prieto, 2015). This study with a soil affected by a moderate to highly severe wildfire showed that mulching resulted in a short-lived stimulation of gross mineralisation and gross immobilisation of ammonium (NH 4 + ).…”