Effects of forest residue mulching on organic matter and nutrient exports after wildfire in North-Central Portugal.

The quick implementation of emergency stabilization measures is vital for minimizing post-fire soil erosion and the associated fertility loss. Mulching has proven to be highly effective in minimizing post-fire soil erosion, however few studies have investigated its impacts on organic matter (OM) and nutrient mobilization from burned forest areas. This study evaluates the effectiveness of forest residue mulching at reducing OM, N and P losses within the sediments after a moderate-severity wildfire over a period of 5 years (Ermida, North-central Portugal). Untreated and mulched plots of ca. 100 m2 were bounded with geotextile fabric and sediments were collected from silt fences after a total of 29 periods. During the first five years after the fire, the accumulated OM, N and P exportations in the untreated plots were, respectively, 199, 5.2 and 0.38 g m-2; and mulch significantly reduced these figures in, respectively, 91 %, 94 % and 95 % (p < 0.05). The overall OM content in the sediments of the untreated plots (45 %) was not different from the OM content of the mulched plots (34 %, p = 0.16). However, the N (8.9 g kg-1) and Pav contents (0.62 g kg-1) in the untreated plots were significantly higher than the N (5.6 g kg-1; p < 0.05) and Pav contents (0.36 g kg-1; p < 0.05) in the mulched plots. This effect was especially noticeable in the first year after fire. OM and TN contents in the sediments were highly variable throughout the study period, whereas Pav contents declined sharply in the first post-fire rainfall events, maintaining low values afterwards. The main factors driving nutrient exports were ash and litter cover, whereas no significant relationship was observed for OM exports. The present work has shown that forest residues application can be a sustainable strategy for the conservation of soil carbon and nutrients in fire-affected areas.

Effectiveness of community-based initiatives for mitigation of land degradation after wildfires.

Local communities, informed about the main post-fire environmental threats, applied land restoration techniques after the 2017 Ponte Caldelas wildfire (Galicia, Spain). Volunteers crews, supervised by local researchers/technicians identified high-severity burned areas and applied: post-fire mulch by using corn mulch (chopped corn at a rate of 4 Mg ha-1), corn strip mulch (chopped corn on 1-m wide contourline-strips at the bottom of plots, at 1 Mg ha-1) and wheat mulch (wheat straw, at 2 Mg ha-1), as well as acorn seeding with native oak (Quercus robur) acorns. Furthermore, the mulch effect on seed germination/root elongation was tested in the laboratory using direct topsoil samples and <0.45 microns-filtered soil solutions. Soil erosion during the first post-fire year was 27-11 Mg ha-1 for the Parada-Laforet sites, respectively. Corn, corn strip, and wheat mulching were very effective at reducing soil erosion, leading to 95, 76% and 93% less erosion than in untreated plots. During post-fire year 2 mulching was still effective at reducing soil erosion in the case of corn (87%) and corn strip mulch (73%), but less effective with straw mulch (36%). For the third year no differences were observed. Acorn seedling establishment failed in 96% of the cases. Still, survival rate of the germinated acorns was significantly higher on the mulched (50%) than on the untreated spots (26%; p<0.05). The laboratory experiments also showed higher germination rates, shoot length and biomass in mulched than untreated soils. The results demonstrated that the mulching measures applied by the local volunteers were effective for preventing soil erosion after forest fires. The success of ecosystem restoration with acorn seeding was low due to the low germination and the high predation by rodents, and further research is needed to scale acorn seeding to natural recruitment rates.

Can straw-biochar mulching mitigate erosion of wildfire-degraded soils under extreme rainfall?

High severity wildfires cause a drastic alteration of soil carbon cycling - both oxidising and thermally altering soil organic matter (SOM) - and usually are followed by strong runoff and erosion events. To restore wildfire-degraded soils, SOM needs to be rebuilt while soil erosion is prevented. Post-fire straw mulching has been shown to mitigate soil erosion by providing a protective cover against rainsplash. However, SOM takes many decades or centuries to rebuilt naturally. Biochar, co-applied with straw to the soil surface can replace the SOM of the O-horizon, while the stabilised soil - by straw mulching - may gain in SOM naturally and by downward movement of biochar. We conducted a field study to test if straw-only and straw-biochar co-application could restore soils degraded by wildfire in one high burn severity (HBS) and one moderate burn severity (MBS) study area in southern Portugal and Spain, respectively, by monitoring erosion and SOM for the most intense rainfall period of the first post-fire year. Burned sites were characterized for soil and sediment physical properties, TOC content, SOM quality by thermogravimetry (DTG) and nuclear magnetic resonance (NMR 13C) spectroscopy. Straw-biochar mulching significantly reduced soil erosion by 76% and 65% in the HBS and MBS sites, respectively, in both cases similar to the erosion reduction by straw-only mulching. DTG and NMR 13C indicated that a relatively small proportion of the biochar eroded, i.e. 0.7%, indicating that co-application of straw with biochar may help restore the SOC lost in the wildfire in the medium term. The amount of SOM eroded was lower with straw-biochar mulching than in the untreated plots for both study areas. Straw-biochar mulching mitigates erosion of wildfire-degraded soils under extreme rainfall, while a relatively small proportion of the biochar is lost by erosion. Future studies need to monitor medium term effects.

Predicting the effectiveness of different mulching techniques in reducing post-fire runoff and erosion at plot scale with the RUSLE, MMF and PESERA models.

Wildfires have become a recurrent threat for many Mediterranean forest ecosystems. The characteristics of the Mediterranean climate, with its warm and dry summers and mild and wet winters, make this a region prone to wildfire occurrence as well as to post-fire soil erosion. This threat is expected to be aggravated in the future due to climate change and land management practices and planning. The wide recognition of wildfires as a driver for runoff and erosion in burnt forest areas has created a strong demand for model-based tools for predicting the post-fire hydrological and erosion response and, in particular, for predicting the effectiveness of post-fire management operations to mitigate these responses. In this study, the effectiveness of two post-fire treatments (hydromulch and natural pine needle mulch) in reducing post-fire runoff and soil erosion was evaluated against control conditions (i.e. untreated conditions), at different spatial scales. The main objective of this study was to use field data to evaluate the ability of different erosion models: (i) empirical (RUSLE), (ii) semi-empirical (MMF), and (iii) physically-based (PESERA), to predict the hydrological and erosive response as well as the effectiveness of different mulching techniques in fire-affected areas. The results of this study showed that all three models were reasonably able to reproduce the hydrological and erosive processes occurring in burned forest areas. In addition, it was demonstrated that the models can be calibrated at a small spatial scale (0.5 m2) but provide accurate results at greater spatial scales (10 m2). From this work, the RUSLE model seems to be ideal for fast and simple applications (i.e. prioritization of areas-at-risk) mainly due to its simplicity and reduced data requirements. On the other hand, the more complex MMF and PESERA models would be valuable as a base of a possible tool for assessing the risk of water contamination in fire-affected water bodies and for testing different land management scenarios.

Post-fire overland flow generation and inter-rill erosion under simulated rainfall in two eucalypt stands in north-central Portugal.

The aim of this study was to improve the existing knowledge of the runoff and inter-rill erosion response of forest stands following wildfire, focusing on commercial eucalypt plantations and employing field rainfall simulation experiments (RSE's). Repeated RSE's were carried out in two adjacent but contrasting eucalypt stands on steep hill slopes in north-central Portugal that suffered a moderate severity fire in July 2005. This was done at six occasions ranging from 3 to 24 months after the fire and using a paired-plot experimental design that comprised two pairs of RSE's at each site and occasion. Of the 46 RSE's: (i) 24 and 22 RSE's involved application rates of 45-50 and 80-85 mm h(-1), respectively; (ii) 22 took place in a stand that had been ploughed in down slope direction several years before the wildfire and 24 in an unploughed stand. The results showed a clear tendency for extreme-intensity RSE's to produce higher runoff amounts and greater soil and organic matter losses than the simultaneous high-intensity RSE's on the neighbouring plots. However, there existed marked exceptions, both in space (for one of the plot pairs) and time (under intermediate soil water repellency conditions). Also, overland flow generation and erosion varied significantly between the various field campaigns. This temporal pattern markedly differed from a straightforward decline with time-after-fire and rather suggested a seasonal component, reflecting broad variations in topsoil water repellency. The ploughed site produced less runoff and erosion than the unploughed site, contrary to what would be expected if the down slope ploughing had occurred after the wildfire instead of several years before it. Finally, sediment losses at both study sites were noticeably lower than those reported by other studies involving repeat RSE's, i.e. in Australia and western Spain. This possibly reflected a history of intensive land use in the study region, including in more recent times after the widespread introduction of eucalypt plantations.