Effects of fire occurrence and recurrence on nitrogen and phosphorus losses by overland flow in maritime pine plantations in north-central Portugal

- Recurrent fires are expected to induce marked changes on the N and P cycles.
- Repeated wildfires strongly increased N and P losses by runoff.
- N and P exports were highest after the first post-fire rainfall event.
- N and P losses were dependent on litter and vegetation recovery after fire.

Wildfires have increased in Portugal in the recent decades, raising concerns about the long-term negative effects of fire recurrence on the environment. We studied the impacts of recurrent fires on the nitrogen (N) and phosphorus (P) content of mineral soil in the first year after a fire. Total nitrogen (TN) and phosphorus (TP) losses by runoff were also evaluated within the two years after a fire. Nine sites in a maritime pine forest were selected following a large wildfire in September 2012 that affected roughly 3000 ha of the Viseu municipality. Three sites had been burnt four times in the past 40 years (4 ×), three sites had been burnt once in September 2012 (1 ×), and three control sites had not been burnt (0 ×). Runoff was collected in 9 micro-plots (0.25 m2) at each site after rain events from September 2012 to September 2014. Soil N and P content were significantly higher in both burnt sites relatively to the control sites. Nitrogen as well as phosphorus losses via runoff were significantly higher at the 4 × burnt sites than at both the 1 × burnt and unburnt sites. Nutrient loss was particularly high after heavy rains. Vegetation and litter cover played an important role in reducing runoff and the associated N and P transport at the 4 × burnt sites, since a decrease in both variables was observed with the increase in vegetation cover after fire.

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