31 Chapter 3 Variability of organic matter in fire affected soils

lignite coals (Fakoussa & Hofrichter 1999). Subjecting SOM of fire-affected and unaffected soils from the Sierra de Aznalcóllar, Southern Spain, to biochemical degradation under laboratory conditions over a period of 7 months revealed mean residence times (MRTs) of PyOM which were only five to six times longer than those determined for unburnt SOM (Knicker et al. 2013). Considering further that without additional burnings, degraded SOM and PyOM is replaced by constant input of fresh litter, such short MRTs should lead to a decrease of the aromaticity of SOM with increasing time elapsed since the last fire.

On the other hand, loss of PyOM may also occur due to erosion, as was recently shown in catchments of mountainous areas (Kloss et al. 2012), or due to vertical leaching as indicated after characterizing the SOM in soil profiles under a Brazilian Cerrado (Velasco-Molina et al. 2016). However, irrespective of the involved mechanism, the observed alterations of the post-fire SOM suggests a steady recuperation of a fire-affected ecosystem, although it is unclear to what extent such processes proceed. This is mainly due to the fact that most of the studies on the impact of vegetation fires on soil ecosystems have compared unburnt soils by fire with soil having been burnt just recently before sampling (Certini et al. 2011, Kara & Bolat 2009, Knicker et al. 2006). Only a few investigations have monitored medium-term effects (De la Rosa et al. 2013, Prieto- Fernández et al. 2004, Vergnoux et al. 2011), reporting decreasing microbial activity, pH, C- and nitrate contents, with prolonged post-fire recovery time. However, in order to elucidate lasting impact of fire on SOM and related biochemical cycles, more studies following the changes of post-fire SOM as a function of recuperation time on a medium and long term-scale are needed.

Many of the fire-affected areas in the Mediterranean region affected by fire are mountainous. This implies that the impact of relief on the spatial distribution of soil properties and SOM composition has to be taken into account. In addition, one has to bear in mind that, prior to the burning, fuel distribution may not have been homogeneous. This could cause considerable variation of the quantity and quality of produced PyOM. Certainly, well accepted statistical approaches available (Li 2010a) accounting for such inhomogeneities are available. However, many of those require sample replication numbers which are rarely feasible for many labor- intense or expensive analytical techniques, although the latter could provide information that is urgently needed for a better understanding of the ecological dynamics and the long term measures for recuperation of such areas.

One of those is solid-state 13C NMR spectroscopy. As a non-degradative approach, it represents the only method which allows the quantitative determination of the chemical composition of SOC without a prior extraction (Knicker 2011b). Unfortunately, only a few

32 The impact of soil disturbance on N and C in Mediterranean soils

laboratories offer access to such instruments and the usable instrument time is often extremely limited. As a consequence, in many cases, it is simply not possible to measure the amount of replicates commonly required for an appropriate statistical approach.

Thus, if one does not want to abstain from the valuable information this technique can deliver, sampling approaches which allow the researcher to extract meaningful results from a manageable amount of samples are critically important. In the present study, we elucidated the extent to which relief and pre-fire fuel distribution affects soil properties and composition of SOM in fire-affected and unburnt soils of the hilly area of the Sierra de Aznalcóllar, Andalusia, Spain. Therefore, samples from several spots of various sites with different relief, fire history and vegetation were compared with respect to their C and N contents, their acidity and the composition of their SOM as revealed by means of solid-state 13C NMR spectroscopy. Within this study two sampling methods with reduced numbers of replicates were tested. For the first, at each site, five spots from the corners and the center of a square with a side length of approximately 15 m were probed and each sample was analyzed. The respective results were obtained by the means of those five measurements. For the second approach, for each site, three composite samples of five spots were obtained with the same scheme. Here, the results derived from the average of the analysis of the three replicate plots. With the help of standard errors, we sought to test if this approach can be used to determine differences in soil parameters and SOM composition of unburnt soils and soils which were burnt once or twice.

3.2. Material and methods 3.2.1. Site description

The samples derived from the Sierra de Anzálcollar which is close to Seville in Southern Spain. This mountain area is part of the Sierra Morena and covers the Northwest of Andalusia. The sampled soils are classified as Cambisols (IUSS Working Group WRB 2014) and developed on slate, sandstones, and quartzite with occasional carbonates outcrop. The Sierra de Aználcollar represents a region with a high-to-moderate fire risk during the summer months.

In 2004, this area suffered a very intense fire which destroyed approximately 30 000 ha of forest. Some sampling locations were also affected by an intense fire in 1996. A more detailed description of the sites is given in Table 3.1.

For our study, we used two different sampling methods. For the first, four corners and the center of a randomly chosen square with a side length of 15 m (Fig. 3.1). For the second

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