149 Capítulo 8 Discusión general

possibly because the experimental conditions avoided N-limitation. Under those conditions, the availability of additional Ni did not decrease the efficiency of 15N uptake from unburnt or burnt

OM. Thus, negative priming of Ni was not observed. Based on this we can assume that kind and amount of 15N which is taken up by the plants depends mainly on the bioavailability of the 15N, which again is determined by the overall degradability of the 15N-containing organic source. Although slower than N in unburnt litter, we were able to evidence degradation of BN and thus the mobilization of its BN. In summary, our results point to the conclusion that incorporation of charred organic residues and ash into the soil after a forest fire can indeed alter N-cycling in soil, although this alteration will be mainly effective on a short and medium term. Fertilization with Ni from the ash directly after the fire provides nutrients for a quick recovery of the ground

vegetation and microbial activity, whereas sequestration of N in PyOM allows a slow release of N for biomass growth on a medium term scale. This prevents N-losses from the ecosystem and provides additional N for the next vegetation cycles. Considering the importance of a fast development of a new plant cover after fire events to avoid soil erosion and desertification, our study clearly underlined the important ecological role of BN in fire-prone ecosystems, a fact that certainly asked for an improved consideration of BN in environmental research.

However, going into more detail with respect to the fate of BN in soils by analyzing the recovery of its N in soil amino acids (AAs), some impact of charcoal input on N-cycling was

observed. It seems that after the fire events, the newly synthesized peptides have a lower resistance against acid hydrolysis than peptides immobilized in soil organic matter formed without fire impact. Addition of fresh litter seems to shift the higher extractability of AAs towards the unburnt soil, possibly because the first provides additional labile AAs and allows for a higher microbial activity leading to new biomolecules with low resistance against acid hydrolysis. According to our results, at least some N of BN was recycled for the built-up of peptides in newly synthesized microbial biomass. With this step, the BN-derived nitrogen has transformed into a biogenic N source and is expected to behave as such during N cycling within the soil organic nitrogen (SON) pool. Our studies indicated further that inorganic N, organic N of PyOM and OM are simultaneously used as N supply. Thus, the presence of easily bioavailable N is not hindering the synthesis of new soil peptides from N in bound charred organic residues.

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

8.3. References

Glaser B, Amelung W (2003): Pyrogenic carbon in native grassland soils along a climosequence in North America. Global Biogeochemical Cycles 17, n/a-n/a

González-Pérez JA, González-Vila FJ, Almendros G, Knicker H (2004): The effect of fire on soil organic matter—a review. Environment International 30, 855-870

Guénon R, Vennetier M, Dupuy N, Ziarelli F, Gros R (2011): Soil organic matter quality and microbial catabolic functions along a gradient of wildfire history in a Mediterranean ecosystem. Applied Soil Ecology 48, 81-93

Skjernstad J O, Taylor J A, Smernik R J (1999): Estimation of charcoal (char) in soils. Communications in Soil Science and Plant Analysis 30, 2283-229

Vasilyeva NA, Abiven S, Milanovskiy EY, Hilf M, Rizhkov OV, Schmidt MWI (2011): Pyrogenic carbon quantity and quality unchanged after 55 years of organic matter depletion in a Chernozem. Soil Biology and Biochemistry 43, 1985-1988

Capítulo 9.

Conclusiones

Chapter 9.

CONCLUSIONS

The examination of the soil organic matter (SOM) in fire-affected and unburnt soils in the Sierra de Aznalcóllar clearly confirmed that the longevity of pyrogenic organic matter (PyOM) in their soils is by far shorter than commonly assumed for Black Carbon (BC). Although being longer than fresh litter, mean residence times in the range of humified SOM were recently determined for PyOM (Knicker et al. 2013). Possibly, the mild climatic conditions of the Mediterranean areas during winter seasons together with soil conditions, which are favorable for microbial activity, support a fast turnover rate not only of naturally formed SOM but also of PyOM. However, the relatively fast loss of PyOM by biodegradation during post-fire recovery time can solve the contradiction observed by (Masiello & Druffel 2003). They stated that “If Black Carbon has been produced since the last glacial maximum via biomass burning at the same rate as it is now produced, BC should account for 25 – 125% of the total soil organic carbon pool”.

The fact that in topsoil, the majority of PyOM does not survive for millennia has been also observed in other studies (De la Rosa et al. 2013, Velasco-Molina et al. 2016), and asks for a more detailed analysis of the role of BC as an important long-term carbon (C) sink within the global C cycling. The fact that at archeological sites and sediments considerably old PyOM has

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

been discovered suggests that longevity of PyOM is closely related to the conditions under which it is produced and accumulated. However, the involved mechanisms still need to be examined in more detail.

Comparable to other studies, most of the surviving PyOM after seven years of the fire was recovered with the particulate organic fractions (POM). This seems in contrast to a suggested protection of partially degraded PyOM from microbial degradation by interaction with the mineral phase as it was proposed for Australian soils (Qi et al. 2017). However, it may be speculated that the unprotected PyOM in the POM fraction may be finally degraded whereas the protected PyOM in the mineral fractions will accumulate. According to this scenario, only the latter may be still present on a long-term time scale.

The organic nitrogen (N) in PyOM was commonly neglected and only recently the scientific community started to become interested in this issue. In contrast to views, in which heterocyclic aromatic N represents biochemically recalcitrance forms, the present study confirmed that Black Nitrogen (BN) can be degraded and its N can be used both by microorganism and by plants for the built-up of new biomass. Here, one has to consider that the incorporation rate of N from BN was considerably lower than those from inorganic N or unburnt organic N, however, in fire- affected areas it plays certainly an important role for N-storage. Whereas, after the fire Ni released during combustion of OM represents an N source which is directly available for the recovery of the vegetation, BN can act as a slow-release fertilizer preventing considerable N losses due to leaching of nutrients after the first post-fire rains. From this point of view, BN accumulating after fires on the topsoil may play a more important role for ecosystem recovery than formerly thought which certainly needs more attention not only for a better understanding of fire-prone ecosystems but also for the development of improved post-fire management strategies.

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