Fire effects on C and H isotopic composition in plant biomass and soil: Bulk and particle size fractions.

This work studies carbon (C) and hydrogen (H) isotope composition of plant biomass and soil organic matter (SOM) in an attempt to assess both, changes exerted by fire and possible inputs of charred materials to the soil after a wildfire. Isotope composition of bulk soil, soil particle size fractions and biomass of the dominant standing vegetation in the area (Quercus suber) from Donana National Park (SW-Spain) were studied by isotope ratio mass spectrometry (IRMS). SOM C isotope composition indicates the occurrence of two SOM pools with different degree of alteration. Coarse soil fractions (>0.5 mm) were found C-13 depleted with delta O-13 values close to those in leaf biomass, pointing to a predominance of poorly transformed SOM. Conversely, fine fractions (<0.1 mm) were found enriched in C-13 as corresponds to a more humified SOM. The fire produced no changes in this trend, although a consistent C-13 enrichment (c. 1 parts per thousand) was observed in all soil fractions with decreasing size. Concerning H isotopes, the coarse fractions (>0.5 mm) displayed significant lower delta H-2 values than the intermediate and fine ones (<0.5 mm), again similar to those in leaf biomass (c.-80 parts per thousand), whereas the fine fractions were found deuterium (H-2)-enriched with significant higher delta H-2 values (c. 50 parts per thousand), suggesting physical speciation of H depending on soil particle size. The fire produced a significant H-2 depletion (Delta H-2 c. -10 parts per thousand) in the finer fractions (<0.1 mm). The study of stable isotope analysis added new information and complements the results obtained by other proxies to better understand the effect of fire on SOM. (C)2020 Elsevier B.V. All rights reserved.