Comparative study of the effects of wildfire and land use change on soil organic carbon decomposition rate in aggregate size fraction in the Northern Zagros Oak Forest

Forest fires and land use change have become a major global concern due to their significant role in environmental degradation and climate change. Meanwhile, the soil of forests and pastures has always been considered due to its relatively high organic matter and suitable structure. But fires and changes in the management of these ecosystems and the application of tillage have had detrimental effects on soil structure and loss of organic matter of the natural ecosystems. Therefore, this study investigates the effect of wildfires and land use change to vineyard on soil organic carbon (SOC) content and its soil organic carbon decomposition rate (SDR) in aggregate size fraction in an Oak Forest in Northern Zagros.

For this purpose, topsoil (0-5 cm depth) was sampled in a natural oak forest from both inside (FI) and outside (FO) sprout clumps, a portion of the forest burned three years earlier from areas undergone high (BHI), moderate (BMI), or low (BLO) severity, and a vineyard planted thirty years earlier in place of the forest from both under the foliage of vines (VI) and outside it (VO). All soil samples were air-dried and sieved to obtain four aggregate size fractions (8–2, 2–1, 1–0.25, and

The results of this study showed that cultivation and fire, especially its severe type, cause significant damage to large aggregates and increase the fine aggregates. So that, the highest and lowest amounts of coarse aggregates (2–8 mm) were obtained in FI (61.6%) and VI (11.1%) treatments, respectively. Whereas, the highest and lowest amounts of fine aggregates (< 0.25 mm) were obtained in VI (12.4%) and FI (5.7%) treatments, respectively. The results also showed that in all the studied treatments, the amount of SOC increased with decreasing of the soil aggregates size. Furthermore, in compared to the virgin forest, cultivation in the vineyard resulted in a significant reduction in SOC, while no changes in this parameter were observed in the fire treatments. The results also showed that in all the studied treatments, the amount of SDR index increased with decreasing of the soil aggregates size and this index followed the trend: FI (7.2) > BHI (5.4) > FO (5.2) > BMI (4.7) > BLO (2.9) > VI (1.4) > VO (0.4).

Overall, thirty years of farming were more impacting on soil aggregates degradation than a single fire, although severe; nevertheless, severe fire appears to have been much more impacting on soil aggregates degradation than every single yearly plowing. The results also showed that the SDR in soils that are rich in organic matter (FI and BHI) could be more sensitive to warming and intensify global warming more strongly than arable lands (VI and VO).