Investigation of firing effect in rangelands on soil organic carbon changes using remotely sensed based indices

Rangelands are one of the natural ecosystems that have an important part of soil carbon reservoirs and also, as very diverse genetic reservoirs guarantee the dynamics of the ecosystem. Fire is a natural factor in rangelands burning most of the existing natural cover. Rangeland fires directly alter soil microbial activity by burning soil microorganisms and indirectly by reducing organic matter, altering soil organic matter quality and other soil properties. Investigating the positive and negative effects of fire on ecosystems, especially on soil properties, has led researchers to look for alternative methods, instead of direct methods, which are generally very costly and time-consuming. One of the new methods and technologies that are very useful in the field of natural resources is satellite remote sensing. The purpose of this study was to investigate the short-term effect of fire on organic carbon, acidity, and electrical conductivity of rangeland soils in the Gonbad region of Hamadan, and to investigate the capability of remotely sensed data in the indirect estimation of soil surface carbon in semi-arid rangelands after the fire.  

 In this study, 20 soil samples were taken from each site from a depth of 0-10 cm (40 samples in total) and the coordinates of each sampling point were recorded with a GPS device. Sampling was performed 15 to 20 days after the fire in early October. After transferring to the laboratory, the samples were used to measure the amount of soil organic carbon. Then, the statistical relationship between non-burned areas and burned areas was examined and analyzed by an independent t-test. Indirect estimation of soil surface organic carbon at non-burned and burned sites was also investigated and their changes were evaluated using remote sensing satellite imagery. For this purpose, after performing the pre and post-processing on satellite data, the corresponding values ​​of spectral reflectance of each pixel with sampling points at different wavelengths and spectral indices were extracted, and the correlation and regression equation of indices with the Carbon reservoirs were analyzed.

 The results of the Pearson correlation test showed that among all spectral indices, only the HI index was correlated with soil organic carbon in the short time and in the non-burned site. Besides, among all indices, BI, NDBI, NDVI, SAVI, VCI, and VHI indices were correlated with the EC value in the non-burned site. At the non-burned site, there was a significant correlation between most spectral indices and soil EC, which was eliminated after the fire at the burned site. Regarding the correlation between pH and spectral indices, it was observed that there is a correlation between some spectral indices and pH. As a matter of fact, it can be concluded that the fire has caused a large change in the rate of reflection and propagation of waves from the soil surface so that in the non-burned site, the indices were correlated with EC, but in the burned site, the correlation between indices and EC was completely eliminated, and instead, a correlation has been established between the indices and the pH. Furthermore, none of the spectral indices in April 2017 at the non-burned site had a significant positive or negative correlation with soil organic carbon, and the results showed that after six months of the fire, the soil carbon changes were not such that the spectral indices could be examined its process. Comparing the results of October 2016 with the results of April 2017 on the non-burned site, it was found that after six months, the NBR index has found a significant correlation with the EC rate, but the BI and VHI indices have lost their correlation. According to the NBR index and the SWIR2 band, it seems that after six months from the occurrence of the fire, changes have occurred in the control site, which has led to a correlation between this index and soil EC. Since the amount of reflected energy from the earth's surface depends on several factors such as soil moisture, changes in soil organic matter content, and surface cover, so the effect of these factors on the soil reflectance should be considered in the growing season. Failure to change these results after six months can prove that the positive and negative effects of the fire have not disappeared in a short period of six months and a longer time is needed for the situation to return to normal.

 According to the results, it was found that soil organic carbon reservoirs in burned rangelands in comparison with non-burned rangelands is not significantly different. Deformation and stabilization of soil organic matter due to fire have been studied by many researchers, but the transformation of soil organic matter by fire has often led to heterogeneous and different results. At a depth of 10-20 cm, the fire was found to have no effect on soil organic carbon content, but other researchers found that 6 months after the fire, the amount of carbon in the burned soils increased compared to the non-burned soils. It was also found that the percentage of soil organic carbon decreased significantly three months after the fire. Moreover, in another study on the effect of fire on soil organic carbon, it was found that in the area affected by the fire compared to the control area in one year and two years after the fire, the amount of soil organic carbon has decreased significantly. Since the effect of fire on the physical and chemical properties of soil is strongly influenced by fire intensity, soil moisture, climate, and vegetation, so all these factors have led to different results in investigating the effect of fire on soil organic carbon. Due to environmental conditions, climate, the slope of the area, soil texture and structure, and factors related to fire such as its intensity and duration, the amount of soil carbon has changed. For example, in the event of a medium-sized fire, the conditions for vegetation regrowth are faster, but in the event of a severe fire, the entire organic layer of the soil surface is generally removed and carbon is reduced over time. Also, in examining the correlation between spectral indices and soil organic carbon, it was found that only the HI index with soil organic carbon was significant at the non-burned site, but no correlation was observed at the burned site. This can be examined by examining the spectrum of visible blue and green wavelengths in the mathematical relationship of this index because only in this index the green and blue wavelength spectrum have been used. According to the results of other researchers, it seems that estimating soil organic carbon using remote sensing has certain complexities. Since soil organic carbon has the greatest impact on soil color, it is difficult to estimate it using remotely sensed data if its amount is low. The occurrence of fire in the region has a major impact on the spectral reflectance of surface soil so that after the fire in a short time the correlation of HI index with soil organic carbon is lost. According to the results of the present research, it seems that the main point about the impact of fire on soil organic carbon is the time and the opportunity for soil to change.