5:15 PM - 6:45 PM
[HTT16-P07] Effects of fire on phosphate oxygen isotope ratio in soil: A experimental study
Introduction
Fire can significantly change the physical, chemical, and biological properties of soils. The effects of fire are complex and are influenced by factors such as fire intensity, duration, and soil properties (Alcañiz et al., 2018). Phosphorus (P) in soils can be a limiting factor for plant growth in terrestrial ecosystems and is primarily existing in organic form. Previous studies have reported that fire increases bioavailable inorganic phosphate (PO4) through thermal mineralization of organic PO4 (Alcañiz et al., 2018). However, it is difficult to evaluate impacts of fire on P dynamics in soils due to the large spatial heterogeneity of fire spread and soil properties.
Phosphate oxygen isotope ratios (δ18OPO4) of inorganic PO4 could be a useful tool to assess fire-induced transformations in soils P. During mineralization of organic PO4, oxygen atoms are incorporated into inorganic PO4 from possible oxygen sources (e.g. water, carbonate, and air), resulting in isotopic shift. Therefore, δ18OPO4 analysis should be possible to evaluate fire-induced P transformation. However, the response of δ18OPO4 in soils to fire is not clear due to a lack of studies.
The purpose of this study was to validate the utility of the δ18OPO4 analysis to assess fire-induced changes in P dynamics. We examined the response of δ18OPO4 to temperature through soil burning experiments.
Material and Methods
A surface soil sample was collected from cropland in the Yogo Town, Nagahama City, Shiga Prefecture, central Japan. The soil sample was heated in an electric furnace at 50°C, 150°C, 350°C, and 550°C for 3 hours. Silver phosphate (Ag3PO4) with a known δ18OPO4 value (10.7‰) was also heated.
After heating, the soil P was fractionated using Hedley's sequential extraction method (Hedley et al., 1982) into labile P, aluminum- and iron-bound P, and calcium-bound P. The extracted inorganic PO4 from each fraction was purified following Ishida et al., (2022) and the δ18OPO4 values were measured using a thermal conversion elemental analyzer with glassy carbon connected to an isotope ratio mass spectrometer (Delta-V advantage via ConFlo IV, Thermo Fisher Scientific) at the Research Institute for Humanity and Nature in Japan. The analytical precision (±standard deviation) was ±0.4‰.
Results and Discussion
The response of δ18OPO4 values to temperature for soils and Ag3PO4 samples were different. For the soil sample, δ18OPO4 values in all P fractions decreased above 350°C. On the other hand, the δ18OPO4 values of Ag3PO4 sample decreased only at 550°C. The decrease in δ18OPO4 values of Ag3PO4 sample suggests isotopic exchange between PO4 and possible oxygen source at 550°C because of the absence of organic PO4 in Ag3PO4 sample. In addition, the result indicates that the exchange between PO4 and the oxygen sources does not occur below 350°C. Therefore, the decrease in the δ18OPO4 value of the soils heated at 350°C can be attributed to thermal mineralization of organic PO4. At the 550°C heating, thermal mineralization and isotope exchange should be involved in the isotope shift in soils.
We successfully demonstrated that δ18OPO4 analysis is a promising tool for assessing fire-induced P transformation in soils by characterizing the response of δ18OPO4 to temperatures. In future studies, identification of the oxygen sources to PO4 is important for predicting the isotopic shift by fire. We also need to clarify the isotopic fractionation with temperature. The δ18OPO4 analysis would be a valuable tool to elucidate the complexity of P dynamics due to the interaction between the high spatial heterogeneity of fire and soil properties.
Fire can significantly change the physical, chemical, and biological properties of soils. The effects of fire are complex and are influenced by factors such as fire intensity, duration, and soil properties (Alcañiz et al., 2018). Phosphorus (P) in soils can be a limiting factor for plant growth in terrestrial ecosystems and is primarily existing in organic form. Previous studies have reported that fire increases bioavailable inorganic phosphate (PO4) through thermal mineralization of organic PO4 (Alcañiz et al., 2018). However, it is difficult to evaluate impacts of fire on P dynamics in soils due to the large spatial heterogeneity of fire spread and soil properties.
Phosphate oxygen isotope ratios (δ18OPO4) of inorganic PO4 could be a useful tool to assess fire-induced transformations in soils P. During mineralization of organic PO4, oxygen atoms are incorporated into inorganic PO4 from possible oxygen sources (e.g. water, carbonate, and air), resulting in isotopic shift. Therefore, δ18OPO4 analysis should be possible to evaluate fire-induced P transformation. However, the response of δ18OPO4 in soils to fire is not clear due to a lack of studies.
The purpose of this study was to validate the utility of the δ18OPO4 analysis to assess fire-induced changes in P dynamics. We examined the response of δ18OPO4 to temperature through soil burning experiments.
Material and Methods
A surface soil sample was collected from cropland in the Yogo Town, Nagahama City, Shiga Prefecture, central Japan. The soil sample was heated in an electric furnace at 50°C, 150°C, 350°C, and 550°C for 3 hours. Silver phosphate (Ag3PO4) with a known δ18OPO4 value (10.7‰) was also heated.
After heating, the soil P was fractionated using Hedley's sequential extraction method (Hedley et al., 1982) into labile P, aluminum- and iron-bound P, and calcium-bound P. The extracted inorganic PO4 from each fraction was purified following Ishida et al., (2022) and the δ18OPO4 values were measured using a thermal conversion elemental analyzer with glassy carbon connected to an isotope ratio mass spectrometer (Delta-V advantage via ConFlo IV, Thermo Fisher Scientific) at the Research Institute for Humanity and Nature in Japan. The analytical precision (±standard deviation) was ±0.4‰.
Results and Discussion
The response of δ18OPO4 values to temperature for soils and Ag3PO4 samples were different. For the soil sample, δ18OPO4 values in all P fractions decreased above 350°C. On the other hand, the δ18OPO4 values of Ag3PO4 sample decreased only at 550°C. The decrease in δ18OPO4 values of Ag3PO4 sample suggests isotopic exchange between PO4 and possible oxygen source at 550°C because of the absence of organic PO4 in Ag3PO4 sample. In addition, the result indicates that the exchange between PO4 and the oxygen sources does not occur below 350°C. Therefore, the decrease in the δ18OPO4 value of the soils heated at 350°C can be attributed to thermal mineralization of organic PO4. At the 550°C heating, thermal mineralization and isotope exchange should be involved in the isotope shift in soils.
We successfully demonstrated that δ18OPO4 analysis is a promising tool for assessing fire-induced P transformation in soils by characterizing the response of δ18OPO4 to temperatures. In future studies, identification of the oxygen sources to PO4 is important for predicting the isotopic shift by fire. We also need to clarify the isotopic fractionation with temperature. The δ18OPO4 analysis would be a valuable tool to elucidate the complexity of P dynamics due to the interaction between the high spatial heterogeneity of fire and soil properties.