13:45 〜 15:15
[HTT15-P05] Isotopic evidence for fire-induced changes in phosphorus dynamics in soils.
キーワード：焼畑、土壌養分、安定同位体比
1. Introduction
Fire is a common process that is universally experienced in many ecosystems and is introduced by natural processes such as natural fires or anthropogenic processes such as slash-and-burn. Fire significantly alters vegetation structures and soil properties. Many previous studies have shown that soil nutrients increase after fire. On the other hand, some studies have reported that fire did not increase soil nutrients (Alcañiz et al. 2018). This discrepancy may be attributed to the large heterogeneity in soil and fire effects. To effectively evaluate the effect of fire on soil properties, qualitative indicators would be useful.
Understanding the effects of fire on phosphorus (P) dynamics is important because P can be a limiting factor for plants in many ecosystems. Phosphate oxygen isotope (δ18OPO4) could be useful tools to examine fire-induced P transformation. The δ18OPO4 is a stable isotope ratio of oxygen (18O/16O) bound to phosphate (PO4). When inorganic PO4 is formed from organic P by the fire, it obtains oxygen atoms from the surrounding water (Sarancha et al. 2022). Since the oxygen isotope value in ambient water is often negative, the δ18OPO4 of produced P by the fire should be a low value. This characteristic of the δ18OPO4 could be used to determine how much P is transformed by the fire. However, there are no studies using the δ18OPO4 to assess the effects of fire on P dynamics in soils.
The purpose of this study is to evaluate fire-induced changes in soil P using the δ18OPO4 analysis.
2. Materials and methods
The study was conducted in Yogo area, Shiga Prefecture, central Japan. In this region, slash-and-burn is practiced on the mountain slopes. We collected soil samples from three sampling points immediately before (Pre1, Pre2, Pre3) and after (Post1, Post2, Post3) the fire. The samples were collected from the surface to 10 cm depth at 2 cm intervals.
The P in the soil sample was sequentially extracted using the Hedley procedure (Hedley et al., 1982). The Hedley procedure was performed by shaking at a soil–solution ratio of 3.3:200 for 16 h using 0.5 M NaHCO3, 0.1 M NaOH and 1 M HCl. The inorganic P concentration (InP) of each P fraction was measured on a spectrophotometer (Multiskan GO, Thermo Fisher Scientific) using the molybdenum blue method (Murphy and Riley 1962). The total P concentration in each P fraction was also measured by the molybdenum blue method after oxidized by adding potassium peroxodisulfate and then digested in an autoclave (JIS, 1993). The organic P (Org-P) concentration was obtained as the difference between total P and InP.
The δ18OPO4 analysis was applied to the extracts of Hedley procedure. The PO4 in extract solutions was concentrated using the magnesium-induced coprecipitation. The concentrated PO4 was converted to an Ag3PO4 solid sample through the ZrME method (Ishida et al., 2022). The δ18OPO4 of Ag3PO4 samples was measured using a thermal conversion elemental analyzer 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.
3. Results and discussions
Before the fire, the P concentrations extracted by Hedley procedure varied little with depth. The HCl-OrgP was the predominant P form from the surface to 10 cm depth. The NaHCO3-InP was dominant in the surface of Pre3. After the fire, the high P concentrations were found in the surface of Post1 and Post2. In Post3, the concentrations varied little with depth.
Before the fire, δ18OPO4 values varied little with depth in all fractions, except HCl-InP of Pre2, which was low value in the surface. In contrast, after the fire the lowest δ18OPO4 value were found in the surface of all sites and fractions. This low δ18OPO4 may indicate the presence of P produced by fire, suggesting that P produced by the fire could be detected by the δ18OPO4.
Fire is a common process that is universally experienced in many ecosystems and is introduced by natural processes such as natural fires or anthropogenic processes such as slash-and-burn. Fire significantly alters vegetation structures and soil properties. Many previous studies have shown that soil nutrients increase after fire. On the other hand, some studies have reported that fire did not increase soil nutrients (Alcañiz et al. 2018). This discrepancy may be attributed to the large heterogeneity in soil and fire effects. To effectively evaluate the effect of fire on soil properties, qualitative indicators would be useful.
Understanding the effects of fire on phosphorus (P) dynamics is important because P can be a limiting factor for plants in many ecosystems. Phosphate oxygen isotope (δ18OPO4) could be useful tools to examine fire-induced P transformation. The δ18OPO4 is a stable isotope ratio of oxygen (18O/16O) bound to phosphate (PO4). When inorganic PO4 is formed from organic P by the fire, it obtains oxygen atoms from the surrounding water (Sarancha et al. 2022). Since the oxygen isotope value in ambient water is often negative, the δ18OPO4 of produced P by the fire should be a low value. This characteristic of the δ18OPO4 could be used to determine how much P is transformed by the fire. However, there are no studies using the δ18OPO4 to assess the effects of fire on P dynamics in soils.
The purpose of this study is to evaluate fire-induced changes in soil P using the δ18OPO4 analysis.
2. Materials and methods
The study was conducted in Yogo area, Shiga Prefecture, central Japan. In this region, slash-and-burn is practiced on the mountain slopes. We collected soil samples from three sampling points immediately before (Pre1, Pre2, Pre3) and after (Post1, Post2, Post3) the fire. The samples were collected from the surface to 10 cm depth at 2 cm intervals.
The P in the soil sample was sequentially extracted using the Hedley procedure (Hedley et al., 1982). The Hedley procedure was performed by shaking at a soil–solution ratio of 3.3:200 for 16 h using 0.5 M NaHCO3, 0.1 M NaOH and 1 M HCl. The inorganic P concentration (InP) of each P fraction was measured on a spectrophotometer (Multiskan GO, Thermo Fisher Scientific) using the molybdenum blue method (Murphy and Riley 1962). The total P concentration in each P fraction was also measured by the molybdenum blue method after oxidized by adding potassium peroxodisulfate and then digested in an autoclave (JIS, 1993). The organic P (Org-P) concentration was obtained as the difference between total P and InP.
The δ18OPO4 analysis was applied to the extracts of Hedley procedure. The PO4 in extract solutions was concentrated using the magnesium-induced coprecipitation. The concentrated PO4 was converted to an Ag3PO4 solid sample through the ZrME method (Ishida et al., 2022). The δ18OPO4 of Ag3PO4 samples was measured using a thermal conversion elemental analyzer 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.
3. Results and discussions
Before the fire, the P concentrations extracted by Hedley procedure varied little with depth. The HCl-OrgP was the predominant P form from the surface to 10 cm depth. The NaHCO3-InP was dominant in the surface of Pre3. After the fire, the high P concentrations were found in the surface of Post1 and Post2. In Post3, the concentrations varied little with depth.
Before the fire, δ18OPO4 values varied little with depth in all fractions, except HCl-InP of Pre2, which was low value in the surface. In contrast, after the fire the lowest δ18OPO4 value were found in the surface of all sites and fractions. This low δ18OPO4 may indicate the presence of P produced by fire, suggesting that P produced by the fire could be detected by the δ18OPO4.