10:45 〜 12:15
[AHW18-P05] Assessment of the phosphorus leaching from phosphorus-saturated orchard soils
キーワード：富栄養化、安定同位体比、地下水
1. Introduction
Long-term phosphorus (P) fertilization results in abundant P accumulation in agricultural soils (legacy P). Legacy P increases the risk of P leaching from agricultural lands into coastal ecosystems through surface water and groundwater. However, the impact of legacy P on groundwater systems is unclear, especially in clayey soils with high P adsorption capacity. Soil P storage capacity (SPSC) and the oxygen stable isotope ratio of phosphate (δ18OPO4) can be useful tools to examine legacy P leaching at the field level. The δ18OPO4 can be used to estimate P input from sources with unique isotopic signatures and the degree of biological processes (Paytan and McLaughlin, 2012). The SPSC is an indicator of the “safe” soil P storage capacity that considers the P sorption capacity in soil (Nair and Harris, 2014).
This study aims to evaluate the legacy P leaching into groundwater using a δ18OPO4 technique and the concept of soil SPSC.
2. Material and Methods
The study was conducted in the Kubi area of Osakishimojima, a small island in the Seto Inland Sea in Hiroshima Prefecture, western Japan. Citrus cultivation, the main industry in the area, began in about 1900. The use of P fertilizers in Japan has increased since the 1960s with significant developments in the economy and agricultural technologies.
We collected the water samples of shallow groundwater (shallow GW, 16 sites), deep groundwater (deep GW, 2 sites) and river water (2 sites) and the source samples of lithological sediments (1 site), forest soils (3 sites), orchard soils (4 sites) and fertilizers (organic fertilizer and chemical fertilizer).
The P concentration and SPSC values in the soils were determined by Mehlich-3 extractable P, Al, and Fe. The δ18OPO4 analysis was applied to the shallow GW, river water, lithological sediment, orchard soil, and fertilizer. The δ18OPO4 samples were prepared by ZrME method (Ishida et al., 2022) and 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. Result and discussion
Shallow groundwater had oxic conditions with high PO4 concentrations (>0.1 mgP L−1). Soil P was highly accumulated in orchard soils (498–1167 mgP kg−1) compared to forest soils (2.9–14 mgP kg−1). SPSCs were positive for all forest soils and negative for all orchard soils, indicating that forest soils were P sinks and orchard soils were P sources. The δ18OPO4 values in shallow groundwater (15.2‰–17.5‰) were higher than the expected biological equilibrium values, indicating significant P sources with high d18OPO4 values for the groundwater. The negative SPSC and high δ18OPO4 values (19.7‰–23.7‰) in orchard soils indicated that P leaching from orchard soils was the major P enrichment mechanism in shallow groundwater. The amount of high-risk P, easily leached from the soil to water bodies, was estimated to be 1691–3500 kgP ha−1. The values are equivalent to 29.6–61.3 years of annual surplus P. This study demonstrates that groundwater can be an important pathway of legacy P runoff, even in clayey soils, and that there is an urgent need to optimize agricultural practices according to the actual nutrient status of agricultural soils.
Long-term phosphorus (P) fertilization results in abundant P accumulation in agricultural soils (legacy P). Legacy P increases the risk of P leaching from agricultural lands into coastal ecosystems through surface water and groundwater. However, the impact of legacy P on groundwater systems is unclear, especially in clayey soils with high P adsorption capacity. Soil P storage capacity (SPSC) and the oxygen stable isotope ratio of phosphate (δ18OPO4) can be useful tools to examine legacy P leaching at the field level. The δ18OPO4 can be used to estimate P input from sources with unique isotopic signatures and the degree of biological processes (Paytan and McLaughlin, 2012). The SPSC is an indicator of the “safe” soil P storage capacity that considers the P sorption capacity in soil (Nair and Harris, 2014).
This study aims to evaluate the legacy P leaching into groundwater using a δ18OPO4 technique and the concept of soil SPSC.
2. Material and Methods
The study was conducted in the Kubi area of Osakishimojima, a small island in the Seto Inland Sea in Hiroshima Prefecture, western Japan. Citrus cultivation, the main industry in the area, began in about 1900. The use of P fertilizers in Japan has increased since the 1960s with significant developments in the economy and agricultural technologies.
We collected the water samples of shallow groundwater (shallow GW, 16 sites), deep groundwater (deep GW, 2 sites) and river water (2 sites) and the source samples of lithological sediments (1 site), forest soils (3 sites), orchard soils (4 sites) and fertilizers (organic fertilizer and chemical fertilizer).
The P concentration and SPSC values in the soils were determined by Mehlich-3 extractable P, Al, and Fe. The δ18OPO4 analysis was applied to the shallow GW, river water, lithological sediment, orchard soil, and fertilizer. The δ18OPO4 samples were prepared by ZrME method (Ishida et al., 2022) and 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. Result and discussion
Shallow groundwater had oxic conditions with high PO4 concentrations (>0.1 mgP L−1). Soil P was highly accumulated in orchard soils (498–1167 mgP kg−1) compared to forest soils (2.9–14 mgP kg−1). SPSCs were positive for all forest soils and negative for all orchard soils, indicating that forest soils were P sinks and orchard soils were P sources. The δ18OPO4 values in shallow groundwater (15.2‰–17.5‰) were higher than the expected biological equilibrium values, indicating significant P sources with high d18OPO4 values for the groundwater. The negative SPSC and high δ18OPO4 values (19.7‰–23.7‰) in orchard soils indicated that P leaching from orchard soils was the major P enrichment mechanism in shallow groundwater. The amount of high-risk P, easily leached from the soil to water bodies, was estimated to be 1691–3500 kgP ha−1. The values are equivalent to 29.6–61.3 years of annual surplus P. This study demonstrates that groundwater can be an important pathway of legacy P runoff, even in clayey soils, and that there is an urgent need to optimize agricultural practices according to the actual nutrient status of agricultural soils.