Microbial iron reduction compensates for phosphorus limitation in paddy soils

Area of Science:

• Agricultural Science
• Soil Science
• Biogeochemistry

Background:

• Low phosphorus (P) availability is a major constraint for rice growth in tropical and subtropical soils due to high iron (Fe) content.
• Ferric iron-bound P (Fe(III)-P) can be a P source in paddy soils through reductive dissolution to Fe(II), releasing phosphate (H2PO4-).
• The contribution of Fe(III)-P reductive dissolution to plant and microbial phosphorus uptake remains unclear.

Purpose of the Study:

• To quantify the contribution of Fe(III)-P to plant and microbial P uptake in rice paddies.
• To investigate the role of microbial activity and carbon availability in Fe(III)-P mobilization.

Main Methods:

• Utilization of 32P-labeled ferrihydrite to trace phosphorus uptake in rice paddy soil mesocosms.
• Monitoring of 32P recovery in rice plants and microbial biomass over 33 days.
• Measurement of microbial biomass carbon (MBC) and dissolved organic carbon (DOC) to assess carbon limitation.

Main Results:

• Rice plants recovered nearly 2% of added 32P, accounting for 12% of total P in shoots and roots after 33 days.
• Microbial biomass 32P recovery decreased significantly from 0.5% to 0.08% between 10 and 33 days.
• A substantial decrease in MBC (8-54%) and DOC (68-77%) indicated carbon limitation for microbial Fe(III) reduction, particularly in the rhizosphere.

Conclusions:

• Fe(III)-P serves as a valuable, albeit small, phosphorus source for rice, partially meeting plant P demands.
• Phosphorus fertilization strategies should account for P mobilization from Fe (oxyhydr)oxides in flooded paddy soils.
• Enhanced carbon availability in the rhizosphere stimulates P mobilization, crucial during early rice growth stages.