Labile carbon and soil texture control nitrogen transformation in deep vadose zone

Area of Science:

• Environmental Science
• Soil Science
• Geochemistry

Background:

• Nitrogen (N) and carbon (C) cycling in the deep vadose zone is poorly understood.
• Characterizing deep vadose zone organic and inorganic N and C pools is crucial for managing groundwater nitrate contamination.

Purpose of the Study:

• To investigate inorganic N storage and organic/inorganic C and N pools in deep vadose zones.
• To understand the role of these pools in N transformations beneath croplands.

Main Methods:

• Sampling and characterization of soil N and C pools at 27 cropland sites with varying vadose zone thicknesses (6-45 m).
• Measurement of inorganic N (nitrate, ammonium), total Kjeldahl nitrogen (TKN), hot-water extractable organic carbon (EOC), soil organic carbon (SOC), and stable carbon isotopes (δ13C).

Main Results:

• Inorganic N stocks ranged from 21.7-1043.6 g m⁻²; thicker vadose zones stored significantly more N.
• Significant reservoirs of TKN and SOC, likely paleosols, were found at depth, indicating potential sources of organic C and N.
• Evidence suggests N mineralization, nitrification, and denitrification occur in the deep vadose zone, influenced by soil texture and water-filled pore space (WFPS).

Conclusions:

• The deep vadose zone can store substantial inorganic N.
• Paleosols represent important deep C and N reservoirs.
• Microbial N transformations are possible in the deep vadose zone, controlled by labile C availability and soil properties.