Nitrate and Phosphate Leaching from Aridisols and Entisols: Laboratory Studies and Field Observations

Abstract

Nitrate (NO₃ ⁻) and Phosphate (H₂PO₄ ⁻) leaching was studied under laboratory and field conditions to assess the effect of soil structure development on nutrient leaching losses. Laboratory leaching studies were conducted using undisturbed columns excavated from calcareous soils varying in clay content and structure development, i.e., Shahdara (Fluventic Camborthid), Sultanpur (Typic Torrifluvent), Lyallpur (Ustalfic Haplargid), and Pacca (Ustolic Camborthid). The NO₃ ⁻ and H₂PO₄ ⁻ breakthrough curves (BTCs) were fitted to deterministic equilibrium and non-equilibrium convective dispersive equation (CDE) and transport parameters were determined. In addition, a field study was conducted in Shahdara and Lyallpur soils, sown with a maize (Zea mays L)-maize-potato (Solanum tuberosum L) sequence to assess the leaching losses of surface applied nitrogen and phosphorus at high rates. The NO₃ ⁻ and H₂PO₄ ⁻ concentrations of the soil profile were monitored over a period of three years.

In all Lyallpur and Pacca columns, both NO₃ ⁻ and H₂PO₄ ⁻ appeared in the effluent almost immediately after pulse application and peaked after 0.045 m of cumulative drainage. In contrast, appearance of NO₃ ⁻ in effluent was delayed until 0.055 m cumulative drainage in the Sultanpur columns and until 0.073 m cumulative drainage in Shahdara columns and peaked at ≈0.14 m cumulative drainage. The appearance of H₂PO₄ ⁻ in the effluent was further delayed until 0.10 and 0.13m cumulative drainage, respectively. The deterministic equilibrium CDE domain fitted well (r² = 0.92—0.98) only to the Shahdara and Sultanpur BTCs whereas a deterministic non-equilibrium fit was equally good for all four soils (r² = 0.90–0.99). In situ, there was an obvious increase in NO₃ ⁻ concentration in the plots receiving recommended and higher fertilizer rates to a depth of 0.50 m but thereafter a small increase was observed only in plots receiving higher rates of fertilizer. The increase in NO₃ ⁻ concentration was more obvious in Shahdara soil than that of Lyallpur soil. The increase in phosphate concentration in Shahdara and Lyallpur soil profiles was comparable to that of NO₃ ⁻ but the magnitude was less. Both laboratory and field studies highlighted the role of soil structure variability in loss of applied fertilizer.

Keywords:

• calcareous soils
• field observations
• leaching
• nitrogen
• phosphorus
• undisturbed column

Acknowledgment

The research work was financially supported by the Pakistan Agricultural Research Council through the Agricultural Linkages Program. We thank Ghulam Haider and Ishfaq Ahmad for assistance in laboratory and field work and Mr. Qamar Masood for his help in the field experimentation.