Nitrate and phosphate leaching from Aridisols and Entisols: Laboratory studies and field observations

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.