ABSTRACT

Fertilization and irrigation (FI) are exerting an important impact on the fluxes of carbon, both between and within ecosystems in an agricultural area. However, the impacts of urea, diammonium phosphate ((NH₄)₂HPO₄) and animal manure, as the most common fertilizers in Northern China, and irrigation on carbonate weathering in soil are still relatively unknown due to the complicated biogeochemical process of these added fertilizers. In this study, four soil columns with different treatments (control (CK), fresh cow manure (FM) urea (NF) and diammonium phosphate (PF)), were established to explore the influence of fertilization and irrigation on carbonate weathering in an agricultural area in Northern China. The pH, K⁺, Ca²⁺, Na⁺, Mg²⁺, NH₄⁺, NO₃^–, NO₂^–, free CO₂, HCO₃^–, Cl^–, SO₄^2–, COD and HPO₄^2– in leachates from the soil columns were analyzed. The results showed that Ca²⁺ and NO₃^–, HCO₃^–, SO₄^2– were the dominant cations and anions (>25% in meq%) in the leachate from the CK and FM columns, respectively, while Ca²⁺ and NO₃^– were the dominant cations and anions in NF and PF treatments, respectively. The addition of fresh cow manure had no distinct effect on the hydro-chemical characteristics in the soil solution, and the weathering rate of carbonate, whereas an increase in the weathering rate of carbonate, CO₂ concentration, Ca²⁺, Mg²⁺ and NO₃^– was observed after the addition of urea and (NH₄)₂HPO₄, which was mainly attributed to the release of protons from the nitrification reaction. The co-precipitation of phosphate ions with calcium carbonate had a blockage effect on the leaching of phosphate, but has no apparent effect on the weathering rate of carbonate when (NH₄)₂HPO₄ was added to the carbonate-enriched soil column. The widespread application of urea and (NH₄)₂HPO₄ along with irrigation in the agricultural area of Northern China increases the rate of carbonate weathering and soil CO₂ emission. The enhanced Ca²⁺, Mg²⁺ and DIC (free CO₂ + HCO₃^– + CO₃^2–) due to the addition of urea and (NH₄)₂HPO₄ as well as irrigation should not be disregarded when evaluating CO₂ consumption by mineral weathering in the agricultural area.