[Dissolved Ion Concentrations and Isotope Values in Agricultural Fertilizer Locally Applied in Henan Province]

Agricultural fertilizers (AFs) have provided vegetation with necessary nutrients, but unabsorbed constituents have been retarded in soil, potentially affecting the quality of adjacent surface water and groundwater. AFs element contents and stable isotope compositions have often been utilized to assess and calculate AFs pollution to nitrate and sulfate in surface water and groundwater; however, due to various AFs applied, the dissolved ion concentrations and isotope ratios are still unknown. This study collected commercial AF widely utilized in Henan province, China, to constrain their ion concentrations and isotope values. The dissolved ions (1 g AFs dissolved in 1 L ultrapure water), sulfate sulfur, and oxygen isotope values(δ³⁴S and δ¹⁸O) were analyzed, and total nitrogen (TN) contents coupled with nitrogen isotope values(δ¹⁵N) in solid AFs were determined to elucidate their elemental and isotopic compositions. These characteristics provided a scientific basis for further assessing their contributions to surface water and groundwater contaminations. The results indicated that pH values in the AFs solutions varied from 3.6 to 10.2, with a mean value of 6.7±1.5 (n=30, 1σ). Sulfate (SO₄^2-) and nitrate (NO₃^-) concentrations ranged from 4.38 mg·L^-1 to 827.29 mg·L^-1 and from 1.34 mg·L^-1 to 208.90 mg·L^-1, with median values of 192.80 mg·L^-1 and 13.51 mg·L^-1 and average values of (256.19±239.83) mg·L^-1 (n=30) and (37.07±53.21) mg·L^-1 (n=29), respectively. Dissolved sulfate δ³⁴S and δ¹⁸O values in AFs varied from -3.5‰ to 19.0‰ and from 6.7‰ to 18.5‰, with median values of 4.1‰ and 10.1‰ and mean values of (5.8±5.5)‰ (n=22, 1σ) and (10.7±2.7)‰ (n=22, 1σ), respectively. TN and δ¹⁵N values in AFs ranged from 0.5% to 38.9% and from -2.7‰ to 3.4‰, with median values of 13.3% and 0.0‰ and average values of (14.8±9.3)% (n=25) and 0.0±1.5‰ (n=24, 1σ), respectively. The lower averaged δ³⁴S values and positive averaged δ¹⁸O values potentially resulted from sulfuric acids added as raw materials, giving rise to a negative relationship between pH values and SO₄^2- concentrations (P<0.05). The δ¹⁵N values of AFs were close to that of air N₂, corresponding to the fact that NO₃^--N and NH₄⁺-N were synthesized via air N₂. Our results revealed the dissolved ion concentrations of SO₄^2-, NO₃^-, and NH₄⁺ and their δ³⁴S, δ¹⁸O, and δ¹⁵N values of typically applied AFs in Henan province, which provided the scientific basis for studying the AFs contributions to SO₄^2- and NO₃^- pollutions in surface water and groundwater surroundings.