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SOIL BIOLOGY & BIOCHEMISTRY

Long-Term Fertilization Influences on Clay Mineral Composition and Ammonium Adsorption in a Rice Paddy Soil

^a State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China
^b College of Resources and Environ.Sciences, Nanjing Agricultural Univ., Nanjing 210095, China
^c Laboratoire de Géologie, CNRS URA 8538 Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris, France

^* Corresponding author (bzhang{at}issas.ac.cn).

Increasing NH₄⁺ adsorption can be an effective alternative in building an available N pool in soils to optimize crop recovery and minimize losses into the environment. We hypothesized that long-term fertilization may change NH₄⁺ adsorption due to changes in the chemical and mineralogical properties of a rice (Oryza sativa L.) paddy soil. The objectives of this study were (i) to determine clay minerals in two soil clay size fractions using classical x-ray diffraction methods and a numerical diagram-decomposition method, (ii) to measure NH₄⁺ adsorption isotherms before and after H₂O₂ oxidation of organic matter, and (iii) to investigate whether NH₄⁺ adsorption is correlated with changes in soil chemical and mineral properties. The 23-yr long-term fertilization treatments caused little change in soil organic C (SOC) but a large variation in soil mineral composition. The whole-clay fraction (<5 µm) corresponded more to the fertilization treatment than did the fine-clay fraction (<1 µm) in terms of illite peak area percentage. The total vermiculite–chlorite peak area percentage was significantly correlated with the total illite peak area percentage (R = –0.9, P < 0.0001). Different fertilization treatments gave significantly different results in NH₄⁺ adsorption. The SOC oxidation test showed positive effects of SOC on NH₄⁺ adsorption at lower NH₄⁺ concentration (200 mg L^–1) and negative effects at higher NH₄⁺ concentration (300 mg L^–1). The NH₄⁺ adsorption by soil clay minerals after SOC oxidization accounted for 60 to 158% of that by unoxidized soils, suggesting a more important role of soil minerals than SOC on NH₄⁺ adsorption. The NH₄⁺ adsorption potential was significantly correlated to the amount of poorly crystallized illite present (P = 0.012). The availability of adsorbed NH₄⁺ for plant growth needs further study.

Abbreviations: CEC, cation exchange capacity • ML, mixed layer mineral • PCI, poorly crystallized illite • SOC, soil organic carbon • V/CH, vermiculite–chlorite • WCI, well crystallized illite