Land use effects on gross soil nitrogen transformations in karst desertification area

PURPOSE: Investigating soil nitrogen (N) cycling to evaluate inorganic N supply can guide land resource utilization. In this study, four typical land uses including grassland, Eucalyptus, corn, and pitaya plantations were chosen in a karst desertification area, all of which are the main plants in the local area. The corn and pitaya plantations experienced greater human disturbance than the grassland and Eucalyptus plantations; the latter two were not fertilized and tilled. We explored how land use change affects the gross N transformation rates and inorganic N supply in karst soils. METHODS: Soils were sampled from four land uses, and a ¹⁵ N-tracer incubation experiment containing two ¹⁵ N treatments (¹⁵NH₄NO₃ and NH₄¹⁵NO₃ at 10 atom% ¹⁵ N excess) was conducted at 25 °C under 60% water-holding capacity. Gross N transformation rates in the soils were qualified by a N cycle model (Müller et al., Soil Biol Biochem 39:715–726, 2007). RESULTS: Compared to grassland, pitaya cultivation did not affect heterotrophic nitrification (ONₒᵣg) but increased the rates of the mineralization of organic N to NH₄⁺ (MNₒᵣg), NH₄⁺ oxidation to NO₃⁻ (ONH₄), and microbial NO₃⁻ immobilization (INO₃), resulting in increased inorganic N supply and turnover. By contrast, corn cultivation lowered the inorganic N supply by inhibiting MNₒᵣg and ONH₄ rates, while increasing ONₒᵣg. Compared to corn and pitaya plantations, the Eucalyptus plantation further lowered the inorganic N supply by inhibiting ONH₄ rates while increasing the rates of INH₄ and NH₄⁺ adsorption on cation-exchange sites. Lower clay content, alkyl-C, aromatic‐C, alkyl‐C/O‐alkyl‐C, and aromaticity levels but higher O‐alkyl‐C and carbonyl‐C levels were found in the grassland and pitaya soils than the Eucalyptus and corn soils, indicating the clayey texture and low labile organic matter in the latter two soils. The rates of MNₒᵣg, ONH₄, and INO₃ were significantly negatively related to the soil clay content, alkyl-C/O-alkyl-C and aromaticity, suggesting that soil texture and the stability of organic matter were the important factors affecting inorganic N supply. CONCLUSIONS: These results highlight the significant effect of land uses on N transformation rates. Compared to natural grassland, cash crop plantations such as pitaya can increase inorganic N supply capacity, while Eucalyptus and corn plantations reduce it, in karst rocky desertification areas. Our results indicate that the application of active organic fertilizer to agricultural plantations may be an effective practice for increasing labile organic C and improving the soil structure to accelerate N cycling and inorganic N supply.