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 ((NH4NO3)-N-15 and (NH4NO3)-N-15 at 10 atom% (15) N excess) was conducted at 25 degrees C under 60% water-holding capacity. Gross N transformation rates in the soils were qualified by a N cycle model (Muller et al., Soil Biol Biochem 39:715-726, 2007). Results Compared to grassland, pitaya cultivation did not affect heterotrophic nitrification (O-Norg) but increased the rates of the mineralization of organic N to NH4+ (M-Norg), NH4+ oxidation to NO3- (O-NH4), and microbial NO3- immobilization (I-NO3), resulting in increased inorganic N supply and turnover. By contrast, corn cultivation lowered the inorganic N supply by inhibiting M-Norg and O-NH4 rates, while increasing O-Norg. Compared to corn and pitaya plantations, the Eucalyptus plantation further lowered the inorganic N supply by inhibiting O-NH4 rates while increasing the rates of I-NH4 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 M-Norg, O-NH4, and I-NO3 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.