In response to the growing demand for fruits, farmers in the Taihu Lake region are rushing to convert paddy fields into fruit orchards in recent years. Changes in land-use and management may affect or alter physico-chemical properties of the soil, and hence cycling of soil N and fate of N fertilizer. Up to date, little has been reported on quantification of effects of changes in land use on soil N gross transformation rate, besides some works that have been mainly focused on effects of the conversion of non-agricultural land into agricultural land, and rarely on the effects of the conversion from one type to another type of agricultural land use. In the Taihu Lake region, paddy fields under rice-wheat crop rotation and orchards coverted from paddy fields are the two typical types of agricultural land-use, which differ sharply in water regimes (periodically waterlogged for paddy fields and water-unsaturated for orchards) and fertilizer management (no input of organic manure for paddy fields and combined application of chemical fertilizer and organic manure for orchards). Therefore, gross N processes (e.g., nitrification and denitrification) in the soils under the two types of land use also differ sharply, as affected by their different aeration conditions and fertilizer managements. The paddy field under rice-wheat crop rotation and the vineyard converted from paddy field under study are located in the upper-streams of the Zhushan Bay Catchment in the Taihu Lake Region of China. Gross transformation rates of soil N under the two types of land use were measured using the ¹⁵N tracing technique combined with the Markov Chain Monte Carlo (MCMC) algorithm-based numerical optimization model, and effects of the conversion on soil N supply and N retention capacity were investigated. Results show that the conversion reduced soil pH (from 5.74 in paddy field to 5.14 in vineyard, on average) and contents of soil organic C and total N, though not much. In the soils of the paddy field and vineyard, the inorganic-N pools were dominated with nitrate, with NH₄⁺/NO₃⁻ being 0.26 and 0.06, respectively, and the gross N mineralization rate (mineralization of labile and recalcitrant soil organic matter) was N 3.90 mg kg^-1 d^-1 and 4.52 mg kg^-1 d^-1, respectively. Obviously the differences between the two were not very sharp. In the paddy field, the gross NH4+ assimilation rate was 0.56 mg kg^-1 d^-1, accounting for only 14% of the total NH4+ produced, while in the vineyard it was almost negligible. The gross N autotrophic nitrification rate in the vineyard was 15.85 mg kg^-1 d^-1, significantly higher than that (13.65 mg kg^-1 d^-1) in the paddy field, while the gross heterotrophic nitrification rate and NO₃⁻ assimilation rate were both negligible in both soils. Through fitting with the MCMC algorithm-based numerical optimization model, consumption of NO₃⁻in the soils was found to have two pathways, namely assimilation of NO₃⁻ and dissimilatory reduction of NO₃⁻ to NH₄⁺ (DNRA). However, in both of the soils, NO₃⁻assimilation was not detected, turning DNRA into the major pathway of NO₃⁻ consumption, moreover, the two soils did not differ much in DNRA rate. The ratio of total nitrification to gross NH₄⁺ assimilation (N/NA) in the soil was 24 in the paddy field and 793 in the vineyard, indicating that ammonia oxidizing bacteria are stronger than heterotrophic nitrifiers in competition for NH4+, and hence autotrophic nitrification is the dominant fate of NH₄⁺, especially in the vineyard. On the whole, the conversion of paddy field into vineyard significantly affects soil autotrophic nitrification, increasing the N autotrophic nitrification rate in the soil, but its influence on NH₄⁺ assimilation rate was almost negligible, thus making autotrophic nitrification the only fate for NH₄⁺ in the vineyard. The decreased NH₄⁺ assimilation rate and the increased autotrophic nitrification rate in the vineyard enhanced NO₃⁻ accumulation in the soil, which may in turn increase the risk of N leaching and losing with runoff. It is recommended that nitrification inhibitor and/or organic manure high in C/N ratio should be applied to mitigate the risk.