| Summary: | Determining the availability and supply capacity of soil inorganic nitrogen (N) can effectively guide the appropriate application of N fertilizers during crop cultivation. However, the mechanism underlying soil inorganic N production remains unknown for cash crops in karst regions. In this study, the rates of organic N mineralization to ammonium (NH<sub>4</sub><sup>+</sup>) and NH<sub>4</sub><sup>+</sup> nitrification to nitrate (NO<sub>3</sub><sup>−</sup>) were determined using a <sup>15</sup>N tracing technique to evaluate the supply capacity of inorganic N in soils from woodland and pitaya plantations with different cultivation years (3, 9, and 15 years) in the subtropical karst region of China. The conversion of woodland to pitaya plantations significantly decreased the content of soil organic carbon (SOC), total N, calcium (Ca), and magnesium (Mg), along with the soil pH and cation-exchange capacity (CEC), but significantly increased the content of available potassium, available phosphorus, iron, and aluminum, in a more pronounced fashion with the increasing length of pitaya cultivation. The conversion of woodland to pitaya plantations has not significantly changed soil NH<sub>4</sub><sup>+</sup> and NO<sub>3</sub><sup>−</sup> content, but this land use has resulted in divergent effects on mineralization and nitrification rates. Compared to woodland (5.49 mg N kg<sup>−1</sup> d<sup>−1</sup>), pitaya cultivation significantly reduced the mineralization rate to 0.62–2.38 mg N kg<sup>−1</sup> d<sup>−1</sup>. Conversely, the nitrification rate significantly increased from 4.71 mg N kg<sup>−1</sup> d<sup>−1</sup> in soil under woodland to 9.32 mg N kg<sup>−1</sup> d<sup>−1</sup> in soil under 3-year pitaya cultivation, but this rate decreased to 1.74 mg N kg<sup>−1</sup> d<sup>−1</sup> under 15-year cultivation. Furthermore, the mean residence time of inorganic N was significantly higher in long-term than in short-term pitaya plantations, indicating the decline in inorganic N turnover with the increasing length of pitaya cultivation. Taken together, long-term pitaya cultivation could significantly decrease the supply capacity and turnover of inorganic N in soil. The Ca, Mg, SOC, and total N content, as well as CEC, were significantly and positively related to the mineralization rate, but negatively related to the mean residence time of NH<sub>4</sub><sup>+</sup> and NO<sub>3</sub><sup>−</sup>, suggesting that the incorporation of organic matter can accelerate the soil inorganic N supply and turnover for long-term pitaya plantation in subtropical regions.
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