Determination of rates of mineralization of organic nitrogen (N) into ammonium-N (NH₄⁺-N) and nitrification of NH₄⁺-N into nitrate-N (NO₃⁻-N) could be used to evaluate inorganic N supply capacity, which, in turn, could guide N fertilizer application practices in crop cultivation systems. However, little information is available on the change of mineralization and nitrification in soils under fruit cultivation systems converted from forestlands in karst regions. In a ¹⁵N-tracing study, inorganic N supply capacity in forest soils and three typical fruit crop soils under long-term cultivation was investigated, in addition to factors influencing the supply, in calcareous soils in the karst regions in southwestern China. Long-term fruit crop cultivation decreased soil organic carbon (SOC), total N, and calcium concentrations, cation exchange capacity (CEC), water holding capacity (WHC), pH, and sand content, significantly, but increased clay content. Compared to that of forests, long-term fruit crop cultivation significantly decreased mineralization and nitrification rates to 0.61–1.34 mg N kg⁻¹ d⁻¹ and 1.95–5.07 mg N kg⁻¹ d⁻¹, respectively, from 2.85–6.49 mg N kg⁻¹ d⁻¹ and 8.17–15.5 mg N kg⁻¹ d⁻¹, respectively, but greatly increased the mean residence times of NH₄⁺-N and NO₃⁻-N. The results indicate that long-term fruit crop cultivation could decrease soil inorganic N supply capacity and turnover in karst regions. Both mineralization and nitrification rates were significantly and positively correlated with SOC and total N concentrations, CEC, and WHC, but negatively correlated with clay content, suggesting that decreased soil organic matter and increased clay content were responsible for the decline in mineralization and nitrification rates in soils under long-term cultivation of fruit crops. The results of the present study highlight the importance of rational organic fertilizer application in accelerating soil inorganic N supply and turnover under long-term cultivation of fruit crops in karst regions.

中文翻译：

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西南喀斯特地区石灰岩土壤长期种植果园可降低矿化和硝化速率

测定有机氮（N）成铵态氮（NH ₄ ⁺ -N）和硝化NH ₄ ⁺ -N成硝态氮（NO ₃ ^-- N）的速率可以用来评估无机氮的供应能力进而可以指导作物种植系统中的氮肥施用实践。但是，关于从喀斯特地区林地转变而来的水果栽培系统下土壤中矿化和硝化变化的信息很少。在¹⁵除了影响氮素供应的因素外，还研究了西南喀斯特地区钙质土壤的氮素追踪研究，森林土壤和三种典型的长期种植的水果作物土壤中的无机氮供应能力。长期的果树栽培显着降低了土壤有机碳（SOC），总氮和钙浓度，阳离子交换量（CEC），持水量（WHC），pH和沙含量，但增加了粘土含量。与森林相比，长期的果树种植显着降低了矿化和硝化速率，从2.85–6.49 mg分别降至0.61–1.34 mg N kg ^-1 d ^-1和1.95–5.07 mg N kg ^-1 d ^-1。牛顿^-1 d^-1和8.17-15.5毫克N-千克^-1 d ^-1，分别，但大大提高NH的平均停留时间₄ ⁺ -N和NO ₃ ^--N。结果表明，长期果树栽培会降低喀斯特地区土壤无机氮的供应能力和周转量。矿化和硝化速率与SOC和总氮含量，CEC和WHC呈显着正相关，与粘土含量呈负相关，表明土壤有机质的降低和粘土含量的增加是造成矿化和硝化速率下降的原因。土壤长期种植水果作物。本研究的结果突出了在喀斯特地区长期种植水果作物的情况下，合理施用有机肥料对加速土壤无机氮供应和更新的重要性。