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 (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.

中文翻译：

---

喀斯特荒漠化区土地利用对土壤总氮转化的影响

目的

调查土壤氮 (N) 循环以评估无机氮供应可以指导土地资源利用。本研究在喀斯特荒漠化地区选择了草地、桉树、玉米、火龙果等四种典型土地利用方式，均是当地的主要植物。玉米和火龙果人工林比草原和桉树人工林遭受更大的人为干扰；后两者没有施肥和耕种。我们探讨了土地利用变化如何影响岩溶土壤中的总氮转化率和无机氮供应。

方法

土壤进行从四个土地用途采样，一个¹⁵ 含有两个N示踪培养实验¹⁵  （N处理¹⁵ NH ₄ NO ₃和NH ₄ ¹⁵ NO ₃以10原子％^15倍 Ñ过量）在25℃下60％进行持水能力。土壤中的总氮转化率由氮循环模型限定（Müller 等人，Soil Biol Biochem 39:715–726, 2007）。

结果

与草地相比，火龙果种植不影响异养硝化作用（O _Norg），但增加了有机氮矿化为NH ₄ ⁺ ( M _Norg )、NH ₄ ⁺氧化为NO ₃ ⁻ ( O _NH4 )和微生物NO的速率₃ ^-固定化（I _NO3），导致无机氮供应和周转增加。相比之下，玉米种植通过抑制M _Norg和O _NH4率降低了无机氮的供应，同时增加了O _Norg. 与玉米和火龙果种植园相比，桉树种植园通过抑制O _NH4速率，同时增加I _NH4和 NH ₄ ⁺在阳离子交换位点上的吸附速率，进一步降低了无机氮的供应。与桉树和玉米相比，草地和火龙果土壤中的粘土含量、烷基-C、芳香-C、烷基-C/O-烷基-C 和芳香性水平较低，但 O-烷基-C 和羰基-C 水平较高土壤，表明后两种土壤的粘土质地和低不稳定有机质。M _Norg、O _NH4和I _NO3的比率 与土壤粘土含量、烷基-C/O-烷基-C和芳香性呈显着负相关，表明土壤质地和有机质稳定性是影响无机氮供给的重要因素。

结论

这些结果突出了土地利用对氮转化率的显着影响。在喀斯特石漠化地区，与天然草地相比，火龙果等经济作物种植园可以增加无机氮供应能力，而桉树和玉米种植园则降低无机氮供应能力。我们的研究结果表明，在农业种植园施用活性有机肥可能是增加不稳定有机碳和改善土壤结构以加速氮循环和无机氮供应的有效做法。