不同有机/无机氮添加对草原土壤氮素分配和转化特征的影响

氮沉降增加已成为全球变化的重要现象之一,已显著影响草地土壤氮素循环。以内蒙古额尔古纳草甸草原为研究对象,进行了6年不同形式氮添加试验,设置无机氮和有机氮比例分别为：10∶0 （N₁）,7∶3 （N₂）,5∶5 （N₃）,3∶7 （N₄）,0∶10 （N₅）和对照处理0∶0 （CK）。通过土壤有机质物理分组及室内矿化培养的方法,从氮素形态、氮素组分及氮素潜在矿化三方面研究不同比例有机、无机氮添加对草原土壤氮素分配和转化特征的影响。结果表明,土壤全氮含量未受氮素添加形式的影响;氮添加显著提高了0~20 cm土层矿质氮含量,尤其是土壤硝态氮,其中N₄（无机氮∶有机氮=3∶7）混合氮肥处理下硝态氮含量增幅最大,较对照处理增加1332%。不同形式氮添加没有影响氮素在土壤颗粒态有机氮（轻组）及矿物结合态有机氮（重组）中的占比;N₁（无机氮∶有机氮=10∶0）处理显著提高了0~10 cm土层颗粒态有机氮（轻组）及0~20 cm土层矿物结合态有机氮（重组）中的氮素相对含量,较对照分别增加了91%和44%。氮添加增加了10~20 cm次表层土壤硝化速率的同时降低了氨化速率,但土壤净氮矿化速率不受氮添加形式的影响。因此,有机/无机氮添加比例变化对草原土壤氮素形态和周转的影响也更加复杂。

Effects of different organic::in o r g anic nitrogen addition ratios on nitrogen distribution and transformation in a grassland soil

Increase in nitrogen （N） deposition has become one of the most important phenomena of global change, and has significantly affected the soil nitrogen cycle in many grasslands. This experiment studied the effects of inorganic versus organic forms of N addition on the soil N cycle, in a six-year N addition experiment carried out in meadow steppe grassland in Inner Mongolia. Inorganic N （IN） was added as NH₄NO₃, while organic N （ON） was added as urea and glycine mixed in equal proportions, based on their N concentrations. The ratios of IN∶ON used as treatments were 10∶0 （N₁）, 7∶3 （N₂）, 5∶5 （N₃）, 3∶7 （N₄）, 0∶10 （N₅） and 0∶0 （CK）. Soil N was fractionated into light fractions and heavy fractions. Additionally, a short-term incubation was conducted to assess potential N mineralization. Our results showed that soil total N content was not affected by the form of N addition. Nitrogen addition significantly increased soil nitrate and mineral N concentrations in the 0-20 cm soil layer, and this effect was most pronounced in the N₄ （IN∶ON=3∶7） treatment. Compared with the control treatment, soil mineral N increased by 1332% in the N₄ treatment. The N concentration of light fractions in the 0-10 cm soil horizon was increased by 91% and the N concentration of heavy fractions in the 0-20 cm horizon was increased by 44% both under N₁ treatment （IN∶ON=10∶0）. However, the percentage of N concentrations in light fractions and heavy fractions was not affected by different forms of N addition. The data also indicated that N addition increased the nitrification rate while decreasing the ammonization rate in the 10-20 cm soil horizon, but it did not affect the soil net N mineralization rate. Therefore, the effects of changing the IN∶ON addition ratio on the form and turnover of nitrogen in these grassland soils are complex.