草甸草原土壤与植物系统自然丰度氮稳定同位素的年际变化及其指示作用

氮是陆地生态系统生产力的首要限制性养分,利用自然丰度δ¹⁵N(¹⁵N/¹⁴N)可以有效指示生态系统氮循环过程。本试验研究了内蒙古草甸草原土壤与植物系统自然丰度δ¹⁵N、土壤净氮矿化潜势的年际变化。结果表明: 2017—2020年,土壤NO₃^--N含量(9.83～14.79 mg·kg^-1)均显著高于NH₄⁺-N含量(3.92～5.00 mg·kg^-1);土壤NH₄⁺的δ¹⁵N值(13.3‰～18.3‰)显著高于NO₃^-的δ¹⁵N值(3.76‰～6.14‰),土壤NO₃^-的δ¹⁵N值与土壤NO₃^-含量呈显著负相关;干旱年NH₄⁺的δ¹⁵N值相对较高,降水较高或较低年NO₃^-的δ¹⁵N值显著降低。干旱年土壤净氮矿化速率、净氨化速率显著高于湿润年,而土壤硝化速率与年降水量无显著相关性。植物δ¹⁵N值与土壤δ¹⁵N值无显著相关性,但与植物N含量呈显著负相关;豆科植物与非豆科植物δ¹⁵N值、N含量均呈显著正相关,在一定程度上表明豆科植物对非豆科植物的N吸收具有促进作用。研究结果可为草原土壤-植物系统氮循环过程及其对降水变化的响应提供数据支撑。

Interannual variations in natural abundance of nitrogen stable isotopes in soil and plant and their implications in a meadow steppe

Nitrogen is the most limiting nutrient for ecosystems. The natural abundance of δ¹⁵N (¹⁵N/¹⁴N) can efficiently indicate ecosystem nitrogen cycling processes. We investigated the interannual variations in natural abundance of δ¹⁵N in soil-plant system and soil net nitrogen mineralization in a meadow steppe of Inner Mongolia. Results across the four sampling years (2017-2020) showed that the content of soil NO₃^--N (9.83-14.79 mg·kg^-1) was significantly higher than that of NH₄⁺-N (3.92-5.00 mg·kg^-1) and that δ¹⁵N value of soil NH₄⁺ (13.3‰-18.3‰) was significantly higher than that of NO₃^-(3.76‰-6.14‰). The δ¹⁵N value of soil NO₃^- was negatively correlated with soil NO₃^- content. The δ¹⁵N value of soil NH₄⁺ was relatively higher in the dry years, while the δ¹⁵N value of soil NO₃^- significantly decreased in the wetter and drier years. Soil net mineralization and ammonification rates were significantly higher in the dry years than that of the wet years, while soil nitrification rates showed no correlation with annual precipitation. The δ¹⁵N values of plants were not related to that of soils, but nega-tively correlated with plant nitrogen content. Both δ¹⁵N values and nitrogen contents were significantly and positively correlated between the leguminous and non-leguminous plants, suggesting that legume could facilitate nitrogen uptake of non-leguminous plants. These results could provide supporting data for nitrogen cycling and their responses to changes in precipitation in grassland soil-plant systems.