Abstract:

The freeze-thaw cycle (FTC) is an important factor that affects the physicochemical and biological properties of alpine grassland soils. The deposition and degradation of grazing livestock dung during freeze-thaw period may also be affected by the complex and significant effects of FTCs. However, the effects and mechanisms of FTCs on dung degradation and nutrient changes in alpine grassland remain unclear. In this study, a 30-day indoor freeze-thaw simulation experiment was conducted by using yak and Tibetan sheep dung collected from the seasonal freeze-thaw alpine grassland in the northern Tibetan plateau. Dung samples were collected to analyze the physicochemical properties and nutrient concentration after 0, 5, 15, and 30 days of FTCs, respectively, so as to investigate the detailed influences of repeated FTCs on dung degradation and major nutrient dynamics. The results showed that 1) the moisture content and dry matter content of yak dung significantly decreased during the first 15 days of FTCs (P<0.05), and the moisture content of sheep dung decreased by 82.6% in the first 5 days of FTCs. This decrease could be attributed to the repeated FTCs of fresh dung that easily damage the physical structure of dung pats at the initial stage and therefore accelerate the loss of moisture content and dry matter content. 2) The concentration of ammonium nitrogen (NH₄⁺-N) of yak dung reached its maximum after 5 days FTCs (415 mg·kg⁻¹), while the nitrate nitrogen (NO₃⁻-N) concentration of yak and sheep dung significantly decreased across the first 15 days FTCs (P<0.05). The increased organic nitrogen mineralization and enhanced denitrification rate at the early stage of FTCs may be the primary reasons for the brief increase in NH₄⁺-N concentration and sustained decrease in NO₃⁻-N concentration. 3) The concentration of AP in yak dung significantly decreased by 21.2% (P<0.05) from 15 to 30 days of FTCs, while the concentration of AP in Tibetan sheep dung fluctuated throughout the entire experimental period, indicating that yak dung pat AP was variously influenced by later-stage FTCs, whereas particulate sheep dung AP was subject to more complex and variable influences from FTCs across the whole experiment. In summary, repeated FTCs tend to induce changes in the block structure, physicochemical properties, and microbial characteristics of yak and sheep dung to various degrees, thereby resulted in significant variation in the nitrogen and phosphorus migration, transformation, and active nitrogen and phosphorus nutrient concentrations. The conclusions of this study can provide theoretical references for optimizing the management of livestock dung in grassland during the seasonal freeze-thaw period and promoting the healthy development of grassland ecosystems.

Key words: freeze-thaw action, livestock dung, mineralized nitrogen, available phosphorus, nutrient changes, northern Tibetan Plateau

摘要：

冻融是影响高寒草地土壤理化和生物学特性的重要因素，冻融期放牧牲畜粪便沉积及其降解特性也可能受到冻融作用复杂且不容忽视的重要影响，而目前关于冻融对高寒草地牲畜粪便降解及养分变化的影响效应和作用机制尚不清楚。选取季节性冻融藏北高原牦牛和藏绵羊粪便开展30 d室内冻融模拟试验，分别在冻融0、5、15和30 d后采集两种粪便样品进行理化和养分指标测定，探究不同冻融时间对牛羊粪便降解及其主要养分变化的影响。结果表明，1）冻融试验期内牛粪含水量与干物质量在前15 d冻融循环中均显著降低（P<0.05），羊粪含水量在前5 d冻融中下降了82.6%；反复冻融易破坏粪便降解初期的物理结构，加速粪便含水量及牛粪干物质量的损耗。2）牛粪铵态氮（NH₄⁺-N）含量在冻融后第5 d达到最大值（415 mg·kg⁻¹），且牛羊粪硝态氮（NO₃⁻-N）含量在0－15 d冻融过程中显著降低（P<0.05），冻融初期有机氮矿化增加和反硝化速率增强可能是导致NH₄⁺-N含量短暂增加和NO₃⁻-N含量持续下降的主要原因。3）牛粪有效磷（AP）含量在15－30 d冻融中显著降低21.2%（P<0.05），而羊粪AP在整个试验期呈现反复增加-降低的波动变化特性，表明斑块状牛粪AP受后期冻融的影响较大，而颗粒状羊粪AP受冻融作用的影响更为复杂多变。综上，反复冻融通过不同程度改变牛羊粪便斑块结构、理化和微生物特性，使得氮磷迁移转化及活性氮磷养分含量呈现较大的差异性变化特征。研究结论可为优化季节性冻融期草地牲畜粪便管理模式和促进草地生态系统健康发展等提供理论参考。

关键词: 冻融作用, 牲畜粪便, 矿化氮, 有效磷, 养分变化, 藏北高原