冻融循环作用下植被混凝土团聚结构变化对养分固持能力的影响北大核心CSCD

植被混凝土生态修复技术是当前国内用于裸露陡边坡植被恢复的典型技术之一,具备肥力持续供给能力是植被混凝土有别于其他建筑材料的基本属性。冻融循环作用下物理结构剧变导致养分固持能力减弱是限制植被混凝土在高寒地区应用的关键因素,但养分固持能力变化的深层原因尚不清楚。通过控制性试验,以初始含水率和冻融循环频次为变量,测定了植被混凝土水稳性团聚体粒径分布、团聚特征参数、主要养分含量及其淋溶流失率的变化规律。结果表明:随初始含水率提高,植被混凝土中水稳性微团聚体向大团聚体转化,尤其以≥1~2mm粒组增幅最多,团聚特征参数变化也反映出团聚体稳定性随之提高;冻融循环导致水稳性团聚体平均粒径不断减小,但会随冻融频次增长逐步趋于稳定。初始含水率的提高促使各养分含量略有增加;冻融循环作用下有机质、铵态氮、有效磷、速效钾含量仍有增长,但硝态氮含量不断降低。同时,冻融循环还会导致各养分淋溶流失率不断增大,最大增幅可超过90%,并随冻融频次增长趋于稳定。这说明冻融循环对养分固持能力的影响会逐步减弱,而且侧面反映出团聚结构与养分固持能力间存在紧密联系。Pearson相关性分析进一步表明,团聚特征参数与各养分淋溶流失率均达到显著相关水平,综合考虑显著性水平与相关性系数绝对值,认为团聚特征参数中几何平均直径与各养分淋失率相关程度最高,最适合用于表征植被混凝土的养分固持能力。上述研究结果证实,冻融循环作用下团聚效应减弱是导致植被混凝土养分固持能力降低的深层原因。

Effect of aggregate structure change in vegetation concrete on nutrient retention ability under freeze-thaw cycles

Vegetation concrete （VC） ecological protection technology is an effective solution for the vegetation recovery of bare steep slopes， which has been increasingly applied in cold regions in recent years. When the technology is implemented， the nutrient retention ability of VC substrate is essentially concerned. Under the actions of freeze-thaw cycles， fertility of the VC substrate as well as natural soil is thought to degrade gradually. It has been recognized that the nutrient retention ability of soil is significantly correlated with its physical structure. Similarly， the nutrient retention ability of VC substrate could be supposed to be physical structure-dependent. To enhance the comprehensive performance of VC substrate in cold regions， the investigation of nutrient retention ability is required， which nevertheless is still little identified. In this study， a series of freeze-thaw cycle experiments for VC substrate were conducted， and the effects of initial water contents and freeze-thaw cycles on characteristic parameters of water-stable aggregates and leaching loss rates of major nutrient substances were studied. A freeze-thaw cycle for specimen treatment， performed by a fast air freeze-thaw test machine， was defined as the freezing process of 12 hours at -20 ℃ in addition to the thawing process of 12 hours at +20 ℃. Except for the non-treatment， namely without freeze-thaw cycle， 7 treatments were considered to prepare the specimens， including 1 cycle， 2 cycles， 4 cycles， 8 cycles， 16 cycles， 32 cycles and 64 cycles. According to the field experience in practice， the lower and upper initial water contents of specimens were designated to be 18% and 24%， respectively. The results showed that the water-stable aggregates of the VC substrate were mainly composed of the particles with size ranging from 0.05 mm to 0.25 mm， which contained the proportions over 50% of total mass for all specimens. With increasing initial water content， the water-stable micro-aggregates transformed into the macro-aggregates， among which the particles of ≥1~2 mm were found to hold the maximum increase rate in proportion. Other parameters， which could quantitatively represent the characteristics of aggregate structure， also showed that the aggregate stability increased with initial water content. In addition， the contents of particles smaller than 0.25 mm were positively related to freeze-thaw cycles， while that of the particles larger than 0.25 mm showed the inverse trend. This indicated that the average value of aggregate particle sizes decreased with freeze-thaw cycles. It was noticed that the dispersion rates of aggregate increased with initial water content， which showed that destructive action to aggregate caused by freeze-thaw cycles was greater than the reinforcement provided by the increasing cement hydration products. Furthermore， the freeze-thaw cycles required for the aggregate characteristic parameters of VC to reach the stable state were more than that for natural soil. It may be due to that natural soil would go through the repetitive process of decomposition and aggregation， while destruction process of cement hydration products was irreversible. For the fertility， a high initial water content was associated with the increasing contents of major nutrient substances. Contents of organic matter， ammonium nitrogen （NH₄⁺-N）， available phosphorus （PO₄^3--P） and potassium （K⁺） still increased with freeze-thaw cycles， while content of nitrate nitrogen （NO₃^--N） decreased. Moreover， the leaching losses of these nutrient substances increased with freeze-thaw cycles obviously. From the Pearson correlation analysis， the leaching loss rates of major nutrient substances were found to correlate closely with the aggregate characteristic parameters. In consideration of significance levels and absolute values of correlation coefficients， geometric mean diameter （GMD） could be suggested as the reasonable index to describe the nutrient retention ability of VC substrate. The results may contribute to illustrate the underlying reason for VC substrate fertility degradation under freeze-thaw cycles and provide theory basis for countermeasure.