三峡水库消落带不同土地利用对土壤团聚体稳定性及其碳氮分布的影响

团聚体稳定性是维持土壤生产力与土壤健康的关键因素,研究不同土地利用方式对团聚体稳定性及其碳氮分布的影响,对消落带生态环境的恢复与重建具有十分重要的意义。以三峡水库土质消落带3种典型的土地利用方式(玉米地、水田、撂荒草地)为研究对象,采用湿筛法测定0~10、10~25 cm土层团聚体粒径分布与团聚体稳定性,并测定全土及各粒径土壤团聚体有机碳、氮含量,利用结构方程模型(SEM)分析团聚体有机胶结物、土壤基本理化性质与水稳性团聚体稳定性的关系。结果表明:(1)三峡水库消落带中,土地利用方式在不同土壤深度对团聚体稳定性及其碳氮含量的影响不同;(2)0~10 cm土层中,农田撂荒后显著提升了土壤大团聚体比重与团聚体稳定性;撂荒草地的各粒径土壤团聚体有机碳、氮含量显著高于农田;>0.25 mm粒径团聚体有机碳、氮对全土有机碳、氮含量的相对贡献率显著提升;(3)10~25 cm土层中,农田撂荒后土壤团聚体及其有机碳、氮含量并没有得到明显改善;(4)结构方程模型分析发现,土壤有机碳(路径系数为0.76)与全磷(0.58)对团聚体的稳定具有显著的正面影响,pH(-0.42)对团聚体的稳定具有显著的负面影响,全钾(0.23)与全氮(0.15)通过促进土壤有机碳的形成而间接促进土壤团聚体稳定性,土壤容重(-0.11)通过影响土壤有机碳而间接削弱土壤团聚体稳定性。研究结果可为三峡水库消落带土壤结构稳定性评价及生态恢复规划提供理论依据。

连续种植不同绿肥作物的耕层土壤团聚体中氮分布及其固持特征

为探讨连续种植绿肥对土壤团聚体中氮分布的影响以及其中氮的固持特征,以箭筈豌豆(Vicia sativa L.)、肥田萝卜(Raphanus sativus L.)、蓝花苕子(Vicia cracca L.)、毛叶苕子(Vicia villosa Roth)、光叶苕子(Vicia villosa var.)为对象开展研究。结果表明:光叶苕子能够显著提高土壤全氮含量。肥田萝卜、蓝花苕子提升耕层土壤团聚体各粒径中氮含量的效果较为明显,光叶苕子提升> 2 mm粒径土壤团聚体中氮含量的效果较为明显。随着粒径减小土壤团聚体氮含量也呈现下降的趋势,各粒径中氮的富集系数在1.24与0.89之间。在> 1 mm粒径中,几乎所有处理富集系数均大于1,处于积累状态,≥0.25 mm粒径的团聚体氮贡献率占了85%以上,其中> 5 mm粒径的氮贡献率最大,0.5~0.25 mm粒径的氮贡献率最小。连续种植绿肥,土壤全氮的积累随着各粒径土壤团聚体中氮含量的增加而增加,全氮与> 0.5 mm粒径的团聚体中氮含量显著正相关,且5~2 mm粒级土壤团聚体中氮的增加对土壤全氮的影响较为突出,连续种植绿肥促进了氮在大团聚体中的固持。

Enzyme Activities and Microbial Communities in Subtropical Forest Soil Aggregates to Ammonium and Nitrate-Nitrogen Additions

A laboratory incubation experiment was established to examine the impacts of nitrate and ammonium nitrogen additions on soil microbial attributes of a subtropical Pinus elliottii forest ecosystem in southern China. Soils were subjected to three different treatments： the control with no nitrogen addition （CK）, the ammonium nitrogen addition （NH4^＋-N）, and the nitrate nitrogen addition （NO23^-N）. Samples from bulk and two different size fractions （macroaggregate （〉250 pm） and microaggregate （53-250 μm）） were analyzed for soil properties, enzyme activities and microbial communities on day 7 and 15 of the incubation. Our study demonstrated that NH4^＋-N had a 9rearer influence on soil microbial activities than NO3-N. NH4^＋-N additions resulted in significant increases in 13-1,4-glucosidase （βG） and β-1,4-N-acetyl glucosaminidase （NAG） enzyme activities in bulk, macroaggregate and microag- gregate soils after 7 and 15 days incubation. NO3^-N additions only significantly increased in βG and NAG enzyme activities in bulk, macroaggregate soils after 7 and 15 days incubation, but not in microaggregate. All NH4^＋-N and NO3-N additions resulted in significant increases in gram-positive bacterial PLFAs in microaggregates. Only a significant correlation between soil nutrient contents and enzyme activities in macroaggregates was founded, which suggests that the soil aggregation structure played an important role in the determining enzyme activities.