河套灌区典型农田防护林网对土壤水盐变化的影响

Hetao Irrigation Area is the largest irrigation area of Yellow River region, water scarcity and saline condition are severe, farmland shelterbelt play an important role in the soil water and salt regulation. Since the growth of shelterbelt decays in the large area, it is necessary to recover and rebuild the shelterbelt system，however, the studies on the effect of farmland shelterbelt on the soil water and salt in this area are relatively less, therefore, there is insufficient theoretical support for how to scientifically construct the shelterbelt system under the saline soil condition. According to the current situation, to illustrate the regulation mechanism of soil water and salt of farmland shelterbelt with different structure, we will carry out the study through methods of field investigation, spot monitoring, laboratory analysis, numerical simulation, and obtain the data of fractal porosity of shelterbelt，trees transpiration, soil evaporation, soil water and salt content, salt content of trees, root biomass of trees and crops, groundwater level and wind speed, air temperature within one grid of shelterbelt. On the basis, the shelterbelt structure and its thermal dynamic functions, effect of shelterbelt on soil water and salt balance, and spatio-temporal variation of soil water and salt will be studied. The effect of shelterbelt on water-saving and salt reduction can be evaluated, the scope of saline soil in horizontal and vertical scale can be defined, the parameters of the optimal shelterbelt structure with function of salt and water regulation can be determined, the study can provide theoretical basis for the construction of new type farmland shelterbelt with the function of water saving and salt reduction.

河套灌区是我国最大的引黄灌区，区内水资源匮乏，次生盐渍化严重，农田防护林系统在灌区土壤水盐调控中发挥着重要作用。目前，区内农田防护林出现大面积衰退，亟需进行更新建设，然而，关于该区农田防护林对土壤水盐影响的研究较少，土壤盐渍化条件下如何科学构建防护林系缺少理论支持。针对该状况，本项目围绕不同结构农田防护林网对土壤水盐的调控机理这一关键问题，通过野外调查与定位观测、数值模拟、测试分析等研究方法，获取林带疏透度、林木蒸腾量、土壤蒸发散、土壤水盐含量、林木盐离子含量、根系生物量、田间地下水位、以及林网内风速、气温、地温、辐射等基础数据。在此基础上，开展不同结构防护林体系的热动力效应、以及防护林对土壤水盐平衡和时空变化的影响几方面的研究。最终评定防护林节水减盐效果、界定林网内土壤盐渍化范围、确定水盐调控最优林带结构参数，旨在为构建灌区新型节水减盐农田防护林体系提供科学理论依据。

目前，水资源短缺和土壤盐渍化已成为河套灌区农业发展面临的重要问题，农田防护林在调节田间水盐方面发挥着重要作用。本项目以农田防护林网作为研究对象，采用野外观测和数值模拟相结合方法，开展了不同结构防护林体系的热动力效应、农田防护林对土壤水盐平衡的影响、农田防护林对土壤水盐时空变化的影响和乌兰布和沙漠东北缘绿洲化过程中土壤特性变化等四个方面的研究。结果表明：防护林具有明显的防风效益，可以有效降低光照强度，明显降温增湿；不同配置农田防护林影响田间土壤水盐空间变异；绿洲化进程中土壤性质得到优化。具体表现为：在所研究密度和风速下，均匀分布模式的保护效果最大，随机分布模式的降低风速效果最低。不同结构的林带中，2行小美旱杨和2行二白杨的混交林防风效益最好。采用数值模拟的方法揭示了两种配置结构林带下的防风机理。樟子松树液流量保持在154mm/年的相对恒定水平，樟子松可以显著降低浅层土壤蓄水量和深层土壤补给。柽柳群落生长和分布影响土壤含水量和土壤电导率的时空变化。气象因素对紫穗槐树液流的特征有显著影响，也受到土壤相对可提取水的有效性的强烈调节，与夜间液流相比，白天液流占主导地位。BPF（埋有2mm孔的塑料覆盖物）是一种很有前途的脱盐替代品。冻融时期农田防护林内土壤盐分运移机制比水分复杂程度更高。不同配置的农田防护林网内水平方向上土壤含水量与林带距离呈正向关系，垂直方向上0-80cm土壤水分从表层向下逐渐增加。基于不同防护林模式土壤水分储存和土壤含盐量分布存在时空差异，四行和八行模式的水分储存相对较低；两行和五行模式在较深的土层和远离防护林边缘的位置存在较好的土壤水分储存。林带边缘植被细根主要分布在0-100cm，林缘附近土壤含水量表现为亏缺状态，土壤有机碳处于碳汇状态。绿洲化进程中随着农田耕作年限的延长，土壤颗粒组成、质地类型和土壤水力特性得到优化，土壤水分、盐碱度环境得以改善。