The ameliorating effect of deep banding of nutrient-rich organic amendments, termed subsoil manuring, on improving physical structure of sodic high-clay subsoils, has been often attributed to organic amendments per se. However, this cannot explain the transformation of soil physical properties between the rip-lines, away from the amendments. This study assessed the effect of deep-banding nutrient-rich amendments on aggregation and dispersion of a clay subsoil in the presence and absence of wheat (Triticum aestivum) roots under controlled environment conditions. A specially-designed dual-column was set up to simulate a soil profile where a well-structured topsoil overlaid a sodic clay subsoil with an exchangeable sodium percentage (ESP) of 21%. The five amendments include a control (zero amendments), fertilizer nutrients (NPKS), wheat straw + fertilizer nutrients (straw/NPKS), poultry litter (PL) and poultry litter + controlled-release fertilizer (PL/mac). All amendments were added to the centre of the subsoil, 6 cm below the base of the topsoil. Our results showed that the presence of deep-placed nutrient-rich amendments, such as straw/NPKS and PL/mac, greatly enhanced deep root proliferation in this sodic clay subsoil, and resulted in the rapid build-up of large (>2000 μm) water-stable macroaggregates. There was a significant (P < 0.05) positive linear relationship between the root length density and the formation of large macroaggregates in the subsoil adjacent to and below the amendment. The stimulation of microbial growth by root exudates or by mucilage, as indicated by a significantly higher bacterial and fungal abundance (P < 0.05) in the planted than unplanted soils, is likely to have contributed to the formation of these macroaggregates. The effectiveness of wheat straw/NPKS in promoting the formation of macroaggregates in the unplanted soil could be attributed to the ‘straw effect’ which induced a marked increase in fungal growth (P < 0.05). Soil electrical conductivity (EC) and aggregate size were the key determinants of clay dispersion in the aggregated subsoil. Plant roots showed a contrasting effect on clay dispersion: increasing clay dispersion by reducing soil EC while suppressing clay dispersion via root-induced increases in large macroaggregates. We argue that the degree of slaking or disaggregation is likely to determine the net effect of roots on clay dispersion, and that root effect on increasing dispersion of macroaggregates in wet subsoil is limited. The major finding of the study is that increased aggregation in a dispersive clay subsoil can occur when wheat roots grow actively in these layers, in response to deep-placed nutrient-rich amendments.

富含营养的有机改良剂的深层带改善底土处理，改善了钠基高黏土的物理结构，其改善效果通常归因于有机改良剂本身。但是，这不能解释除修正外土壤在裂隙线之间的物理性质的转变。这项研究评估了存在和不存在小麦的情况下，深层营养丰富的改良剂对黏土底土聚集和分散的影响（普通小麦））根系在可控的环境条件下。设置了一个专门设计的双柱来模拟土壤剖面，其中结构良好的表层土覆盖了钠交换比为21％的钠粘土底土。这五种修正包括对照（零修正），肥料养分（NPKS），麦秆+肥料养分（秸秆/ NPKS），家禽垫料（PL）和家禽垫料+控释肥料（PL / mac）。所有修正都添加到了表土底部下方6厘米处的地下土壤中心。我们的结果表明，深层营养丰富的改良剂（例如稻草/ NPKS和PL / mac）的存在大大增强了这种钠黏土深层土壤的深层根部增殖，并导致了大块土壤（> 2000μm）的快速堆积。 ）耐水的大型骨料。有一个显着的（P <0.05）根长长度密度与邻近和低于修正值的地基中大型宏观团聚体的形成之间呈正线性关系。根系分泌物或黏液对微生物生长的刺激作用表明， 种植土壤中细菌和真菌的丰度明显高于未种植土壤（P <0.05），这可能有助于这些大型聚集体的形成。小麦秸秆/ NPKS促进未种植土壤中大团聚体形成的有效性可归因于“稻草效应”，该效应导致真菌生长显着增加（P <0.05）。土壤电导率（EC）和团粒大小是决定黏土在团聚地下土壤中分散性的关键因素。植物根系对粘土的分散显示出相反的效果：通过减少土壤EC来增加粘土的分散，同时通过根系诱导的大型团聚体的增加来抑制粘土的分散。我们认为，崩解或分解的程度可能决定根对粘土分散的净效应，而根对增加大团聚体在湿土中的分散的根效应是有限的。该研究的主要发现是，当小麦根部在这些层中积极生长时，对深层营养丰富的改良剂做出反应，会增加分散性粘土底土中的聚集。