Plant roots and deep-banded nutrient-rich amendments influence aggregation and dispersion in a dispersive clay subsoil

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 mu m) 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.