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Title:   Author  item
                                            Wuest, Stewart
  Submitted to: Soil & Tillage Research
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 5/3/2007
                                    Publication  Date: 9/5/2007
                                    Citation: Wuest, S.B. 2007. Surface versus incorporated residue effects on water-stable  aggregates. Soil & Tillage Research 96 (2007) 124-130.Interpretive Summary: Reduced tillage methods currently being promoted for field crop production result in less disruption of soil structure, and often at the same time result in increased amounts of crop residue maintained on the soil surface.  The combination of these two factors causes increased surface soil aggregation.  This study was conducted to determine whether surface residue by itself improves soil aggregation within a single crop season.  Two pot studies were conducted and compared to field plots where, for the past seven years, wheat crop residues were either incorporated through tillage or removed before tillage and replaced on the surface after tillage.  The field study also included plots where wheat was grown with no tillage.  In the pots, there was no effect of the surface residue treatment on aggregate mean weight diameter, measured monthly for four winter months.  In fact, the no-till field was the only treatment to demonstrate significantly greater aggregation.  Our conclusion is that surface residue by itself is insufficient to increase aggregation of surface soil, and that other factors such as live and decaying roots, greater fungal activity and reduced physical disturbance with no-till, as well as time, are important in producing greater macroaggregate levels under no-till.
                                Technical Abstract: Reduced tillage methods for field crop production result in less disruption of soil structure and often increased amounts of crop residue maintained on the soil surface.  The combination of these two factors produces increased surface soil aggregation.  This study was conducted to determine whether surface residue by itself improves soil aggregation within a short period of time.  The soil was a silt loam loess deposit in the Pacific Northwest, USA, where summers are hot and dry, and most precipitation (420 mm) is received during the mild winters.  Two pot studies were conducted, one under a shelter with controlled irrigations, and the other outdoors receiving only natural precipitation.  In both pot studies 640 g m^-2 wheat (Triticum aestivum L.) residue was either placed on the surface of the soil or thoroughly mixed into the soil. A third study was conducted on field plots where, for the past seven years, wheat crop residues were either incorporated through tillage or removed before tillage and replaced on the surface after tillage.  The field study also included plots where wheat was grown with no tillage.  In the pots, there was no significant effect due to residue treatment on aggregate mean weight diameter, measured monthly for four winter months.  This was true despite dissolved organic carbon being leached from the surface residue.  In the seven-year-old field plots, replacing surface residue resulted in slightly greater mean weight diameter of aggregates at 5- to 10-cm depth compared to the mixed residue treatment.  The no-till plots had significantly greater mean weight diameter at 0- to 5-cm depth than either tilled treatment.  Our conclusion is that surface residue by itself failed to increase aggregation of tilled surface soil within the first rainy season after tillage.


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