Changes in Soil Aggregate Size and Stability over Time Under Riparian Vegetation.

Grass filters are a conservation practice designed to protect water quality and prevent soil erosion by improving soil quality, including soil aggregation, in the riparian zone.  USDA Conservation Reserve Program (CRP) grass filter contracts span 10-15 years.  This study was conducted to establish whether aggregation continues to improve after typical CRP contracts expire.  Aggregate mean weight diameter (MWD) and percent water-stable macroaggregates (%WSA, >0.25 mm) were measured in 2010-2011 under a chisel-plowed corn (Zea mays L.)-soybean (Glycine max) field and two 20-year-old grass filters in central Iowa, USA on Clarion loam (fine-loamy, mixed, superactive, mesic Typic Hapludoll) and Coland clay loam (fine-loamy, mixed, superactive, mesic Cumulic Endoaquoll).  One grass filter contains smooth brome (Bromus inermis); the other grass filter originally contained switchgrass (Panicum virgatum L. ‘Cave-in-Rock’), but has since been invaded by smooth brome.  Results were compared with data collected in 1997, seven years after grass filter establishment.  %WSA and MWD under the former switchgrass increased 45.8% and 120.5%, respectively, since 1997; under smooth brome, %WSA and MWD increased 17.9% and 34.3%, respectively, but %WSA and MWD decreased by 37.0% and 35.2% in the crop field.  %WSA and MWD measured in 1997 in the same locations ranked in order of cool-season grass (67% and 1.98 mm, respectively) > switchgrass (48% and 0.83 mm, respectively) = crop (46% and 0.71 mm, respectively).  These results suggest switchgrass takes longer to increase aggregation.  Smooth brome increases aggregation in short time periods, but lacks other attributes that make switchgrass attractive in a grass filter.