Background/Question/Methods Despite their yield, grain quality, and other ecosystem functioning implications such as biodiversity maintenance, very little research has been done on weed communities in the northern Great Plains region of the United States.  The objectives of this study were first, evaluate the effects of management systems on the weed aboveground and seedbank abundance, species composition, and species diversity.  Second, assess the relationship between weed seedbanks and aboveground weed communities in small grain production systems.  Weed communities were sampled over two years in organic and conventional no-tillage spring wheat production fields near Big Sandy, Montana.  Aboveground weed communities were characterized along three 100 m transects per field by measuring percentage weed cover by species in 100 1m by 0.33m quadrats.  The spatial pattern of weed distribution was compared across systems using a combination of aggregation measures and quadrat variance techniques.  Eight 1m by 0.33 m quadrats were further selected in each field to encompass a wide range of measured aboveground weed diversities.  Three soil cores of 4 cm in diameter were taken in each quadrat to a depth of 20 cm and split in half at 10 cm to assess the impact of cropping systems on weed seedbank communities.

Results/Conclusions

Aboveground weed cover was aggregated in both the conventional and the organic systems but the patterns of aggregation were different for the two systems.  The number of seeds recovered from the top ten cm of soil and the 10-20 cm depth were significantly affected by sampling year and an interaction between cropping system and sampling year.  A multivariate ordination indicated that while year played a significant role in determining weed seedbank communities, management system had a role only during 2006.  Finally, we failed to detect strong correlations between the aboveground and belowground weed communities.  Results of this study suggest that first, the processes causing aggregation of aboveground weed communizes in the two systems may be different between organic and conventionally managed crop fields.  Second, in the studied region, yearly fluctuations in environmental factors have significant impacts on weed seedbank communities.