Crop management legacies in organic systems affect environmental conditions, but not ground beetle communities

In an effort to build soil organic matter while controlling weeds, organic farmers employ various types of tillage practices and crop rotations while minimizing synthetic inputs.  These practices produce diverse model systems in which to assess effects of disturbance regimes, soil and vegetative properties in animal communities over time.  To assess both immediate and possible legacy effects of organic agricultural practices, we analyzed carabid community response to four diverse cropping systems between 2008-2011.  The four systems assessed were: (1) Conventional (inversion) tillage, high cover crop diversity followed by 2 years of alfalfa, (2) Conventional tillage, low cover crop diversity followed by 2 years alfalfa/grass, (3) Conventional tillage cereal rye/hairy vetch followed by minimal tillage triticale/hairy vetch and maize, (4) Minimal tillage cereal rye/hairy vetch followed by conventional tillage cereal rye and maize.  All systems were planted in soybeans in 2011 to provide a standard crop in which to ascertain legacy effects.  For each plot we annually recorded the number of disturbances (plowing, mowing, harvesting, etc.), and collected crop biomass, pitfall samples of carabid species, and soil cores to assess 17 soil chemical and physical properties.  We assessed whether: (1) Carabid community composition, species richness, species evenness, and mean beetle size varied by systems over time, (2) Environmental variables varied by systems over time, (3) Carabid community measurements were directly correlated with environmental variables, (4) Whether system effects were still detectable in carabid communities and environmental variables in the standardized soybean plots after 3 years of diverse cropping practices.

All measurements of carabid communities varied significantly over time and among systems, but no measurements significantly differed among systems in 2011, when crop type was standardized.  Environmental factors that significantly correlated with carabid communities included number of soil disturbances within the year of each insect collection, soil moisture, soil phosphorus, permanganate oxidizable carbon, and residue nitrogen and phosphorus.  All of these environmental variables significantly varied in systems over time.  Ordination analysis of these environmental variables showed some enduring divergence in the 4 systems in 2011.  These data suggest that different organic farming practices can strongly influence environmental variables that affect carabid communities, and that there are some legacy effects in the environmental factors themselves; however, these effects are too weak in relation to current crop type to produce legacy effects in carabid communities.