Grasslands as benchmarks for agricultural sustainability: nematode and insect communities

Background/Question/Methods

Annual crops cover approximately 70% of global cropland and provide a large proportion of human food and fiber. To maintain annual exports of nutrients, annual crops often rely on nutrient addition (e.g. N) and pest management (e.g. insect, weed). However, annually hayed prairie meadows have comparable annual nutrient exports but do not receive fertilizer or pest management. In order to better understand how hayed prairie meadows can maintain comparable nutrient export to that of annual crop fields without fertilization and pest management, we examined two biotic components of annual crop and hayed prairie meadow production systems: nematodes and above-ground insects. Specifically, we asked: how do nematode and above-ground insect assemblages differ between hayed prairie meadows and annual grain fields?
We sampled above- and below-ground nematodes and above-ground insects during June 2006 and 2007 for five sites in five northern central Kansas, USA counties. For each site there was a pair of an annually hayed prairie meadow (PM), and an adjacent annual grain crop field field (AG). The prairie meadows have been in continuous hay production for at least 75yrs and have never been tilled. Over a similar time period, the annual grain crop fields have been primarily used for wheat production. Nematodes were sampled by bulking five cores that were 4cm in diameter and 100cm deep (separated by depth: duff, 0-10, 10-20, 20-40, 40-60, 60-80, 80-100cm ) within each PM and AG field. From a 100g soil subsample, the first 200 nematodes were identified to family or genus. Insects were sampled using a sweep net along 11 transects 22m long (242 sweeps/ 484m²) within each PM and AG field. Adult insects were separated from the samples and sorted to morphospecies within family.

Results/Conclusions

Differences in nematode assemblages in PM and AG soils were driven by differences in soil food web structure, taxa richness and, relative abundance of omnivores, predators, and bacterivores for both years and over all soil depths. Proportion and total abundance of bacterivores in AG was significantly greater than in PM (p< 0.01) while total number of omnivores and predators was significantly greater in PM (p<0.05). The number of insects did not differ between PM and AG (p> 0.1) but there were more insect morphospecies in PM than AG (p= 0.04). The number of insect herbivores, and predators and parasites did not differ between production system but there were greater numbers of pollinators (p=0.059) and saprophages (p=0.066) in PM.