Organic amendment effects on nematode distribution within aggregate fractions in agricultural soils

To evaluate the effect of organic amendments on soil nematode community composition and diversity within aggregate fractions, a study was initiated in agricultural soils with four-year organic amendments. Soil samples were collected from the plow layer (0-20 cm) under three cornfield management scenarios: 1) conventional cropping (CK, corn straw removal and no organic manure application); 2) straw retention (SR, incorporation of chopped corn stalk); and 3) manure application (MA, chicken manure input). The soil samples were fractionated into four aggregate sizes, i.e., >2 mm (large macroaggregates), 1-2 mm (macroaggregates), 0.25-1 mm (small macroaggregates), and <0.25 mm (microaggregates, silt and clay fractions). The composition and diversity of soil nematode communities were determined within each aggregate fraction. The results showed that both SR and MA treatments significantly increased the percentage of macroaggregates (>1 mm) and only MA treatment strongly increased the mean weight diameter compared to the CK (P<0.05). The abundance of total nematodes and four trophic groups were affected significantly by the aggregate fractions and their higher abundance occurred in the larger aggregates. The effects of aggregate size on most nematode genera were significant. Bacterivores in the small macroaggregates and microaggregates, and fungivores in the large macroaggregates were significantly different among treatments. The percentage of bacterivores increased after the application of organic materials, while that of fungivores decreased. It can be concluded that organic management significantly affects soil aggregation and soil characteristics within aggregates, and the aggregate size subsequently influences the distribution of nematode communities.