Are Long-Term Changes in Soil Organic Matter Reflected in the Soil Communities?.

The ‘Highfield ley-arable experiment’ at Rothamsted Research, UK, has grass, arable and bare-fallow plots which have been under continuous management for over 50 years. Consequently, soil organic matter contents and soil structure differ between plots. Soil microbial biomass, phospholipid fatty acids (PLFA), soil DNA, mesofauna and heterotrophic bacterial abundance followed the trend: fallow<arable<pasture. The Shannon diversity index (H′) for PLFAs in the fallow soil was significantly lower than in pasture (but not arable) soil. Biolog™ substrate utilisation by heterotrophic soil bacteria varied between the three soils but H′ did not. The number of DGGE bands representing different 16S rRNA genes (and hence bacterial phyla) differed between soils: bare-fallow>pasture>arable; this order was reflected in H′. The active bacterial population (16S rRNA) was most diverse in the pasture and least in the arable soil. There were no differences in utilisation of added substrate between soils. ATP concentrations in the biomass were reasonably constant between all substrate amended and unamended soils at around 8 -14 µmol ATP g^-1 biomass C. To summarise preliminary results, the microbial community in fallow soil survives on highly recalcitrant, humified, soil organic matter and small substrate inputs from weed growth between tillings and, despite decreased abundance, shows no decrease in phylum richness or potential activity. A new long-term field experiment with these soils will determine effects on the different soil communities of converting the fallow to arable and grassland and vice versa. The first experimental phase is now underway with areas of the fallow planted to either grass or arable; areas of the grass plots being made fallow or planted with arable; areas of the arable plots being made fallow or planted with grass. Changes in the biology, chemistry and physics of the field soils will be followed over the next few years and the long-term.