The influence of plant species, fertilization and elevated CO2 on soil aggregate stability

We tested the effects of plant species, fertilization and elevated CO2 on water-stable soil aggregation. Five annual grassland species and a plant community were grown in outdoor mesocosms for 4 years, with and without NPK fertilization, at ambient or elevated atmospheric CO2 concentrations. Aggregate stability (resistance of aggregates to slaking) in the top 0.15 m of soil differed among plant species. However, the more diverse plant community did not enhance aggregate stability relative to most monocultures. Species differences in aggregate stability were positively correlated with soil active bacterial biomass, but did not correlate with root biomass or fungal length. Plant species did not affect aggregate stability lower in the soil profile (0.15-0.45 m), where soil biological activity is generally decreased. Elevated CO2 and NPK fertilization altered many of the factors known to influence aggregation, but did not affect water-stable aggregation at either depth, in any of the plant treatments. These results suggest that global changes will alter soil structure primarily due to shifts in vegetation composition.