Changes in soil organic matter quality during long-term bare fallow do not affect microaggregate stability

Organic substances of diverse origins are known to promote the formation of microaggregates in soils. However, their contribution to the resistance of microaggregates against mechanical stress remains unclear. This study tests for possible effects of plant- and microbial-derived organic matter on the stability of microaggregates against ultrasonic dispersion, taking advantage of a 14-year field experiment with either continuous (cropland) or minimum (bare fallow) organic inputs. The idea was that minimum input will result in the depletion of organic matter and, consequently, in decreased microaggregate stability. Microaggregates were separated into three size fractions (<20, 20-53, 53-250 µm) by wet sieving and subjected to ultrasonic disturbance at various energies. The contents of organic C, total N and neutral and amino sugars in microaggregates were determined by thermal combustion and biomarker analyses, and X-ray photoelectron spectroscopy of intact and crushed microaggregates was used to analyse the spatial distribution and oxidative alteration of organic matter. The results show that most microaggregate samples under bare fallow showed little to no decline in organic C concentrations, while bulk soil C decreased from 1.2 to 0.9 %. Amino and neutral sugars, however, decreased significantly, indicating decreased contribution of microbial products. This finding is in conflict with the missing plant C input, which should have promoted microbial processing of organic matter, resulting in declining contents of organic C with increased contributions of microbially derived compounds. Microaggregate surfaces were significantly enriched in C, with no decrease under bare fallow, which might indicate that microaggregates are not built around organic cores but are structural units collecting organic matter from their surroundings. This agrees with the finding that more oxidised and microbially processed material is stored within microaggregates, while organic matter on the outer surfaces is less oxidised, i.e. less strongly processed and thus fresher. This may explain why microaggregates lost very little organic C during fallow, as degrading plant material could have provided organic matter, substituting the loss of mineralized microbial organic matter. All microaggregate size fractions showed little and rather similar resistance against mechanical stress, achieving near complete dispersion after the application of 25 J/ml. Microaggregate stability was, in agreement with organic C contents, similar for both treatments but showed no indication that the varying contribution of amino and neutral sugars was of relevance to microaggregate stability. We conclude that, despite the clear effect of bare fallow on the organic matter composition, it had little effect on microaggregate organic C contents and their resistance to mechanical stress. This indicates that the composition of organic matter may not be the primary factor for the mechanical stability of microaggregates.