Abstract

HELFRICH, M. , LUDWIG, B., POTTHOFF, M. and FLESSA, H. (2008).

Soil Biol. Biochem. 40: 1823-1835

We investigated the effect of plant residue decomposability and fungal biomass on the dynamics of macroaggregate (250-2000 mm) formation in a three months' incubation experiment and determined
the distribution of residue-derived C and N in the microbial biomass and in aggregate size fractions (250-2000 mm, 53-250 mm and <53 mm) using 13C and 15N data. A silty loam soil (sieved <250 mm) was incubated with and without addition of 15N labeled maize leaves (C/N ¼ 27.4) and roots (C/N ¼ 86.4). Each treatment was carried out with and without fungicide application. The addition of maize residues enhanced soil respiration and microbial biomass C and N and resulted in increased macroaggregate formation with a higher and more rapid maximum macroaggregation in the soil amended with maize leaves than in that with addition of roots. Fungicide application led to a significant decline of microbial biomass C and mineralization of the added residues compared to untreated soils, which demonstrates a successful suppression of part of the active microbial biomass by the fungicide. However, this was not confirmed by a generally lower ergosterol concentration. Consequently, ergosterol was no reliable fungal biomarker in periods of rapid decline of the fungal biomass. A single addition of fungicide was insufficient for continued inhibition of the fungal biomass. Yet, a significant delay (28-42 days) in macroaggregation in fungicide treated compared to untreated samples highlighted the importance of the fungal biomass in macroaggregate formation. Macroaggregates were enriched in maize-derived 13C and 15N compared to microaggregates or the fraction < 53 mm. They turned over rapidly with decreasing substrate availability, which entailed a transfer of maize-derived C and N stored within macroaggregates during the first weeks of incubation to microaggregates with proceeding incubation time. Our results indicate that this transfer happened within macroaggregates, because no considerable amount of free particulate organic matter (POM) was released upon macroaggregate breakdown. We conclude that substrate decomposability and fungal activity are key factors determining extent and dynamics of macroaggregation during decomposition processes. Macroaggregate formation implied rapid incorporation
and thereby short-term protection of maize-derived C and N. Moreover, macroaggregates allowed a transfer of maize-derived organic matter into microaggregates within macroaggregates, which
prevented the release of significant amounts of free POM upon macroaggregate breakdown. Consequently, macroaggregates constitute to the transfer of recently added C into more stable soil organic
matter fractions.