Higher rates of manure application lead to greater accumulation of both fungal and bacterial residues in macroaggregates of a clay soil

Abstract

Microbial residues represent a significant soil organic matter pool and participate in soil aggregation. The addition of organic manure is known to modify soil aggregation and strongly influence soil microbial residues. How manure application influences the spatial distribution of microbial residues in soil aggregates is largely unknown. This study attempts to determine the effect of manure application at various rates on the content and distribution of microbial residues among aggregates of different sizes. We used a long-term manure application experiment in a Mollisol in northeastern China, where manure has been applied since 2001 at rates of 0, 7.5, 15, and 22.5 Mg ha−1 yr−1 (dry weight). The abundance of microbial residues was indicated by amino sugar analysis. Glucosamine and muramic acid were used as biomarkers for fungal and bacterial residues, respectively. Amino sugars were examined within four aggregate fractions: large macroaggregate (>2000 μm), small macroaggregate (250–2000 μm), large microaggregate (53–250 μm) and small microaggregate (<53 μm). Application of manure at 15 and 22.5 Mg ha−1 yr−1 provided significantly higher proportions of macroaggregates and mean weight diameter (MWD) than non-manure treatment and manure applied at 7.5 Mg ha−1 yr−1. Manure application, especially at higher rates, significantly stimulated the accumulation of total amino sugars in both macroaggregates and large microaggregates and more amino sugars were found in >250 μm macroaggregates compared with microaggregates. However, effects of manure application rates on amino sugar accumulation in larger aggregates were limited when manure rate was increased from 15 to 22.5 Mg ha−1 yr−1. The response of fungal- and bacterial-derived amino sugars to manure application rates differed among aggregate fractions, i.e., glucosamine associated with macroaggregates increased more than that of microaggregates, whereas the enhancement of muramic acid was prominent in both macroaggregates and large microaggregates. The mass proportions of macroaggregates and MWD showed significant positive correlations with amino sugar contents, indicating that these microbial residues are involved in the formation and stabilization of aggregates. Manure applications greatly increased the contribution of microbial residues to soil organic C (SOC) in small macroaggregates and large microaggregates (P ≤ 0.05). We conclude that higher manure input may promote soil aggregation and higher SOC storage, which is closely related to a greater microbial residues-mediated improvement of soil aggregate stability. Our results also suggest that measurement of amino sugar content is a useful approach to assess fungal and bacterial contributions to soil aggregation.