Linking soil aggregation to organic matter chemistry in a Calcic Cambisol: evidence from a 33-year field experiment

Soil aggregation contributes to soil organic C (SOC) sequestration, but it is still unclear how the organic functional groups and microbial necromass operate to influence soil aggregation. This issue was explored after 33 years of manure (i.e. farmyard manure at agronomic and elevated rates) and chemical fertilizer application in a Calcic Cambisol on the Loess Plateau. The application of manure increased SOC and the mass proportion of macroaggregates by 1.1- to 1.8-fold and 1.5- to 2.3-fold, respectively, compared with the unfertilized control. The aliphaticity, aromaticity and hydrophobicity indices of SOC as characterized by C-13 nuclear magnetic resonance spectroscopy were higher in the manure-amended soils than in the chemical fertilizer-treated soils. The application of manure increased the microbial necromass by 1.3- to 2.6-fold compared with the control. Fungal necromass was enriched in the macroaggregates, while bacterial necromass was enriched in the microaggregates. Compared with the control, manure application increased (P < 0.05) the relative contribution of fungal necromass to SOC, which was positively correlated with the chemical recalcitrance of the SOC (i.e. aliphaticity, aromaticity and hydrophobicity) (P < 0.05). Furthermore, the chemical recalcitrance and relative contribution of microbial necromass, particularly that of fungal necromass, in the bulk soil (P < 0.05) and soil aggregates (P < 0.01) were positively correlated with the mass proportion of macroaggregates. Therefore, the accumulation of recalcitrant C components and fungal necromass contributes to soil aggregation in the Calcic Cambisols of the Loess Plateau.