Long-term effects of organic fertilizers and humic compounds on soil aggregate stability in a silt loam soil

Aggregate stability plays a pivotal role in physical protection of organic molecules and therefore soil carbon sequestration. Little is known about the effects of individual components of organic matter on aggregate stability either in quantitative or qualitative terms. In the present study the effects of humic substances on aggregate breakdown were investigated in a field experiment in north-eastern Italy comparing long-term application (40 yrs) of organic (farmyard manure and liquid manure), mineral and mixed (organic + mineral) fertilizers. Half of the treatments also included crop residue incorporation from corn monocultures. Wet sieve analysis were performed on 1-2 mm soil aggregates, pre-treated with ethanol, benzene, fast and slow wetting to better investigate the underlying mechanisms of breakdown. Aggregate pore distribution in the range 0.007-100 μm were also analysed by mercury porosimetry. Humic substances were characterised using elemental analysis, molecular weight distribution (nominal molecular weight: >100 kDa, 100-10 kDa and <10 kDa) and molecular structure, applying differential thermal analysis (DTA) and diffuse reflectance infrared Fourier transform (DRIFT). The amendment practices clearly had influenced the functional groups of humic substances, so that when the soil did not receive any amendment or mineral fertilizer, the aromatic and oxygenate groups decreased. Aromatic groups generally increased with farmyard manure and mixed fertilizers amendments. This might be mainly due to the incorporation of phenolic groups in humic structure arising from the lignin of the crop residues. Aggregate stability was strongly affected by the amount and type of organic input. Only high C-input treatments differed significantly from the controls, emphasizing the action of two major mechanisms of breakdown. The first one was associated to the hydrophobicity induced by the aromatic carbon groups of the >100 kDa humic C. The second one was associated to the increase of the ultramicro-pores (0.1–5 μm) fraction, which most likely reduced the fast wettability of the aggregates and consequently the slaking effect. The results obtained by the present research have provided additional evidence of the important role played by organic matter on aggregate stability.