CONICET | Buscador de Institutos y Recursos Humanos

Crop residue addition to the soil promotes anincrease in microbial activity and, as a consequence, may improve soilaggregate stability. However, this effect may be different in soils withcontrasting aggregation agents. The aim of this study was to evaluate, in anincubation experiment, the role of microbial activity on soil aggregatestability after crop residue addition in two soils with contrasting claymineralogy. Soybean and maize crop residues were added with three differentfrequencies on a Mollisol and a Vertisol, which were incubated during a120-days period. It was studied the dynamics of soil aggregate stability afterapplication of a fast wetting (MWDfw),slow wetting (MWDsw)and stirring after prewetting treatment (MWDst) which evaluates three different breakdown mechanism,i.e., slacking, microcrackingand mechanical breakdown, respectively. Microbial activity dynamicwas evaluated through measurements of soil respirationrate, microbial biomass-C (MBC) and hot-water extractable carbohydrate-C(HWEC). The Vertisol showed higher resistance to slaking (59% higher) but lowerresistance to mechanical breakdown (92% lower) and microcracking (102% lower)than the Mollisol, with a scarce effect of residue quality and additionfrequency. The discrete changes recorded in MWDfw throughout the experiment, were positivelyassociated with changes in cumulative respiration (P < 0.001), MBC (P < 0.05) y HWEC (P < 0.05) in the Mollisol,and only with changes in MBC in the Vertisol (P < 0.05). Also, the smallchanges in MWDst wereweakly and positively associated (P < 0.05) with changes in cumulativerespiration in the Mollisol but not in the Vertisol. However, changes in MWDsw were closely associated with changes incumulative soil respiration rate (P < 0.0001) and MBC (P < 0.001) in both soils,and with HWEC only in the Mollisol (P < 0.0001), with a higher effect on thesevariables than the other aggregate stability test. However, while in theVertisol the increase in MWDsw asa function of cumulative soil respiration was lineal, in the Mollisol it wasdetected an evident increase in MWDsw upto a threshold of 500 μg C-CO2 gsoil-1 of cumulative soil respiration. Similarly, changesin MWDsw associated with changes in MBC were higher in theMollisol than in the Vertisol (i.e. 0.018 v. 0.004 mm of MWDsw per unit of MBC). Thus, this research added newevidence about the contrasting role of transitory aggregation agents that areprovided by microbial activity on aggregate stability in two contrasting soilssuch as a Mollisol and a Vertisol.

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