Microbial activity effect on aggregate stability after residue addition in a Mollisol and a Vertisol in the Pampas, Argentina

Abstract

Crop residue addition to the soil promotes an increase in microbial activity and, as a consequence, may improve soil aggregate stability. However, this effect may be different in soils with contrasting aggregation agents. The aim of this study was to evaluate, in an incubation experiment, the role of microbial activity on soil aggregate stability after crop residue addition in two soils with contrasting clay mineralogy. Soybean and maize crop residues were added with three different frequencies on a Mollisol and a Vertisol, which were incubated during a 120-days period. It was studied the dynamics of soil aggregate stability after application of a fast wetting (MWDfw), slow wetting (MWDsw) and stirring after prewetting treatment (MWDst) which evaluates three different breakdown mechanism, i.e., slacking, microcracking and mechanical breakdown, respectively. Microbial activity dynamic was evaluated through measurements of soil respiration rate, microbial biomass-C (MBC) and hot-water extractable carbohydrate-C (HWEC). The Vertisol showed higher resistance to slaking (59% higher) but lower resistance to mechanical breakdown (92% lower) and microcracking (102% lower) than the Mollisol, with a scarce effect of residue quality and addition frequency. The discrete changes recorded in MWDfw throughout the experiment, were positively associated 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 small changes in MWDst were weakly and positively associated (P < 0.05) with changes in cumulative respiration in the Mollisol but not in the Vertisol. However, changes in MWDsw were closely associated with changes in cumulative 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 these variables than the other aggregate stability test. However, while in the Vertisol the increase in MWDsw as a function of cumulative soil respiration was lineal, in the Mollisol it was detected an evident increase in MWDsw up to a threshold of 500 μg C-CO2 g soil−1 of cumulative soil respiration. Similarly, changes in MWDsw associated with changes in MBC were higher in the Mollisol than in the Vertisol (i.e. 0.018 v. 0.004 mm of MWDsw per unit of MBC). Thus, this research added new evidence about the contrasting role of transitory aggregation agents that are provided by microbial activity on aggregate stability in two contrasting soils such as a Mollisol and a Vertisol.