Microbial activity effect on aggregate stability after residue addition in a Mollisol and a Vertisol in the Pampas, Argentina Leonardo E. Novelli a,b,c, ⁎, Walter L. Hass a , Silvia M. Benintende a , Octavio P. Caviglia a,c a Facultad de Ciencias Agropecuarias, Universidad Nacional de Entre Ríos, Ruta 11, Km 10.5, 3101 Oro Verde, Argentina b INTA EEA Paraná, Ruta 11, Km 12.5, 3101 Oro Verde, Argentina c CONICET, Argentina abstract article info Article history: Received 30 December 2019 Received in revised form 23 October 2020 Accepted 24 October 2020 Available online xxxx Keywords: Hot-water extractable carbohydrate-C Microbial biomass C Soil aggregation agents Mollisol Vertisol 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 pe- riod. It was studied the dynamics of soil aggregate stability after application of a fast wetting (MWD fw ), slow wet- ting (MWD sw ) and stirring after prewetting treatment (MWD st ) 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 extract- able 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 MWD fw 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 MWD st were weakly and positively associated (P < 0.05) with changes in cumulative respiration in the Mollisol but not in the Vertisol. However, changes in MWD sw were closely associated with changes in cumulative soil res- piration 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 MWD sw as a function of cumulative soil respiration was lineal, in the Mollisol it was detected an ev- ident increase in MWD sw up to a threshold of 500 μg C-CO 2 g soil -1 of cumulative soil respiration. Similarly, changes in MWD sw associated with changes in MBC were higher in the Mollisol than in the Vertisol (i.e. 0.018 v. 0.004 mm of MWD sw 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. © 2020 Elsevier B.V. All rights reserved. 1. Introduction The remarkable land-use change recorded in the last two decades in several agricultural areas of South America has dramatically changed the dynamics of vegetal residue inputs to the soils as compared with previous productive systems (Nosetto et al., 2012). Such changes have been driven by the expansion of the area under agriculture in this region and by the increase in the proportion of soybean [Glycinemax (L.) Merr.] in the crop sequences (Novelli et al., 2011), with drastic consequences on soil quality (Wingeyer et al., 2015). The dynamics of vegetal residues input to the soil, characterized by the amount, quality and frequency, may affects several soil properties such as microbial activity and, in turn, soil organic C (SOC) and aggregate stability, both closely related with soil productivity and quality (Amézketa, 1999; Ball et al., 2005; Lauber et al., 2008). Shortly after a crop residue is added to the soil, microbial activity is rapidly increased due to the decomposition of easily available com- pounds, decreasing later when these compounds are depleted and more recalcitrant compounds become predominant (Duong et al., 2009). This microbial activity promotes the release of transitory aggre- gation agents (Tisdall and Oades, 1982) such as hot-water extractable carbohydrate-C (HWEC), a pool of C from microbial origin (Haynes Geoderma Regional 23 (2020) e00346 ⁎ Corresponding author at: Facultad de Ciencias Agropecuarias, UNER, Ruta 11, km 10.5, 3101 Oro Verde, Argentina. E-mail address: leonardo.novelli@fca.uner.edu.ar (L.E. Novelli). https://doi.org/10.1016/j.geodrs.2020.e00346 2352-0094/© 2020 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geoderma Regional journal homepage: www.elsevier.com/locate/geodrs