Physicogenic and biogenic aggregates under different management systems in the Cerrado region, Brazil

Abstract

An important strategy for the sustainable management of Cerrado soils is no-till (NT) systems, which may improve soil properties, particularly aggregation. Soil aggregates can be categorized according to their formation pathways into physicogenic (Phy) and biogenic (Bio). This study aimed (i) to quantify the relative proportion of physicogenic and biogenic aggregates and (ii) evaluate the levels of total organic carbon and their respective humic and physical fractions in the aggregates' formation pathways. The following managed and unmanaged sites were evaluated: a 6-year no-till site (6NT), an 18-year no-till site (18NT), a conventional tillage site (CT), and a reference Cerrado site (RS). Retained aggregates were analyzed morphologically, separated into Phy and Bio, and quantified. Subsequently, aggregates were subjected to total organic carbon (TOC) determination, fulvic acid carbon, humic acid carbon, humin carbon, particulate organic carbon (POC), mineral-associated organic carbon, and free light fraction carbon (FLFC). The proportion of Bio aggregates increased with decreasing management intensity. When TOC and humic acid carbon levels were compared between sites, it was found that Bio aggregates from 18NT and 6NT contained higher carbon content than Bio aggregates from CT. Particulate organic carbon and FLFC differed between aggregate types, with higher POC values observed in Bio aggregates from CT and 18NT and higher FLFC values in Bio aggregates from CT, 6NT, and 18NT. The practices adopted in the conservation management system favored biogenic aggregation in the Cerrado region, which can be proven through the study of the fractions of soil organic matter contained in these aggregates. The biogenic aggregation changed the SOM dynamics. Principal component analysis showed a clear distinction between conventional and conservationist management systems.

Biogenic and physicogenic aggregates: formation pathways, assessment techniques, and influence on soil properties

“…Soil aggregates are still largely identified, classified, and evaluated by the method used to visualize or isolate these structural units (Lavelle et al, 2020). Studies that identify, classify, and evaluate aggregates based on morphological characteristics (Bullock et al, 1985) have gained prominence in the last two decades, especially those related to the origin or formation pathways of aggregates (Pulleman et al, 2005;Velasquez et al, 2007;Batista et al, 2013;Loss et al, 2014Loss et al, , 2017Mergen Junior et al, 2019a,b;Melo et al, 2019a;Ferreira et al, 2020;Pinto et al, 2021). The separation of aggregates according to their morphology is a simple and low-cost method based on visual characteristics as it does not require sophisticated equipment, and can be applied by non-scientific operators, academics, technicians, or farmers (Lavelle et al, 2020), as well as scholars and connoisseurs of soil science.…”

“…In this study, Pulleman et al (2005), based on descriptions by Bullock et al (1985), described, methodologically, what biogenic and physicogenic aggregates are and highlighted for the first time the class of intermediate aggregates. In Brazil, some studies used aggregates ranging from 19 to 8 mm (Melo et al, 2019a); 9.7 to 4.0 mm (Batista, 2015;Ferreira et al, 2020); 9.5 to 4.0 mm (Velasquez et al, 2007;Loss et al, 2017;Ventura et al, 2018;Mergen Junior et al, 2019a,b); 9.7 to 8.0 mm (Batista et al, 2013;Loss et al, 2014;Rossi et al, 2016;Fernandes et al, 2017;Lima et al, 2020;Pinto et al, 2021). Pulleman et al (2005) point out that small aggregates (1 to 2 mm) are very fragile, which makes separation hard; however, larger aggregates (19 to 8 mm) makes separation easier, which decreases the error associated with aggregation pathway classification (Jouquet et al, 2009;Melo et al, 2019a).…”

“…The relative contribution of the aggregates can be determined after classification, and the results obtained are expressed in mass. For this purpose, all the aggregates (biogenic, intermediate and physicogenic) that have been identified can be weighed and thus quantify the fraction of aggregates according to the formation pathway in relation to the initial mass (Loss et al, 2017;Mergen Junior et al, 2019a,b;Pinto et al, 2021). Another possibility to assess the percentage of each fraction can be done by weighing a certain mass of the total aggregates before morphological separation, for example, 100 g (Batista et al, 2013;Batista, 2015;Ferreira et al 2020);200 g (Melo et al, 2019a) or 500 g (Pulleman et al, 2005) and, after identification, the percentage of each type of aggregate in relation to the initial mass is calculated.…”

“…Figure 6 shows examples of these units, identified and classified in areas subjected to constant application of these types of waste, highlighting the swine manure on the Argissolo Vermelho-Amarelo (Ultisol; Acrisol) in Braço do Norte (Santa Catarina, Brazil) and poultry and swine manure on the typical Latossolo Vermelho Eutroférrico (Oxisol; Ferralsol) in Londrina (Paraná, Brazil). Some practices such as the maintenance of plant residues on the soil surface and the absence of disturbance of this layer in association with the addition of organic waste can provide favorable physical and chemical conditions for soil fauna, which are primarily responsible for the formation of biogenic aggregates (Batista et al, 2013;Mergen Junior et al, 2019b;Pinto et al, 2021).…”

“…The presence and contribution of roots in the formation of biogenic aggregates can be clearly observed (Figures 7a, 7b, 7c and 7d) compared to physicogenic aggregates, which have angular and prismatic shapes typical (Figures 7e, 7f, 7g and 7h). It is worth noting that soil and water conservation management systems provide a more balanced distribution between the percentage amounts of physicogenic and biogenic aggregates in the soil surface layer (Melo et al, 2019a;Pinto et al, 2021). , Brazil).…”