Carbon and nitrogen mineralization in hierarchically structured aggregates of different size
Graphical abstract
Introduction
Soil aggregation depends on the soil fauna, microorganisms, roots, organic and inorganic binding agents, and environmental and physical forces (Oades, 1993, Six et al., 2004). Neutral to alkaline soils with C/N ratios <15, derived from calcareous parent material with mull humus form, are characterized by low proportions of organic matter present as plant debris (Oades, 1984, Oades, 1988). Under these slightly alkaline conditions, calcium and magnesium cations preferentially form bridges between negatively charged clay colloids and negatively charged soil organic matter (SOM), enhancing the stability of the aggregate system (Baldock et al., 1997, Baldock and Skjemstad, 2000, Bockenhoff et al., 1997, Oades, 1988, Rashad et al., 2010). These polyvalent metal ions ensure the integrity of microaggregates and other persistent organic materials (Elliott, 1986, Tisdall and Oades, 1982).
Previous studies on aggregate carbon mineralization have focused on land use (Akinsete and Nortcliff, 2014, Gupta and Germida, 1988, Rabbi et al., 2015) and tillage systems (Beare et al., 1994, Fernandez et al., 2010, Nyamadzawo et al., 2009, Six et al., 2000). Carbon mineralization has been altered by management practices. In a 100-day incubation study, the carbon mineralization per unit of soil organic carbon (SOC) in pastures (0.005%) was double that of arable cropping systems (0.002%) (Curtin et al., 2014). This difference is attributed to a continuous supply of labile carbon in the form of coarse organic matter from perennial vegetation, which is greatly reduced under arable cropping (Baker et al., 2007, Curtin et al., 2014). However, the stability of SOC with undisturbed natural aggregates has been little investigated (Álvarez et al., 2007). The simultaneous evaluation of carbon and nitrogen mineralization behavior in aggregates has been undertaken only recently (Curtin et al., 2014, Oorts et al., 2006), and reports of SOC and nitrogen in undisturbed aggregates remain rare. The carbon and nitrogen mineralization of aggregates is commonly evaluated in complementary laboratory studies, wherein the soils are segregated into aggregate size classes and incubated (Six and Paustian, 2014). In the laboratory experiment of Curtin et al. (2014), carbon and nitrogen mineralization was calculated by the increased surface area with decreasing aggregate size.
Nevertheless, despite some studies (Gupta and Germida, 1988, Sainju et al., 2009, Zhang et al., 2012) researching the distribution of microbial biomass carbon (MBC) and nitrogen (MBN) in different aggregate classes, and how this biomass relates to SOM mineralization in each aggregate class, this topic still remains poorly understood. Miller and Dick (1995) found a qualitative difference in the microbial communities occupying macro- and microaggregates. Gupta and Germida, 1988, Gupta and Germida, 2015 emphasized the importance of microbial biomass in the formation of macroaggregates and as a primary source of SOM for carbon mineralization. They identified a crushing effect of the macroaggregates, which enhances carbon mineralization. This disruption releases the microbial biomass mucilage that binds the macroaggregates, which contributes to the mineralizing SOM. Therefore, by collecting data on MBC and MBN and the community structure across aggregate size classes, we can begin to elucidate carbon and nitrogen cycling (Six and Paustian, 2014) and their coupling as determined by the soil structure.
To better understand the coupled regulation of carbon and nitrogen mineralization in undisturbed aggregates, we conducted an incubation experiment. We studied mineralization in two aggregate size classes of a dolomitic grassland soil (Haplic Cambisol), composed of different hierarchically organized substructures. Specifically, we asked the following questions: (1) Do different aggregate classes mineralize at similar rates when their water and air availabilities are comparable? (2) Do aggregate size classes differ in their substructures, external surface areas, and bioavailability of SOM? (3) Do aggregate classes of different sizes exhibit diverse heterotrophic respiration efficiencies? Thus, the present study seeks to identify the role of different intact structural units to the overall soil mineralization of carbon and nitrogen as controlled by bioavailability in aggregate substructures.
Section snippets
Site characteristics and origin of soil material
The sampling site Graswang is located near Garmisch–Partenkirchen in a foothill valley (877 m above sea level) of the German limestone Alps in southern Bavaria. This area is designated as permanent grassland (Unteregelsbacher et al., 2013). Soil material for the incubation study was collected from the Ah horizon (10–15 cm) of a Haplic Cambisol (Calcaric, Humic, Siltic) (IUSS Working Group WRB, 2014) underneath the main rooting zone in the fall of 2009. The parent material is dolomitic alluvial
Characterization of incubated aggregate size classes
The coarse and fine aggregate size classes showed distinctly different substructures of micro- and macroaggregates (Fig. 2). The coarse aggregates comprised macroaggregates >630 μm (43%), whereas fine aggregates comprised mainly microaggregates <20 μm (53%). On the contrary, 90% of the fine aggregates were microaggregates <63 μm, whereas microaggregates <63 μm comprised only 40% of the coarse aggregates.
After (pore-size dependent) dewatering on suction plates to field capacity (pF 2.5) prior to
Architecture of macroaggregates
Both coarse and fine aggregates were hierarchically structured; that is, the smaller subunits (the microaggregates) were accumulated into larger macroaggregates. In coarse aggregates, these macroaggregates (200 μm–6.3 mm) were not disrupted by ultrasonication at 60 J ml−1, indicating that the macroaggregate structures were stabilized therein. By contrast, fine aggregates exposed to this energy were almost entirely broken down into microaggregates (Fig. 2). Some of the microaggregates were naturally
Conclusions
The incubation of intact aggregate classes differing in size revealed distinct carbon and nitrogen bioavailabilites due to a specific aggregate architecture. Future studies on carbon and nitrogen mineralization in natural coarse and fine aggregates should consider (1) the different aggregate architectures of the size classes and (2) the increased surface area of smaller aggregate diameters. These physical characteristics are taken into account by referring mineralization on unit soil mass.
Acknowledgements
We thank Sigrid Hiesch for laboratory assistance. Two anonymous reviewers and the editor are gratefully acknowledged for their helpful comments. This study was supported by a grant from the Bavarian State Ministry of Education, Science and the Arts.
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