Elsevier

Soil and Tillage Research

Volume 160, July 2016, Pages 23-33
Soil and Tillage Research

Carbon and nitrogen mineralization in hierarchically structured aggregates of different size

https://doi.org/10.1016/j.still.2015.12.011Get rights and content

Highlights

Abstract

Soil organic matter pools are turned over at different rates, but uncertainty persists regarding how far the hierarchically organized soil structure controls the mineralization dynamics. To better understand carbon and nitrogen mineralization in undisturbed aggregate classes (coarse aggregates: 2–6.3 mm, fine aggregates: <2 mm), we conducted a long-term (224-days) laboratory incubation experiment. A grassland soil (Haplic Cambisol) was chosen since its aggregates were not disturbed by tillage. The field-moist aggregate size classes were separated by a gentle dry-sieving method. We monitored the CO2–C and NH3–N emissions, nitrogen mineralization, pool sizes of total and salt extractable (0.5 M K2SO4) organic carbon and nitrogen, and microbial biomass carbon and nitrogen. By this approach, we could distinguish between the carbon and nitrogen mineralization processes of two soil aggregate size classes, relative to undisturbed bulk soil. The classes showed different aggregate architectures with variously sized subunits, but confirmed the aggregate hierarchy. For both aggregate classes, the recombined sum of respired CO2–C per unit of soil organic carbon equaled the bulk soil, proving that our aggregate separation preserved the original aggregates as intact functional units. Both aggregate size classes and the bulk soil respired only 4% soil organic carbon throughout the incubation period. The coarse aggregates, which mostly comprised small macroaggregates, mineralized more carbon per unit soil organic carbon than the fine aggregates (composed of microaggregates), indicating a higher bioavailability of soil organic matter in the coarse aggregates. Accordingly microbial metabolic efficiency was higher in coarse than in fine aggregates. Nitrogen mineralization was higher in fine aggregates than in coarse aggregates, but was impaired by carbon limitation as the incubation experiment proceeded. We conclude that bioavailability of soil organic matter was affected by different aggregate architectures in the different aggregate size classes. The lower bioavailability of soil organic matter in fine aggregates is due to an enhanced stabilization through cation bridging of the dolomitic soil material at the microaggregate level, whereas the higher soil organic matter bioavailability in coarse aggregates is explained by labile organic components at the macroaggregate level.

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.

References (56)

  • V.V.S.R. Gupta et al.

    Distribution of microbial biomass and its activity in different soil aggregate size classes as affected by cultivation

    Soil Biol. Biochem.

    (1988)
  • V.V.S.R. Gupta et al.

    Soil aggregation: Influence on microbial biomass and implications for biological processes

    Soil Biol. Biochem.

    (2015)
  • X. Jiang et al.

    Long-term tillage effects on the distribution patterns of microbial biomass and activities within soil aggregates

    Catena

    (2011)
  • V.K. La Mer

    Coagulation symposium introduction

    J. Colloid Sci.

    (1964)
  • F. Lafuma et al.

    Bridging of colloidal particles through adsorbed polymers

    J. Colloid Interface Sci.

    (1991)
  • M. Miller et al.

    Dynamics of soil C and microbial biomass in whole soil and aggregates in 2 cropping systems

    Appl. Soil Ecol.

    (1995)
  • G. Nyamadzawo et al.

    Soil microbial biomass and mineralization of aggregate protected carbon in fallow-maize systems under conventional and no-tillage in Central Zimbabwe

    Soil Till. Res.

    (2009)
  • J.M. Oades

    The role of biology in the formation, stabilization and degradation of soil structure

    Geoderma

    (1993)
  • K. Oorts et al.

    C and N mineralization of undisrupted and disrupted soil from different structural zones of conventional tillage and no-tillage systems in northern France

    Soil Biol. Biochem.

    (2006)
  • S.M.F. Rabbi et al.

    Aggregate hierarchy and carbon mineralization in two Oxisols of New South Wales, Australia

    Soil Till. Res.

    (2015)
  • M. Rashad et al.

    Dissolved organic matter release and retention in an alkaline soil from the Nile River Delta in relation to surface charge and electrolyte type

    Geoderma

    (2010)
  • J. Six et al.

    Soil macroaggregate turnover and microaggregate formation: a mechanism for C sequestration under no-tillage agriculture

    Soil Biol. Biochem.

    (2000)
  • J. Six et al.

    A history of research on the link between (micro) aggregates, soil biota, and soil organic matter dynamics

    Soil Till. Res.

    (2004)
  • J. Six et al.

    Aggregate-associated soil organic matter as an ecosystem property and a measurement tool

    Soil Biol. Biochem.

    (2014)
  • J. Tian et al.

    Aggregate size and their disruption affect 14C-labeled glucose mineralization and priming effect

    Appl. Soil Ecol.

    (2015)
  • E.D. Vance et al.

    An extraction method for measuring soil microbial biomass C

    Soil Biol. Biochem.

    (1987)
  • S.X. Zhang et al.

    Effects of conservation tillage on soil aggregation and aggregate binding agents in black soil of Northeast China

    Soil Till. Res.

    (2012)
  • S.J. Akinsete et al.

    Storage of total and labile soil carbon fractions under different land-use types: a laboratory incubation study. Soil Carbon

  • Cited by (0)

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