Understanding Soil Aggregate Dynamics and Its Relation With Land Use and Climate Change

CHAPTER 13

Understanding Soil Aggregate Dynamics and Its Relation With Land Use and Climate Change Pratap Srivastava1, , Rishikesh Singh2, , Rahul Bhadouria3,4, Sachchidanand Tripathi5, Hema Singh3 and Akhilesh Singh Raghubanshi2 1

Department of Botany, Shyama Prasad Mukherjee Post-graduate College, University of Allahabad, Allahabad, India 2 Institute of Environment and Sustainable Development (IESD), Banaras Hindu University, Varanasi, India 3 Ecosystems Analysis Laboratory, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India 4 Natural Resource Management Laboratory, Department of Botany, University of Delhi, New Delhi, India 5 Department of Botany, Deen Dayal Upadhyaya College, University of Delhi, New Delhi, India

Contents 13.1 Introduction 13.2 Soil Aggregates: The State of the Art 13.2.1 Biophysical Interaction in Soil Aggregate Development 13.2.2 Aggregate Characteristics 13.3 Importance of Soil Aggregates: Relative Proportion 13.4 Aggregate Stability 13.5 Practices Influencing Soil Aggregate Dynamics 13.6 Management of Belowground Interaction Using Soil Aggregate Dynamics as Surrogate 13.7 Conclusion Acknowledgment References Further Reading

331 333 337 339 340 342 344 345 347 348 348 354

13.1 INTRODUCTION Soil is the crucial component of the Earth’s biosphere (Ellert et al., 1997; Coleman et al., 2004). It is at the core of sustainable human development



Authors contributed equally

Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00021-9

Copyright © 2019 Elsevier Inc. All rights reserved.

331

332

Climate Change and Agricultural Ecosystems

Figure 13.1 Soil as the core of sustainability development.

(see Fig. 13.1) and is responsible for the rise and downfall of civilizations. Soil is the product of variable spatiotemporal interactions between climate, organisms (i.e., plant roots, animals, and microorganisms), parent materials, and topography (Ellert et al., 1997; Coleman et al., 2004). Moreover, the biotic interaction in soil is invariably affected by the soil’s physical and chemical properties (Milleret et al., 2009). These intricate biotic abiotic interactions affect the ecosystem’s properties due to their impact on principal ecosystem processes such as nutrient cycling, primary production (Ellert et al., 1997), and plant community dynamics (Frank et al., 2015). In particular, these interactions are crucial for the functioning of global biogeochemical processes as they regulate the accessibility and decomposition of soil organic matter (Siddiky, 2011). It is consistent with the current findings that not only the chemical recalcitrance but also the biophysical interactions of soil (as observed in aggregate development, determining its occluded C characteristics), are the major determinants of soil organic carbon (SOC) turnover and sequestration (Schmidt et al., 2011). Tisdall and Oades (1982) demonstrated that the SOC level closely relates to the soil aggregate formation and stability. These factors together represent the integrative effects of soil type, environment, plant species, and soil management (Martens and Frankenberger, 1992; Nyamangara et al., 1999; Martens, 2000). It has been established that the loss of SOC and aggregate stability (or turnover) represents the unsustainable nature of soil management (Amezketa, 1999; Carter, 2002). Therefore, in order to understand the soil carbon dynamics of an agro-ecosystem and to restore soil

Understanding Soil Aggregate Dynamics

333

Figure 13.2 Illustration showing integrative nature of soil aggregate dynamics

multifunctionality, soil aggregate dynamics and its response to management needs to be assessed closely (Fig. 13.2). In this chapter, we collate studies showing the development in soil aggregate and its importance in the present ecological context. Therefore we discuss the recent advances in soil aggregates (role of biophysical factors and its characteristics) followed by aggregate relative proportion in soil nutrient dynamics and its stability under different management practices, and conclude by proposing a model showing soil aggregate as a surrogate to manage the belowground interactions in the soil system. Moreover, the emerging management practices which have a significant role in soil aggregate and related nutrient dynamics (in relation to nutrient stoichiometry) have also been reviewed to derive a holistic management perspective. The recent advances in this field suggest that although soil aggregates and related dynamics is an old topic, it has considerable impact under the present ecological perspective in terms of soil health management and carbon sequestration.

13.2 SOIL AGGREGATES: THE STATE OF THE ART Recently, focus has been given on identifying the key indicators of soil functions and processes. In this context, soil structure (aggregate structure

334

Climate Change and Agricultural Ecosystems

and pore spaces) has been regarded as a key indicator for determining soil functioning and processes (Rabot et al., 2018). Aggregates are the fundamental, although virtual, unit of soil structure. It represents a threedimensional matrix of soil particles, organic matter, air, and water with distributed biological components (Six et al., 2000, 2002). It is important to understand that soil aggregate develops and is visible only when the soil is dried and mechanically stressed to a degree that it ruptures through a plane of weakness. It is a function of microbial behavior which helps in aggregation due to its particle orientation property. Therefore it is primarily affected by soil management practices which have a considerable effect on microbial activity. Like an organism, community, and ecosystem, aggregate develops hierarchically (see Box 13.1) in a management-specific manner. The qualitative (size distribution and stability) and quantitative (carbon pools and its characteristics) attributes of soil aggregate can be the functional traits which could be widely used to monitor the effect of management practices on agricultural sustainability (Fig. 13.2). Moreover, it could be used as surrogate to understand the fundamental principle of

BOX 13.1 Aggregate Development Concepts Fundamental assumptions of soil aggregate development that are used for understanding soil organic carbon dynamics include: (1) aggregate hierarchy concept, which describes a spatial scale dependence of mechanisms involved in micro- and macroaggregate formation (Tisdall and Oades, 1982); and (2) the formation of microaggregates occurring within macroaggregates (Oades, 1984). Tisdall and Oades (1982) proposed the aggregate hierarchy concept based on the infuence of soil organic matter as a binding agent. It was considered that binding agents act through three major mechanisms: temporary (mainly polysaccharides), transient (roots and fungal hyphae), and persistent (humic substances, polyvalent metal cation complexes, and oxides). In this model, fine particles (,20 µm) bind together by persistent binding agents forming microaggregates (53 250 µm). These microaggregates, in turn, bind together into macroaggregates by means of roots, hyphae, or fresh organic residues ( . 250 µm). However, Six et al. (2000) reported a contrasting theory that microaggregates are formed within macroaggregates by the mineral encapsulation of particulate organic matter (POM) and the microbial byproducts produced due to its decomposition. In addition, inorganic binding agents (such as oxides and carbonates) have also been reported to play an important role in aggregate formation (Six et al., 2004).

Understanding Soil Aggregate Dynamics

335

Figure 13.3 Factors affecting soil aggregate development.

how physical and biological forces interact to define the efficiency of soil ecosystem in the utilization and conservation of energy and resources. Soil aggregate stability depends on pedoclimatic characteristics such as temperature and precipitation, and on several soil properties including texture, clay mineralogy, cation content, aluminum, and iron oxides as well as soil organic matter content (Bronick and Lal, 2005; Abiven et al., 2009). The factors affecting soil aggregate development are shown in Fig. 13.3. However, considering soil aggregate as a measure of soil function and processes is still a debatable issue as its quantitative estimate is dependent on the methods used for its estimation (Rabot et al., 2018). Therefore most of the studies suggest soil pore space measurement as the potential indicator for soil functioning (Rabot et al., 2018). SOC is a primary factor influencing soil structure and aggregate stability (Kay, 1998), and is itself influenced strongly by the dynamics of soil structure (i.e., aggregation stability and distribution) (Elliott and Cambardella, 1991). SOC is an intrinsic component of soil aggregate, therefore, considering soil aggregates (qualitative and quantitative characteristics) could be of vital importance to understand its management activities. An illustration of how the bidirectional interaction between SOC and aggregate development plays a crucial role in agricultural sustainability is provided in Fig. 13.4. Based on the studies in the literature, aggregates have been classified into three major size classes (see Box 13.2). Studies suggest that the availability of the organic matter source and management regulates the soil aggregate formation and soil carbon sequestration potential mediated by the soil aggregate development (Six and Paustian, 2014; Castellano et al., 2015; Toosi et al., 2017a,b). For example, the presence of fresh organic matter acts as a binding agent for

336

Climate Change and Agricultural Ecosystems

Persistent SOC (Humic substance)

Temporary SOC (Polysaccharide) Soil chemical attributes

Soil biological attributes

Soil physical attributes

Soil physical occlusion

Soil organic Carbon

Soil chemical recalcitrance

Soil aggregate characteristics

Soil quality

Agricultural sustainability

Figure 13.4 Complicated aggregate development and its relation with global environmental crisis.

BOX 13.2 Aggregate Size Classes Soil crumbs are divided in aggregate size fractions using different sieve sizes (Tisdall and Oades, 1982; Six et al., 2002). The choice of sieve size and number depends on the experiment and idea to be tested. Generally, sieves of 2 mm, 250 µm, and 53 µm are used in tandem to get fractions of the respective size classes because the major variation under different land use and management practices across spatiotemporal scales have mostly been reported on these size fractions. These size fractions so obtained are, respectively, termed as macroaggregate ( . 2 mm), mesoaggregate (2 mm 250 µm), microaggregate (250 53 µm), and silt and clay (mineral) fractions (,53 µm). However, due to their similar behavior irrespective of management regime, macro- ( . 2 mm) and mesoaggregates (2 mm 250 µm) are sometimes lumped together as macroaggregates ( . 250 µm).

Understanding Soil Aggregate Dynamics

337

the formation of macroaggregates (Tisdall and Oades, 1982; Six et al., 2000) whereas decomposition of soil organic matter within macroaggregates resulted in the formation of stable microaggregates (Gale et al., 2000). However, qualitative (aggregate associated carbon content) and quantitative (percent distribution) properties of the aggregates are of higher relevance for understanding soil nutrient dynamics and carbon sequestration potential. Garland et al. (2018) suggested that the soil aggregate-associated nutrients (especially soil P) showed more stability and closed-loop cycling under tropical soils. Moreover, macroaggregates showed fast response to changes in management practices related with organic matter input (Toosi et al., 2017b). Furthermore, organic matter decomposition is regulated by the pore spaces; Toosi et al. (2017a,b) observed that the pore space ,32 and .136 µm showed less organic matter decomposition within the macroaggregate under natural or cover-cropping systems. Moreover, change in the microbial community structure by the addition of soil ameliorant led to the differential behavior of soil organic matter decomposition and soil aggregate stability (Quilliam et al., 2013; Whitman et al., 2016). Zheng et al. (2018) observed an increase in microbial biomass carbon and a decrease in metabolic quotient under the biochar applied soils showing the increased stability of soil aggregates and microbial soil carbon sequestration within aggregates. Therefore we will critically review the role of various factors (physical, chemical, and biological) influencing aggregate development and their nutrient dynamics.

13.2.1 Biophysical Interaction in Soil Aggregate Development Soil microorganisms and plant root secretions have been found to play a significant role in soil carbon management affecting soil structural dynamics. Microbial secretions serve various purposes like attachment, nutrient capture, and desiccation resistance (Rillig, 2004; Rillig and Mummey, 2006). Bacteria and fungi either act synergistically or antagonistically in soil aggregate development through their polymeric secretions. Fungusderived polysaccharides have been found to contribute significantly in soil aggregation (Chenu, 1989). Fungal metabolic products are either secreted outside of, or contained in, the hyphal wall, which has been implicated as an important mechanism in soil aggregation (Tisdall and Oades, 1982). For example, biopolymers like hydrophobins and glomalins help to enhance the aggregation in addition to hyphal enmeshment (Miller and Jastrow, 2000). Root exudates stimulate the formation of aggregates by

338

Climate Change and Agricultural Ecosystems

providing carbon into the soil which constitute about a third of the photosynthetic production. For example, root mucilage can cause short-term stabilization of aggregates by sticking the particles together (Morel et al., 1991). The marked impact of differential microbial dominance is found on soil carbon sequestration. Fungi seem relatively more important than bacteria in carbon sequestration. Comparatively, fungi possess unique carbon metabolic pathways and secrete more recalcitrant organic compounds than bacteria in the soil. Also, they are more carbon-efficient, and, therefore, sequester more carbon in biomass than they respire (Malik et al., 2016). Despite seemingly higher fungal contribution in soil structural dynamics and carbon sequestration, an extensive study on this aspect has not yet been possible due to some technical limitations in experimentation. Fungi can modify the physical environment to their advantage. Fungi have been underestimated in the regulation of aggregate turnover compared to bacteria. Fungi possess particle orientation capability, also termed as physical ecosystem engineering (Jones et al., 1997) which is the alteration in pore architecture and microbial habitation leading to modification in soil structural dynamics. Further, soil structural dynamics controls the compositional and functional attributes of the bacterial community through biotic and abiotic means such as predator prey relations and the availability of substrate, nutrient, oxygen, and water. Fungi (particularly arbuscular mycorrhizal fungi) which have been mostly studied in soil structural aspects, exert a strong influence at the scale of macroaggregates. However, bacteria and archaea modify the formation and stability of microaggregates. Moreover, arbuscular mycorrhizal fungi-facilitated alteration in prokaryotic communities (bacteria and archaea) may have considerable impact on soil carbon sequestration via a top down effect on soil structural dynamics (Rillig and Mummey, 2006). Fungi bacteria interaction is also important in soil structural dynamics. Fungi alter the physiological states of hyphae-associated microbes through microenvironmental changes (Budi et al., 1999; Bezzate et al., 2000; Mansfeld-Giese et al., 2002). Arbuscular mycorrhizal fungi can directly affect bacterial community characteristics via fungal metabolic secretions containing bacterio- or fungi-static agents (Rillig, 2004; Rillig and Mummey, 2006) which can modify growth and activity of a specific microbial group/species (Rillig, 2005; Rillig and Mummey, 2006). Fungal hyphae and roots help in aggregate development via various physical means in addition to its metabolic secretions and entanglement.

Understanding Soil Aggregate Dynamics

339

Fungal hyphae are considered as tunneling “machines” (Wessels, 1999) which create a considerable amount of pressure on adjacent soil particles while invading the soil matrix (Money, 1994). This pressure leads to the formation of the microaggregate, enabling organic matter and clay particles to be more coherently attached in a similar way that roots do. Enmeshment by fungal hypha stabilizes the aggregate not only because of its primary involvement as it also brings various other factors into play through this mechanism. It has the ability to align the primary particles (especially clay particles) along its hyphae (Tisdall, 1991; Chenu et al., 2002). Roots induce aggregate formation by the temporal dry wet cycle in the rhizosphere. It contributes to increased binding between clay and root exudates under the pressure built up due to the localized dry wet cycle which helps in microaggregate development (Reid and Goss, 1982). A similar mechanism operates during fungi-mediated soil aggregate development, though at a smaller scale. The production of bacterial polysaccharide changes under localized drying wetting, compaction, and nutrient availability in the soil. It affects the soil structural development and dynamics considerably. Extreme drying wetting cycles have a strong impact for soil aggregation due to the differential response of soil microorganisms (Six et al., 2004). Decreased soil moisture typically enhances the contact points between primary particles and soil organic matter, which increases cohesion and strength in the soil aggregates (Horn and Dexter, 1989; Horn et al., 1994). Soil moisture affects bacterial growth and activity more than osmotolerant fungi. However, it affects fungal growth dynamics and metabolism affecting hyphal turgidity which is needed to penetrate through the soil matrix, especially during the dry season. It has been found that the sensitivities to water and nitrogen addition vary among microbial groups within soil aggregates (Wang et al., 2017a,b). Fungal growth dynamics and invasion is crucial for soil carbon sequestration due to its physical nature. It helps to disperse the carbon in the soil from distinct islands (like plant roots and surface layers), as it possesses the unique ability to link spatiotemporal dimensions. It has been found that the management in agroecosystems, which favors a fungal-dominated community, shows quantitative and qualitative improvements of SOC (Six et al., 2006).

13.2.2 Aggregate Characteristics Assessments of the soil organic matter’s chemical characteristics are commonly used to infer its potential reactivity (Kögel-Knabner et al., 2008)

340

Climate Change and Agricultural Ecosystems

which is an important aspect in soil carbon sequestration studies. Several studies (Saroa and Lal, 2003; Mikha and Rice, 2004; Kong et al., 2005; Lagomarsino et al., 2009) showed higher concentration of C and N in macroaggregates than microaggregates because microaggregates are bound together by soil organic matter to form the macroaggregate. However, microaggregates hold stabilized SOC formed within stable macroaggregates (Six and Paustian, 2014). Moreover, soil aggregates withhold soil nutrients on its surface rather than allowing their availability in soil mineral fractions (Garland et al., 2018). Puget and Chenu (1995) suggested that greater C content in macroaggregates could be due to the less-decomposable soil organic matter associated with this fraction. However, in some studies, the SOC mineralization rate and protection has been found similar in macroaggregates and microaggregates (Rabbi et al., 2014). Furthermore, the stability of organic carbon in aggregate fractions is highly regulated by the pore structure (Toosi et al., 2017a). The distribution of microbial biomass and community (composition and function) are found to differ considerably in different aggregate size classes (Gupta and Germida, 1988; Mummey et al., 2006) under varied management systems (Singh and Singh, 1996). It was found that microbial communities inhabiting aggregates dynamically adjust their attributes in response to changes in soil moisture and other external conditions (Ebrahimi and Or, 2016), which determines the nature of soil biogeochemical activity and gas fluxes emitted from soil profiles. Therefore it is imperative to understand the importance of soil aggregates and their stability under different management practices.

13.3 IMPORTANCE OF SOIL AGGREGATES: RELATIVE PROPORTION Two well-known hypotheses regarding soil structural development follow opposite courses, being (1) bottom up and (2) top down development. The functional importance of micro- and macroaggregates, respectively, has been postulated in these two models of soil structure development. Elliott and Coleman (1988) proposed that macroaggregate characteristics define the microaggregate characteristics via anaerobic means (i.e., developing an anaerobic environment inside the macroaggregate). The redistribution of carbon from macroaggregates to microaggregates has been reported using tracer techniques (Angers et al., 1997). Oades (1984) also advocated the concept of microaggregate formation within

Understanding Soil Aggregate Dynamics

341

macroaggregates (i.e. top-down model). On the other hand, the formation of microaggregates has been proposed to define the functional characteristics of macroaggregates in the other model (i.e. bottom-up model). This is due to the fact that microaggregates stick together with the help of cementing agents of transient or permanent natures to form the macroaggregate (Tisdall and Oades, 1982; Six et al., 2000). Moreover, the relative functional importance of micro- and macroaggregate might be a function of the ecosystem, successional stage, and management, which may define the course of soil structural development mediating scale-dependent microbial behavior and its interaction (i.e., biophysical) with soil abiotic factors (Fig. 13.2). Appropriate soil structural development is essential for enhancing soil fertility and soil C stocks (Bronick and Lal, 2005). Soil aggregate occludes the SOC physicochemically from microbial decomposition (Garland et al., 2018). Though, organic matter in aggregate fractions is beneficial for soil C storage, a potential trade-off between yield and long-term C sequestration exists (Cates and Ruark, 2017). A negative correlation between macroaggregate C and crop yield has been reported in the literature (Cates and Ruark, 2017). Aggregate size distribution and nutrient distribution among these fractions determine the microbial activity (such as denitrification) and carbon utilization rates with a change in soil moisture behavior (Ebrahimi and Or, 2016). Thus, the size of the aggregate can directly define the decomposability of organic material (Abiven et al., 2009) and nutrient availability. It has been reported that macroaggregates can physically protect the original and recently added organic matter from microbial attack and mineralization (Oorts et al., 2006; Razafimbelo et al., 2008). In contrast, these larger soil aggregates have also been suggested to quickly develop suitable internal conditions for microbial activity inside the aggregates (Diba et al., 2011), thus possibly helping in C humification and protection. However, these macroaggregates have been found to be comparatively more sensitive to climate warming (Fang et al., 2016) which has been attributed to the differential impact of climate warming on soil microbial communities and enzyme activities in various aggregate fractions that may have important implications for C cycling. This suggests that macroaggregates (including mesoaggregates) may have an important role in soil C sequestration which could be affected drastically under changing climate conditions. Organic materials are protected by the heterogeneity of the soil microenvironment which limits the access of decomposers and their enzymes to

342

Climate Change and Agricultural Ecosystems

organic matter (Schmidt et al., 2011; Ananyeva et al., 2013). Aggregates not only protect soil organic matter, but also influence other biotic and abiotic factors which act secondarily in SOC protection such as an alteration in the microbial community structure (Mummey and Stahl, 2004), limitation on oxygen diffusion (Sexstone et al., 1985) and water movements (Prove et al., 1990), and nutrient adsorption and desorption (Linquist et al., 1997). All these factors may affect the microbial processes, which could be accessed directly or indirectly to correlate with soil carbon efflux and sequestration and to understand the nature of soil structural development. In some recent studies, the soil nitrification process has been suggested to associate with soil aggregate development and macroaggregate characteristics, which could be used as an indicator of soil carbon dynamics (Srivastava et al., 2016b,c). Agricultural management practices such as tillage and nitrogen fertilization regulate greenhouse gas (GHG) production in macroaggregates through changes in the substrates’ C/N ratio and microbial activity (Plaza-Bonilla et al., 2014). Moreover, changes in nutrient stoichiometry under different management scenarios also have considerable impact on aggregate formation, stability, and its dynamics (Srivastava et al., 2016c; Toosi et al., 2017b). In a recent experiment, Lenka and Lal (2013) suggested that the aggregate hierarchy theory could be extended to describe the effect of soil aggregation on GHG emission from the soil. An illustration of the complicated mechanisms of soil aggregate development and its close relationship with the current global environment crisis is provided in Fig. 13.5. It has been suggested that the response of soil organic matter decomposition to higher temperatures due to the changing climate might be closely associated with soil aggregation, which may complicate the responses of ecosystem C budgets to future warming scenarios (Wang et al., 2015). This may be attributed to the interactive effect of temperature and aggregation on soil organic matter decomposition. Therefore macroaggregate attributes need to be studied in interaction with the changing climate and N availability for better soil management.

13.4 AGGREGATE STABILITY According to Tisdall and Oades (1982) and Chan (2001) macroaggregate stability is controlled by the temporary form of soil organic carbon which is highly sensitive to management practices. For example, soil macroaggregate stability is higher in reduced than conventional tillage, and disintegrates into microaggregates and silt and clay fractions (Six et al., 2002).

Understanding Soil Aggregate Dynamics

343

Macroaggregate Bottom-up model

Top-down model

Microaggregate

Differential impact across size classes

Physical pore architecture

Aggregate chemical attributes

Microbial biogeography

Soil quality

Sustainable agriculture

Figure 13.5 Role of bidirectional interaction between soil organic C and aggregate development in agricultural sustainability.

The quality of added organic residues as defined by specific biochemical characteristics (N, lignin, and polyphenol contents), may influence the rate of macroaggregate turnover (Six et al., 2001; Six and Paustian, 2014; Toosi et al., 2017b). Contrary to low-quality organic residues, high-quality residues induce a faster macroaggregate turnover (Six et al., 2001). This is associated with the enhanced mineralization of carbon and nutrients leading to their loss from the system (Haynes and Beare, 1997). Moreover, the addition of low-quality organic residues provides a short-term increase of macroaggregate SOC and N compared to high-quality residues (Chivenge et al., 2011). However, the simultaneous addition of N fertilizers has been found to nullify such effects of low-quality residues. Similarly, the addition of N fertilizers without a C source enhances macroaggregate turnover due to the enhanced decomposition of C-rich binding agents by microbes (Harris et al., 1963). Manure application has been found to improve aggregation in coarsetextured soils (Nyamangara et al., 1999); however, manure decreased the

344

Climate Change and Agricultural Ecosystems

aggregate stability in fine-textured soils (Pare et al., 2000). Similarly, biochar has also been suggested to improve aggregate stability and, hence, could be a strategy for enhanced agro-production (Li et al., 2017). Aggregate stability has been found to reduce the carbon output from the soil as particulate organic C (POC) and dissolved organic C (DOC) (Chaplot and Cooper, 2015). Recently, a mechanistic linkage has been proposed to exist between soil’s physicochemical (such as pH, porosity, relative availability of inorganicN pools) properties and aggregate formation (Regelink et al., 2015; Srivastava et al., 2016b,c).

13.5 PRACTICES INFLUENCING SOIL AGGREGATE DYNAMICS Management practices in agriculture have a significant impact on soil aggregate stability and dynamics (Bhattacharyya et al., 2010; Huang et al., 2010) which have been identified as the major culprits in declining soil fertility and changing climate. Simultaneously, soil aggregate stability and dynamics have also received attention as an integrative soil functional trait, which may have some management implications. Management disturbances in agricultural soils play a defining role in the global carbon cycle and soil carbon dynamics affecting soil microstructures, which contains two thirds of terrestrial carbon remains sequestered within it. Therefore agronomic practices influence aggregation and, thus, SOC content. Soil aggregation is found to be affected by a variety of factors in agroecosystems, such as tillage regime (Shirani et al., 2002; Zotarelli et al., 2005), crop rotation system (Holeplass et al., 2004; Zotarelli et al., 2005), crop species (Wright and Hons, 2005), residue management (Saroa and Lal, 2003), cropping duration, and fertilization regime (Holeplass et al., 2004). Soil aggregate dynamics, particularly its size distribution and carbon characteristics is management-specific (Singh and Singh, 1995). The latter defines the extent and direction of soil aggregate development and, thus, efficiency of SOC protection from decomposition. Long-term studies under varied management practices (Tiessen and Stewart, 1983; Christensen, 1986; Dalal and Mayer, 1986; Yu et al., 2012) have shown differential aggregate size distribution and carbon characteristics. Manure application increases SOC concentration, aggregate stability. and soil biological activities which have been found to be associated with improvement in soil structure (Martens and Frankenberger, 1992; Haynes and Naidu, 1998; Nyamangara et al., 1999; Aoyama et al., 2000).

Understanding Soil Aggregate Dynamics

345

Continuous cultivation affects the distribution and stability of soil aggregates and reduces organic carbon stock in the soil (Six et al., 2002). The inter-microaggregate binding agents are the principle component of soil organic carbon which is lost when the soil in cultivated (Tisdall and Oades, 1982). Whalen et al. (2003) and Jiao et al. (2006) suggested that significant quantities of carbon from farmyard manure was retained in whole soil and its aggregate fractions. Several researchers reported that increasing C input in soil leads to enrichment in the aggregate-associated C (Kong et al., 2005; Manna et al., 2006). Bhattacharyya et al. (2010) showed that the relationship between carbon input and carbon sequestration is dominated by the increase in SOC associated with the macro- as well as microaggregates. Aoyama and Kumakura (2001) found that the application of animal manure increased soil organic matter and the formation of macroaggregate. Also, increased soil organic matter has been attributed to the increased accumulation of macroaggregate-protected carbon. Organic carbon input into the soil generally improves the mass proportion of macroaggregate at the expense of microaggregate and the free silt and clay fraction (Liao et al., 2006; Yu et al., 2012). In contrast, long-term mineral fertilizer application decreases SOC concentration as well as mass distribution of macroaggregate. However, some long-term studies on mineral fertilization have shown an increase of SOC without significant improvement in aggregation (Yu et al., 2012). Moreover, Aoyama et al. (1999) reported that manure addition has no significant effect on SOC storage over chemical fertilization in smaller fractions. Biochar improves the resistance of aggregates to stresses, actively promotes soil C storage and, thus, provides a scientific strategy for sustainable agricultural production (Li et al., 2017; Wang et al., 2017a,b). Additionally, biochar types have been found to greatly impact soil chemical properties and microbial communities (Zheng et al., 2018). However, comparative studies on temporal change in soil aggregate dynamics under various kinds of nutrient amendments in relation to soil carbon dynamics are almost absent for tropical agroecosystems. Such studies may further provide some important understanding regarding the unifying concept of soil C accumulation.

13.6 MANAGEMENT OF BELOWGROUND INTERACTION USING SOIL AGGREGATE DYNAMICS AS SURROGATE The complex interaction in spatio-temporal dimensions is fundamental to a biological or ecological system (such as organism or ecosystem) as it provides stability, efficiency (in use of energy and nutrients), resilience, and

346

Climate Change and Agricultural Ecosystems

sustainability to the system. Such dynamic and nonlinear interactions among the soil biotic and physicochemical environments play crucial roles in the ecosystem engineering and functioning. Currently, scientists firmly believe that reliance on agrochemicals (fertilizers and pesticides) in agroecosystems could be considerably reduced by managing the dynamic and nonlinear interactions in the soils (Médiène et al., 2011). However, the present human ability to directly manage and control these intricate microscale interactions is highly limited which is clearly visible by the escalating environmental problems due to human mismanagement of the biosphere. However, the identification of integrative macroscale indicators representing the overall function of interactions happening inside soil may help us to manage them indirectly. It would be analogous to “managing the lock (integrative macro-scale indicator) to understand and use the key (micro-scale and dynamic interaction)” functionally. The integrative indicatorswould ultimately help the agricultural and soil scientists to reap the indispensable ecological subsidies provided by the inherent interactions in the soil ecosystem (Srivastava et al., 2016a). A macroscale indicator which integrates the ecosystem interactions as a whole in itself may well indicate the sustainable or unsustainable nature of interactions in soil. For example, quantitative and qualitative characteristics of soil aggregate fractions which are regulated by microbial behavior, pedoclimatic conditions, and management have the potential to become one such (integrative and surrogate) variable (Srivastava et al., 2016b,c) (Fig. 13.5). Soil aggregate development holds crucial importance in contemporary climate change and decline in soil quality due to its primary role in soil C sequestration (see Fig. 13.5) (Holeplass et al., 2004) as it primarily controls the physical, chemical, and biological attributes (Yang and Wander, 1998). On the other hand, it has also been found to be strongly affected by the soil’s physicochemical and biological properties. Beauchamp and Seech (1990) reported that a high variability in denitrification activity was associated with aggregate size and its water stability. Soil aggregate size and stability reduces soil erosion determining the physical protection of soil organic matter from decomposition (Paul et al., 2013). Therefore aggregate stability and soil organic matter relate strongly with each other and impart synergistic effects on soil health. Therefore both are considered as indicators of soil health and agricultural sustainability with their losses representing an unsustainable management system (Carter, 2002). Soil aggregate dynamics protect and maintain the multifaceted function of soils; however, its potential to do so depends on management practices

Understanding Soil Aggregate Dynamics

347

(Srivastava et al., 2016a). Soil aggregate development and dynamics integrates the spatiotemporally variable interactions among soils (Miller and Jastrow, 1990). It has been found that the main difference between microand macroaggregates might be attributed to the microbial community structure within each fraction, which could affect soil nutrient dynamics, especially N availability (Noguez et al., 2008). Although enhanced N availability has been found to have significant, but variable, effect on microbial biomass C and N, DOC, and DOC to inorganic N across aggregate size fractions (Yin et al., 2016; Zhong et al., 2017). However, how the interaction of microorganisms with the physical environment during soil aggregate development affects nutrient turnover and availability has hardly been explored. Such an understanding may have a significant future prospect in soil C sequestration in agroecosystems. Recently, qualitative and quantitative characteristics of soil aggregates have been found to be associated with microbial community characteristics and the consequent relative availability of inorganic N pools (Srivastava et al., 2016b,c). The relative availability of inorganic N pools has been identified as an integrative variable of significant importance in soil carbon dynamics because it indirectly indicates the microbial behavior inside the soil under complex physical constrains (Srivastava et al., 2016b,c). Therefore we propose that combining physical fractionation and chemical characterization with soil relative availability of inorganic N pools across aggregate size fractions under various management systems may help to better understand the mechanism of soil carbon sequestration, which could be helpful in the mitigation of climate change. Moreover, it may help to identify novel and integrative (physicochemical) signatures/markers of soil carbon dynamics for cost-effective and simple, onsite management of the agroecosystems for improved C sequestration. Therefore, such integrative characteristics (having a crucial role in soil carbon dynamics) as a whole, or in appropriate combinations may help us to track and manage the complex belowground interactions in an indirect manner for site-specific management of soil carbon dynamics and fertility to achieve sustainable agriculture on the one hand and the mitigation of climate change on the other.

13.7 CONCLUSION Soil multifunctionality seems to be a function of the level of physical engineering in soil aggregate development which is primarily linked with climate change due to its close association with soil carbon sequestration.

348

Climate Change and Agricultural Ecosystems

Soil aggregate characteristics reflect dynamic and variable spatiotemporal interactions between the physical, chemical, and biological properties of the soil. Agro-management practices distinctly affect these soil biophysical interactions at each level of the hierarchy and, thus, define soil aggregate development by modifying the soil chemical environment. Some recent studies have shown that the resource (C and N) conservation mechanisms might be linked with the change in soil chemical environment (such as stoichiometric availability of inorganic-N pools), which strongly correlates with soil aggregate characteristics. In addition, various emerging soil ameliorants like biochar have a potential impact over soil aggregate dynamics which needs to be explored in relation to soil aggregate nutrient dynamics. Therefore, we strongly propose that soil aggregate quantitative and qualitative characteristics should be studied in relation to soil inorganic N pool dynamics and carbon accumulation under various land uses and management for effective climate change mitigation strategies. This would also help in the identification of some novel physicochemical signatures/markers of soil carbon sequestration related to soil aggregate characteristics. These markers as a whole (or in various combinations) can be potentially used as a surrogate indicator to indirectly manage the belowground interactions in a cost-effective and site-specific manner to increase carbon sequestration by providing dual benefits of increased soil fertility and mitigation of climate change.

ACKNOWLEDGMENT The authors gratefully acknowledge financial support from University Grants Commission (UGC), Council of Scientific and Industrial Research (CSIR) and DST-SERB, New Delhi, India. The authors also extend their gratitude to the handling editor and three anonymous reviewers for their constructive suggestions for the improvement of the manuscript.

REFERENCES Abiven, S., Menasseri, S., Chenu, C., 2009. The effects of organic inputs over time on soil aggregate stability a literature analysis. Soil Biol. Biochem. 41, 1 12. Amezketa, E., 1999. Soil aggregate stability: a review. J. Sust. Agric. 14, 83 151. Ananyeva, K., Wang, W., Smucker, A., Rivers, M., Kravchenko, A., 2013. Can intraaggregate pore structures affect the aggregate’s effectiveness in protecting carbon? Soil Biol. Biochem. 57, 868 875. Angers, D., Recous, S., Aita, C., 1997. Fate of carbon and nitrogen in waterstable aggregates during decomposition of 13C15N-labelled wheat straw in situ. Eur. J. Soil Sci. 48, 295 300.

Understanding Soil Aggregate Dynamics

349

Aoyama, M., Kumakura, N., 2001. Quantitative and qualitative changes of organic matter in an Ando soil induced by mineral fertilizer and cattle manure applications for 20 years. Soil Sci. Plant. Nutr. 47, 241 252. Aoyama, M., Angers, D., N’dayegamiye, A., Bissonnette, N., 1999. Protected organic matter in water-stable aggregates as affected by mineral fertilizer and manure applications. Can. J. Soil Sci. 79, 419 425. Aoyama, M., Angers, D., N’dayegamiye, A., Bissonnette, N., 2000. Metabolism of 13 Clabeled glucose in aggregates from soils with manure application. Soil Biol. Biochem. 32, 295 300. Beauchamp, E.G., Seech, A.G., 1990. Denitrification with different sizes of soil aggregates obtained from dry-sieving and from sieving with water. Biol. Fert. Soils 10, 188 193. Bezzate, S., et al., 2000. Disruption of the Paenibacillus polymyxa levansucrase gene impairs its ability to aggregate soil in the wheat rhizosphere. Environ. Microbiol. 2, 333 342. Bhattacharyya, R., et al., 2010. Long term effects of fertilization on carbon and nitrogen sequestration and aggregate associated carbon and nitrogen in the Indian subHimalayas. Nutr. Cycl. Agroecosyst. 86, 1 16. Bronick, C.J., Lal, R., 2005. Soil structure and management: a review. Geoderma 124, 3 22. Budi, S.W., van Tuinen, D., Martinotti, G., Gianinazzi, S., 1999. Isolation from the Sorghum bicolor mycorrhizosphere of a bacterium compatible with arbuscular mycorrhiza development and antagonistic towards soilborne fungal pathogens. Appl. Environ. Microbiol. 65, 5148 5150. Carter, M.R., 2002. Soil quality for sustainable land management. Agronomy J. 94, 38 47. Castellano, M.J., Mueller, K.E., Olk, D.C., Sawyer, J.E., Six, J., 2015. Integrating plant litter quality, soil organic matter stabilization, and the carbon saturation concept. Glob. Change Biol. 21, 3200 3209. Cates, A.M., Ruark, M.D., 2017. Soil aggregate and particulate C and N under corn rotations: responses to management and correlations with yield. Plant Soil 415, 521 533. Chan, K., 2001. Soil particulate organic carbon under different land use and management. Soil Use Manag. 17, 217 221. Chaplot, V., Cooper, M., 2015. Soil aggregate stability to predict organic carbon outputs from soils. Geoderma 243, 205 213. Chenu, C., 1989. Influence of a fungal polysaccharide, scleroglucan, on clay microstructures. Soil Biol. Biochem. 21, 299 305. Chenu, C., Stotzky, G., Huang, P., Bollag, J., Sensi, N., 2002. Interactions between microorganisms and soil particles: an overview. Interactions Between Soil Particles and Microorganisms: Impact on the Terrestrial Ecosystem. IUPAC. John Wiley & Sons, Ltd, Manchester, pp. 1 40. Chivenge, P., Vanlauwe, B., Gentile, R., Six, J., 2011. Organic resource quality influences short-term aggregate dynamics and soil organic carbon and nitrogen accumulation. Soil Biol. Biochem. 43, 657 666. Christensen, B.T., 1986. Straw incorporation and soil organic matter in macro-aggregates and particle size separates. J. Soil Sci. 37, 125 135. Coleman, D.C., Crossley, D., Hendrix, P.F., 2004. Fundamentals of Soil Ecology. Academic Press. Dalal, R., Mayer, R., 1986. Long term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland. II. Total organic carbon and its rate of loss from the soil profile. Soil Res. 24, 281 292. Diba, F., Shimizu, M., Hatano, R., 2011. Effects of soil aggregate size, moisture content and fertilizer management on nitrous oxide production in a volcanic ash soil. Soil Sci. Plant. Nutr. 57, 733 747.

350

Climate Change and Agricultural Ecosystems

Ebrahimi, A., Or, D., 2016. Microbial community dynamics in soil aggregates shape biogeochemical gas fluxes from soil profiles upscaling an aggregate biophysical model. Glob. Change Biol. 22, 3141 3156. Ellert, B., Clapperton, M., Anderson, D., 1997. An ecosystem perspective of soil quality. Dev. Soil Sci. 25, 115 142. Elliott, E., Cambardella, C., 1991. Physical separation of soil organic matter. Agric. Ecosyst. Environ. 34, 407 419. Elliott, E., Coleman, D., 1988. Let the soil work for us. Ecol. Bull. 39, 23 32. Fang, X., Zhou, G., Li, Y., Liu, S., Chu, G., Xu, Z., et al., 2016. Warming effects on biomass and composition of microbial communities and enzyme activities within soil aggregates in subtropical forest. Biol. Fert. Soils 52, 353 365. Frank, D.A., Pontes, A.W., Maine, E.M., Fridley, J.D., 2015. Fine-scale belowground species associations in temperate grassland. Mol. Ecol. 24, 3206 3216. Gale, W.J., Cambardella, C.A., Bailey, T.B., 2000. Surface residue- and root-derived carbon in stable and aggregates. Soil Sci. Soc. Am. J. 64, 196 201. Garland, G., Bünemann, E.K., Oberson, A., Frossard, E., Snapp, S., Chikowo, R., et al., 2018. Phosphorus cycling within soil aggregate fractions of a highly weathered tropical soil: a conceptual model. Soil Biol. Biochem. 116, 91 98. Gupta, V., Germida, J., 1988. Distribution of microbial biomass and its activity in different soil aggregate size classes as affected by cultivation. Soil Biol. Biochem. 20, 777 786. Harris, R., Allen, O., Chesters, G., Attoe, O., 1963. Evaluation of microbial activity in soil aggregate stabilization and degradation by the use of artificial aggregates. Soil Sci. Soc. Am. J. 27, 542 545. Haynes, R., Beare, M., 1997. Influence of six crop species on aggregate stability and some labile organic matter fractions. Soil Biol. Biochem. 29, 1647 1653. Haynes, R., Naidu, R., 1998. Influence of lime, fertilizer and manure applications on soil organic matter content and soil physical conditions: a review. Nutr. Cycl. Agroecosyst. 51, 123 137. Holeplass, H., Singh, B., Lal, R., 2004. Carbon sequestration in soil aggregates under different crop rotations and nitrogen fertilization in an inceptisol in southeastern Norway. Nutr. Cycl. Agroecosyst. 70, 167 177. Horn, R., Dexter, A., 1989. Dynamics of soil aggregation in an irrigated desert loess. Soil Tillage Res. 13, 253 266. Horn, R., Taubner, H., Wuttke, M., Baumgartl, T., 1994. Soil physical properties related to soil structure. Soil Tillage Res 30, 187 216. Huang, S., Peng, X., Huang, Q., Zhang, W., 2010. Soil aggregation and organic carbon fractions affected by long-term fertilization in a red soil of subtropical China. Geoderma 154, 364 369. Jiao, Y., Whalen, J.K., Hendershot, W.H., 2006. No-tillage and manure applications increase aggregation and improve nutrient retention in a sandy-loam soil. Geoderma 134, 24 33. Jones, C.G., Lawton, J.H., Shachak, M., 1997. Positive and negative effects of organisms as physical ecosystem engineers. Ecology 78, 1946 1957. Kay, B., 1998. Soil structure and organic carbon: a review. Soil Processes Carbon Cycle 169 197. Kögel-Knabner, I., et al., 2008. An integrative approach of organic matter stabilization in temperate soils: linking chemistry, physics, and biology. J Plant Nutr. Soil Sci. 171, 5 13. Kong, A.Y., Six, J., Bryant, D.C., Denison, R.F., Van Kessel, C., 2005. The relationship between carbon input, aggregation, and soil organic carbon stabilization in sustainable cropping systems. Soil Sci. Soc. Am. J. 69, 1078 1085.

Understanding Soil Aggregate Dynamics

351

Lagomarsino, A., Grego, S., Marhan, S., Moscatelli, M., Kandeler, E., 2009. Soil management modifies micro-scale abundance and function of soil microorganisms in a Mediterranean ecosystem. Eur. J. Soil Sci. 60, 2 12. Lenka, N.K., Lal, R., 2013. Soil aggregation and greenhouse gas flux after 15 years of wheat straw and fertilizer management in a no-till system. Soil Tillage Res. 126, 78 89. Li, Q., Jin, Z., Chen, X., Jing, Y., Huang, Q., Zhang, J., 2017. Effects of biochar on aggregate characteristics of upland red soil in subtropical China. Environ. Earth Sci. 76, 372. Liao, J., Boutton, T., Jastrow, J., 2006. Storage and dynamics of carbon and nitrogen in soil physical fractions following woody plant invasion of grassland. Soil Biol. Biochem. 38, 3184 3196. Linquist, B., Singleton, P., Cassman, K., 1997. Inorganic and organic phosphorus dynamics during a build-up and decline of available phosphorus in an ultisol. Soil Sci. 162, 254 264. Malik, A., Chowdhury, S., Schlager, V., Oliver, A., Puissant, J., Vazquez, P.G.M., et al., 2016. Soil fungal:bacterial ratios are linked to altered carbon cycling. Front. Microbiol. 7, 1247. Manna, M., et al., 2006. Soil organic matter in a West Bengal inceptisol after 30 years of multiple cropping and fertilization. Soil Sci. Soc. Am. J. 70, 121 129. Mansfeld-Giese, K., Larsen, J., Bødker, L., 2002. Bacterial populations associated with mycelium of the arbuscular mycorrhizal fungus Glomus intraradices. FEMS Microb. Ecol. 41, 133 140. Martens, D.A., 2000. Management and crop residue influence soil aggregate stability. J. Environ. Qual. 29, 723 727. Martens, D., Frankenberger, W., 1992. Modification of infiltration rates in an organicamended irrigated. Agron. J. 84, 707 717. Médiène, S., et al., 2011. Agroecosystem management and biotic interactions: a review. Agron. Sustain. Dev. 31, 491 514. Mikha, M.M., Rice, C.W., 2004. Tillage and manure effects on soil and aggregateassociated carbon and nitrogen. Soil Sci. Soc. Am. J. 68, 809 816. Miller, R., Jastrow, J., 1990. Hierarchy of root and mycorrhizal fungal interactions with soil aggregation. Soil Biol. Biochem. 22, 579 584. Miller, R., Jastrow, J., 2000. Arbuscular Mycorrhizas: Physiology and Function. Springer, pp. 3 18. Milleret, R., Le Bayon, R.C., Gobat, J.M., 2009. Root, mycorrhiza and earthworm interactions: their effects on soil structuring processes, plant and soil nutrient concentration and plant biomass. Plant Soil 316, 1 12. Money, N., 1994. Growth, Differentiation and Sexuality. Springer, pp. 67 88. Morel, J.L., Habib, L., Plantureux, S., Guckert, A., 1991. Influence of maize root mucilage on soil aggregate stability. Plant Soil 136, 111 119. Mummey, D., Stahl, P., 2004. Analysis of soil whole-and inner-microaggregate bacterial communities. Microb. Ecol. 48, 41 50. Mummey, D.L., Rillig, M.C., Six, J., 2006. Endogeic earthworms differentially influence bacterial communities associated with different soil aggregate size fractions. Soil Biol. Biochem. 38, 1608 1614. Noguez, A.M., Escalante, A.E., Forney, L.J., Nava-Mendoza, M., Rosas, I., Souza, V., et al., 2008. Soil aggregates in a tropical deciduous forest: effects on C and N dynamics, and microbial communities as determined by t-RFLPs. Biogeochemistry 89, 209 220.

352

Climate Change and Agricultural Ecosystems

Nyamangara, J., Piha, M., Kirchmann, H., 1999. Interactions of aerobically decomposed cattle manure and nitrogen fertilizer applied to soil. Nutr. Cycl. Agroecosyst. 54, 183 188. Oades, J.M., 1984. Soil organic matter and structural stability: mechanisms and implications for management. Plant Soil 76, 319 337. Oorts, K., Nicolardot, B., Merckx, R., Richard, G., Boizard, H., 2006. 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. 38, 2576 2586. Pare, T., Dinel, H., Schnitzer, M., 2000. Carbon and nitrogen mineralization in soil amended with non-tabletized and tabletized poultry manure. Can. J. Soil Sci. 80, 271 276. Paul, B., et al., 2013. Medium-term impact of tillage and residue management on soil aggregate stability, soil carbon and crop productivity. Agric. Ecosyst. Environ. 164, 14 22. Plaza-Bonilla, D., Cantero-Martínez, C., Álvaro-Fuentes, J., 2014. Soil management effects on greenhouse gases production at the macroaggregate scale. Soil Biol. Biochem. 68, 471 481. Prove, B., Loch, R., Foley, J., Anderson, V., Younger, D., 1990. Improvements in aggregation and infiltration characteristics of a krasnozem under maize with direct drill and stubble retention. Soil Res. 28, 577 590. Puget, P., Chenu, C., 1995. Total and young organic matter distributions in silty cultivated. Eur. J. Soil Sci. 46, 449 459. Quilliam, R.S., Glanville, H.C., Wade, S.C., Jones, D.L., 2013. Life in the ‘charosphere’—does biochar in agricultural soil provide a significant habitat for microorganisms? Soil Biol. Biochem. 65, 287 293. Rabbi, S.F., Wilson, B.R., Lockwood, P.V., Daniel, H., Young, I.M., 2014. Soil organic carbon mineralization rates in aggregates under contrasting land uses. Geoderma 216, 10 18. Rabot, E., Wiesmeier, M., Schlüter, S., Vogel, H.J., 2018. Soil structure as an indicator of soil functions: a review. Geoderma 314, 122 137. Razafimbelo, T.M., et al., 2008. Aggregate associated-C and physical protection in a tropical clayey soil under Malagasy conventional and no-tillage systems. Soil Tillage Res. 98, 140 149. Regelink, I.C., Stoof, C.R., Rousseva, S., Weng, L., Lair, G.J., Kram, P., et al., 2015. Linkages between aggregate formation, porosity and soil chemical properties. Geoderma 247, 24 37. Reid, J., Goss, M., 1982. Interactions between soil drying due to plant water use and decreases in aggregate stability caused by maize roots. J. Soil Sci. 33, 47 53. Rillig, M.C., 2004. Arbuscular mycorrhizae, glomalin, and soil aggregation. Can. J. Soil Sci. 84, 355 363. Rillig, M.C.A., 2005. Connection between fungal hydrophobins and soil water repellency? Pedobiologia 49, 395 399. Rillig, M.C., Mummey, D.L., 2006. Mycorrhizas and soil structure. New Phytol. 171, 41 53. Saroa, G., Lal, R., 2003. Soil restorative effects of mulching on aggregation and carbon sequestration in a Miamian soil in central Ohio. Land Degrad Dev. 14, 481 493. Schmidt, M.W., et al., 2011. Persistence of soil organic matter as an ecosystem property. Nature 478, 49 56. Sexstone, A.J., Revsbech, N.P., Parkin, T.B., Tiedje, J.M., 1985. Direct measurement of oxygen profiles and denitrification rates in soil aggregates. Soil Sci. Soc. Am. J. 49, 645 651.

Understanding Soil Aggregate Dynamics

353

Shirani, H., Hajabbasi, M., Afyuni, M., Hemmat, A., 2002. Effects of farmyard manure and tillage systems on soil physical properties and corn yield in central Iran. Soil Tillage Res. 68, 101 108. Siddiky, M.R.K., 2011. Soil biota interactions and soil aggregation, Freie Universität Berlin. Singh, S., Singh, J., 1995. Microbial biomass associated with water-stable aggregates in forest, savanna and cropland soils of a seasonally dry tropical region, India. Soil Biol. Biochem. 27, 1027 1033. Singh, S., Singh, J., 1996. Water-stable aggregates and associated organic matter in forest, savanna, and cropland soils of a seasonally dry tropical region, India. Biol. Fert. Soils 22, 76 82. Six, J., Paustian, K., 2014. Aggregate-associated soil organic matter as an ecosystem property and a measurement tool. Soil Biol. Biochem. 68, A4 A9. Six, J., Elliott, E., Paustian, K., 2000. Soil macroaggregate turnover and microaggregate formation: a mechanism for C sequestration under no-tillage agriculture. Soil Biol. Biochem. 32, 2099 2103. Six, J., et al., 2001. Impact of elevated CO2 on soil organic matter dynamics as related to changes in aggregate turnover and residue quality. Plant Soil 234, 27 36. Six, J., Conant, R., Paul, E.A., Paustian, K., 2002. Stabilization mechanisms of soil organic matter: implications for C-saturation of soils. Plant Soil 241, 155 176. Six, J., Bossuyt, H., Degryze, S., Denef, K., 2004. A history of research on the link between (micro) aggregates, soil biota, and soil organic matter dynamics. Soil Tillage Res. 79, 7 31. Six, J., Frey, S., Thiet, R., Batten, K., 2006. Bacterial and fungal contributions to carbon sequestration in agroecosystems. Soil Sci. Soc. Am. J. 70, 555 569. Srivastava, P., Singh, R., Tripathi, S., Raghubanshi, A.S., 2016a. An urgent need for sustainable thinking in agriculture an Indian scenario. Ecol. Indic. 67, 611 622. Srivastava, P., Singh, R., Bhadouria, R., Tripathi, S., Singh, P., Singh, H., et al., 2016b. Organic amendment impact on SOC dynamics in dry tropics: a possible role of relative availability of inorganic-N pools. Agric. Ecosyst. Environ. 235, 38 50. Srivastava, P., Singh, P.K., Singh, R., Bhadouria, R., Singh, D.K., Singh, S., et al., 2016c. Relative availability of inorganic N-pools shifts under land use change: an unexplored variable in soil carbon dynamics. Ecol. Indic. 64, 228 236. Tiessen, H., Stewart, J., 1983. Particle-size fractions and their use in studies of soil organic matter: II. Cultivation effects on organic matter composition in size fractions. Soil Sci. Soc. Am. J. 47, 509 514. Tisdall, J., 1991. Fungal hyphae and structural stability of soil. Soil Res. 29, 729 743. Tisdall, J., Oades, J.M., 1982. Organic matter and water-stable aggregates in soils. J. Soil Sci. 33, 141 163. Toosi, E.R., Kravchenko, A.N., Guber, A.K., Rivers, M.L., 2017a. Pore characteristics regulate priming and fate of carbon from plant residue. Soil Biol. Biochem. 113, 219 230. Toosi, E.R., Kravchenko, A.N., Mao, J., Quigley, M.Y., Rivers, M.L., 2017b. Effects of management and pore characteristics on organic matter composition of macroaggregates: evidence from characterization of organic matter and imaging. Eur. J. Soil Sci. 68, 200 211. Wang, Q., Wang, D., Wen, X., Yu, G., He, N., Wang, R., 2015. Differences in SOM decomposition and temperature sensitivity among soil aggregate size classes in a temperate grasslands. PLoS One 10, e0117033. Wang, D., Fonte, S.J., Parikh, S.J., Six, J., Scow, K.M., 2017a. Biochar additions can enhance soil structure and the physical stabilization of C in aggregates. Geoderma 303, 110 117.

354

Climate Change and Agricultural Ecosystems

Wang, R., Dorodnikov, M., Dijkstra, F.A., Yang, S., Xu, Z., Li, H., et al., 2017b. Sensitivities to nitrogen and water addition vary among microbial groups within soil aggregates in a semiarid grassland. Biol. Fert. Soils 53, 129 140. Wessels, J.G., 1999. Fungi in their own right. Fungal Genet. Biol. 27, 134 145. Whalen, J.K., Hu, Q., Liu, A., 2003. Compost applications increase water-stable aggregates in conventional and no-tillage systems. Soil Sci. Soc. Am. J. 67, 1842 1847. Whitman, T., Pepe-Ranney, C., Enders, A., Koechli, C., Campbell, A., Buckley, D.H., et al., 2016. Dynamics of microbial community composition and soil organic carbon mineralization in soil following addition of pyrogenic and fresh organic matter. ISME J. 10, 2918 2930. Wright, A.L., Hons, F.M., 2005. Soil carbon and nitrogen storage in aggregates from different tillage and crop regimes. Soil Sci. Soc. Am. J. 69, 141 147. Yang, X.M., Wander, M.M., 1998. Temporal changes in dry aggregate size and stability: tillage and crop effects on a silty loam Mollisol in Illinois. Soil Tillage Res 49, 173 183. Yin, J., Wang, R., Liu, H., Feng, X., Xu, Z., Jiang, Y., 2016. Nitrogen addition alters elemental stoichiometry within soil aggregates in a temperate steppe. Solid Earth 7, 1565 1575. Yu, H., Ding, W., Luo, J., Geng, R., Cai, Z., 2012. Long-term application of organic manure and mineral fertilizers on aggregation and aggregate-associated carbon in a sandy loam soil. Soil Tillage Res. 124, 170 177. Zheng, H., Wang, X., Luo, X., Wang, Z., Xing, B., 2018. Biochar-induced negative carbon mineralization priming effects in a coastal wetland soil: roles of soil aggregation and microbial modulation. Sci. Total Environ. 610, 951 960. Zhong, X.L., Li, J.T., Li, X.J., Ye, Y.C., Liu, S.S., Hallett, P.D., et al., 2017. Physical protection by soil aggregates stabilizes soil organic carbon under simulated N deposition in a subtropical forest of China. Geoderma 285, 323 332. Zotarelli, L., et al., 2005. Impact of tillage and crop rotation on aggregate-associated carbon in two oxisols. Soil Sci. Soc. Am. J. 69, 482 491.

FURTHER READING Kandeler, E., Tscherko, D., Spiegel, H., 1999. Long-term monitoring of microbial biomass, N mineralisation and enzyme activities of a Chernozem under different tillage management. Biol Fert. Soils 28, 343 351. Majumder, B., Mandal, B., Bandyopadhyay, P., Chaudhury, J., 2007. Soil organic carbon pools and productivity relationships for a 34 year old rice wheat jute agroecosystem under different fertilizer treatments. Plant Soil 297, 53 67. Poll, C., Thiede, A., Wermbter, N., Sessitsch, A., Kandeler, E., 2003. Micro-scale distribution of microorganisms and microbial enzyme activities in a soil with long-term organic amendment. Eur. J. Soil. Sci. 54, 715 724. Sohi, S.P., et al., 2001. A procedure for isolating soil organic matter fractions suitable for modeling. Soil Sci. Soc. Am. J. 65, 1121 1128. Stemmer, M., Gerzabek, M.H., Kandeler, E., 1998. Organic matter and enzyme activity in particle-size fractions of soils obtained after low-energy sonication. Soil Biol. Biochem. 30, 9 17.