Soil Quality as Affected by Intensive Versus Conservative Agricultural Managements
Summary and Keywords
Soils, the earth’s skin, are at the intersection of the lithosphere, hydrosphere, atmosphere, and biosphere. The persistence of life on our planet depends on the maintenance of soils as they constitute the biological engines of earth. Human population has increased exponentially in recent decades, along with the demand for food, materials, and energy, which have caused a shift from low-yield and subsistence agriculture to a more productive, high-cost, and intensive agriculture. However, soils are very fragile ecosystems and require centuries for their development, thus within the human timescale they are not renewable resources. Modern and intensive agriculture implies serious concern about the conservation of soil as living organism, i.e., of its capacity to perform the vast number of biochemical processes needed to complete the biogeochemical cycles of plant nutrients, such as nitrogen and phosphorus, crucial for crop primary production. Most practices related to intensive agriculture determine a deterioration even in the short-middle term of their physical, chemical, and biological properties, which all together contribute to soil quality, along with an overexploitation of soils as living organisms. Recent trends are turning toward styles of agriculture management that are more sustainable or conservative for soil quality.
Usually, use of soils for agricultural purposes deflect them at various degrees from the “natural” soil development processes (pedogenesis), and this shift may be assumed as a divergence from soil sustainability principles. For decades, the misuse of land due to intensive crop management has deteriorated soil health and quality. A huge plethora of microorganisms inhabits soils, thus acting as “the biological engine of the earth”; indeed, this microbiota serves the soil ecosystem, performing several fundamental functions. Therefore, management practices might be planned looking at the safeguard of soil microbial diversity and resilience. In addition, each unexpected alteration in numberless soil biochemical processes, being regulated by microbial communities, may represent an early and sensible signal of soil homeostasis weakening and, consequently, warn about soil conservation. Within the vast number of soil biochemical processes and connected features (bioindicators) virtually effective to measure the sustainable soil exploitation, those related to the mineralization or immobilization of the main nutrients (C and N), including enzyme activity (functioning) and composition (diversity) of microbial communities, exert a fundamental role because of their involvement in soil metabolism. Comparing the influence of many cropping factors (tillage, mulching and cover crops, rotations, mineral and organic fertilization) under both intensive and sustainable managements on soil microbial diversity and functioning, through both chemical and biological soil quality indicators, makes it possible to identify the most hazardous diversions from soil sustainability principles.
Sustainable agriculture lies in the effective conservation of natural resources for agricultural productivity to fulfill increasing human demands while keeping or even improving the environmental safety (Gomiero, Pimentel, & Poletti, 2011). Therefore, sustainable agriculture implies a land use withstanding resource exhaustion, thus assuring the environmental quality by definition (Horrigan, Lawrence, & Walker, 2002). Sustainable agriculture is based on the reduced use of pesticides and synthetic fertilizers since they may contaminate groundwater, thus reaching the human food chain and causing health hazards.
Soils perform several ecosystem functions, in turn crucial for most terrestrial life. They provide suitable microbial habitats, reservoirs of plant nutrients, and ecosystem services such as water depuration and storage. Moreover, soils carry out decay and recycling of organic residues from plants, animals, and microorganisms; they also mitigate climate change by sequestering carbon into soil organic matter (Lal, 2004).
Organic matter (SOM) is a fundamental constituent of soil as it represents a “revolving nutrient reserve,” improves soil structure while minimizing erosion, and maintains crop production. The most urgent worry for sustainable farming agro-systems is the persistent lowering of SOM until its contents are inadequate for profitable crop yields (Rasmussen et al., 1998). Furthermore, SOM is per se a dynamic entity. Its quantity and quality depend on several factors such as climatic conditions, crop types, farming management, soil fertility, anthropic disturbance, and land planning. In particular, soil organic carbon stocks are generally reduced by long-term agriculture under intensive tillage systems and the removal of crop residues for animal feed (Bossuyt, Sic, & Hendrix, 2002).
The world’s soils store more carbon than is present in biomass and in the atmosphere. Little is known, however, about the factors controlling the stability of soil organic carbon stocks and the response of the soil carbon pools to climate change (Bellamy, Loveland, Bradley, Lark, & Kirk, 2005). Fontaine et al. (2007) investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubation, and radiocarbon dating. The supply of fresh plant-derived carbon to the subsoil (0.6–0.8 m in depth) stimulated the microbial mineralization of 2,567 ± 226-year-old carbon, supporting their previous suggestion (Fontaine & Barot, 2005) that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. They concluded that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature (Fontaine et al., 2007). Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile, such as deep plowing versus conservation tillage (CT), use of drought-resistant crops with deep root systems (Lal, 2004), could, however, stimulate the loss of ancient buried carbon. Enhancing SOM levels may be achieved by avoiding practices that speed up its mineralization by microbial communities and/or by increasing organic residues inputs. A reduction in tillage intensity is well accepted as a valuable procedure to slow down the SOM loss rate (West & Post, 2002). Conventional tillage (CT) systems are meant to accelerate SOM mineralization and consequently increase CO2 flux from soil to the atmosphere. Plowing favors organic residues mixing throughout soil, thus not only allowing soil microorganisms to reach crop residues with higher probability but also ameliorating soil microclimatic conditions for their degradation (e.g., optimal soil humidity, oxygenation, and temperature; Goh, 2004). In contrast, no-tillage systems (NT) reduce soil microbiological metabolism and, therefore, SOM decay (Murage, Voroney, Kay, Deen, & Beyaert, 2007). Indeed, NT facilitates the compaction of soil structure, with higher bulk density and decreased porosity, thus reducing oxygen supply for microbial heterotrophic activities. As a consequence, a number of scientific reports emphasize that NT may enhance SOM content across the soil profile compared to CT, although, other studies do not mention any change in SOM. In the latter case, scientists stress that NT only stratifies the SOC, since a topsoil enrichment in SOC is usually offset by a concomitant impoverishment in the subsurface soil layers (Yang, Drury, Reynolds, & Tan, 2008).
Both organic (manure, compost, etc.) and inorganic (industrially manufactured) fertilizers are largely used by farmers to preserve soil quality and enhance crop productivity, respectively. Organic amendments and inorganic fertilizers, especially when coupled together, may indirectly enhance soil C inputs through the returned crop residues and rhizodepositions while directly controlling C outputs via soil microbial activity (Gong, Yan, Wang Hu, & Gong, 2009). The introduction into the soil of fresh organic matter with a low C/N ratio, such as by green manuring, enhances SOM content and quality, thus sustaining a high potential microbial activity and biomass (Edmeades, 2003). Reversing CT to sustainable agriculture usually decreases soil bulk density, enhances SOM content as favors the immobilization of C and N, and improves most soil microbial quality indicators (Badalucco et al., 2010). In general, soil C sequestration, i.e., the mitigation of greenhouse gas emissions such as carbon dioxide, through conversion to conservative and sustainable land farming procedures is more remarkable under cooler and wetter than hotter and drier climates (Jones & Donnelly, 2004).
The concept of soil quality fits between conservation management practices and the achievement of sustainable agriculture. Hence, the assessment of soil quality and health, also over time, is a prerequisite for planning a sustainable land management. In other words, soil quality indicators involve quantifiable soil features affecting the aptitude of soils to grow crops and/or carry out environmental services. Soil functioning depends on its physical, chemical, and biological properties. Doran and Parkin (1996) suggested a minimum data set for assessing soil quality, and this data set included physical (texture, rooting depth, infiltration rate, bulk density, water retention capacity), chemical (pH, total carbon content, electrical conductivity, nutrients level), and biological (C and N microbial biomass, potentially mineralizable N, soil respiration) properties. In addition, they proposed considering other supplementary soil properties to better assess soil quality, chosen according to the local climatic, geographic, and socio-economic conditions. Physical and chemical properties generally change only when the soil is subjected to a drastic disturbance (Schoenholtz, Vam Miegroet, & Burger, 2000), while several biological and biochemical properties are generally reliable and sensitive parameters, including after slight changes that the soil can undergo under the action of most stressing agents (Gil-Sotres, Trasar-Cepéda, Leiròs, & Seoane, 2005). Soil microorganisms can react and respond quickly due to their fast metabolism, and thus they may signal a hazardous environment; therefore, they should be preferentially considered when monitoring soil quality and health (Bastida, Zsolnay, Hernández, & García, 2008).
The great number of soil microbiological components and biochemical pathways are reflected in the multipurpose properties of soil microbial communities within soil ecosystem. Consequently, the pivotal indicators are still under debate. Thiele-Bruhn, Bloem, de Vries, Kalbitz, and Wagg (2012) highlighted the importance of the diversity of the whole soil biota (i.e., including fauna) as a prerequisite for ecosystem stability and services.
However, sustainable agriculture does not necessarily coincide with subsistence farming and permanent poor yields. Indeed, crop yield can be increased in a sustainable way relying on inoculating microorganisms that improve both soil and plant health (Avis, Gravel, Antoun, & Tweddel, 2008). For example, diverse groups of bacteria colonizing the rhizosphere (the soil surrounding plant roots) are able to stimulate, directly or indirectly, the plant development (plant growth promoting bacteria [PGPB]; Vessey, 2003). The bacteria may directly speed up plant nutrient uptake in several different ways, such as by fixing atmospheric nitrogen, solubilizing phosphorus-rich minerals, producing compounds with high affinity for iron (siderophores), and synthesizing phytohormones (auxins, cytochinins, and gibberellins), thus enhancing plant biomass production (Pii et al., 2015). Indirectly, the bacteria defeat various harmful effects of pathogenic organisms through various mechanisms, such as induction of host resistance (Beneduzi, Ambrosini, & Passaglia, 2012). The large-scale application of PGPR to crops as inoculants is ever-increasing as it may reduce the use of chemical fertilizers and pesticides.
In this article we will concentrate on the effects of the most widespread intensive and conservative agricultural management practices on SOM dynamics and related soil quality indicators. Moreover, we focus on the need for considering soil functioning under a multidisciplinary perspective (physical, chemical, biological) given that soils are among the most complex and non-renewable terrestrial ecosystems.
Agricultural Management Practices
Conservation agriculture (CA), consisting of all minimal soil disturbance (NT), permanent soil cover (mulch), and rotations, is a recent agricultural management increasingly popular worldwide since it is considered as the most reliable practice to enable farmers to achieve the goal of sustainable agricultural production (Kassam, Friedrich, Derpsch, & Kienzle, 2015; Kassam, Friedrich, Shaxson, & Pretty, 2009). However, the Oxford English dictionary defines cultivation as “the tilling of land,” “the raising of a crop by tillage,” or “to loosen or break up soil” as well as “improvement or increase in (soil) fertility.” Definitely, traditional cultivation is synonymous with tillage or plowing. For millennia soil tillage has been a main feature of farming practices (Lal, 2001). With the start of Industrial Revolution in the 19th century, mechanized tractors appeared to perform tillage operations, and in the early 21st century a plethora of machines is on the market for agriculture management. There are at least six main reasons for tillage use: (1) seedbed preparation to obtain a uniform seed germination; (2) allowing the crop to grow early in its life cycle without weed competition for light, water, and nutrients, with consequent higher yield; (3) tillage-assisted release of plant nutrients for crop growth through microbial mineralization after exposure of soil native organic matter to oxygen; (4) crop loose residues may block seeding equipment by raking and clogging if not incorporated into the soil—tillage prevents this drawback, also allowing soil fertilization (organic or inorganic) or amendment (e.g., liming); (5) tillage temporarily alleviates soil compaction using tools to shatter below-ground compaction layers; and (6) tillage may be a crucial management practice to control soil-borne diseases and some insects. However, the above list of tillage benefits concerns directly only the farmer, due to costs to the environment and natural resources on which farming inexorably depends. Indeed, advantages of plowing were quickly questioned after the 1930s by the farming community, which started supporting reduced tillage systems using fewer fossil fuels, reducing runoff and soil erosion, and inverting the loss of soil organic matter (Baker et al., 2006). The conservation tillage movement was born in that way. Today, a remarkable extent of agricultural land is cropped using CT. Nevertheless, CT observes some of the principles of CA but still involves some soil disturbance. Ultimately, the UN Food and Agriculture Organization (FAO, 2015) has defined CA as follows: “CA maintains a permanent or semi-permanent organic soil cover. This can be a growing crop or dead mulch. Its function is to protect the soil physically from sun, rain and wind and to feed soil biota. The soil micro-organisms and soil fauna take over the tillage function and soil nutrient balancing. Mechanical tillage disturbs this process. Therefore, zero or minimum tillage and direct seeding are important elements of CA. A varied crop rotation is also important to avoid disease and pest problems.”
Up to 2013 no-tillage had been adopted worldwide over about 157 M ha (Kassam, Friedrich, Derpsch, & Kienzle, 2015), and conventional tillage is still among the most used agricultural practices. Specifically, moldboard plowing has caused major losses of SOM through release of CO2 and N oxides greenhouse gases with global warming as a side effect (Lal, 1997; Linquist, Groeningen, Adviento-Borbe, Pittelkow, & Kessel, 2012). CT contributes a reduction in the consumption of fossil fuels, thus decreasing both greenhouse gas emissions and breakdown of soil macroaggregates, by just limiting the number of tillage operations. Reducing intensity of tillage is an effective means to enhance SOM content (Halvorson, Weinhold, & Black, 2002). West and Post (2002), interpreting the findings from 67 long-term crop experiments concluded that the replacement of conventional with no-tillage can promote the sequestration of 60 g C m−2 yr−1. As already discussed, this depends on several soil processes; for example, under no-tillage, the lack of soil disturbance produces a decrease in soil porosity, basically due to an opposite increase in soil bulk density, that may lead to a more limited oxygen supply for microbial activity, with consequent diminution of heterotrophic decomposition of SOM (Carof, de Tourdonnet, Coquet, Hallaire, & Roger-Estrade, 2007; Kay & Van den Bygaart, 2002). However, that no-tillage compared to moldboard plowing always increases SOM is somehow controversial since no net change in SOM content has been reported. Indeed, Yang et al. (2008) suggested that no-tillage only stratified SOM through soil profile because while SOM increased at topsoil, concomitantly it decreased in the subsoil. Other studies, carried out in Spain and southern Italy, compared conventional tillage to minimum or no-tillage within different crop rotation systems and showed significantly larger SOM concentrations under no-tillage in the topsoil (Álvaro-Fuentes, López, Cantero-Martínez, & Arrúe, 2008; Barbera, Poma, Gristina, Novara, & Egli, 2012). By contrast, Luo, Wang, and Sun (2010) meta-analyzing all data from 69 paired worldwide experiments suggested that cultivation for periods longer than 5 years induced a soil organic C (SOC) decrease larger than 20 tons ha−1 in the upper 60 cm soil profile, but conventional and no-tillage did not differ significantly. Moreover, the conversion from conventional to no-tillage significantly altered the vertical distribution of C across the soil profile, with a SOC increase in the 0- to 10-cm layer and a decline at a depth of 10–40 cm. These findings confirm the doubts of Baker, Ochsner, Venterea, and Griffis (2007) that “the widespread belief that conservation tillage favors C sequestration may simply be an artifact of sampling methodology” in some cases.
Tillage-induced changes in SOC distribution within soil profile depend on two causes: redistribution of SOC in topsoil and alteration of root system development. Primarily, topsoil layers hold the majority of SOC, and under conventional tillage plowing transfers it substantially into subsoil. Second, deep plowing, usually down to 50 cm, changes soil physical conditions and encourages substantial crop root development toward the deepest loose soil layers, thus increasing C inputs as rhizodepositions (Luo et al., 2010). In contrast, conservation tillage, by reducing soil disturbance, leads to larger soil cover while enhancing soil compaction. It either narrows root expansion into deeper soil layers (Martínez, Fuentes, Silva, Valle, & Acevedo, 2008) or the downward transfer of topsoil SOC. Moreover, residues on the ground surface under no-tillage bring about a decrease in soil temperature, especially during the summer, thus leading to a decrease in SOC decomposition by microbial biomass (Holland, 2004; Spedding, Hamel, Mehuys, & Madramootoo, 2004).
Tillage promotes the oxidative decomposition of SOM in two ways: (1) tillage crushes soil macroaggregates, thus increasing the exposed surface area of physically aggregate-protected SOC to microbial attack (Bossuyt et al., 2002; Mikha & Rice, 2004); (2) crop residues after incorporation into the soil reach better moisture and temperature conditions for decay (Coppens, Garnier, Findeling, Merckx, & Recous, 2007), thus supplying nutrients and fuel to microorganisms and further intensifying SOC decomposition, more recalcitrant humic C included (Fontaine et al., 2007). These two contrary effects induced by crop residue incorporation (i.e., C input enhancement counterbalanced by the stimulation of native soil C decay) may partly neutralize each other. Therefore, the net global effect of tillage on SOC reservoirs may be driven and tuned up by crop system types, which establish both quantity and quality of crop residues (as organic C entering the soil), but also by physico-chemical soil conditions affecting the decomposition rate of the integrated crop residues (Luo et al., 2010).
However, as a general rule, tillage-induced changes in SOM conditions, usually occurring over relatively short periods (e.g., 1–5 years), are hard to quantify because of large background amounts of rather stable soil organic matter already present—i.e., “native” SOM (von Lützow et al., 2006). By contrast, because of their simpler chemical structure (i.e., higher degradability), labile fractions of SOM, such as microbial biomass C (MBC), light fraction C, and easily extractable or mineralizable C pools are able to respond quickly to changes in organic C supply (Haynes, 2005). The above components have, therefore, been suggested as early indicators of the effects of soil agricultural practices and farming systems on SOM quality (Gregorich, Carter, Angers, Monreal, & Ellert, 1994) and are also considered to be key indicators of soil health (Doran & Zeiss, 2000).
Soil microbial biomass (SMB) is the very small portion of SOM constituted by living microorganisms smaller than 5–10 μm3 (Jenkinson & Ladd, 1981). MBC amounts to approximately 1–5% of SOC (Anderson & Domsch, 1989), whereas microbial N (MBN) accounts for 2–6% of the total organic nitrogen (Jenkinson, 1988). Usually, MBC content varies from 100 to more than 1,000 mg C kg−1 soil (Paul, Harris, Klug, & Ruess, 1999). SMB shows a turnover time of less than one year (Paul, 1984) and, consequently, responds to stress/disturbance agents much more rapidly than the whole SOM, which may require decades to appreciably change its content. Because of its dynamic nature, SMB content throughout time is not able to reflect the trend of the SOM content (i.e., if it is increasing, decreasing, or at equilibrium) because SMB content is strongly dependent on seasonal trends, which in turn interacts with above-ground plant communities (Waldrop & Firestone, 2006). SMB serves contemporarily as a dynamic store of nutrients and as a crucial player in organic matter degradation. The role of SMB is pivotal, therefore, to allow nutrient fluxes within and between the earth’s ecosystems (Smith & Paul, 1990). Jenkinson, Hart, Rayner, and Parry (1987) defined the SMB as “the eye of the needle through which all organic matter needs to pass.”
The impact of tillage on SMB have been widely investigated. SMB has been found larger with NT than with traditional tillage by 7–36%, especially in topsoil, while frequent tillage tended to decrease either total and active MBC (Alvarez & Alvarez, 2000). Usually, enhancement of SMB occurs rapidly within a few years after conversion to reduced tillage (Ananyeva, Demkina, Jones, Cabrera, & Steen, 1999). Rising SMB normally favors soil particle aggregation, accelerates nutrient cycling through slow release of organically stored nutrients, and helps to control pathogenic microorganisms by competition (Ghorbani, Wilcockson, Koocheki, & Leifert, 2008). Gonzalez-Chavez et al. (2010) reported that, after 28 years of no-tillage, SMB nearly doubled compared to with conventional tillage. Long-term no-tillage in a subtropical environment increased MBC in 0- to 5-, 5- to 10-, and 10- to 20-cm soil layers under soybean/wheat, maize/wheat, and cotton/wheat rotations (from 11 to 98%) compared with values of the respective layers of soil subjected to conventional tillage (Balota, Colozzi-Filho, Andrade, & Dick, 2004). Both MBC and MBN under minimum tillage increased by about 50% compared to conventional tillage in a semi-arid Mediterranean environment, regardless of a doubling of compost amendment (Laudicina, Badalucco, & Palazzolo, 2011). In no-tilled or minimally tilled soils, compared to conventionally tilled ones, crop residues left on the surface can lower soil temperature and increase water and SOM contents while improving soil structure. These conditions boost the growth and maintenance of microorganisms, particularly close to the surface soil (Diaz-Zorita, Duarte, & Grove, 2002; Doran, 1980). Inevitably, tillage dismantles soil pore networks, including those holding mycorrhizal hyphae, crucial protagonists for phosphorus availability in most soils (Kabir, 2005). Zero-tillage, thus, is conductive to a better balance between microbes and other organisms for a healthier soil (Helgason, Walley, & Germida, 2009).
Mulching and Cover Crops
Any crop not harvested (i.e., deliberately forgotten on the field) will produce crop residues similarly to cover crops (legume or non-legume) planned for providing mulch. Mulch can also be applied as composts or manures, although transport costs to the field may limit their use to profitable crops only, such as early vegetables.
Raindrops crashing on bare soil by kinetic energy destroy soil aggregates and clog up soil pores with rapid reduction in water infiltration, causing runoff and soil erosion. Mulch intercepts this energy and prevents the above hazards (Jordán, Zavala, & Gil, 2010). NT converted to mulch prevents topsoil crusting, helps water infiltration, minimizes runoff, and fosters higher yield than tilled soils (Thierfelder, Amezquita, & Stahr, 2005). Similarly, the surface residue, anchored or loose, protects the soil from wind erosion (Nelson, 2002). Mulching helps reduce water losses from the topsoil by evaporation and also helps moderate soil temperature, thus promoting biological activity and enhancing nitrogen mineralization (Kumar & Goh, 1999).
Cover crops favor the accumulation of SOM in the upper soil horizons (Madari, Machado, Torres, de Andrade, & Valencia, 2005), and this result is even enhanced if combined with NT. Mulching also contributes to recycling of nutrients, particularly in the presence of legume cover crops, by either the symbiosis with root-hosted N-fixing microorganisms or furnishing C sources for microbial biomass through rhizodepositions (Thorup-Kristensen, Magid, & Jensen, 2003). In sub-Saharan Africa, the establishment of natural or improved fallow systems (agroforestry) showed the greatest potential for increasing SOC, with an achievable C storage rate of 0.1–5.3 Mg C ha−1 yr−1 (Vagen, Lal, & Singh, 2005). Moreover, in those croplands organic amendment with crop residues or manure together with NT can achieve C accumulation rates up to 0.36 Mg C ha−1 yr−1. Lal (2005) estimated that in developing countries, enhancing SOC by 1 Mg ha−1 yr−1 can increase food grain production by 32 million Mg yr−1.
Amendments, such as crop residues and manures, promote SMB, whereas burning and removal of residues decrease it (Galdos, Cerri, & Cerri, 2009; Graham, Haynes, & Meyer, 2002). Cover crops encourage biologically affected soil tillage through their rooting; the surface mulch provides substrates, nutrients, and energy for macro-fauna (earthworms, arthropods, etc.) and soil-adapted microbial communities that live exploring soil biological space. Resorting to deep-rooted cover crops and biological agents (earthworms, etc.) can also alleviate compaction following zero-tillage management (Roger-Estrade, Anger, Bertrand, & Richard, 2010). Bacteria, actinomycetes, fungi, earthworms, and nematodes are usually more abundant in no-tilled and residue-mulched fields than in those tilled and with residues buried (Buck, Langmaack, & Schrader, 2000; Frey, Elliot, & Paustian, 1999; Ouellet, Lapen, Topp, Sawada, & Edwards, 2008).
Mulching and cover crops trigger soil biological diversity both below and above ground. The population of beneficial ground insects is largest with cover crops and/or mulch, and these help keep insect pests under control (Frank & Liburd, 2005). Interactions and exchanges between root systems and rhizosphere bacteria affect not only soil quality but also crop health and yield. Root exudates by plants activate and support specific rhizobacterial communities that accelerate nutrient cycling and nitrogen fixation while favoring biocontrol of plant pathogens, plant disease resistance, and plant growth stimulation (Pii et al., 2015; Sturz & Christie, 2003). Ground cover is expected to increase biological diversity, which, in turn, may enhance the agro-system homeostasis.
Crop rotation as an agricultural management practice claims ancient origins. Among its various benefits is the cultural control of plant diseases, thereby lessening the need for expensive pesticides and herbicides (Ghorbani et al., 2008). The rotation of different crops with different rooting patterns combined with minimal soil tillage promotes a more extensive network of root channels and macropores through the soil, with enhanced water infiltration and C storage in depth, as SOM becomes less susceptible to degradation processes (Peters, Sturz, Carter, & Sanderson, 2003). Rotation brings about biological and microbial diversity, and thus the risk of pests from pathogenic organisms is alleviated through controlling them by competition mechanisms (Carter, Peters, Noronha, & Kimpinski, 2009; Leake, 2003; Petersen, 2000). Agricultural good practices recommended by CA essentially promote soil biological activity and the minimal use of toxic pesticides. Advantages from CA are even raised by those dealing with integrated pest management (IPM), which resorts to alternative pest control methods not affecting the functional diversity of soil biota (Lamine, 2011; Leake, 2003).
Moreover, the inclusion of symbiotic nitrogen-fixing varieties in a rotation enriches soil in nitrogen, thus reducing the need for energy-intensive production of nitrogen fertilizers (Herridge, Paoples, & Bodder, 2008). Also, legume-based rotations are considered crucial for maintaining soil fertility as they can promote C sequestration in soil, especially in semi-arid environments. For example, Gregorich, Drury, and Baldock (2001) compared a conventional continuous maize cropping with a legume-based rotation. The maize monoculture after 35 years held 20 t C ha−1 less than legume-based rotation, with the SOM present below the plowed layer in the rotation being biologically more stabilized. A positive effect on SOC (increase by 4 t ha−1 throughout 21 cm topsoil depth) was also found in a 15-year study with legumes and alternate cattle grazing in a semi-arid environment of Argentina (Miglierina, Iglesias, Landriscini, Galantini, & Rosell, 2000). Nevertheless, legume-based rotations seem less efficient in storing soil C than cereals alone. For example, Curtin, Wang, Selles, McConkey, and Campbell (2000) showed the higher efficiency of cereals than legumes in achieving maximum soil C sequestration rates. Indeed, in a semi-arid area of Canada, they found that annual C input to soil ranged from 1.6 tons C ha−1 in average for black lentils while it was two to three times higher for wheat, possibly because leguminous crop residues can potentially emit more CO2 than wheat due to their lower C/N ratio (Chaves, De Neve, Hofman, Boeckz, & Van Cleemput, 2004). In other words, the lower the crop residue C/N ratio, the higher the residue decomposition rate (Al-Kaisi & Yin, 2005). As a consequence, legume residues (green manure) can improve soil quality and crop productivity (Rochester, Peoples, Hulugalle, Gault, & Constable, 2001).
Plant residues provide fresh resources for incorporation into SOM. However, in agricultural systems, since plants are harvested, only about 20% of primary production is on average accumulated into the soil organic fraction (Schimel, 1995). There is disagreement about the actual quantities of residue returned to the soil since it depends on crop type, growth conditions, and agricultural management (Gregorich et al., 2001). In a semi-arid environment of Canada, the conversion of residues to SOC was reported to be 9% under fallow while it increased up to 29% under continuous crops (Campbell et al., 2000). All below-ground plant biomass, except for root crops, is available for incorporation into SOM. Rhizodepositions are considered the major source of SOM, although tillage may substantially reduce the net accumulation of C from roots (Badalucco & Nannipieri, 2007). In cool climates, below-ground C input from roots alone can generally maintain soil C levels, but this does not occur in warmer or semi-arid regions where residues are oxidized much more quickly, as long as enough moisture is available (Laudicina, Novara, Barbera, Egli, & Badalucco, 2015). Consequently, with continuous cropping in semi-arid environments, especially under the pending global warming scenario, failed return of above-ground plant residues will invariably lead to SOM decrease (Álvaro-Fuentes & Paustian, 2011).
Crop rotations including leguminous crops foster a higher soil MBC/SOC ratio (microbial quotient) than monoculture crops (Anderson & Domsch, 1989). This difference is sustained by the higher biochemical variety of returned organic residues under crop rotation systems (Corbeels, O’Connell, Grove, Mendham, & Rance, 2003; Kaschuk, Alberton, & Hungria, 2010). Similarly, increases in soil MBC were found in soybean fields previously cultivated with legumes (lupins), compared to those previously cultivated with wheat (Franchini, Crispino, Souza, Torres, & Hungria, 2007). Moreover, crop rotations involving a higher percentage of legumes in relation to non-legumes showed higher soil microbial quotients. However, effects of legume crops on soil MBC sometimes are uncertain. Indeed, Franchini et al. (2007) suggested that differences in microbiological properties of soil under crop rotations could be detectable in long-term trials only, even under no-tillage. Findings from crop rotations may depend on the quality of organic compounds released as rhizodepositions (i.e., root both exudates and debris) that either raise or constrain microbial activities (Badalucco & Kuikman, 2001; Hinsinger, Bengough, Vetterlein, & Young, 2009).
Mineral and Organic Fertilization
In contrast to nutrients from organic fertilizers, which must be mineralized by microbial activity before being available for plant nutrition, the nutrients from inorganic fertilizers can be directly taken up by plant roots. This is why inorganic fertilizers directly affect crop yields and, consequently, the main reason for applying them. Substantial SOM increase after both manuring and inorganic fertilization has often been attributed to their induced amelioration of crop yields (Gong et al., 2009; Purakayastha, Rudrappa, Singh, Swarup, & Bhadraray, 2008; Verma & Sharma, 2007). However, other studies have showed that inorganic fertilization alone had no or negative effects not only on total SOM accumulation but also on its fractions (Manna et al., 2006; Rudrappa, Purakayastha, Singh, & Bhadraray, 2006). Without doubt the N management impacts SOC dynamics (Neff et al., 2002). On the one hand, N fertilization can increase the SOC content through enhancing biomass production and thus C inputs to soil (Russell, Laid, Parkin, & Mallarino, 2005). On the other hand, N fertilization affects CO2 fluxes from soil and hence C outputs (Ding, Meng, Yin, Cai, & Zheng, 2007). As a result, N fertilization may affect the SOC balance. Khan, Mulvaney, Ellsworth, and Boast (2007) suggested that the loss of SOC from soils may occur in response to synthetic N fertilizers as they promote the decomposition of crop residues and native SOM.
Cereal production now sustaining a world population of about 7 billion has tripled during the past 40 years, with a concomitant increase from 12 to 104 Tg yr−1 of synthetic N applied largely as ammonium fertilizers. They have represented a cost-effective form of warranty against low yields but disregard the inherent effect of mineral N in triggering microbial C utilization. Indeed, a net loss of soil organic C was observed for the Morrow Plots, America’s oldest experiment field, after 40–50 years of synthetic N fertilization that substantially exceeded grain N removal (Mulvaney, Khan, & Ellsworth, 2009). A similar expected decline in total soil N was reported in the same site due to the prevalent organic occurrence of soil N. The same decline is observed in numerous long-term chemical-based cropping systems through a variety of soil types, geographic regions, and tillage practices. Undoubtedly, the loss of organic N decreases soil productivity and the agronomic efficiency of fertilizer N. In order to increase N input crop efficiencies, the fate of applied ammonium-N through the soil-plant system should be accurately monitored. Long-term sustainability may require agricultural diversification, with a gradual transition from intensive synthetic N inputs to legume-based crop rotations (Mulvaney et al., 2009). However, there is still a heated debate on this crucial topic (Powlson et al., 2010; Reid, 2008).
The application of mineral N generally increases CO2 fluxes from soil (Iqbal et al., 2009), although at high N fertilization rates soil respiration may decrease (Al-Kaisi, Kruse, & Sawyer, 2008), possibly because of many soil enzymes being inactivated or denaturated by the increased soil solution ionic strength due to dissolved N fertilizers (Fog, 1988).
Long-term inorganic fertilizations (NPK), common in conventional and intensive agriculture, may have either deleterious or beneficial effects on SMB and microbial activity (Enwall et al., 2007; Ge et al., 2010). These contradictory results may depend on the different influence of the numerous experimental factors, such as rate, type and balance of fertilizer additions, soil type, crop residue management, tillage regime, experimental duration, and soil management before the experiment start (Su, Wang, Suo, Zhang, & Du, 2006). Among different reports, comparable care is not always devoted to complete information.
In general, rising nutrient inputs into terrestrial ecosystems result in benefits not only for plant communities but also for associated soil microbial communities. Studies carried out in predominantly undisturbed ecosystems such as natural forests report that increasing N inputs tends to decrease soil microbial biomass (Wallenstein, McNulty, Fernandez, Boggs, & Schlesinger, 2006); less clear results are available about long-term fertilizer impacts in managed systems such as agro-ecosystems (Geisseler & Scow, 2014). A meta-analysis based on 107 datasets from 64 long-term trials carried out worldwide highlighted that mineral fertilizer application caused a 15.1% increase in the SMB in comparison with unfertilized control treatments. Mineral fertilization also increased SOC content, with it being a pivotal factor contributing to the overall increase in SMB. Remarkably, the magnitude of the effect of fertilization on SMB was pH dependent. While fertilization tended to reduce SMB in soils with a pH below 5, it significantly enhanced SMB at higher soil pH values. SMB increase was most pronounced in studies carried out for at least 20 years. The input of N per se did not seem to negatively affect SMB under cropping. The application of urea and ammonia fertilizers, however, could temporarily increase pH, osmotic potential, and ammonia concentrations to levels inhibitory to microbial communities. How specific functional microbial groups respond to repeated applications of mineral fertilizers, however, varies considerably and seems to depend on factors related to both crop management and environment (Geisseler & Scow, 2014).
Manure and other organic materials from various sources (farmyard, stable, compost, etc.) are usually applied to the soil in order to (1) enhance the availability of plant nutrients and (2) improve the physical, chemical, and biological soil properties that affect long-term soil fertility.
Organic manure application directly increases SOC. Several reports show that spreading out manure acts synergistically with inorganic fertilizers in increasing SOM (Gong et al., 2009; Purakayastha et al., 2008; Rudrappa et al., 2006).
Among the major advantages of manure application is the formation and stabilization of soil macro-aggregates (Whalen & Chang, 2002) and particulate organic matter (Yan, Wang, & Yang, 2007), with a significant improvement in soil structure. Manure is more resistant to microbial decomposition than plant residues. Consequently, with equal C inputs, soil C storage is higher after manure application than crop residues (Jenkinson et al., 1987). Laudicina et al. (2011) investigated the combined effects of compost input and tillage intensity on soil chemical and biochemical properties under semi-arid Mediterranean conditions. Compost was the main factor affecting SOC content (56% of total variance explained). Indeed, in plots amended at high input of compost (30 tons ha−1), TOC was almost twice as large as found for the respective plots amended at low input (15 tons ha−1). Compost at low input was suitable to improve soil fertility, but under reduced tillage only. In a long-term study carried out in Kenya (i.e., at optimal both temperature and moisture conditions for microbial decomposition), Kapkiyai, Karanja, Qureshi, Smithson, and Woomer (1999) showed that SOM declined even when manure was applied and maize residues returned to soil.
However, application of high manure rates could cause several problems such as accumulation of cations (K+, Na+, and NH4+) and production of water-repellent substances by decomposer fungi (Haynes & Naidu, 1998). Organic manure had a great influence on MBC as well as CO2, CH4, and other greenhouse gas emissions from soil (Owen & Whendee, 2015). Most studies carried out to investigate the effects of organic manure on soil MBC report increases in both MBC and MBC/SOC ratio as well as an increase in soil respiration. This increase in CO2 emission is due to the high presence of more rapidly decomposable (labile) organic compounds in manure, especially when liquid (Bol et al., 2003; Chantigny, Rochette, & Angers, 2001).
When plant residues and other organic amendments are incorporated into soil, mineral N should be applied as well, especially when the C/N ratio of organic input is high, in order to favor an adequate degradation rate by soil microbial communities from one side while meeting the crops N demand from the other (Chivenge, Vanlauwe, & Six, 2011). However, as observed by Iqbal et al. (2009), when straw (high C/N) is applied with inorganic N fertilizer, a concomitant SOC loss occurs due to microbial triggered CO2 flux from soil. Similarly, the addition of readily decomposable C by organic amendments, also without a concomitant inorganic N input, is prone to increase the decomposition of native SOM through a priming effect reducing the N deficiency for microbial biomass (Chen et al., 2014).
Future agricultural activities will allow us to increasingly produce food from decreasingly available cultivable land without neglecting the more efficient use of natural resources and the need for ever-lower impact on the environment. Only by keeping this in mind will food supply keep up with demand and land productivity be maintained for future generations. The use of productive but more sustainable management practices has become mandatory. Crop and soil management systems that help improve soil quality and health indicators (physical, biological, and chemical) as well as reduce farmer costs are crucial. Although conventional and conservation agricultural systems are based on contrasting priorities, both bring about considerable agro-environmental advantages and also face considerable agronomic challenges. Due to the unequivocal difficulty for conservation agriculture to be successful worldwide, the adoption of moderate-intensity farming coupled to integrated farming seems a good choice to provide satisfactory conditions for crop yield while adequately sustaining environmental quality and health. This choice could equip both farmers and policymakers with environmentally sound solutions for sustainable land management. It is very likely that, due to the continuously growing human population together with the accelerated climatic change and shrinking of natural resources, moderate-intensity and integrated farming systems will become increasingly attractive.
In light of this trend, CA should be increasingly pursued since it has the potential to fulfill several crucial benefits at both regional and global scales that cannot be further postponed due to the relentless exhaustion of the earth’s resources. Among the regional ones are (1) soil stabilization and protection from erosion, (2) reduction in toxic contamination of surface water and groundwaters, (3) more regular river flows and reduced flooding, and (4) re-emergence of dried wells. Among the global ones are (1) improvement in soil fertility and moisture retention, with long-term yield increase and greater food security, (2) reduction of greenhouse gas emissions to the atmosphere, (3) conservation of terrestrial and soil-based biodiversity, and (4) reduction in air pollution resulting from soil tillage machines.
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