Current Concepts and Technologies

A sustainable use of soil means its exploitation in a way and at a rate that preserves at the long term its multitude of functions and protects or improves its quality, thereby maintaining its potential to meet the likely needs and aspirations of present and future generations (Van-Camp et al. 2004).

Soil organic matter (SOM) plays a fundamental role in plant nutrients status; maintenance of soil functions; and release of CO2, methane, and other gases in the atmosphere. Two factors influence SOM content: natural (climate, soil parent

Dipartimento di Scienza del Suolo, della Pianta, dell'Ambiente e delle Produzioni Animali, Universita di Napoli Federico II, Naples, Italy e-mail: [email protected]

A. Piccolo (ed.), Carbon Sequestration in Agricultural Soils,

DOI 10.1007/978-3-642-23385-2_1, © Springer-Verlag Berlin Heidelberg 2012

material, land cover and/or vegetation and topography) and human-induced factors (land use, management, and degradation). In nature, uniform moisture conditions and comparable vegetation, the average total organic matter and nitrogen increase regularly from two to three times for each 10° C fall in mean temperature (Buckman and Brady 1960). Conversely, cultivation significantly affects OM content of soil by exposing fresh topsoil to rapid surface drying and air oxidation. Therefore, organic compounds are released to the atmosphere as a result of their biotic and abiotic degradation, while soil aggregates concomitantly break down due to progressive mineralization of binding humic materials. Unless OM is maintained or quickly replenished, the soil system is in a state of degradation, leading eventually to unsustainability (World Bank 1993). For example, a decline in OM content is accompanied by a decrease in soil fertility and biodiversity, and loss of structure, which together exacerbate overall soil degradation.

The current rapid depletion of OM in soils under farm land makes them sources of organic carbon rather than sinks. Organic carbon sequestration in soils is a potential tool for reducing greenhouse gas (GHG) emissions. The potential contribution of the agricultural sector to tackling climate change issues is now being acknowledged both under a strategic (i.e., in policy making) and practical standpoint. The Kyoto Protocol highlights that carbon sequestration in agricultural soils by land management practices can contribute to mitigating climate change. For example, estimates for Europe indicate that organic carbon sequestration in farm soils can account for about 20% of the total reduction required during the first commitment period (8% reduction required between 2008 and 2012 from a 1990 base) (EU Soil Thematic Strategy 2004). The role of soil, both as an emitter and a sink for carbon, is particularly important in this context. At global scale, research indicates that the soil carbon pool of 2,500 billion ton includes about 1,550 billion ton of soil organic carbon, which is 3.3 times the size of the atmospheric pool (760 billion ton) and 4.5 times the size of the biotic pool (560 billion ton) (Lal 2004). Between 1850 and 1998, the emission from terrestrial ecosystems was 136 ± 55 billion ton. The latter includes 78 ± 12 billion ton from soil, of which about one-third is attributed to soil degradation and accelerated erosion and two-thirds to OM mineralization (IPCC 2000). The European Union endorsed the need to link soil sustainability and its role in mitigating climate change, by calling for "a robust approach to address the interaction between soil protection and climate change from the viewpoints of research, economy and rural development, so that policies in this areas are mutually supportive" (EC 2006).

Management options available to sequester carbon in cropland include reduced and zero tillage, set-aside, perennial crops and deep rooting crops, more efficient use of organic amendments, improved rotations, irrigation, bioenergy crops, intensification of organic farming, and conversion of arable land to grassland or woodland (Smith et al. 2000, 2008). Due to advances in weed control methods and farm machinery which allow many crops to be grown with minimum tillage (reduced tillage) or without tillage (no till), these practices, which limit soil disturbance and consequently soil C losses through reduced microbial decomposition, are now usually believed to increase SOC sequestration in cropland soils. However, there are no solid scientific bases to justify this belief (Cerri et al. 2004; Smith and Conen 2004; Gregorich et al. 2005; Plaza-Bonilla et al. 2010; Mancinelli et al. 2010). Moreover, a long-term application is usually required for reduced tillage practices to produce a significant and steady improvement of OC content in cultivated soils (West and Post 2002). Reduced tillage is also advocated to affect N2O emissions but the net effect is inconsistent and depends on soil and climatic conditions (Marland et al. 2004). Additionally, the reduced tillage practices do not ensure a persistent organic carbon sequestration, since, as tillage is resumed (possibly by lack of sufficient incentives to farmers), the fixed carbon is rapidly lost again from soil. In fact, the incorporation of biolabile components derived from plant material is limited to soil surface (Six et al. 2000; Jacobs et al. 2010; Mishra et al. 2010), and their rapid decomposition is accelerated, if soil management is reversed to conventional tillage. Carbon sequestration in cropland by adopting reduced-tillage practices has been estimated (Fig. 1.1) to be rather small (<0.5 ton C ha-1 year-1) and extremely variable (>50% error), thereby showing their little use in off-setting GHG emissions in Europe (Freibauer et al. 2004; Smith et al. 2007).

The shortcomings of current management practices for soil carbon sequestration based on reduced tillage are (1) reduced crop productivity; (2) small, inconsistent and variable carbon fixation; (3) temporary sequestration until traditional tillage practice is resumed. These shortcomings clarify that reduced- or no-till agriculture do not consistently result in soil C and N gain, and, in addition, it is not well quantified globally. Therefore, there is a clear and unmet need to find better alternatives to current soil management practices for organic carbon sequestration in agriculture.

H 20

| Soil carbon sequestration potential (t C ha-1 per year

I Total soil carbon sequestration potential for EU15 (MtC per year)

Agronomic Practices

Fig. 1.1 Carbon sequestration potentials limited only by availability of land, biological resources and land suitability, and the potentials estimated to be realistically achievable by 2012

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