Carbon sequestration in soil
Introduction
Generically, carbon (C) sequestration in soil refers to capture and secure by storage of atmospheric CO2 with pedosphere in a manner that also increases its mean residence time (MRT) and minimizes sinks of re-emission [1••]. Among numerous objectives of soil C sequestration are: (i) off-setting anthropogenic emissions by fossil fuel combustion, cement production and deforestation, (ii) reducing net increase in atmospheric concentration of CO2 (which reached 400 ppmv in 2013) and pool (∼800 PgC), (iii) improving soil organic C (SOC) concentration (and pool) to above the threshold level of 1.5–2.0%, (iv) restoring soil quality and its ecosystem functions and services, (v) improving water and nutrient retention capacity, (vi) enhancing use efficiency of inputs in soils of managed ecosystems, (vii) reducing risks of accelerated erosion and non-point source pollution (NPSP), (viii) creating climate-smart soils and agroecosystems, (ix) improving use efficiency of inputs, and strengthening soil's disease-suppressive characteristics, and (x) increasing and sustaining agronomic productivity, and advancing food and nutritional security. Because of numerous co-benefits, there is a strong interest in the definition, concepts, experimental approaches, procedures of laboratory analyses, and methods of determining SOC sequestration rates. In this context, SOC sequestration is defined as, ‘process of transferring CO2 from the atmosphere into the soil of a land unit through units plants, plant residues and other organic solids, which are stored and retained in the unit as part of the soil organic matter (humus)’. Retention time of sequestered carbon in soil (terrestrial pool) can range from short-term (immediately released back to the atmosphere) to long-term (millennia) storage. The sequestered SOC processes should increase the new SOC storage during and at the end of a study to above the previous pre-treatment baseline [2]. Minister of Agriculture of France, Mr Stephane Le Foll, is proposing to UNFCCC-COP21 in Paris in December 2015 SOC sequestration at the rate of ‘4 per 1000’ to offset anthropogenic emissions.
Some concerns have been expressed about the concepts [3, 4], and the magnitude of SOC sink capacity [5] or the merits of pool vs. depth distribution of SOC in relation to management [6, 7], the short-term vs. long-term effects of soil management on SOC [8], and the fate of erosion-induced transport of SOC as a source or sink of atmospheric CO2 [9••, 10]. Therefore, the objective of this article is to address potential and challenges of SOC sequestration to mitigate climate change, improve soil quality and advance food security and nutritional security.
Section snippets
Soil inorganic carbon pool
The soil inorganic carbon (SIC) pool comprises about 950 Pg C in the top 1 m [11]. The predominant SIC forms are carbonate minerals either derived from the soil parent material (primary, lithogenic) or synthesized during soil formation (secondary, pedogenic). Secondary carbonates are formed through the reaction of dilute carbonic acid with Ca2+ or Mg2+ brought in by dust, runoff, manure, ocean drift and sediment from outside [1••]. This process is a principal mechanism for SIC sequestration in
Depth distribution
Soils hold C not only close to the soil surface but also to the full soil profile depth [14]. For example, more than 50% of the SIC and SOC stocks to 1-m depth are recorded in subsoil horizons below 0.3-m depth [15••]. Increasing C sequestration within soil profiles may be possible by adapting adequate soil management techniques [16]. While SOC concentrations in surface soils are generally higher than those at deeper depth, subsoil may be very far from being saturated with SOC [17]. Where deep
Soil carbon dynamics
In general, SOC is extremely dynamic, because its highly reactive, a source of energy for all microorganisms and other biota in the soil, and is preferentially removed by erosional processes because it has low density and is located in vicinity of the soil surface. Therefore, SOC pool is in a dynamic equilibrium with its environment. Its magnitude and the rate of change depend on the balance between gains of biomass-C or input (Eq. (1)) and losses of biomass-C or output (Eq. (2)):where
Physical protection
Soil aggregate formation is among the mechanisms for SOC stabilization. Understanding physical mechanisms of SOC sequestration and stabilization in soils have received much research interests for decades [8, 18•, 19•, 20, 21•, 22]. Tillage disintegrates soil aggregates fractions and can substantially transfer protected SOC pools with mean residence times of decades to active pools with mean residence times of only weeks [23, 24, 25].
Like tillage practices, subsequent drying–rewetting cycles may
Croplands
Globally, croplands occupy about 1500 million hectares (Mha). Conversion of natural ecosystems into agro-ecosystems depletes the SOC pool because of: (i) lower return of biomass-C, (ii) higher losses of SOC by erosion, mineralization and leaching, and (iii) stronger variations in soil temperature and moisture regimes. Depletion of SOC pool from croplands is also exacerbated by degradation processes (e.g., erosion, salinization, nutrient depletion, decline in soil structure and aggregation).
Developing countries
Land governance involves the rules, processes, and structures through which decisions are made about access to land and its use, the manner in which the decisions are implemented and enforced, and the way that competing interests in land are managed [53]. Developing countries are facing complex problems regarding insecurity of food, feed, water, and energy which are aggravated by soil degradation, climate changes, and lack of land tenure security. Soil degradation exacerbates challenge of
Conclusions
Soil organic carbon is the essence of all terrestrial life, and is critical to human well-being and nature conservancy. Through its impact on soil quality and several key pedospheric processes, it is the source of numerous ecosystem goods and services. Being the largest terrestrial reservoir of C to 3-m depth (∼4000 Pg), total soil C pool (both organic and inorganic) can be a source or sink of atmospheric CO2 depending on land use and management. World soils have been the source of CO2 and
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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