Soils play an essential role in the global cycling of carbon and understanding the stabilisation mechanisms behind the preservation of soil organic carbon (SOC) pools is of globally recognised significance. Until recently, research into SOC stabilisation has predominantly focused on acidic soil environments and the interactions between SOC and aluminium (Al) or iron (Fe). The interactions between SOC and calcium (Ca) have typically received less attention, with fewer studies conducted in alkaline soils. Although it has widely been established that exchangeable Ca (CaExch) positively correlates with SOC concentration and its resistance to oxidation, the exact mechanisms behind this relationship remain largely unidentified. This synthesis paper critically assesses available evidence on the potential role of Ca in the stabilisation of SOC and identifies research topics that warrant further investigation. Contrary to the common view of the chemistry of base cations in soils, chemical modelling indicates that Ca2+ can readily exchange its hydration shell and create inner sphere complexes with organic functional groups. This review therefore argues that both inner- and outer-sphere bridging by Ca2+ can play an active role in the stabilisation of SOC. Calcium carbonate (CaCO3) can influence occluded SOC stability through its role in the stabilisation of aggregates; however, it could also play an unaccounted role in the direct sorption and inclusion of SOC. Finally, this review highlights the importance of pH as a potential predictor of SOC stabilisation mechanisms mediated by Al- or Fe- to Ca, and their respective effects on SOC dynamics.

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Calcium-mediated stabilisation of soil organic carbon

Global climate change has resulted in changes to the earth's geological, ecological, and biological ecosystems, which pose a severe threat to the existence of human civilization and sustenance of agricultural productivity vis-a-vis food security. In the last several decades, climate change has been linked to erratic rainfall distribution patterns and large variations in diurnal temperatures, because of a rise in atmospheric CO2 concentration. This, in turn, is thought to make world agricultural production systems more prone to failure. Soil organic carbon (SOC) is an important component for the functioning of agro-ecosystems, and its presence is central to the concept of sustainable maintenance of soil health. Soil is the largest terrestrial carbon sink and contains 2- and 3-times more carbon than the carbon in the atmosphere and vegetation, respectively. Therefore, a meager change in soil carbon sequestration will have a drastic impact on the global carbon cycle and climate change. The SOC has different pools and fractions including total organic carbon (TOC), particulate organic carbon (POC), microbial biomass carbon (MBC), dissolved organic carbon (DOC), permanganate oxidizable carbon (KMnO4-C), and mineral associated organic carbon (MOC). Each has a varying degree of decomposition rate and stability. Researchers have identified many ways to offset the effect of climate change through modification of carbon sequestration in the soil. Identification of location-specific, suitable land use and management practices is one of the options to mitigate the impact of the climate change. It can be done by re-balancing different carbon pools and emission fluxes. Labile organic carbon pools including MBC, POC, and KMnO4-C are the most sensitive indicators for assessing soil quality after the adoption of alternate land use and management practices. Information on soil aggregation and SOC stabilization helps for long-term sequestration of carbon in the soil. Here we review the progress of work on SOC dynamics in the major ecosystems of the world. The information should enrich understanding of carbon sequestration and climate change mitigation strategies.

Soil organic carbon dynamics: Impact of land use changes and management practices: A review

Net effect of liming on soil organic carbon stocks: A review

Soil acidification can be accelerated by intensive farming or prevented by sustainable management practices. Soil acidification in a managed agricultural production system is caused by the transformation of carbon (C), nitrogen (N), and sulfur (S), which releases protons (H+) to soil solution. Soil acidification decreases soil pH, causing adverse effects on plants and soil microorganisms. Acidification, coupled with aluminum, manganese, and iron toxicities, and phosphorus, calcium, magnesium, and potassium deficiencies, can lead to low soil fertility. Soil acidity influences soil C and N cycles by controlling activities of microorganisms involved in the transformations of these two elements. Traditionally, lime materials are added to neutralize acidic soils and to overcome the problems associated with soil acidification, but they also influence C and N cycles, thereby affecting greenhouse gas (GHG) flux in soils. For example, liming has been shown to decrease nitrification-induced nitrous oxide (N2O) emission from many agricultural lands. However, there are concerns that liming increases the availability of soil nitrate ( N O 3 − ), which is a substrate for N2O emission through denitrification. The dissolution of liming materials can act as either a net source or sink for carbon dioxide (CO2). Lime-derived CO2 reacts with microbial respiration-derived carbonic acid in soils to yield carbonate material, serving as a sink of CO2 in soil. In calcareous soils with high pH, agricultural lime (CaCO3) serves as a net sink for CO2 whereas in acid soils it serves as a net source of CO2. In acid soils, increased availability of aluminum (Al3+) ions inhibits activity of methane (CH4) oxidizers. Adding lime to soils has shown to increase CH4 oxidation and reduce GHG emission. The present review brings together basic concepts of soil acidification and recent developments on the implications of liming in relation to C and N transformations and cycling, particularly GHG emissions from soils. Given the major influence of lime addition on soil microorganisms relating to C and N cycles, future research should focus on the role of liming on soil microbial communities to provide insight into combined mitigation of N2O, CO2, and CH4 gases from agricultural soils.

Functional Relationships of Soil Acidification, Liming, and Greenhouse Gas Flux

A B S T R A C T Liming is a common agricultural practice worldwide, used for increasing productivity in acid agricultural soils. Liming reduces Al saturation and toxicity and/or increases pH up to values where the availability of nutrients is higher. The effect of this practice on soil properties has been extensively studied, with focus of most studies upon pH, exchangeable cations and productivity. In turn, the effects of liming on soil organic C (SOC) stocks still remain poorly known. The net effect on SOC can be the result of several factors: first, liming increases the soil biological activity, thus favoring the mineralization of organic matter, which should result in CO2 losses and a decrease of the SOC stocks. Second, liming ameliorates soil structure, increasing the stability of clay assemblages and clay-organic matter bonds, which should bring an increase in SOC physical and physicochemical protection. Finally, as liming ameliorates soil conditions to plant growth, plant productivity increases and also the return of C inputs to soil, thus potentially increasing SOC concentrations. The net effect of these processes is not well understood yet. Still, some overall trends can be deduced from data currently available in the literature. Liming does modify SOC stocks, increasing them in most cases, what seems to be caused by higher C inputs to limed soils due to increased productivity. Reductions in SOC have also been reported, probably in connexion with increased mineralization, whereas the role of improved soil structure remains unclear. Overall, these insights are deduced from published data which are still scarce, so we encourage the scientific community to synthesize unpublished SOC data from existing in situ experiments, in order to enlarge the span of experimental conditions and gain knowledge about the role of such a widespread agricultural practice on SOC stocks.

Net effect of liming on soil organic carbon stocks: a review

Soil organic matter pools and carbon-protection mechanisms in aggregate classes influenced by surface liming in a no-till system

Changes in Organic Matter Pools and Increases in Carbon Sequestration in Response to Surface Liming in an Oxisol under Long-Term No-Till

O objetivo do trabalho foi avaliar o efeito do modo de aplicacao do fertilizante nitrogenado e fosfatado na cultura do milho no sistema plantio direto. O delineamento experimental foi o de blocos ao acaso com quatro repeticoes. Os tratamentos foram constituido pelos modos de aplicacao do fertilizante nitrogenado e fosfatado: aplicacao a lanco em area total, localizado no sulco de semeadura e testemunha sem fertilizante. Foi avaliado a altura de requeima das folhas e a produtividade de graos. A aplicacao do fertilizante a lanco, apresentou diferenca significativa em relacao a testemunha e ao tratamento com aplicacao localizada. O incremento em relacao a testemunha sem fertilizante foi de 42% e representou 3.175 kg ha -1 , e 16% em relacao a aplicacao localizada no sulco e equivalente a 1.477 kg ha -1 . A aplicacao do fertilizante nitrogenado e fosfatado a lanco mostrou-se mais eficiente que a aplicacao localizada no sulco nos parâmetros estudados.

Jaqueline De Fátima Navarro

Papers

Soil organic matter pools and carbon-protection mechanisms in aggregate classes influenced by surface liming in a no-till system

Changes in Organic Matter Pools and Increases in Carbon Sequestration in Response to Surface Liming in an Oxisol under Long-Term No-Till

Modos de aplicação de nitrogênio e fósforo na cultura de milho em plantio direto.