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  Carbon storage in soils

  Soils are a pivotal component in the global carbon cycle, while carbon storage in soils is a natural phenomenon involving organic carbon. Maintaining or increasing soil carbon levels is beneficial for many ecosystem services. Soil carbon is also a soil condition indicator and a key focus of several Sustainable Development Goals. This chapter describes the forms of carbon in soils, the quantification of carbon stocks and storage, the processes underlying the heterogeneous distribution of carbon stocks across the planet and their dynamics, land-use changes and practices that affect soil carbon stocks, as well as the socioeconomic benefits of soil carbon storage.
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  Carbon sequestration capacity in no-till soil decreases in the long-term due to saturation of fine silt plus clay-size fraction

  The capacity of soils to stabilize carbon (C) may decrease over time, limiting the potential of no-till soil to act as a C sink in the long-term. Our objectives were to evaluate the effects of long-term no-till cropping systems on (i) C storage in soil, (ii) C stabilization in the fine silt plus clay-size (<20 μm) fraction and its relationship with the decrease of C saturation deficit (CSD) in this fraction, and (iii) on C accumulation in labile fractions of soil organic matter (SOM) in 0–2.5, 2.5–5, 5–10 and 10–20 cm layers of a subtropical Acrisol. The study was based on a long-term (36 years) no-till experiment where five cropping systems, with variable annual C inputs, were assessed: [i] bare-soil, [ii] black oat/maize, [iii] black oat + vetch/maize + cowpea, [iv] lablab + maize and [v] pigeon pea + maize. Cropping systems including maize and tropical legumes (lablab, pigeon pea and cowpea) with high C input led to the highest C storage in the top layers (up to 10 cm depth) of this no-till soil. Also, a decrease of CSD in fine silt plus clay-size fraction was observed in all soil layers to 20 cm depth, but the most expressive impact on CSD occurred in the topsoil (0–2.5 cm), where the capacity to further stabilize more carbon decreased by 90–97% when compared to bare soil. Considering the full C saturation level of the silt plus clay-size fraction and the current C contents in the soil, the remaining capacity of C sequestration up to 20 cm was estimated as ranging from 22.5 to 32.8 Mg C ha−1, and much of it (58–75%) was in the 10–20 cm layer. Our results highlight the importance of diversified cropping systems with high input (quantity and quality) crop residues to C sequestration in soil. Moreover, although the mineral-associated SOM of the top layer reached a C stabilization limit, C accumulation continues in non-saturated labile fractions, and in non-saturated fine silt plus clay-size fraction in deeper layers of subtropical no-till soils.
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  Will fungi solve the carbon dilemma?

  Soils are hotspots of diversity and sustain many globally important functions. Here we focus on the most burning issue: how to keep soils as carbon sinks while maintaining their productivity. Evidence shows that life in soils plays a crucial role in improving soil health yet soil ecological processes are often ignored in soil sciences. In this review, we highlight the potential of fungi to increase soil carbon sequestration while maintaining crop yield, functions needed to sustain human population on Earth and at same time keep the Earth livable. We propose management strategies that steer towards more fungal activity but also high functional diversity of fungi which will lead to more stable carbon sources in soil but also affects the structure of the soil food web up to ecosystem level. We list knowledge gaps that limit our ability to steer soil fungal communities such that stabilising carbon in top soils becomes more effective. Using the natural capacity of a biodiverse soil community to sequester carbon delivers double benefit: reduction of atmospheric carbon dioxide by storing photosynthesized carbon in soil and increasing agricultural yields by restoring organic matter content of degraded soils.