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Mechanisms for C stabilization in soils have received much interest recently due to their relevance in the global C cycle. Here we review the mechanisms that are currently, but often contradictorily or inconsistently, considered to contribute to organic matter (OM) protection against decomposition in temperate soils: (i) selective preservation due to recalcitrance of OM, including plant litter, rhizodeposits, microbial products, humic polymers, and charred OM; (ii) spatial inaccessibility of OM against decomposer organisms due to occlusion, intercalation, hydrophobicity and encapsulation; and (iii) stabilization by interaction with mineral surfaces (Fe-, Al-, Mn-oxides, phyllosilicates) and metal ions. Our goal is to assess the relevance of these mechanisms to the formation of soil OM during different stages of decomposition and under different soil conditions. The view that OM stabilization is dominated by the selective preservation of recalcitrant organic components that accumulate in proportion to their chemical properties can no longer be accepted. In contrast, our analysis of mechanisms shows that: (i) the soil biotic community is able to disintegrate any OM of natural origin; (ii) molecular recalcitrance of OM is relative, rather than absolute; (iii) recalcitrance is only important during early decomposition and in active surface soils; while (iv) during late decomposition and in the subsoil, the relevance of spatial inaccessibility and organo-mineral interactions for SOM stabilization increases. We conclude that major difficulties in the understanding and prediction of SOM dynamics originate from the simultaneous operation of several mechanisms. We discuss knowledge gaps and promising directions of future research.

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In the next decades, many soils will be subjected to increased drying/wetting cycles or modified water availability considering predicted global changes in precipitation and evapotranspiration. These changes may affect the turnover of C and N in soils, but the direction of changes is still unclear. The aim of the review is the evaluation of involved mechanisms, the intensity, duration and frequency of drying and wetting for the mineralization and fluxes of C and N in terrestrial soils. Controversial study results require a reappraisal of the present understanding that wetting of dry soils induces significant losses of soil C and N. The generally observed pulse in net C and N mineralization following wetting of dry soil (hereafter wetting pulse) is short-lived and often exceeds the mineralization rate of a respective moist control. Accumulated microbial and plant necromass, lysis of live microbial cells, release of compatible solutes and exposure of previously protected organic matter may explain the additional mineralization during wetting of soils. Frequent drying and wetting diminishes the wetting pulse due to limitation of the accessible organic matter pool. Despite wetting pulses, cumulative C and N mineralization (defined here as total net mineralization during drying and wetting) are mostly smaller compared with soil with optimum moisture, indicating that wetting pulses cannot compensate for small mineralization rates during drought periods. Cumulative mineralization is linked to the intensity and duration of drying, the amount and distribution of precipitation, temperature, hydrophobicity and the accessible pool of organic substrates. Wetting pulses may have a significant impact on C and N mineralization or flux rates in arid and semiarid regions but have less impact in humid and subhumid regions on annual time scales. Organic matter stocks are progressively preserved with increasing duration and intensity of drought periods; however, fires enhance the risk of organic matter losses under dry conditions. Hydrophobicity of organic surfaces is an important mechanism that reduces C and N mineralization in topsoils after precipitation. Hence, mineralization in forest soils with hydrophobic organic horizons is presumably stronger limited than in grassland or farmland soils. Even in humid regions, suboptimal water potentials often restrict microbial activity in topsoils during growing seasons. Increasing summer droughts will likely reduce the mineralization and fluxes of C and N whereas increasing summer precipitation could enhance the losses of C and N from soils.

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The effects of nitrogen (N) deposition on plant litter and soil organic matter decomposition differ depending on the stage of decomposition (early, late, and final stages). The effects can be divided further into direct and indirect ones. Direct effects: additions of ammonium and nitrate to fresh, newly shed litter stimulate the initial decomposition of celluloses and solubles. By contrast, addition of the same compounds to humus (final stages) clearly suppresses activity. This was seen in all studies reviewed and for several types of humus. Indirect effects: long-term deposition leads to increases in litter concentrations of N and other nutrients. This N in litter forms "natural" organic compounds and the resulting effects are similar to those resulting from natural variation among litter types. Thus, initial decomposition is generally higher for N (nutrient) rich plant litters than for litters with a lower N (nutrient) content. In later stages, at which lignin-degradation rates regulate litter decomposition, N has a retarding effect on decomposition. Significant negative correlations have also been found between N concentrations in humus and respiration rate. There probably is a sink for deposited N in the humus. We may conclude that N storage in humus is regulated by a positive feedback mechanism. Raised levels of N resulting from N deposition cause more humus to be left in the system, and the resulting lower levels of Mn further retards humus decomposition, thus leading to an increased storage of N in humus. Thus, when calculating critical loads it would be incorrect to assume that N pools in the humus remain at a steady state.

Freezing and thawing of soils may affect the turnover of soil organic matter and thus the losses of C and N from soils. Here we review the literature with special focus on: (i) the mechanisms involved, (ii) the effects of freezing temperature and frequency, (iii) the differences between arable soils and soils under natural vegetation, and (iv) the hypothesis that freeze-thaw events lead to significant C and N losses from soils at the annual scale. Changes in microbial biomass and populations, root turnover and soil structure might explain increased gaseous and solute fluxes of C and N following freeze-thaw events, but these mechanisms have seldom been addressed in detail. Effects of freeze-thaw events appear to increase with colder frost temperatures below 0°C, but a threshold value for specific soils and processes cannot be defined. The pool of C and N susceptible to freeze-thaw events is rather limited, as indicated by decreasing losses with shortterm repeated events. Elevated nitrate losses from soils under alpine and/or arctic and forest vegetation occurred only in the year following exceptional soil frost, with greatest reported losses of about 13 kg N ha À1 . Nitrate losses are more likely caused by reduced root uptake rather than by increased N net mineralization. N 2 O emissions from forest soils often increased after thawing, but this lasted only for a relatively short time (days to 1-2 months), with the greatest reported cumulative N 2 O emissions of about 2 kg N 2 O-N ha À1 . The emissions of N 2 O after freeze-thaw events were in some cases substantially greater from arable soils than from forest soils. Thus, freeze-thaw events might induce gaseous and/or solute losses of N from soils that are relevant at the annual time scale. While a burst of CO 2 after thawing of frozen soils is often found, there is strong evidence that, at the annual time scale, freeze-thaw cycles either have little effect or will even reduce soil C losses as compared with unfrozen conditions. On the contrary, a milder winter climate with fewer periods of soil frost may result in greater losses of C from soils that are presently influenced by extended frost periods. DefinitionWe realise that the 'true' freezing point of soil is at or close to 0°C, depending on whether there is a significant amount of solute in the soil water. In this paper, we refer to studies in which soils and related materials have been cooled, naturally or artificially, well below this temperature. For the sake of clarity and simplicity, we refer to the temperature to which the soil has been cooled or exposed either as the frost temperature or the freezing temperature. Likewise a decrease in freezing or frost temperature means that conditions became colder, an increase means that they became warmer.

There is growing evidence of the importance of extramatrical mycelium (EMM) of mycorrhizal fungi in carbon (C) cycling in ecosystems. However, our understanding has until recently been mainly based on laboratory experiments, and knowledge of such basic parameters as variations in mycelial production, standing biomass and turnover as well as the regulatory mechanisms behind such variations in forest soils is limited. Presently, the production of EMM by ectomycorrhizal (EM) fungi has been Plant Soil (2013) at~140 different forest sites to be up to several hundreds of kg per ha per year, but the published data are biased towards Picea abies in Scandinavia. Little is known about the standing biomass and turnover of EMM in other systems, and its influence on the C stored or lost from soils. Here, focussing on ectomycorrhizas, we discuss the factors that regulate the production and turnover of EMM and its role in soil C dynamics, identifying important gaps in this knowledge. C availability seems to be the key factor determining EMM production and possibly its standing biomass in forests but direct effects of mineral nutrient availability on the EMM can be important. There is great uncertainty about the rate of turnover of EMM. There is increasing evidence that residues of EM fungi play a major role in the formation of stable N and C in SOM, which highlights the need to include mycorrhizal effects in models of global soil C stores.