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.

Summary This paper reports the range and statistical distribution of oxidation rates of atmospheric CH4 in soils found in Northern Europe in an international study, and compares them with published data for various other ecosystems. It reassesses the size, and the uncertainty in, the global terrestrial CH4 sink, and examines the effect of land‐use change and other factors on the oxidation rate. Only soils with a very high water table were sources of CH4; all others were sinks. Oxidation rates varied from 1 to nearly 200 μg CH4 m−2 h−1; annual rates for sites measured for ≥1 y were 0.1–9.1 kg CH4 ha−1 y−1, with a log‐normal distribution (log‐mean ≈ 1.6 kg CH4 ha−1 y−1). Conversion of natural soils to agriculture reduced oxidation rates by two‐thirds –‐ closely similar to results reported for other regions. N inputs also decreased oxidation rates. Full recovery of rates after these disturbances takes > 100 y. Soil bulk density, water content and gas diffusivity had major impacts on oxidation rates. Trends were similar to those derived from other published work. Increasing acidity reduced oxidation, partially but not wholly explained by poor diffusion through litter layers which did not themselves contribute to the oxidation. The effect of temperature was small, attributed to substrate limitation and low atmospheric concentration. Analysis of all available data for CH4 oxidation rates in situ showed similar log‐normal distributions to those obtained for our results, with generally little difference between different natural ecosystems, or between short‐and longer‐term studies. The overall global terrestrial sink was estimated at 29 Tg CH4 y−1, close to the current IPCC assessment, but with a much wider uncertainty range (7 to > 100 Tg CH4 y−1). Little or no information is available for many major ecosystems; these should receive high priority in future research.

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.

Soil moisture affects microbial decay of SOM and rhizosphere respiration (RR) in temperate forest soils, but isolating the response of soil respiration (SR) to summer drought and subsequent wetting is difficult because moisture changes are often confounded with temperature variation. We distinguished between temperature and moisture effects by simulation of prolonged soil droughts in a mixed deciduous forest at the Harvard Forest, Massachusetts. Roofs constructed over triplicate 5 Â 5 m 2 plots excluded throughfall water during the summers of 2001 (168 mm) and 2002 (344 mm), while adjacent control plots received ambient throughfall and the same natural temperature regime. In 2003, throughfall was not excluded to assess the response of SR under natural weather conditions after two prolonged summer droughts. Throughfall exclusion significantly decreased mean SR rate by 53 mg C m À2 h À1 over 84 days in 2001, and by 68 mg C m À2 h À1 over 126 days in 2002, representing 10-30% of annual SR in this forest and 35-75% of annual net ecosystem exchange (NEE) of C. The differences in SR were best explained by differences in gravimetric water content in the Oi horizon (r 2 5 0.69) and the Oe/Oa horizon (r 2 5 0.60). Volumetric water content of the A horizon was not significantly affected by throughfall exclusion. The radiocarbon signature of soil CO 2 efflux and of CO 2 respired during incubations of O horizon, A horizon and living roots allowed partitioning of SR into contributions from young C substrate (including RR) and from decomposition of older SOM. RR (root respiration and microbial respiration of young substrates in the rhizosphere) made up 43-71% of the total C respired in the control plots and 41-80% in the exclusion plots, and tended to increase with drought. An exception to this trend was an interesting increase in CO 2 efflux of radiocarbon-rich substrates during a period of abundant growth of mushrooms.Our results suggest that prolonged summer droughts decrease primarily heterotrophic respiration in the O horizon, which could cause increases in the storage of soil organic carbon in this forest. However, the C stored during two summers of simulated drought was only partly released as increased respiration during the following summer of natural throughfall. We do not know if this soil C sink during drought is transient or long lasting. In any case, differential decomposition of the O horizon caused by interannual variation of precipitation probably contributes significantly to observed interannual variation of NEE in temperate forests.