Reappraisal of drying and wetting effects on C and N mineralization and fluxes in soils

Abstract

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.

Impacts of drying–wetting cycles on rhizosphere respiration and soil organic matter decomposition

“…Surface soils often undergo gradual drying by evapotranspiration followed by rapid wetting as a result of precipitation or irrigation. These dryingewetting cycles can influence soil aggregation (Denef et al, 2001;Cosentino et al, 2006), microbial activity and community structure (Gordon et al, 2008;Tiemann and Billings, 2011;Evans and Wallenstein, 2012), and C and N mineralization (Birch, 1958;Fierer and Schimel, 2003;Borken and Matzner, 2009). In the coming decades, many soils will likely be subjected to more frequent and intense dryingewetting cycles (Huntington, 2006), which can impact SOM decomposition and its feedback to climate change.…”

“…Following the early work of Birch (1958), many lab incubations of root-free soils (e.g. Fierer and Schimel, 2003;Xiang et al, 2008;Guo et al, 2012) have been used to investigate the impact of dryingewetting on microbial stresse response physiology (Schimel et al, 2007) and microbial decomposition of SOM (Borken and Matzner, 2009). However, soil CO 2 efflux in natural ecosystems consists of two componentsdmicrobial decomposition of native SOM, and rhizosphere respiration by roots and associated microbes using root-derived substrates which is omitted in previous root-free soil incubation studies.…”

“…We partitioned cumulative soil respiration during an entire dryingewetting cycle into rhizosphere respiration and SOM decomposition using a continuous 13 C-labeling technique (Cheng and Dijkstra, 2007), estimated cumulative net N mineralization during the whole experimental period using an N-budgeting method (Cheng, 2009), and calculated rhizosphere priming effect as the difference in SOM decomposition between the planted treatment and the unplanted treatment (Kuzyakov, 2002;Cheng and Kuzyakov, 2005). Given that the intensity of drying can affect cumulative C and N mineralization in root-free soils (Borken and Matzner, 2009;Unger et al, 2010), we used two plant species with different transpiration rates to simulate two dryingewetting regimes. Sunflower (Helianthus annuus L.) has relatively high transpiration rate and causes severe soil drying, while soybean (Glycine max L.…”

Diffusion based modelling of temperature and moisture interactive effects on carbon fluxes of mineral soils

“…Some of the most relevant observations these models have failed to reproduce include: changes (typically a dampening) of temperature sensitivities of decomposition over time (Hamdi et al, 2013), non-linear responses to soil moisture content 10 (Borken and Matzner, 2009), and changes in decomposition rates in response to variations in concentrations of organic matter (Blagodatskaya and Kuzyakov, 2008). Such model shortcomings, which reflect missing or wrongly simulated processes, create a difficult to quantify uncertainty in global long term predictions of soil C and its feedback to climate change.…”

A study of soil-dynamics based on a simulated drought in an alpine meadow on the Tibetan Plateau

“…For example, soil water loss in summer can reduce soil CO 2 efflux and change its seasonality (Joos et al 2010). The increasing droughts in summer can also reduce the fluxes and mineralization of nitrogen (Borken and Matzner 2009). The drought as shown in a simulated case in summer does not only delay microbial N mineralization but also P mineralization (van Meeteren et al 2008).…”