Snail Mucus Increases the CO2 Efflux of Biological Soil Crusts

Geoderma, 2017

In Mediterranean ecosystems, the soil biological crust (hereafter biocrust) plays a crucial role in maintaining ecosystem functioning. In these ecosystems, soil water content can often be a stronger driver of soil CO 2 efflux than soil temperature, or at least comparable. However, little is known on the contribution of the biocrust to soil CO 2 efflux or how the respiration of the biocrust responds to soil water content and temperature. A manipulative experiment was performed in a Mediterranean shrubland ecosystem in Sardinia (Italy) to assess the contribution of the biocrust to soil CO 2 efflux and to identify the main environmental drivers of the CO 2 efflux. For 19 months, in situ soil CO 2 efflux was measured over two different surfaces: soil deprived of biocrust (hereafter Soil) and intact soil (hereafter Soil + BC), and estimated by subtraction in a third surface: biocrust (hereafter BC). CO 2 efflux emitted by Soil, BC and Soil + BC was uniquely driven by soil moisture and temperature: BC respiration was mainly controlled by soil moisture at 5 cm depth, whereas both soil temperature and water content at 20 cm depth determined Soil CO 2 efflux. Soil temperature and water content at 5 cm depth drove Soil + BC respiration. We also found that biocrusts can contribute substantially (up to 60%) to the total soil respiration depending on its moisture content. This contribution persists even in periods in which deeper soil layers are inactive, as small water pulses can activate the metabolism of carbon in soils through lichens, mosses and cyanobacteria associated with the biocrust, while deeper soil layers remain dormant. The important differences observed in CO 2 efflux between Soil and Soil + BC suggest that carbon models and budgets may underestimate soil CO 2 efflux in spatially heterogeneous Mediterranean areas. Our results highlight the importance of accounting for the biocrust contribution to soil respiration and its response to environmental drivers. We provide an accurate estimation of this key component of the carbon cycle at the ecosystem level in water limited ecosystems.

Geoderma, 2022

Soil biocrusts, formed from communities of microbes and their extracellular products are a common feature of dryland soil surfaces. Biocrust organisms are only intermittently metabolically active, but due to their ubiquity they make a significant contribution to the carbon cycle. Quantification of the controls and insights into the interlinked process of photosynthesis and respiration are essential to enhancing our understanding of the carbon cycle in the world's drylands. Yet, there have been relatively few field studies investigating controls on both biocrust photosynthesis and respiration. We undertook field-based experiments at two dune sites during the dry season in Diamantina National Park in Queensland, Australia to determine how biocrust hydration and illumination affect soil CO 2 flux and photosynthesis. Static chambers and an infra-red gas analyser were used to quantify soil CO 2 flux, and a fluorometer and a CFImager were used to determine a range of photosynthetic parameters in the field and laboratory respectively. When dry, biocrust photosynthetic activity was not detected and soil CO 2 flux was very low irrespective of biocrust cover. Hydration led to a large and immediate increase in CO 2 flux, which was more pronounced in the presence of biocrusts and on the dune with thinner biocrusts. Hydration also initiated the onset of photosynthesis in some biocrusts, which was greatest under low light conditions and sustained with further hydration. There were only infrequent periods of net CO 2 uptake to the soil, occurring when CO 2 uptake due to photosynthetic activity was less than background soil CO 2 flux. Chlorophyll fluorescence imaging indicated biocrust spatial heterogeneity was evident at the cm scale where microtopography creates a myriad of environments for different crust organisms. Our findings demonstrate that biocrusts are highly spatially heterogenetic at both landscape and small scale, which suggests the maintenance of biocrust spatial diversity is likely to be key to imparting resilience to changing climate and disturbance. As well as reaffirming the importance of biocrusts for the carbon cycle in dryland dune soils the study demonstrates that biocrust respiration and photosynthesis respond differently to hydration and shading. This adds an unpredictability to the distribution of soil carbon stocks and the gaseous exchanges of CO 2 between the surface and atmosphere. Future changes to precipitation and increased temperatures are likely to reduce soil moisture across much of the Australian interior and consequently biocrusts may experience a decline in biomass, structure, and function which could have significant repercussions beyond carbon stocks.

Soil Biology and Biochemistry, 2017

Increasing N inputs and changing rainfall regimes will lead to drastic changes in multiple ecosystem functions such as nutrient cycling, organic matter decomposition and gas exchange in dryland ecosystems. As fundamental components of drylands, biological soil crusts (biocrusts) play important roles in the regulation of responses of multiple ecosystem functions to global environmental changes. Biocrusts are home to highly functional microbial communities; however little is known on the role of microbial communities associated with different biocrust species in regulating the response of multiple ecosystem functions to global change. Here, we conducted a microcosm experiment to evaluate the roles of biocrust-forming lichens (Diploschistes thunbergianus, Psora crystallifera and Xanthoparmelia reptans) in mediating the effects of simulated changes in rainfall frequency and nitrogen (N) addition on soil multifunctionality involving nutrient availability, greenhouse gas flux and enzyme activities. The three biocrust species supported different levels of soil bacterial diversity, and specific community composition as revealed by MiSeq sequencing. Biocrust species always promoted multiple functions related to carbon, nitrogen and phosphorus cycling compared to bare ground, with X. reptans having the highest effect on multifunctionality. Most importantly, the relative abundance of specific microbial communities associated with different lichen species modulates the response of multifunctionality to impacts of water frequency (negative) and N addition (positive). Our results suggest that biocrust species could regulate global change impacts on soil multifunctionality in drylands, although the strength and direction vary among the biocrust species. These findings highlight the importance of preserving biocrusts as hotspots of microbial genetic resources and ecosystem functioning in drylands.

Functional Ecology, 2015

Applied Soil Ecology, 2020

It has been suggested that, in addition to consuming litter and releasing C via respiration, soil fauna may also indirectly increase overall C loss from soil by causing the priming effect. This is based on the assumption that fauna increases the amount of liable C rather than N in litter leachate. An increase in the labile C would increase soil respiration via the priming effect, while an increase in N in leachates would presumably have the opposite effect (negative priming, decrease of respiration). Here we used a microcosm experiment to study the effects of the presence or absence of two species of soil fauna (the collembolan Folsomia candida and the enchytraeid Enchytraeus crypticus) used alone or in combination, on: 1) the litter respiration of five plants (Alnus glutinosa, Calamagrostis epigejos, Quercus robur, Salix caprea, and Picea omorika), 2) the loss of C and N by leaching from the same litter during a one-year laboratory experiment, and 3) the effect of these leachates on microbial respiration of the soil underlying the litter layer. Litter-based respiration in microcosms was unaffected by soil fauna. The fauna did not affect the leaching of C, but significantly increased the leaching of N. These effects were strongest in the early stages of decomposition. Leachates collected from fauna and no fauna treatment were then applied to the soil underlying the litter. The soil supplied by leachate from fauna treatments showed significantly lower respiration than the soil supplied by no fauna treatments leachates. This suggests that the fauna effect on litter leachate may reduce soil respiration.

Ecosystems, 2011

Biological soil crusts (BSCs) are a key biotic component of dryland ecosystems worldwide. However, most studies carried out to date on carbon (C) fluxes in these ecosystems, such as soil respiration, have neglected them. We conducted a 3.5-year field experiment to evaluate the spatio-temporal heterogeneity of soil respiration in a semiarid Stipa tenacissima steppe and to assess the contribution of BSC-dominated areas to the annual soil respiration of the whole ecosystem. We selected the six most frequent microsites in the study area: Stipa tussocks (ST), Retama sphaerocarpa shrubs (RS), and open areas with very low (<5% BSC cover, BS), low, medium and high cover of well-developed BSCs. Soil respiration rates did not differ among BSCdominated microsites but were significantly higher and lower than those found in BS and ST microsites, respectively. A model using soil temperature and soil moisture accounted for over 85% of the temporal variation in soil respiration throughout the studied period. Using this model, we estimated a range of 240.4-322.6 g C m -2 y -1 released by soil respiration at our study area. Vegetated (ST and RS) and BSC-dominated microsites accounted for 37 and 42% of this amount, respectively. Our results indicate that accounting for the spatial heterogeneity in soil respiration induced by BSCs is crucial to provide accurate estimations of this flux at the ecosystem level. They also highlight that BSC-dominated areas are the main contributor to the total C released by soil respiration and, therefore, must be considered when estimating C budgets in drylands.

2016

49 Soil organic matter (SOM) is key to maintaining soil fertility, mitigating climate change, 50 combatting land degradation, and conserving aboveand below-ground biodiversity and 51 associated soil processes and ecosystem services. In order to derive management options for 52 maintaining these essential services provided by soils, policy makers depend on robust, predictive 53 models identifying key drivers of SOM dynamics. Existing SOM models and suggested 54 guidelines for future SOM modelling are defined mostly in terms of plant residue quality and 55 input and microbial decomposition, overlooking the significant regulation provided by soil fauna. 56 The contribution of soil fauna activities to SOM decomposition can be as high as 40%, as they 57 control almost any aspect of organic matter turnover, foremost by regulating the activity and 58 functional composition of soil microorganisms and their physico-chemical connectivity with soil 59 organic matter. We suggest that inclusion ...

Plant and Soil, 2013

Aims Human activities are causing imbalances in the nutrient cycles in natural ecosystems. However, we have limited knowledge of how these changes will affect the soil microbial functional diversity and the nitrogen (N) cycle in drylands, the biggest biome on Earth. Communities dominated by lichens, mosses and cyanobacteria (biocrusts) influence multiple processes from the N cycle such as N fixation and mineralization rates. We evaluated how biocrusts modulate the effects of different N, carbon (C) and phosphorus (P) additions on theN availability, the dominance of different available N forms and the microbial functional diversity in dryland soils. Methods Soil samples from bare ground (BG) and biocrust-dominated areas were gathered from the center of Spain and incubated during seven or 21 days under different combinations of N, C and P additions (N, C, P, N+C, N+P, P+C, and C+N+P). Results The relative dominance of dissolved organic N (DON) and the microbial functional diversity were higher in biocrust than in BG microsites when C or P were added. Changes in the C to N ratio, more than N availability, seem to modulate N transformation processes in the soils studied. In general, biocrusts increased the resilience to N impacts (N, C+N, N+P, C+N+P) of the total available N, ammonium, nitrate and DON when C was present. Conclusions Our results suggest that biocrusts may buffer the effects of changes in nutrient ratios on microbial functional diversity and DON dominance in dryland soils. Thus, these organisms may have an important role in increasing the resilience of the N cycle to imbalances in C, N and P derived from human activities.

Plant and Soil, 2000

We have reviewed the responses of soil fauna to increased concentrations of atmospheric CO2 and the consequent climate change. These will affect several attributes of animal populations and communities including their density, biomass, diversity, activity, rates of consumption, life history parameters and migration ability. Changes in the quality and quantity of litter and global warming are the main factors which are expected to modify soil fauna. Although changes have been observed in several attributes of the soil fauna as a consequence of increased concentrations of atmospheric CO2, no general trend which might allow to the prediction of a general pattern of response has been identified. Because of the complexity of the biological mechanisms and the synergetic action of several factors, the few resulting responses reported in the literature are inconclusive. However, some aspects of the situation deserve more attention. These include the consequences of (1) changes in the food resources for soil fauna in the litter layer and in the rhizosphere, (2) the consumption of low quality litter by the macrofauna, (3) the change in life span in response to temperature elevation, (4) the enhancement of earthworm burrowing activity and (5) the changes in community composition arising because of specific differential resistance to adverse conditions.

Biogeochemistry, 2000

Many forest soils in the Mediterranean basin areshallow and contain high amounts of gravel in theorganic layers. Recent studies on soil organic matteraccumulation have shown high amounts of organic matteroccurring mainly in soils with high levels ofstoniness at the soil surface. The gravel layer mayaffect the microclimatic conditions of the soilsurface and probably the distribution and activity ofsoil fauna. In order to quantify the combined effects soil fauna(epigeic macrofauna and earthworms) and stoniness onthe release of soil CO2, we performed a threefactor field experiment by using a series ofreconstructed soil profiles. Factors 1 and 2 consistedof the exclusion/presence of soil epigeic macrofaunaand earthworms, and factor 3 of the presence/absenceof a gravel layer intermingled with the H horizon. Weincubated 14C straw in the H horizon and carriedout three 40 mm rainfall simulations. Soil respiration primarily depended on the season. Theeffects of soil fauna were generally small and did notcoincide with periods of high faunal activity. Thelargest effects of both earthworms and soil epigeicfauna were found after wetting the soil in summer. Theeffects of the earthworms were concentrated in themineral soil while the effects of the epigeic faunawere concentrated in the H horizon and mainly arosetowards the end of the experiment. This suggests thatthe effects of epigeic fauna may have beenunderestimated due to the length of the experiment.The gravel layer increased the effect of faunaprobably by creating more favorable microclimaticconditions. The accumulation of organic matter insoils with high levels of stoniness cannot beexplained by the effect of gravel on soil microclimatenor by its effect on the activity of soil fauna.