Non-rainfall Moisture: A Key Driver of Microbial Respiration from Standing Litter in Arid, Semiarid, and Mesic Grasslands

Photodegradation in terrestrial ecosystems

The first step in carbon (C) turnover, where senesced plant biomass is converted through various pathways into compounds that are released to the atmosphere or incorporated into the soil, is termed litter decomposition. This review is focused on recent advances of how solar radiation can affect this important process in terrestrial ecosystems. We explore the photochemical degradation of plant litter and its consequences for biotic decomposition and C cycling. The ubiquitous presence of lignin in plant tissues poses an important challenge for enzymatic litter decomposition due to its biological recalcitrance, creating a substantial bottleneck for decomposer organisms. The recognition that lignin is also photolabile and can be rapidly altered by natural doses of sunlight to increase access to cell wall carbohydrates and even bolster the activity of cell wall degrading enzymes highlights a novel role for lignin in modulating rates of litter decomposition. Lignin represents a key functional connector between photochemistry and biochemistry with important consequences for our understanding of how sunlight exposure may affect litter decomposition in a wide range of terrestrial ecosystems. A mechanistic understanding of how sunlight controls litter decomposition and C turnover can help inform management and other decisions related to mitigating human impact on the planet.

Contrasting responses to aridity by different-sized decomposers cause similar decomposition rates across a precipitation gradient

Litter decomposition is expected to be positively associated with precipitation despite evidence that decomposers of varying sizes have different moisture dependencies. We hypothesized that higher tolerance of macro-decomposers to aridity may counterbalance the effect of smaller decomposers, leading to similar decomposition rates across climatic gradients. We tested this hypothesis by placing plant litter baskets of different mesh sizes in seven sites along a sharp precipitation gradient, and by characterizing the macro-decomposer assemblages using pitfall trapping. We found that decomposers responded differently to precipitation levels based on their size. Microbial decomposition increased with precipitation in the winter while macro-decomposition peaked in arid sites during the summer. This led to similar overall decomposition rates across the gradient except in hyper-arid sites. Macro-decomposer richness, abundance, and biomass peaked in arid environments. Our findings highlight the importance of macro-decomposition in arid-lands, possibly resolving the dryland decomposition conundrum, and emphasizing the need to contemplate decomposer size when investigating zoogeochemical processes.

Climate dependence of the macrofaunal effect on litter decomposition-A global meta-regression analysis

Litter decomposition by microorganisms and animals is influenced by climate and has been found to be higher in warm and wet than in cold and dry biomes. We, however, hypothesized that the macrofaunal effect on decomposition should increase with temperature and aridity since larger animals are more tolerant to aridity than smaller organisms. This hypothesis was supported by our global analysis of macrofauna exclusion studies. Macrofauna increased litter mass loss on average by 40%, twofold higher than the highest previous estimation of macrofaunal effect on decomposition. The strongest effect was found in subtropical deserts where faunal decomposition had not been considered important. Our results highlight the need to consider animal size when exploring climate dependence of faunal decomposition, and the disproportionately large role of macrofauna in regulating litter decomposition in warm drylands. This new realization is critical for understanding element cycling in the face of global warming and aridification.

Precipitation modulates the net effect of solar radiation on litter decomposition and CO2 emission - a meta-analysis

Introduction

Solar radiation plays a crucial role in the decomposition of litter and the cycling of nutrients. Previous studies have investigated that the net effect of solar radiation on litter decomposition depends on the balance of its facilitative and inhibitory effects on microbial activity; however, a gap in understanding the mechanism by which precipitation affects the net effect of solar radiation and the mechanism of litter decomposition on a global scale was observed.

Methods

In addressing this gap, a comprehensive meta-analysis of 351 data points from 37 published studies was conducted to estimate the sole radiation effect and interactive effect of solar radiation and precipitation on a global scale, as well as how they vary at different precipitation levels. In addition, the importance of influential factors regulating the net effect of solar radiation on litter decomposition was assessed to identify the key drivers of the response of mass loss to solar radiation at different precipitation levels.

Results

Our findings indicated that solar radiation largely regulates litter decomposition, and the direction and magnitude are potentially dependent on the precipitation regime. In addition, solar radiation significantly increased mass loss and decreased the nutrient remaining. Furthermore, the effects of solar radiation on mass loss, C remaining, and N remaining were found to be similar among areas with precipitation levels below 200 and above 800 mm and greater than in areas with precipitation levels between 200-400 mm and 400-800 mm. The effect of solar radiation on CO₂ emissions varied from 13.97% when precipitation was below 200 mm to -0.707% when precipitation was between 200 and 400 mm.

Conclusion

Climatic factors determine the response ratio of mass loss to solar radiation in arid lands, whereas the initial litter characteristics have a great influence on the response of mass loss to solar radiation in ecosystems that are not moisture limited. The effect of precipitation on the photodegradation mechanism of litter was primarily achieved by influencing the decomposition of lignin, and the main effect of solar radiation on litter decomposition will shift from the positive effect of "photopriming" to the negative effect of "microbial inhibition" with the increase of precipitation. Our findings can provide a comprehensive understanding of litter decomposition patterns on a global scale, and our results showed that CO₂ emissions from photodegradation will be lessened by precipitation, which is important in predicting CO₂ emission and separating sources of CO₂ under future increasing precipitation scenarios, particularly in arid lands.

Dryland mechanisms could widely control ecosystem functioning in a drier and warmer world

Responses of terrestrial ecosystems to climate change have been explored in many regions worldwide. While continued drying and warming may alter process rates and deteriorate the state and performance of ecosystems, it could also lead to more fundamental changes in the mechanisms governing ecosystem functioning. Here we argue that climate change will induce unprecedented shifts in these mechanisms in historically wetter climatic zones, towards mechanisms currently prevalent in dry regions, which we refer to as 'dryland mechanisms'. We discuss 12 dryland mechanisms affecting multiple processes of ecosystem functioning, including vegetation development, water flow, energy budget, carbon and nutrient cycling, plant production and organic matter decomposition. We then examine mostly rare examples of the operation of these mechanisms in non-dryland regions where they have been considered irrelevant at present. Current and future climate trends could force microclimatic conditions across thresholds and lead to the emergence of dryland mechanisms and their increasing control over ecosystem functioning in many biomes on Earth.

Effects of radiation with diverse spectral wavelengths on photodegradation during green waste composting

Composting is currently the best way to dispose of green waste (GW), which contains lignocellulose and other refractory substances that can prolong composting time. Although the natural degradation of litter involves photodegradation, few studies have considered the effects of photodegradation on GW composting. The current research investigated the influence of radiation with different spectral wavelengths (light-transmitting films were used to filter sunlight) on composting efficiency. Among six treatments that differed in the spectral wavelength of radiation, a no-UV-A treatment (the radiation between 320 nm and 380 nm was blocked by light-transmitting film) produced the best-quality compost product in only 34 days. Compared to the control (the full spectrum of light), the no-UV-A treatment increased total porosity, humus coefficient, optimal particle-size, and germination index by 10%, 2%, 3%, and 9%, respectively; increased available phosphorus, available potassium, and nitrate nitrogen by 21%, 17%, and 21%, respectively; decreased electrical conductivity, residual organic matter, and ammonium nitrogen by 9%, 13%, and 14%, respectively; and increased dehydrogenase, cellulase, and laccase activity by 76%, 66%, and 23%, respectively. These results indicated that the no-UV-A treatment resulted in the most complete degradation of lignocelluloses, the best nutrient properties, and the highest level of microbial activity in the GW compost. In addition, the bulk density, water-holding capacity, total porosity, void ratio, particle-size distribution, and coarseness index of the compost product were the closest to ideal ranges with the no-UV-A treatment and indicated that the no-UV-A compost product had the best granular structure in support of aeration, water drainage, and water retention. In a phytotoxicity assay, the compost produced by the no-UV-A treatment had the highest root length, seed germination rate, and germination index, indicating that the compost product was non-phytotoxic, mature, and suitable for use in agriculture and forestry.

Non-rainfall water inputs: A key water source for biocrust carbon fixation

Links between water and carbon (C) cycles in drylands are strongly regulated by biocrusts. These widespread communities in the intershrub spaces of drylands are able to use non-rainfall water inputs (NRWI) (fog, dewfall and water vapour) to become active and fix carbon dioxide (CO₂), converting biocrusts into the main soil C contributors during periods in which vegetation remains inactive. In this study, we first evaluated the influence of biocrust type on NRWI uptake using automated microlysimeters, and second, we performed an outdoor experiment to examine how NRWI affected C exchange (photosynthesis and respiration) in biocrusts. NRWI uptake increased from incipient cyanobacteria to well-developed cyanobacteria and lichen biocrusts. NRWI triggered biocrust activity but with contrasting effects on CO₂ fluxes depending on the main NRWI source. Fog mainly stimulated respiration of biocrust-covered soils, reaching net CO₂ emissions of 0.68 μmol m^-2 s^-1, while dew had a greater effect stimulating biocrust photosynthesis and resulted in net CO₂ uptake of 0.66 μmol m^-2 s^-1. These findings demonstrate the key role that NRWI play in biocrust activity and the soil C balance in drylands.

Long-Term Population Dynamics of Namib Desert Tenebrionid Beetles Reveal Complex Relationships to Pulse-Reserve Conditions

Noy-Meir's paradigm concerning desert populations being predictably tied to unpredictable productivity pulses was tested by examining abundance trends of 26 species of flightless detritivorous tenebrionid beetles (Coleoptera, Tenebrionidae) in the hyper-arid Namib Desert (MAP = 25 mm). Over 45 years, tenebrionids were continuously pitfall trapped on a gravel plain. Species were categorised according to how their populations increased after 22 effective rainfall events (>11 mm in a week), and declined with decreasing detritus reserves (97.7-0.2 g m^-2), while sustained by nonrainfall moisture. Six patterns of population variation were recognised: (a) increases triggered by effective summer rainfalls, tracking detritus over time (five species, 41% abundance); (b) irrupting upon summer rainfalls, crashing a year later (three, 18%); (c) increasing gradually after series of heavy (>40 mm) rainfall years, declining over the next decade (eight, 15%); (d) triggered by winter rainfall, population fluctuating moderately (two, 20%); (e) increasing during dry years, declining during wet (one, 0.4%); (f) erratic range expansions following heavy rain (seven, 5%). All species experienced population bottlenecks during a decade of scant reserves, followed by the community cycling back to its earlier composition after 30 years. By responding selectively to alternative configurations of resources, Namib tenebrionids showed temporal patterns and magnitudes of population fluctuation more diverse than predicted by Noy-Meir's original model, underpinning high species diversity.

Arthropods as the Engine of Nutrient Cycling in Arid Ecosystems

Simple Summary

Nutrient cycling in terrestrial ecosystems requires moisture. Deserts are characterized by low precipitation and extreme surface temperature that limit biological activity. Attempts to resolve how nutrients recycle despite these constraints were focused primarily on abiotic factors. We suggest that macro-arthropods may play a disproportionally large role in resolving this conundrum. Macro-arthropods are prevalent in deserts and many of them are burrowing detritivores that can remain active during long dry periods. Desert arthropods process and transport plant litter nutrients belowground, where the conditions are favorable for decomposer activity. Consequently, arthropods may accelerate the recycling rate of plant litter nutrients to inorganic nutrients that become available for plant use. This generates a vertical nutrient recycling loop (VRL) that may also assist in explaining how desert plants receive nutrients when the shallow soil is dry. Macro-arthropods may also regulate the spatiotemporal distribution of nutrients by transporting them between patches and across ecosystems. Burrowing activity that alters the desert microtopography and reduces soil salinity may contribute further to creating hotspots of productivity and biological diversity in the otherwise poor desert environment. We conclude that macro-arthropods may play a key role in explaining how desert ecosystems function, and that better understanding of this unique role may assist combating desertification and restoring degraded arid lands.

Abstract

Nutrient dynamics in most terrestrial ecosystems are regulated by moisture-dependent processes. In drylands, nutrient dynamics are often weakly associated with annual precipitation, suggesting that other factors are involved. In recent years, the majority of research on this topic focused on abiotic factors. We provide an arthropod-centric framework that aims to refocus research attention back on the fundamental role that macro-arthropods may play in regulating dryland nutrient dynamics. Macro-arthropods are prevalent in drylands and include many detritivores and burrowing taxa that remain active during long dry periods. Macro-arthropods consume and process large quantities of plant detritus and transport these nutrients to the decomposer haven within their climatically buffered and nutritionally enriched burrows. Consequently, arthropods may accelerate mineralization rates and generate a vertical nutrient recycling loop (VRL) that may assist in explaining the dryland decomposition conundrum, and how desert plants receive their nutrients when the shallow soil is dry. The burrowing activity of arthropods and the transportation of subterranean soil to the surface may alter the desert microtopography and promote desalinization, reducing resource leakage and enhancing productivity and species diversity. We conclude that these fundamental roles and the arthropods’ contribution to nutrient transportation and nitrogen fixation makes them key regulators of nutrient dynamics in drylands.

Why Plants Harbor Complex Endophytic Fungal Communities: Insights From Perennial Bunchgrass Stipagrostis sabulicola in the Namib Sand Sea

All perennial plants harbor diverse endophytic fungal communities, but why they tolerate these complex asymptomatic symbioses is unknown. Using a multi-pronged approach, we conclusively found that a dryland grass supports endophyte communities comprised predominantly of latent saprophytes that can enhance localized nutrient recycling after senescence. A perennial bunchgrass, Stipagrostis sabulicola, which persists along a gradient of extreme abiotic stress in the hyper-arid Namib Sand Sea, was the focal point of our study. Living tillers yielded 20 fungal endophyte taxa, 80% of which decomposed host litter during a 28-day laboratory decomposition assay. During a 6-month field experiment, tillers with endophytes decomposed twice as fast as sterilized tillers, consistent with the laboratory assay. Furthermore, profiling the community active during decomposition using next-generation sequencing revealed that 59-70% of the S. sabulicola endophyte community is comprised of latent saprophytes, and these dual-niche fungi still constitute a large proportion (58-62%) of the litter community more than a year after senescence. This study provides multiple lines of evidence that the fungal communities that initiate decomposition of standing litter develop in living plants, thus providing a plausible explanation for why plants harbor complex endophyte communities. Using frequent overnight non-rainfall moisture events (fog, dew, high humidity), these latent saprophytes can initiate decomposition of standing litter immediately after tiller senescence, thus maximizing the likelihood that plant-bound nutrients are recycled in situ and contribute to the nutrient island effect that is prevalent in drylands.

Fungal Communities on Standing Litter Are Structured by Moisture Type and Constrain Decomposition in a Hyper-Arid Grassland

Non-rainfall moisture (fog, dew, and water vapor; NRM) is an important driver of plant litter decomposition in grasslands, where it can contribute significantly to terrestrial carbon cycling. However, we still do not know whether microbial decomposers respond differently to NRM and rain, nor whether this response affects litter decomposition rates. To determine how local moisture regimes influence decomposer communities and their function, we examined fungal communities on standing grass litter at an NRM-dominated site and a rain-dominated site 75 km apart in the hyper-arid Namib Desert using a reciprocal transplant design. Dominant taxa at both sites consisted of both extremophilic and cosmopolitan species. Fungal communities differed between the two moisture regimes with environment having a considerably stronger effect on community composition than did stage of decomposition. Community composition was influenced by the availability of air-derived spores at each site and by specialization of fungi to their home environment; specifically, fungi from the cooler, moister NRM Site performed worse (measured as fungal biomass and litter mass loss) when moved to the warmer, drier rain-dominated site while Rain Site fungi performed equally well in both environments. Our results contribute to growing literature demonstrating that as climate change alters the frequency, magnitude and type of moisture events in arid ecosystems, litter decomposition rates may be altered and constrained by the composition of existing decomposer communities.

Functional importance and diversity of fungi during standing grass litter decomposition

Although microbial participation in litter decomposition is widely known within terrestrial soils, the role and significance of microorganisms during the aerial standing litter phase of decomposition remains poorly investigated. We examined the fungi inhabiting standing leaf litter of Schizachyrium scoparium and Schizachyrium tenerum in a Longleaf Pine savanna ecosystem and estimated their contribution to litter decomposition. We identified fungal phylotypes associated with leaf litter and quantified leaf C mass loss, fungal biomass production, and microbial respiration during decomposition. These data were used to construct budgets estimating C flow into and through fungi. Significant losses in S. scoparium (55%) and S. tenerum (67%) leaf C mass were observed during standing decomposition along with concomitant increases in fungal biomass, which reached a maximum of 36 and 33 mgC/g detrital C, respectively. Cumulative fungal production during decomposition totaled 99 ± 6 mgC/g initial detrital C in S. scoparium and 73 ± 5 mgC/g initial detrital C in S. tenerum, indicating that 18 and 11% of the litter C was converted into fungal biomass, respectively. Corresponding estimates of cumulative fungal respiration totaled 106 ± 7 and 174 ± 11 mgC/g initial detrital C in S. scoparium and S. tenerum, respectively. Next generation sequencing identified several fungal phylotypes, with the majority of sequences belonging to the Ascomycota (Dothideomycetes) and Basidiomycota (Agaricomycetes). Fungal phylotypes were similar between litter species and changed over time, showing a successional pattern. These findings extend our understanding of fungal processes to standing litter in terrestrial ecosystems, and highlight the quantitative importance of fungi in C cycling processes.

Quantifying microbial growth and carbon use efficiency in dry soil environments via 18 O water vapor equilibration

Soil microbial physiology controls large fluxes of C to the atmosphere, thus, improving our ability to accurately quantify microbial physiology in soil is essential. However, current methods to determine microbial C metabolism require liquid water addition, which makes it practically impossible to measure microbial physiology in dry soil samples without stimulating microbial growth and respiration (namely, the "Birch effect"). We developed a new method based on in vivo ¹⁸ O-water vapor equilibration to minimize soil rewetting effects. This method allows the isotopic labeling of soil water without direct liquid water addition. This was compared to the main current method (direct ¹⁸ O-liquid water addition) in moist and air-dry soils. We determined the time kinetics and calculated the average ¹⁸ O enrichment of soil water over incubation time, which is necessary to calculate microbial growth from ¹⁸ O incorporation in genomic DNA. We tested isotopic equilibration patterns in three natural and six artificially constructed soils covering a wide range of soil texture and soil organic matter content. We then measured microbial growth, respiration and carbon use efficiency (CUE) in three natural soils (either air-dry or moist). The proposed ¹⁸ O-vapor equilibration method provided similar results as the current method of liquid ¹⁸ O-water addition when used for moist soils. However, when applied to air-dry soils the liquid ¹⁸ O-water addition method overestimated growth by up to 250%, respiration by up to 500%, and underestimated CUE by up to 40%. We finally describe the new insights into biogeochemical cycling of C that the new method can help uncover, and we consider a range of questions regarding microbial physiology and its response to global change that can now be addressed.