Soil organic matter priming: The pH effects.

Priming of soil organic matter (SOM) decomposition by microorganisms is a key phenomenon of global carbon (C) cycling. Soil pH is a main factor defining priming effects (PEs) because it (i) controls microbial community composition and activities, including enzyme activities, (ii) defines SOM stabilization and destabilization mechanisms, and (iii) regulates intensities of many biogeochemical processes. In this critical review, we focus on prerequisites and mechanisms of PE depending on pH and assess the global change consequences for PE. The highest PEs were common in soils with pH between 5.5 and 7.5, whereas low molecular weight organic compounds triggered PE mainly in slightly acidic soils. Positive PEs up to 20 times of SOM decomposition before C input were common at pH around 6.5. Negative PEs were common at soil pH below 4.5 or above 7 reflecting a suboptimal environment for microorganisms and specific SOM stabilization mechanisms at low and high pH. Short-term soil acidification (in rhizosphere, after fertilizer application) affects PE by: mineral-SOM complexation, SOM oxidation by iron reduction, enzymatic depolymerization, and pH-dependent changes in nutrient availability. Biological processes of microbial metabolism shift over the short-term, whereas long-term microbial community adaptations to slow acidification are common. The nitrogen fertilization induced soil acidification and land use intensification strongly decrease pH and thus boost the PE. Concluding, soil pH is one of the strongest but up to now disregarded factors of PE, defining SOM decomposition through short-term metabolic adaptation of microbial groups and long-term shift of microbial communities.

The effects of litter input and increased precipitation on soil microbial communities in a temperate grassland.

Global warming has contributed to shifts in precipitation patterns and increased plant productivity, resulting in a significant increase in litter input into the soils. The enhanced litter input, combined with higher levels of precipitation, may potentially affect soil microbial communities. This study aims to investigate the effects of litter input and increased precipitation on soil microbial biomass, community structure, and diversity in a temperate meadow steppe in northeastern China. Different levels of litter input (0%, +30%, +60%) and increased precipitation (0%, +15%, +30%) were applied over a three-year period (2015-2017). The results showed that litter input significantly increased the biomass of bacteria and fungi without altering their diversity, as well as the ratio of bacterial to fungal biomass. Increased precipitation did not have a notable effect on the biomass and diversity of bacteria and fungi, but it did increase the fungal-to-bacterial biomass ratio. However, when litter input and increased precipitation interacted, bacterial diversity significantly increased while the fungal-to-bacterial biomass ratio remained unchanged. These findings indicate that the projected increases in litter and precipitation would have a substantial impact on soil microbial communities. In energy-and water-limited temperate grasslands, the additional litter inputs and increased precipitation contribute to enhanced nutrient and water availability, which in turn promotes microbial growth and leads to shifts in community structure and diversity.

Short-term responses of soil enzyme activities and stoichiometry to litter input in Castanopsis carlesii and Cunninghamia lanceolata plantations.

Litter input triggers the secretion of soil extracellular enzymes and facilitates the release of carbon (C), nitrogen (N), and phosphorus (P) from decomposing litter. However, how soil extracellular enzyme activities were controlled by litter input with various substrates is not fully understood. We examined the activities and stoichiometry of five enzymes including ß-1,4-glucosidase, ß-D-cellobiosidase, ß-1,4-N-acetyl-glucosaminidase, leucine aminopeptidase and acidic phosphatase (AP) with and without litter input in 10-year-old Castanopsis carlesii and Cunninghamia lanceolata plantations monthly during April to August, in October, and in December 2021 by using an in situ microcosm experiment. The results showed that: 1) There was no significant effect of short-term litter input on soil enzyme activity, stoichiometry, and vector properties in C. carlesii plantation. In contrast, short-term litter input significantly increased the AP activity by 1.7% in May and decreased the enzymatic C/N ratio by 3.8% in August, and decreased enzymatic C/P and N/P ratios by 11.7% and 10.3%, respectively, in October in C. lanceolata plantation. Meanwhile, litter input increased the soil enzymatic vector angle to 53.8° in October in C. lanceolata plantations, suggesting a significant P limitation for soil microorganisms. 2) Results from partial least squares regression analyses showed that soil dissolved organic matter and microbial biomass C and N were the primary factors in explaining the responses of soil enzymatic activity to short-term litter input in both plantations. Overall, input of low-quality (high C/N) litter stimulates the secretion of soil extracellular enzymes and accelerates litter decomposition. There is a P limitation for soil microorganisms in the study area.

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Litter decomposition is one of the most important ecosystem processes, which plays a critical role in regu-lating nutrient cycling and energy flow in terrestrial ecosystems. The influence of litter inputs on soil microbial community is helpful for understanding the relationship between soil microbial diversity and terrestrial ecosystem function. We conducted a meta-analysis to examine how litter inputs affect soil microbial activity (fungi, bacteria, actinomycetes) and microbial biomass carbon, nitrogen in China. The results showed that compared with non-litter input, soil microbial biomass carbon and nitrogen were significantly increased by 3.9% and 4.4% respectively after litter inputs. Soil fungal PLFA, bacterial PLFA, and total microbial PLFA were increased by 4.0%, 3.1% and 2.4%, respectively. The effects of litter inputs differed significantly with climatic region, annual precipitation, vege-tation type, and soil pH. Under different climate conditions, the responses of soil microbe showed the trend of subtropical monsoon climatic region > temperate monsoon climatic region > temperate continental climatic region, which increased first and then decreased with increasing annual precipitation. Under different vegetation types, the responses of soil microbes showed the trend of broad-leaved forest > grassland ≈ mixed forest > coniferous forest.

Effects of aboveground and belowground litter inputs on the balance of soil new and old organic carbon under the typical forests in subtropical region.

Litter is one of the most important factors controlling the accumulation, stabilization, and turnover of soil organic carbon (SOC) in forests. There is a knowledge gap of the impacts of aboveground and belowground litter inputs on the balance of new and old SOC under different forests in subtropical region. We examined the effects of aboveground and belowground litter inputs on SOC turnover using isotopic tracing technique, based on a 3-year C3 plants/C4 soil replacement experiment in natural forest (NF), Masson pine (Pinus massoniana) plantation (PM) and Chinese fir (Cunninghamia lanceolata) plantation (CL). Our results showed that forest types, litter treatments, and sampling time significantly affected SOC contents, δ13C, new and old SOC contents. Moreover, there were significant interactions between forest types and litter treatments. Litter input increased SOC content and net SOC increment, with higher sensitivity of NF than CL. Litter inputs decreased soil δ13C, with lower values in NF and PM compared to CL. For PM, the new SOC content in belowground litter treatment was significantly higher than that in aboveground litter treatment. The contents of old SOC were lower in belowground litter treatment than aboveground litter treatment in the NF and CL. Above- and below-ground biomass were positively correlated with SOC content and net increment. Belowground litter biomass were positively correlated with soil C/N ratio and new SOC content. Our results implied that belowground litter input had stronger effects on SOC turnover compared to aboveground litter input, with the effects varying among different forests. Our results provided new information on SOC accumulation and on sustainable management of the typical forests in subtropical region.

Fine roots are the dominant source of recalcitrant plant litter in sugar maple-dominated northern hardwood forests.

Most studies of forest litter dynamics examine the biochemical characteristics and decomposition of leaf litter, but fine roots are also a large source of litter in forests. We quantified the concentrations of eight biochemical fractions and nitrogen (N) in leaf litter and fine roots at four sugar maple (Acer saccharum)-dominated hardwood forests in the north-central United States. We combined these results with litter production data to estimate ecosystem biochemical fluxes to soil. We also compared how leaf litter and fine root biochemistry responded to long-term simulated N deposition. Compared with leaf litter, fine roots contained 2.9-fold higher acid-insoluble fraction (AIF) and 2.3-fold more condensed tannins; both are relatively difficult to decompose. Comparatively, leaf litter had greater quantities of more labile components: nonstructural carbohydrates, cellulose and soluble phenolics. At an ecosystem scale, fine roots contributed over two-thirds of the fluxes of AIF and condensed tannins to soil. Fine root biochemistry was also less responsive than leaf litter to long-term simulated N deposition. Fine roots were the dominant source of difficult-to-decompose plant carbon fractions entering the soil at our four study sites. Based on our synthesis of the literature, this pattern appears to be widespread in boreal and temperate forests.

Litter production and aquatic macroinvertebrates on a headwater subtropical stream in Rio Grande do Sul State, Brazil - DOI: 10.4025/actascibiolsci.v31i3.355 / Produção de folhiço e fauna associada de macroinvertebrados aquáticos em curso dágua de cabeceira em Floresta Ombrófila do Estado do Rio Grande do Sul, Brasil - DOI: 10.4025/actascibiolsci.v31i3.355

The aim of the study was to verify the temporal variation of allochthonous material input into a stream, and which structures contribute to the organic material pool, as well as to analyze the occurrence of macroinvertebrates associated with the litter. Samples were taken from October, 2004 to October, 2005 in two transects along the riparian zone. At each transect, three sampling stations were chosen. At each one, submersed and suspended collectors (1.5 m above bottom) were installed. Input and retention of coarse benthic organic material were analyzed and classified as flowers and diaspores, (seeds and fruits) and vegetative structures (leaves and branches). There were no statistical differences between input of material in suspended and submersed collectors. Therefore, no statistical differences were verified for exposure time for both the suspended and submersed collectors. Vegetative and seeds and fruits and flowers structures differed in relation to their vegetal composition input for total exposure times for both collectors. Leaves were the most important structure for biomass. Aquatics insects, crustaceans, mussels and worms were the most common macroinvertebrates associated with litter. Insecta-Diptera, mainly Chironomidae and Oligochaeta were the most abundant taxa associated with the submerged and suspended collectors. The major taxa richness was found in submerged

Os objetivos deste estudo foram verificar a entrada sazonal de material alóctone e a composição de suas estruturas vegetais e da fauna de macroinvertebrados aquáticos associados ao material alóctone em um riacho de cabeceira em clima subtropical. Foram analisadas, trimestralmente, duas transecções longitudinais no curso dágua, onde foram instalados coletores submersos e suspensos (1,5 m acima da calha) de outubro de 2004 a outubro de 2005. Em ambos os coletores, não se verificou diferença para a entrada de material alóctone entre os períodos de coletas, o que sugere ausência definida de sazonalidade. Também não foram verificadas, temporalmente, diferenças para os componentes, flores, frutos e sementes (diásporos), folhas e galhos (vegetativas). O componente foliar foi o item com maior biomassa na formação do material alóctone em ambos os coletores. Larvas de insetos aquáticos, crustáceos, moluscos e vermes oligoquetos correspondem aos grupos com maior representatividade. Chironomidae (Insecta, Diptera) e Oligochaeta foram os taxa mais abundantes em ambos os coletores, porém a maior riqueza foi observada nos coletores submersos.