Changes in Soil Organic C Fractions and C Pool Stability Are Mediated by C-Degrading Enzymes in Litter Decomposition of Robinia pseudoacacia Plantations

Litter decomposition is the main source of soil organic carbon (SOC) pool, regarding as an important part of terrestrial ecosystem C dynamics. The turnover of SOC is mainly regulated by extracellular enzymes secreted by microorganisms. However, the response mechanism of soil C-degrading enzymes and SOC in litter decomposition remains unclear. To clarify how SOC fraction dynamics respond to C-degrading enzymes in litter decomposition, we used field experiments to collect leaf litter and SOC fractions from the underlying layer in Robinia pseudoacacia plantations on the Loess Plateau. Our results showed that SOC, easily oxidizable organic C, dissolved organic C, and microbial biomass C increased significantly during the decomposition process. Litter decomposition significantly decreased soil hydrolase activity, but slightly increased oxidase activity. Correlation analysis results showed that SOC fractions were significantly positively correlated with the litter mass, lignin, soil moisture, and oxidase activity, but significantly negatively correlated with cellulose content and soil pH. Partial least squares path models revealed that soil C-degrading enzymes can directly or indirectly affect the changes of soil C fractions. The most direct factors affecting the SOC fractions of topsoil during litter decomposition were litter lignin and cellulose degradation, soil pH, and C-degrading enzymes. Furthermore, regression analysis showed that the decrease of SOC stability in litter decomposition was closely related to the decrease of soil hydrolase to oxidase ratio. These results highlighted that litter degradation-induced changes in C-degrading enzyme activity significantly affected SOC fractions. Furthermore, the distribution of soil hydrolases and oxidases affected the stability of SOC during litter decomposition. These findings provided a theoretical framework for a more comprehensive understanding of C turnover and stabilization mechanisms between plant and soil.

Genome-wide gene expression dynamics of the fungal pathogen Dothistroma septosporum throughout its infection cycle of the gymnosperm host Pinus radiata

We present genome-wide gene expression patterns as a time series through the infection cycle of the fungal pine needle blight pathogen, Dothistroma septosporum, as it invades its gymnosperm host, Pinus radiata. We determined the molecular changes at three stages of the disease cycle: epiphytic/biotrophic (early), initial necrosis (mid) and mature sporulating lesion (late). Over 1.7 billion combined plant and fungal reads were sequenced to obtain 3.2 million fungal-specific reads, which comprised as little as 0.1% of the sample reads early in infection. This enriched dataset shows that the initial biotrophic stage is characterized by the up-regulation of genes encoding fungal cell wall-modifying enzymes and signalling proteins. Later necrotrophic stages show the up-regulation of genes for secondary metabolism, putative effectors, oxidoreductases, transporters and starch degradation. This in-depth through-time transcriptomic study provides our first snapshot of the gene expression dynamics that characterize infection by this fungal pathogen in its gymnosperm host.

Foliar Litter Decomposition: A Conceptual Model with Focus on Pine (Pinus) Litter—A Genus with Global Distribution

The genus Pinus encompasses c 120 species and has a global distribution. Today we know more about the decomposition of pine needle litter than litter from any other genus. This paper presents a developed conceptual three-phase model for decomposition, based on pine needle litter, starting with newly shed litter and following the process until a humus-near stable residue. The paper focuses on the mass-loss dynamics and factors regulating the process in the early phase, the late one, and the humus-near phase. For the late phase, the hampering influence of N and the rate-enhancing effect of Mn on the decomposition are given extra attention. Empirical factors related to the limit value/stable residue are discussed as well as the decomposition patterns and functions for calculating limit values. The climate-related litter concentrations of N and Mn are discussed as well as their possible influence on the size of the stable residue, which may accumulate and sequester carbon, for example, in humus layers. The sequestration of carbon in humus layers is discussed as well as the effect of tree species on the process. Although the paper focuses on litter of pine species, there are comparisons to studies on other litter genera and similarities and differences are discussed.

The genomes of the fungal plant pathogens Cladosporium fulvum and Dothistroma septosporum reveal adaptation to different hosts and lifestyles but also signatures of common ancestry

We sequenced and compared the genomes of the Dothideomycete fungal plant pathogens Cladosporium fulvum (Cfu) (syn. Passalora fulva) and Dothistroma septosporum (Dse) that are closely related phylogenetically, but have different lifestyles and hosts. Although both fungi grow extracellularly in close contact with host mesophyll cells, Cfu is a biotroph infecting tomato, while Dse is a hemibiotroph infecting pine. The genomes of these fungi have a similar set of genes (70% of gene content in both genomes are homologs), but differ significantly in size (Cfu >61.1-Mb; Dse 31.2-Mb), which is mainly due to the difference in repeat content (47.2% in Cfu versus 3.2% in Dse). Recent adaptation to different lifestyles and hosts is suggested by diverged sets of genes. Cfu contains an α-tomatinase gene that we predict might be required for detoxification of tomatine, while this gene is absent in Dse. Many genes encoding secreted proteins are unique to each species and the repeat-rich areas in Cfu are enriched for these species-specific genes. In contrast, conserved genes suggest common host ancestry. Homologs of Cfu effector genes, including Ecp2 and Avr4, are present in Dse and induce a Cf-Ecp2- and Cf-4-mediated hypersensitive response, respectively. Strikingly, genes involved in production of the toxin dothistromin, a likely virulence factor for Dse, are conserved in Cfu, but their expression differs markedly with essentially no expression by Cfu in planta. Likewise, Cfu has a carbohydrate-degrading enzyme catalog that is more similar to that of necrotrophs or hemibiotrophs and a larger pectinolytic gene arsenal than Dse, but many of these genes are not expressed in planta or are pseudogenized. Overall, comparison of their genomes suggests that these closely related plant pathogens had a common ancestral host but since adapted to different hosts and lifestyles by a combination of differentiated gene content, pseudogenization, and gene regulation.

Release pattern for potassium from decomposing forest needle and leaf litter. Long-term decomposition in a Scots pine forest. IX

A general pattern for the dynamics of K in decomposing leaf and needle litters is described, based on 139 litter decomposition studies carried out in forests of northern and central Europe and the northern United States. We distinguished an initial phase of fast changes in K concentration and a later one in which the litter was close to humus and all K appeared to be immobile. In the initial phase the rate of change in K concentration was related to the initial K concentration: the higher the initial concentration, the faster its decrease and the faster the quantitative release of K. For litters with particularly low initial K concentrations, an increase in concentration was observed from the very start of decomposition. In the late phase a slight increase in concentrations was observed. After ca. 60% accumulated mass loss, K concentrations approached similar levels in various litter types, irrespective of the litter type and initial concentrations. We also estimated a theoretical equilibrium state at which no change in K concentration should be expected. According to our estimates these equilibrium concentrations ranged from approx. 0.8 mg/g dry mass of litter for Scots pine to approx. 1.2 mg/g dry mass for deciduous litter types. The estimate of K concentration in Scots pine litter in late decomposition stages was very close to that measured in the humus at the same site. Based on the wide variety of litter types and sites studied we conclude that the pattern suggested for K dynamics in decomposing litter holds at least for European coniferous forests north of the Alps and the Carpathians. Key words: potassium, dynamics, release, litter, humus, decomposition.