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Soil organic carbon (OC) is pivotal for soil functioning, especially in the domains of elemental cycling, disease control and the regulation of soil structure and water. High OC contents are generally associated with improved soil functioning, and therefore OC content is widely used as a soil health indicator. The stability of OC influences all functions that depend on microbial decomposition (e.g. nutrient provisioning) or carbon retention (e.g. C sequestration). Yet, a standardized indicator for OC stability that can be used in soil health assessments is still in development. Here, we investigate to what extent selected OC stability indicators can predict soil functioning, and which type of OC stability indicator improves predictions most as an additional measurement besides OC content.We collected soil samples from arable fields in the Netherlands on marine clay (n=144) and sand (n=81) soils. For each soil sample, 18 different soil function measurements were performed and grouped into 3 function domains (i.e. elemental cycling, disease control, and the regulation of soil structure and water). In addition, 21 OC stability indicators were analyzed and grouped into 4 types of OC stability indicators (i.e. carbon pools; carbon fractions, thermal stability indicators, and indicators based on elemental ratio). Multiple linear regression was used to determine how much of the variation in each soil function measurement could be predicted by the OC stability indicators and other measured intrinsic soil properties (e.g. texture and pH).We found that OC stability and intrinsic soil properties could significantly predict soil functions better in the domain of regulation of soil structure and water (R2adj=40±18%) than in the other 2 function domains (elemental cycling: R2adj =32±31%; disease control: R2adj =22±11%). OC stability indicators could predict 80±26% of this maximum explainable variation, averaged over all soil functions (clay: 82±18%; sand: 77±32%). This percentage did not differ significantly between the function domains. A regression model with only total OC content explained 18±21% of the maximum explainable variation (clay:23±26%; sand:14±14%). The addition of 1 OC stability indicator increased this percentage to 49±23% (clay: 58±20%; sand: 39±22%). Predictions of soil functioning were most strongly improved by including a thermal stability indicator in addition to total OC content.We conclude that OC stability indicators can predict soil functioning adequately and that thermal stability indicators show particular potential as OC stability indicator in soil health assessments.

Aims Saprophytic fungi are important agents of soil mineralization and carbon cycling. Their community structure is known to be affected by soil conditions such as organic matter and pH. However, the effect of plant species, whose roots provide the litter input into the soil, on the saprophytic fungal community is largely unknown. Methods We examined the saprophytic fungi in a grassland biodiversity experiment with eight plant species belonging to two functional groups (grasses and forbs), combining DNA extraction from plant roots, next-generation sequencing and literature research. Results We found that saprophyte richness increased with plant species richness, but plant functional group richness was the best predictor. Plant functional group was also the main factor driving fungal saprophytic community structure. This effect was correlated with differences in root lignin content and C:N ratio between grasses and forbs. In monocultures, root traits and plant functional group type explained 16% of the variation in community structure. The saprophyte taxa detected in mixed plant communities were to a large extent subsets of those found in monocultures. Conclusions Our work shows that the richness and community structure of the root-associated saprophytic fungi can largely be predicted by plant functional groups and their associated root traits. This means that the effects of plant diversity on ecosystem functions such as litter decomposition may also be predictable using information on plant functional groups in grasslands.

Questions: The rapid climate warming in tundra ecosystems can increase nutrient availability in the soil, which may initiate shifts in vegetation composition. The direction in which the vegetation shifts will co-determine whether Arctic warming is mitigated or accelerated, making the understanding of successional trajectories urgent. One of the key factors influencing the competitive relationships between plant species is their access to nutrients, depending on the depth where they take up most nutrients. However, nutrient uptake at different soil depths by tundra plant species that differ in rooting depth is unclear. Location: Kytalyk Nature Reserve, northeast Siberia, Russia. Methods: We injected 15N to 5 cm, 15 cm and the thaw front of the soil in a moist tussock tundra. The absorption of 15N by grasses, sedges, deciduous shrubs and evergreen shrubs from the three depths was compared. Results: The results clearly show a vertical differentiation of N uptake by these plant functional types, corresponding to their rooting strategy. Shallow-rooting dwarf shrubs were more capable of absorbing nutrients from the upper soil than from deeper soil. Deep-rooting grasses and sedges were more capable of absorbing nutrients from deeper soil than the dwarf shrubs. The natural 15N abundances in control plants also indicate that graminoids can absorb more nutrients from the deeper soil than dwarf shrubs. Conclusions: Our results show that graminoids and shrubs in the Arctic differ in their N uptake strategies, with graminoids profiting from nutrients released at the thaw front, while shrubs mainly forage in upper soil layers. Our results suggest that tundra vegetation will become graminoid-dominated as permafrost thaw progresses and nutrient availability increases in the deep soil.