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Background and Aims: Input of organic matter into soil creates microbial hotspots. Due to the low organic matter content in subsoil, microbial hotspots can improve nutrient availability to plants. Therefore, carbon (C) input of root biomass and rhizodeposition and the microbial utilization of root C by alfalfa and chicory, both deep-rooting taprooted preceding crops, was determined. Methods: Three replicate plots of alfalfa and chicory grown on a Haplic Luvisol were CO pulse labeled after 110 days of growth. C was traced in plant biomass, rhizosphere, bulk soil and in microbial biomass after 1 and 40 days. C stocks and δC signature were quantified in 15 cm intervals down to 105 cm depth. Results: Alfalfa plant biomass was higher and root biomass was more homogeneously distributed between top- (0-30 cm) and subsoil (30-105 cm) compared to chicory. C input into subsoil by alfalfa, including roots and rhizodeposited C, was 8 times higher (3820 kg C ha) into subsoil compared to chicory after 150 days of growth. Microbial biomass in subsoil increased with alfalfa but decreased with chicory. Conclusions: Despite their general ability to build biopores, taprooted preceding crops differ in creating microbial hotspots in subsoil. Higher C input and microbial growth in subsoil under alfalfa cultivation can improve physico-chemical and biological properties, and so enhance root growth and consequently the water and nutrient uptake from subsoil compared to chicory. [ABSTRACT FROM AUTHOR]

Aims: This study analyzed the extent to which root exudates diffuse from the root surface towards the soil depending on topsoil and subsoil properties and the effect of arbuscular mycorrhizal fungal hyphae on root-derived C distribution in the rhizosphere. Methods: Alfalfa was grown in three-compartment pots. Nylon gauze prevented either roots alone or roots and arbuscular mycorrhizal fungal hyphae from penetrating into the rhizosphere compartments. CO pulse labeling enabled the measurement of C-labeled exudates in dissolved (DOC) and total organic carbon (TOC) in the rhizosphere, distributed either by diffusion alone or by diffusion, root hair and hyphal transport. Results: Root exudation and microbial decomposition of exudates was higher in the rhizosphere with topsoil compared to subsoil properties. Exudates extended over 28 mm (DOC) and 20 mm (TOC). Different soil properties and mycorrhization, likely caused by the low arbuscular mycorrhizal colonization of roots (13 ± 4 % (topsoil properties) and 18 ± 5 % (subsoil properties)), had no effect. Conclusions: Higher microbial decomposition compensated for higher root exudation into the rhizosphere with topsoil properties, which resulted in equal exudate extent when compared to the rhizosphere with subsoil properties. Higher C activity used for labeling compared with previous studies enabled the detection of low exudate concentrations at longer distances from the root surface. [ABSTRACT FROM AUTHOR]

The Kobresia pastures of the Tibetan Plateau represent the world's largest alpine grassland ecosystem. These pastures remained stable during the last millennia of nomadic animal husbandry. However, strongly increased herds' density has promoted overgrazing, with unclear consequences for vegetation and soils, particularly for cycles of carbon (C), nutrients and water. Vegetation-free patches of dead root-mat covered by blue-green algae and crustose lichens (crusts) are common in overgrazed Kobresia pastures, but their effect on C turnover processes is completely unknown. We tested the hypothesis that the crusts strongly affect the C cycle by examining: (i) the long-term C stock measured as soil organic matter content; (ii) medium-term C stock as dead roots; (iii) recent C fluxes analyzed as living roots and CO efflux; and (iv) fast decomposition of root exudates. Up to 7.5 times less aboveground and 1.9 times less belowground living biomass were found in crust patches, reflecting a much smaller C input to soil as compared with the non-crust Kobresia patches. A lower C input initially changed the long-term C stock under crusts in the upper root-mat horizon. Linear regression between living roots and CO efflux showed that roots contributed 23% to total CO under non-crust areas (mean July-August 5.4 g C m day) and 18% under crusts (5.1 g C m day). To identify differences in the fast turnover processes in soil, we added C labeled glucose, glycine and acetic acid, representing the three main groups of root exudates. The decomposition rates of glucose (0.7 day), glycine (1.5 day) and acetic acid (1.2 day) did not differ under crusts and non-crusts. More C, however, remained in soil under crusts, reflecting less complete decomposition of exudates and less root uptake. This shows that the crust patches decrease the rates of medium-term C turnover in response to the much lower C input. Very high C amounts recovered in plants from non-crust areas as well as the two times lower uptake by roots under crusts indicate that very dense roots are efficient competitors with microorganisms for soluble organics. In conclusion, the altered C cycle in the overgrazing-induced crustose lichens and blue-green algae crusts is connected with strongly decreased C input and reduced medium-term C turnover. [ABSTRACT FROM AUTHOR]