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Agroforestry systems are characterized by a high complexity between vegetation components and niche partitioning. In a crop-grass-tree agroforestry system, rape, willow, and grasses were in situ pulse labeled separately with 13CO2 for 6 h, and 13C was traced in shoots, roots, topsoil (0–15 cm) and subsoil (15–30 cm), microbial biomass carbon (C), and dissolved organic C, as well as respiration losses (CO2) up to 28 days after labeling to investigate the effects of vegetation components on C allocation belowground. 13C recovery in roots after 28 days was 7.0% of total assimilated C for grassland, which was 3.5- and 5.2-fold higher than that for rape and willow, respectively. The larger C allocation belowground in grassland was ascribed to its higher root/shoot ratio compared to willow and rape. Grassland facilitated higher accumulation of root-derived C in soil compared to rape (9.2% of recovered 13C) and compared to willow (1.6% of 13C). Willow retained more photosynthetic C aboveground and less was allocated to roots compared to rape. Although the C allocated to the top 15-cm soil was similar between willow and rape, willow facilitated C allocation in deeper soil compared to rape (0.6% vs. 0.2%). This could be explained by the lower microbial activity and subsequent weaker decomposition of rhizodeposits in 15–30-cm depth under willow. The net belowground C inputs in grassland, willow, and rape were 0.53, 0.06, and 0.10 g C m−2 month−1 of vegetation period, including rhizodeposition of 0.24, 0.05, and 0.04 g C m−2 month−1, respectively. Overall, integrating trees and grassland within cropland facilitates higher root-derived C input into soil, thus contributing to the soil C sequestration in agroforestry systems. [ABSTRACT FROM AUTHOR]

• 13C-CO 2 was pulsed labeled in an agroforestry system. • PLFA biomarker 16:1ω5c was enriched with 13C in soil under grassland. • 13C incorporation into gram positive bacteria was higher under willow compare to grassland and rape. • Gram negative bacteria utilized the highest relative amount of 13C. • Microbial utilization of 13C depends on vegetation types in an agroforestry system. The release of organic compounds from plant roots (rhizodeposits) and their subsequent utilization by soil microbial communities is a key process linking atmospheric and terrestrial carbon (C) and nutrient cycling. However, the effects of land-use types on rhizodeposit-mediated alteration of microbial community structure and its consequences for soil C cycle are still, largely, unknown. Therefore, we used 13C-PLFA-SIP to evaluate the rhizosphere C fluxes to microorganisms in a monoculture crop (oilseed rape, Brassica napus L.), a mixed grassland (dominated by Lolium perenne L. mixed with clover Trifolium repens L.), and a tree (willow, Salix schwerinii E.L. Wolf and Salix viminalis L.) within an agroforestry system. Though harboring similar total microbial biomass, less 13C was recovered in microorganisms in soil under willow (0.017 % of 13C input) after 14 days as compared to rape (0.03 %) and grassland (0.09 %). Across three land use types, gram negative bacteria incorporated most of the 13C (28–42 % of total 13C labeled PLFAs), followed by fungi and gram positive bacteria (12–18 %) and less abundance groups (e.g. actinomycetes, <5 %). The arbuscular mycorrhizal fungal biomarker 16:1ω5c was enriched with 13C in soil under grassland (8 % of total 13C labeled PLFAs). The 13C incorporation into gram positive bacteria was higher in soil under willow than under grassland and rape. By contrast, 42 % of total 13C labeled PLFAs were attributed to gram negative bacteria in soil under rape. Overall, our results demonstrated that pronounced differences in microbial community structure and subsequent microbial C utilization between land-use types could affect soil C cycles and storage. [ABSTRACT FROM AUTHOR]

Recently fixed plant carbon (C) being released as rhizodeposits is a major resource fueling soil food webs. Soil microorganisms predominate in incorporating root-derived C and subsequently transfer it to higher trophic levels. However, variation in microbial community structure between cropping systems and its consequences for the incorporation of root-derived C into soil microbivores remain unclear. In the present study, we used 13CO 2 to pulse label a crop monoculture (oilseed rape, Brassica napus L.), a mixed grass community (dominated by Lolium perenne L. mixed with clover Trifolium repens L.), and a young tree plantation (willow, Salix schwerinii E.L. Wolf and Salix viminalis L.). During 28 days, we traced the incorporation of root-derived 13C into phospholipid fatty acids (PLFAs) of soil microorganisms and neutral lipid fatty acids (NLFAs) of five Collembola species belonging to three functional groups: epedaphic (surface-dwelling), hemiedaphic (litter-dwelling), and euedaphic (soil-dwelling). The contribution of bacterial and fungal channels to the incorporation of root-derived C into Collembola varied considerably between cropping systems. Collembola incorporated more 13C from the bacterial channel in rape than in grass and willow, where fungi were the major C source. This corresponded to a similarly higher 13C incorporation into bacterial marker PLFAs in rape compared to grass and willow. By contrast, while the proportion of bacterial and fungal biomarkers in Collembola NLFAs was related to the 13C incorporation into microbial PLFAs, it did not correlate with the proportion of microbial PLFAs in the different cropping systems. This suggests that Collembola rely on specific microbial pools, presumably related to recent plant inputs. Within the same cropping system, hemiedaphic species incorporated more root-derived 13C from the bacterial channel compared to euedaphic and epedaphic species. The results demonstrate the remarkable importance of cropping system for the flux of root C into microorganisms and microbivore soil invertebrates. Changes in root C flux into bacterial and fungal resources among cropping systems resulted in differential utilization of these resources by soil microbivores, suggesting that in particular microorganisms fueled by rhizodeposits are vital resources for the nutrition of higher trophic levels in soil food webs. • Microorganisms in the rhizosphere and not in bulk soil were the main food source for Collembola. • Collembola incorporated more root C from bacteria in rape than in grass and willow. • Fungi were the major C source for Collembola in grass and willow. • The incorporation of root C from microbial channels differed between Collembola species with different functional traits. [ABSTRACT FROM AUTHOR]