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Subsoils are increasingly studied as they, first, store a great deal of terrestrial carbon (C) and possibly even more, and second, offer resources like water and nutrients to plants, potentially mitigating negative consequences of global change. As subsoil access is often hampered by compacted soil layers, the key to accessing subsoil resources and storing more C below ground might be in biopores. Appropriately nicknamed 'highways of root growth', biopores are macropores left behind by dying roots and earthworm activities, often enriched with organic matter (OM) and nutrients. They are thought to be the most abundant microbial hotspots in the subsoil, thus possibly accounting for a large part of C turnover, as well as offering pore wall nutrients to subsequent crops. Understanding the multifunctionality and complexities of biopores remains challenging. This contribution aims to showcase analytical ways to deepen our understanding of origin and functioning of biopores and hotspots. Regarding their biogeochemistry, biopore OM quality and its turnover can be better unravelled through compound‐specific analysis to deduct biopore‐specific OM turnover. Biopores can be reliably differentiated by their OM quality. A more profound understanding of subsoil C turnover in very contrasting hotspots is crucially important for managing subsoil functions. Biopores are often assumed to be beneficial in crop sequences. Roots making use of specific biopores can be, for the first time, quantified after radiotracer application, two‐step phosphor imaging, and image processing. Combining radioactive with stable isotopes as well as plant and microbial biomarkers allows to investigate the relevance of individual pore wall nutrients in plant growth in consideration of physical biopore properties. Biopore‐friendly management practices (e.g., reduced tillage, perennial cover cropping) could be part of smart subsoil management. Faster access to subsoil water and concentrated biopore nutrients may safeguard agricultural production—especially in times of rising fertiliser costs (both monetary and environmental) and more frequent droughts. [ABSTRACT FROM AUTHOR]

Comprehensive climate change mitigation necessitates soil carbon (C) storage in cultivated terrestrial ecosystems. Deep-rooted perennial crops may help to turn agricultural soils into efficient C sinks, especially in deeper soil layers. Here, we compared C allocation and potential stabilization to 150 cm depth from two functionally distinct deep-rooted perennials, i.e., lucerne (Medicago sativa L.) and intermediate wheatgrass (kernza; Thinopyrum intermedium), representing legume and non-legume crops, respectively. Belowground C input and stabilization was decoupled from nitrogen (N) fertilizer rate in kernza (100 and 200 kg mineral N ha−1), with no direct link between increasing mineral N fertilization, rhizodeposited C, and microbial C stabilization. Further, both crops displayed a high ability to bring C to deeper soil layers and remarkably, the N2-fixing lucerne showed greater potential to induce microbial C stabilization than the non-legume kernza. Lucerne stimulated greater microbial biomass and abundance of N cycling genes in rhizosphere soil, likely linked to greater amino acid rhizodeposition, hence underlining the importance of coupled C and N for microbial C stabilization efficiency. Inclusion of legumes in perennial cropping systems is not only key for improved productivity at low fertilizer N inputs, but also appears critical for enhancing soil C stabilization, in particular in N limited deep subsoils. [ABSTRACT FROM AUTHOR]

Phosphorus (P) is a major limiting nutrient for plant growth implying an often-intensive competition between microorganisms and plants in the rhizosphere. Increasing the P availability in subsoils may help to mitigate potential future P fertilizer shortages and to overcome P limitations due to droughts, which mainly affect topsoils. Root exudates provide easily available carbon and energy sources for microorganisms to mobilize soil nutrients. Nonetheless, details regarding the distinct processes underlying P mobilization from various P sources (free vs. sorbed PO 4 3−; low molecular vs. complex organic P, e.g. ATP vs. plant litter P) as affected by root exudates are poorly understood, especially in subsoils. This study aimed to identify the controlling factors and microbial processes regulating the availability of organic and inorganic P in top- and subsoils by 33P isotopic labeling. The focus was on the potential key role of root exudates in P mobilization. We found that microbial communities in top- and subsoils used high- and low-available mineral P to a similar extent, but that the subsoil communities were much more efficient in mobilizing and incorporating complex litter-derived organic P. This capability of subsoil communities was even enhanced when root exudates were present. Microbial activity and nutrient-mobilizing mechanisms (e.g., P-related enzymes) clearly increased by root exudate addition, an effect that was generally higher in sub-than in topsoils. We conclude that subsoil communities are well capable of mobilizing and using complex organic P sources, especially if root exudates accelerate overall activity and P cycling. Thus, high root exudation is highly relevant for crops, which depend on subsoil nutrients and litter-derived P. Accordingly, detritusphere P, e.g. in subsoil root channels, is likely to be plant-available because of exudate-induced microbial P (re-)cycling processes. [Display omitted] • Root exudates did not affect growth of the microbial community nor lead to a community shift. • Due to rapid desorption microbes use sorbed P similarly to mineral P added to soil solution. • Low available litter P was mobilized and incorporated more efficient by sub than topsoil communities. • Root exudates increase microbial activity and thus boost P mining process. [ABSTRACT FROM AUTHOR]

Most subsoil carbon (C) turnover occurs in biopore hotspots such as root channels and earthworm burrows. Biopores allocate large C amounts into the subsoil, where a vast capacity for long-term C sequestration is predicted. We hypothesise that organic matter (OM) cycling in biopores depends on their origin. Earthworm and root biopores were induced under field conditions and were sampled from the subsoil (45–75 and 75–105 cm) after two years of biopore formation. The effects of biopore formation on OM decomposition were studied by biomarkers: neutral sugars, cutin and suberin-derived lipids, lignin-derived phenols and free lipids. The degradation stage of OM was biopore type-specific but was only governed by the soil depth in root biopores. Degradation of OM increased from earthworm biopores to root biopores and bulk soil. Hemicelluloses (GM/AX ratio) were more strongly degraded than lignin side-chains (relative change from initial values). Two years of microbial processing during biopore formation increased the GM/AX ratio in earthworm biopores from 0.65 to 1.05 and in root biopores from 0.15 to 1.35 (both relative to source biomasses). Root biopores and bulk soil had the highest GM/AX ratios (1.2–1.3), hinting to rapid processing of plant residues and accumulation of microbial residues. The regular, frequent OM inputs by earthworms stimulated microbial growth and processing of mostly bioavailable OM and, thus, relatively enriched more persistent OM (e.g. lignin). Syringyl subunits of lignin underwent low (ratio changed from 0.35 to 0.55 relative to initial input) and vanillyl subunits underwent almost no processing in earthworm biopores indicating the preferential microbial utilisation of the easily available compounds frequently replenished by earthworm activity. After two years of decomposition of the root detritus, mainly structural plant material was enriched in root biopores. Short periods (6 months) of earthworm activity effectively recharged the highly processed OM in root biopores with fresh OM. In total, deep-rooting catch crops and short-term earthworm activities promote C accumulation in the subsoil followed by biopore-specific microbial processing predominantly governed by the C input frequency. As root biopores are up to 40 times more common than earthworm biopores, they dominate the OM input into subsoils. Such C inputs create several years lasting hotspots for preferential root growth and nutrient mobilisation in the subsoil. We conclude that root- and earthworm-derived biopores are vertical pathways for plant C from the soil surface into the subsoil and for intensive processing of litter C and sequestration of microbial necromass. [ABSTRACT FROM AUTHOR]

Biopores are pores or voids in soil produced by roots, by earthworms, or by the occupation of earthworms in root pores, which are considered important microbial hotspots, especially in subsoil. We hypothesized that earthworms ( Lumbricus terrestris L.) exert stronger effects on microbial activities than decaying plant roots (of Cichorium intybus L.) in the subsoil because of the addition of pre-digested organic material. We tested this hypothesis by analyzing microbial biomass (C), total organic C (C), and activities of eight enzymes (cellobiohydrolase, β-glucosidase, xylanase, acid phosphomonoesterase, leucine aminopeptidase, tyrosine aminopeptidase, chitotriosidase, and n-acetylglucosaminidase) down to 105-cm depth. The C increase was associated with a two- to threefold increase of C content in biopores as compared to bulk soil. The highest percentage of C-to-C (3.7 to 7.3 %) in the drilosphere demonstrated the enhancement of microbial efficiency for organic matter decomposition by earthworms. The availability of organic matter in biopores increased the activities of C- and N-targeting enzymes by 1.2-11.3 times, but reduced acid phosphomonoesterase activity by 10-40 % in biopores versus bulk soil. Introducing earthworms in root biopores caused 1.5-1.8 times higher microbial biomass and 1.2-1.9 times increased enzyme activities compared to the sole effect of earthworms. Soil depth showed a strong effect on the drilosphere, but only slight effects on the biochemical properties of root biopores and bulk soil. In conclusion, biopores are important microbial hotspots of C, N, and P transformations in subsoil. Earthworms exerted stronger effects on biochemical properties of biopores than decaying roots. [ABSTRACT FROM AUTHOR]

In arable fields, biopores are primarily formed by taproots, but may also be bored by earthworms. Irrespective of the pore origin, repeated use by anecic earthworms yields a wall coating that is rich in carbon, nutrients and microorganisms. However, this effect is halted by routine tillage, and it remains unclear how quickly earthworms are able to alter biopore properties in subsoil. We conducted an earthworm incubation field experiment in arable soil to test the capacity of Lumbricus terrestri s to i. increase total nutrient contents including plant available P, ii. alter the microbial community and iii. increase enzyme activities in biopore walls over one vegetation period. Firstly, biopores that contained chicory roots were identified on a plot scale (4.2 × 1.5 m). After two years under fallow, roots were decomposed. We then inserted individual earthworms at 45 cm depth into a subset of these pores, afterwards refilling with topsoil. After six months, earthworms were removed and soil was opened at 45–75 cm and 75–105 cm soil depth layers. The inner pore wall (1 mm) of individual root biopores (‘RBP’) or root biopores modified by earthworms (‘EBP’) as well as the bulk soil were sampled in 6 depth intervals of 10 cm each and analyzed for total C, N, S content, plant available P, microbial biomass, phospholipid fatty acids (PLFA) and enzyme activity. Biochemical properties of bulk soil, RBP and EBP clearly differed after one vegetation period as indicated by principal component analysis. PLFA markers of fungi and protozoa were detected only in biopores. Compared with the bulk soil, total C, N, S were enriched in RBP by a factor of 2.0–3.1, plant available P by a factor of 8–10, and microbial biomass by a factor of 12–36. In EBP, all of these parameters were as in RBP or elevated even further (C, N, S: factor 1.0–1.4, plant available P: factor 1.3–1.5, microbial biomass: factor 1.5–2.0, PLFA markers of fungi: factor 2.6–4.4, PLFA markers of protozoa: factor 9.2–14.2). PLFA markers indicative of the ratio of Gram-positive to Gram-negative bacteria (G+: G−) were 5–10 fold lower in RBP than in bulk soil, the microbial metabolic quotient ( q CO 2 ) was 0.4–0.6 times as high. In EBP, these parameters were further reduced (ratio G+: G−: factor 0.7, q CO 2 : factor 0.7–0.8). RBP were particularly characterized by high contents of 10-methyl branched fatty acid indicators of actinobacteria. Activities of enzymes involved in the C-cycle (xylanase, cellobiohydrolase, ß-glucosidase) and N-cycle (chitinase, chitotriosidase, leucine aminopeptidase) were also elevated in RBP as compared to the bulk soil (factor 1.1–3.6) and further increased in EBP (factor 1.2–3.7). All these effects were more pronounced in the 45–75 cm soil layer. We conclude that, in only six months, L. terrestris in arable fields modified ordinarily nutrient-rich biopores into ‘super-hotspots’ of microbial biomass, enzyme activity and nutrient availabilities. Hence, even short-term promotion of earthworm populations by agricultural management practices can increase microbial biomass and enzyme activity in biopores and its coupling to nutrient mobilization in the subsoil. [ABSTRACT FROM AUTHOR]

Despite the importance of subsoil carbon (C) deposition by deep-rooted crops in mitigating climate change and maintaining soil health, the quantification of root C input and its microbial utilization and stabilization below 1 m depth remains unexplored. We studied C input by three perennial deep-rooted plants (lucerne, kernza, and rosinweed) grown in a unique 4-m deep RootTower facility. 13C multiple pulse labeling was applied to trace C flows in roots, rhizodeposition, and soil as well as 13C incorporation into microbial groups by phospholipid fatty acids and the long-term stabilization of microbial residues by amino sugars. The ratio of rhizodeposited 13C in the PLFA and amino sugar pools was used to compare the relative microbial stability of rhizodeposited C across depths and plant species. Belowground C allocation between roots, rhizodeposits, and living and dead microorganisms indicated depth dependent plant investment. Rhizodeposition as a fraction of the total belowground C input declined from the topsoil (0–25 cm) to the deepest layer (360 cm), i.e., from 35%, 45%, and 36%–8.0%, 2.5%, and 2.7% for lucerne, kernza, and rosinweed, respectively, where lucerne had greater C input than the other species between 340 and 360 cm. The relative microbial stabilization of rhizodeposits in the subsoil across all species showed a dominance of recently assimilated C in microbial necromass, thus indicating a higher microbial stabilization of rhizodeposited C with depth. In conclusion, we traced photosynthates down to 3.6 m soil depth and showed that even relatively small C amounts allocated to deep soil layers will become microbially stabilized. Thus, deep-rooted crops, in particular lucerne are important for stabilization and storage of C over long time scales in deep soil. Image 1 • Rhizodeposition strongly declined with depth across all plant species. • Microbial stabilization of rhizodeposits increases with depth. • Deep-rooted crops are important for storage and stabilization of rhizodeposition. [ABSTRACT FROM AUTHOR]