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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]

Aims: Microbial turnover processes are typically restricted by low substrate availability in the subsoil. We hypothesized that SOM decomposition increases with plant density and decreases with N fertilization: We expected a greater rate of C allocation to the rhizosphere in the topsoil than in the subsoil treatments. Methods: In order to simulate different degrees of rhizodeposition, wheat was planted in pots at four different densities. The plants were continuously labeled with 13C-depleted CO2. Soil CO2 efflux was partitioned for root- and SOM-derived CO2. Moreover, we determined the enzyme kinetics by measuring catalytic efficiency and enzyme stoichiometry in both topsoil and subsoil. Results: Shoot biomass and the shoot to root ratio were significantly higher for plants grown in the topsoil compared with the subsoil, which demonstrated higher relative C allocation to root biomass in the subsoil treatment. Despite the similar size of the rhizosphere, root-derived CO2 was always higher in the topsoil compared with the subsoil treatment, indicating enhanced root exudation. Effect sizes of all enzyme activities showed stronger magnitudes for the subsoil treatments. This was in line with a two-times increase of the effect size of SOM decomposition in the subsoil relative to topsoil. Conclusions: Overall, the plants in the subsoil treatments allocated more C to root biomass, less C to shoot biomass, and substantially less C to root exudates. However, the effect sizes of both SOM decomposition and enzyme activities were higher in the subsoil than in the topsoil, reflecting a stronger sensitivity to C inputs. [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]

• High microbial activity in topsoil causes apparent PE shortly after substrate addition. • The input of C from fertilizers induces apparent PE at the 10–30 cm depth. • The release and decomposition of mineral-bound organic C shortly after substrate addition contributes to positive PE. • Negative PE in topsoil during later incubation stages is linked to reduced microbial biomass and substrate depletion. • In subsoil, positive PE and net C loss was caused by microbial N mining. Fertilization is crucial for increasing crop productivity and it alters soil microbial biomass and activities. These alterations exert implications for soil carbon (C) stocks, primarily through the priming effect (PE). Here, we investigated the mechanisms underlying PE and their impact on soil C stocks in paddy soils subjected to long-term (31 years) fertilization. Soils from four depths within the upper 40 cm layer, subjected to four fertilization types (no fertilizer, mineral fertilizer, mineral + straw, mineral + chicken manure), were incubated for 60 days with or without 13C-labeled glucose. Following substrate addition, an increase in isotopically unlabeled CO 2 efflux may stem from accelerated microbial turnover (apparent PE) or modified soil organic matter (SOM) mineralization (real PE). By analyzing microbial biomass (MBC), CO 2 and dissolved organic C (DOC) dynamics, we explored the partitioning of C from glucose into three pools. Following glucose addition (day 2), the SOM-derived MBC decreased by 4.7 % at 0–10 cm in unfertilized and by 6.3 % in mineral-fertilized soils. SOM-derived MBC also decreased in soils fertilized with straw- and chicken manure at 0–20 and 0–30 cm. Hence, the apparent PE closely correlated with fertilization types. At day 2, the SOM-derived DOC increased with the increase in extractable Fe2+ content, indicating high contribution of the released mineral-bound organic C in the DOC pool. Microbial decomposition of SOM-derived DOC and loss of unlabeled C through apparent PE increased the release of unlabeled CO 2 , resulting in a positive but low PE. After 60 days of incubation, the addition of glucose to the subsoils (30–40 cm) caused a net loss of C (positive PE) of 6.6–19 mg kg−1. In contrast, negative PEs were observed in the topsoil, indicating a decrease in microbial activity and C turnover. These findings suggested that fertilization regimes, whether involving long-term extra C input or not, can alter microbial biomass and turnover rates. Moreover, the C turnover mechanisms in paddy soils are complex, including apparent PE, release of mineral-bound organic C, and negative PE. Collectively, these processes can consequently change soil C stocks. [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]