Rhizodeposition of C and N in peas and oats after C-N double labelling under field conditions

Compounds released by plant roots during growth can make up a high proportion of below-ground plant (BGP) carbon and nitrogen, and therefore influence soil organic matter turnover and plant nutrient availability by stimulating the soil microorganisms. The present study was conducted to examine the amount and fate of C (CdfR) and N rhizodeposits (NdfR), in this study defined as root-derived C or N present in the soil after removal of roots and root fragments, released during reproductive growth. BGP biomass of peas (Pisum sativum L.) and oats (Avena sativa L.) was successfully labelled in situ with a ¹³C-glucose-¹⁵N-urea mixture under field conditions using a stem feeding method. Pea plants were labelled at the beginning of flowering and harvested 36 and 52 days after labelling at pod filling (P_P) and maturity (P_M), respectively. Oat plants were labelled at grain filling and harvested 42 days after labelling at maturity (O_M). CdfR was 24.2% (P_P), 29.6% (P_M) and 30.8% (O_M) of total recovered plant C. NdfR was 32.1% (P_P), 36.4% (P_M) and 30.0% (O_M) of total plant N. Due to higher N assimilation, amounts of NdfR were four times higher in peas in comparison with oats. The results for NdfR in peas were higher than results from other studies. The C-to-N ratio of rhizodeposits was lower under peas (17.3) than under oats (41.9) at maturity. At maturity, microbial CdfR at 0-30 cm soil depth was 37% of the microbial biomass C in peas and 59% in oats. Microbial NdfR was 15% of microbial N in peas and 5% in oats. Furthermore, inorganic NdfR was 34% in peas and 9% in oats at 0-30 cm at maturity. These results show that rhizodeposits of peas provide a more easily available substrate to soil microorganisms, which are incorporated to a greater extent and turned over faster in comparison with oats. Beside the higher amounts of N released from pea roots, this process contributes to the higher N-availability for subsequent crops.     

Microbial immobilisation and turnover of C labelled substrates in two arable soils under field and laboratory conditions

Microbial biomass C immobilisation and turnover were studied under field and laboratory conditions in soils of high yield (HY) and low yield (LY) areas within an agricultural field. We compared the size and activity of soil microbial biomass (SMB) in the soils of the different yield areas under field and laboratory conditions. Soils were amended with ¹³C labelled mustard (Sinapis alba) residues (both experiments) and labelled glucose (laboratory only) at 500 μg C g⁻¹ dry soil. SMB-C, dissolved organic carbon (DOC) and total C content were monitored in the field and the laboratory. CO₂-efflux was also measured in laboratory treatments. Isotope ratios were determined for SMB in both experiments, but other variables only in the laboratory treatments. A positive priming effect was measured in three of four laboratory treatments. Priming was induced after a significant increase of soil derived C in the microbial biomass. Thereafter, the total C loss through priming was always smaller than or equal to the decline in microbial biomass C. In field and laboratory experiments SMB in the HY soil immobilised less of the added substrate C than LY soil SMB. Calculated turnover times in the laboratory glucose amendment were 0.24 (HY) and 0.31 y (LY), in the laboratory mustard treatment 0.58 (HY) and 0.44 y (LY) and in the field mustard amendments 1.09 (HY) and 1.25 y (LY). In both the field mustard and laboratory glucose treatments turnover in the HY soil tended to exceed that in the LY soil. These turnover times as well as the reaction of SMB-C to drying-rewetting and substrate addition, indicated that the HY soil possessed a more active microbial community with a more rapid C turnover than the LY soil. As C turnover is considered to be closely linked to nutrient cycles, faster turnover in the HY soil may involve a better nutrient supply for crops resulting in higher agricultural yield.     

Carbon fluxes in soil food webs of increasing complexity revealed by C labelling and C natural abundance

Soil food webs are mainly based on three primary carbon (C) sources: root exudates, litter, and recalcitrant soil organic matter (SOM). These C sources vary in their availability and accessibility to soil organisms, which could lead to different pathways in soil food webs. The presence of three C isotopes (¹²C, ¹³C and ¹⁴C) offers an unique opportunity to investigate all three C sources simultaneously. In a microcosm experiment we studied the effect of food web complexity on the utilization of the three carbon sources. We choose an incomplete three factorial design with (i) living plants, (ii) litter and (iii) food web complexity. The most complex food web consisted of autochthonous microorganisms, nematodes, collembola, predatory mites, endogeic and anecic earthworms. We traced C from all three sources in soil, in CO₂ efflux and in individual organism groups by using maize grown on soil developed under C₃ vegetation and application of ¹⁴C labelled ryegrass shoots as a litter layer. The presence of living plants had a much greater effect on C pathways than food web complexity. Litter decomposition, measured as ¹⁴CO₂ efflux, was decreased in the presence of living plants from 71% to 33%. However, living plants increased the incorporation of litter C into microbial biomass and arrested carbon in the litter layer and in the upper soil layer. The only significant effect of food web complexity was on the litter C distribution in the soil layers. In treatments with fungivorous microarthropods (Collembola) the incorporation of litter carbon into mineral soil was reduced. Root exudates as C source were passed through rhizosphere microorganisms to the predator level (at least to the third trophic level). We conclude that living plants strongly affected C flows, directly by being a source of additional C, and indirectly by modifying the existing C flows within the food web including CO₂ efflux from the soil and litter decomposition.     

Release of C and N from roots of peas and oats and their availability to soil microorganisms

Nutrient mobilisation in the rhizosphere is driven by soil microorganisms and controlled by the release of available C compounds from roots. It is not known how the quality of release influences this process in situ. Therefore, the present study was conducted to investigate the amount and turnover of rhizodeposition, in this study defined as root-derived C or N present in the soil after removal of roots and root fragments, released at different growth stages of peas (Pisum sativum L.) and oats (Avena sativa L.). Plants were grown in soil columns placed in a raised bed under outdoor conditions and simultaneously pulse labelled in situ with a ¹³C-glucose-¹⁵N-urea solution using a stem feeding method. After harvest, ¹³C and ¹⁵N was recovered in plant parts and soil pools, including the microbial biomass. Net rhizodeposition of C and N as a percentage of total plant C and N was higher in peas than in oats. Moreover, the C-to-N ratio of the rhizodeposits was lower in peas, and a higher proportion of the microbial biomass and inorganic N was derived from rhizodeposition. These results suggest a positive plant-soil feedback shaping nutrient mobilisation. This process is driven by the C and N supply of roots, which has a higher availability in peas than in oats.     

Microbial response to exudates in the rhizosphere of young beech trees (Fagus sylvatica L.) after dormancy

Plants act as an important link between atmosphere and soil: CO2 is transformed into carbohydrates by photosynthesis. These assimilates are distributed within the plant and translocated via roots into the rhizosphere and soil microorganisms. In this study, 3 year old European beech trees (Fagus sylvatica L.) were exposed after the chilling period to an enriched ¹³C–CO₂ atmosphere (δ¹³C = 60‰ – 80‰) at the time point when leaves development started. Temporal dynamics of assimilated carbon distribution in different plant parts, as well as into dissolved organic carbon and microbial communities in the rhizosphere and bulk soil have been investigated for a 20 days period. Photosynthetically fixed carbon could be traced into plant tissue, dissolved organic carbon and total microbial biomass, where it was utilized by different microbial communities. Due to carbon allocation into the rhizosphere, nutrient stress decreased; exudates were preferentially used by Gram-negative bacteria and (mycorrhizal) fungi, resulting in an enhanced growth. Other microorganisms, like Gram-positive bacteria and mainly micro eucaryotes benefited from the exudates via food web development. Overall our results indicate a fast turnover of exudates and the development of initial food web structures. Additionally a transport of assimilated carbon into bulk soil by (mycrorhizal) fungi was observed.     

Turnover of grain legume N rhizodeposits and effect of rhizodeposition on the turnover of crop residues

The turnover of N derived from rhizodeposition of faba bean (Vicia faba L.), pea (Pisum sativum L.) and white lupin (Lupinus albus L.) and the effects of the rhizodeposition on the subsequent C and N turnover of its crop residues were investigated in an incubation experiment (168 days, 15 °C). A sandy loam soil for the experiment was either stored at 6 °C or planted with the respective grain legume in pots. Legumes were in situ ¹⁵N stem labelled during growth and visible roots were removed at maturity. The remaining plant-derived N in soil was defined as N rhizodeposition. In the experiment the turnover of C and N was compared in soils with and without previous growth of three legumes and with and without incorporation of crop residues. After 168 days, 21% (lupin), 26% (faba bean) and 27% (pea) of rhizodeposition N was mineralised in the treatments without crop residues. A smaller amount of 15–17% was present as microbial biomass and between 30 and 55% of mineralised rhizodeposition N was present as microbial residue pool, which consists of microbial exoenzymes, mucous substances and dead microbial biomass. The effect of rhizodeposition on the C and N turnover of crop residues was inconsistent. Rhizodeposition increased the crop residue C mineralisation only in the lupin treatment; a similar pattern was found for microbial C, whereas the microbial N was increased by rhizodeposition in all treatments. The recovery of residual ¹⁵N in the microbial and mineral N pool was similar between the treatments containing only labelled crop residues and labelled crop residues + labelled rhizodeposits. This indicates a similar decomposability of both rhizodeposition N and crop residue N and may be attributable to an immobilisation of both N sources (rhizodeposits and crop residues) as microbial residues and a subsequent remineralisation mainly from this pool.Abbreviations C or N_dec C or N decomposed from residues - C or N_mic microbial C or N - C or N_micres microbial residue C or N - C or N_min mineralised C or N - C or N_input added C or N as crop residues and/or rhizodeposits - dfr derived from residues - dfR derived from rhizodeposition - Ndfr N derived from residues - NdfR N derived from rhizodeposition - N_loss losses of N derived from residues - SOM soil organic matter - WHC water holding capacity     

Microbial use of maize cellulose and sugarcane sucrose monitored by changes in the C/C ratio

An arable soil with organic matter formed from C₃-vegetation was amended initially with maize cellulose (C₄-cellulose) and sugarcane sucrose (C₄-sucrose) in a 67-day laboratory incubation experiment with microcosms at 25 °C. The amount and isotopic composition (¹³C/¹²C) of soil organic C, CO₂ evolved, microbial biomass C, and microbial residue C were determined to prove whether the formation of microbial residues depends on the quality of the added C source adjusted with NH₄NO₃ to the same C/N ratio of 15. In a subsequent step, C₃-cellulose (3 mg C g⁻¹ soil) was added without N to soil to determine whether the microbial residues formed initially from C₄-substrate are preferentially decomposed to maintain the N-demand of the soil microbial community. At the end of the experiment, 23% of the two C₄-substrates added was left in the soil, while 3% and 4% of the added C₄-cellulose and C₄-sucrose, respectively, were found in the microbial biomass. The addition of the two C₄-substrates caused a significant 100% increase in C₃-derived CO₂ evolution during the 5-33 day incubation period. The addition of C₃-cellulose caused a significant 50% increase in C₄-derived CO₂ evolution during the 38-67 day incubation period. The decrease in microbial biomass C₄-C accounted for roughly 60% of this increase. Cellulose addition promoted microorganisms strongly able to recycle N immediately from their own tissue by “cryptic growth” instead of incorporating NO₃⁻ from the soil solution. The differences in quality of the microbial residues produced by C₄-cellulose and C₄-sucrose decomposing microorganisms are also reflected by the difference in the rates of CO₂ evolution, but not in the rates of net N mineralization.     

Distribution and turnover of recently fixed photosynthate in ryegrass rhizospheres

The cycling of root-deposited photosynthate (rhizodeposition) through the soil microbial biomass can have profound influences on plant nutrient availability. Currently, our understanding of microbial dynamics associated with rhizosphere carbon (C) flow is limited. We used a ¹³C pulse-chase labeling procedure to examine the flow of photosynthetically fixed ¹³C into the microbial biomass of the bulk and rhizosphere soils of greenhouse-grown annual ryegrass (Lolium multiflorum Lam.). To assess the temporal dynamics of rhizosphere C flow through the microbial biomass, plants were labeled either during the transition between active root growth and rapid shoot growth (Labeling Period 1), or nine days later during the rapid shoot growth stage (Labeling Period 2). Although the distribution of ¹³C in the plant/soil system was similar between the two labeling periods, microbial cycling of rhizodeposition differed between labeling periods. Within 24 h of labeling, more than 10% of the ¹³C retained in the plant/soil system resided in the soil, most of which had already been incorporated into the microbial biomass. From day 1 to day 8, the proportion of ¹³C in soil as microbial biomass declined from about 90 to 35% in rhizosphere soil and from about 80 to 30% in bulk soil. Turnover of ¹³C through the microbial biomass was faster in rhizosphere soil than in bulk soil, and faster in Labeling Period 1 than Labeling Period 2. Our results demonstrate the effectiveness of using ¹³C labeling to examine microbial dynamics and fate of C associated with cycling of rhizodeposition from plants at different phenological stages of growth.     

Tracing the source and fate of dissolved organic matter in soil after incorporation of a C labelled residue: A batch incubation study

While dissolved organic matter (DOM) in soil solution is a small but reactive fraction of soil organic matter, its source and dynamics are unclear. A laboratory incubation experiment was set up with an agricultural topsoil amended with ¹³C labelled maize straw. The dissolved organic carbon (DOC) concentration in soil solution increased sharply from 25 to 186 mg C L⁻¹ 4 h after maize amendment, but rapidly decreased to 42 mg C L⁻¹ and reached control values at and beyond 2 months. About 65% of DOM was straw derived after 4 h, decreasing to 29% after one day and only 1.3% after 240 days. A significant priming effect of the straw on the release of autochthonous DOM was found. The DOM fractionation with DAX-8 resin revealed that 98% of the straw derived DOM was hydrophilic in the initial pulse while this hydrophilic fraction was 20-30% in control samples. This was in line with the specific UV absorbance of the DOM which was significantly lower in the samples amended with maize residues than in the control samples. The δ¹³C of the respired CO₂ matched that of DOC in the first day after amendment but exceeded it in following days. The straw derived C fractions in respired CO₂ and in microbial biomass were similar between 57 and 240 days after amendment but were 3-10 fold above those in the DOM. This suggests that the solubilisation of C from the straw is in steady state with the DOM degradation or that part of the straw is directly mineralised without going into solution. This study shows that residue application releases a pulse of hydrophilic DOM that temporarily (<3 days) dominates the soil DOM pool and the degradable C. However, beyond that pulse the majority of DOM is derived from soil organic matter and its isotope signature differs from microbial biomass and respired C, casting doubt that the DOM pool in the soil solution is the major bioaccessible C pool in soil.     

Three-source partitioning of CO2 efflux from soil planted with maize by C natural abundance fails due to inactive microbial biomass

A theoretical approach to the partitioning of carbon dioxide (CO₂) efflux from soil with a C₃ vegetation history planted with maize (Zea mays), a C₄ plant, into three sources, root respiration (RR), rhizomicrobial respiration (RMR), and microbial soil organic matter (SOM) decomposition (SOMD), was examined. The δ¹³C values of SOM, roots, microbial biomass, and total CO₂ efflux were measured during a 40-day growing period. A three-source isotopic mass balance based on the measured δ¹³C values and on assumptions made in other studies showed that RR, RMR, and SOMD amounted to 91%, 4%, and 5%, respectively. Two assumptions were thoroughly examined in a sensitivity analysis: the absence of ¹³C fractionation and the conformity of δ¹³C of microbial CO₂ and that of microbial biomass. This approach strongly overestimated RR and underestimated RMR and microbial SOMD. CO₂ efflux from unplanted soil was enriched in ¹³C by 2.0‰ compared to microbial biomass. The consideration of this ¹³C fractionation in the mass balance equation changed the proportions of RR and RMR by only 4% and did not affect SOMD. A calculated δ¹³C value of microbial CO₂ by a mass balance equation including active and inactive parts of microbial biomass was used to adjust a hypothetical below-ground CO₂ partitioning to the measured and literature data. The active microbial biomass in the rhizosphere amounted to 37% to achieve an appropriate ratio between RR and RMR compared to measured data. Therefore, the three-source partitioning approach failed due to a low active portion of microbial biomass, which is the main microbial CO₂ source controlling the δ¹³C value of total microbial biomass. Since fumigation-extraction reflects total microbial biomass, its δ¹³C value was unsuitable to predict δ¹³C of released microbial CO₂ after a C₃-C₄ vegetation change. The second adjustment to the CO₂ partitioning results in the literature showed that at least 71% of the active microbial biomass utilizing maize rhizodeposits would be necessary to achieve that proportion between RR and RMR observed by other approaches based on ¹⁴C labelling. The method for partitioning total below-ground CO₂ efflux into three sources using a natural ¹³C labelling technique failed due to the small proportion of active microbial biomass in the rhizosphere. This small active fraction led to a discrepancy between δ¹³C values of microbial biomass and of microbially respired CO₂.     

Carbon dynamics of rhizodeposits, root- and shoot-residues in a rice soil

A laboratory incubation experiment was conducted to investigate the fates of plant-derived C during the simulated fallow period in a rice soil. The ¹³C labelled soil and plant materials were used to follow the residue decomposition and its effect on soil organic C (SOC) dynamics under the conditions of either incorporation into soil or intact root systems. The soils were incubated at 15 °C for 240 d and destructive sampling was conducted at 60, 150 and 240 d. To observe the temperature effect, one batch of incubation was shifted from 15 to 25 °C during the last 45 d (between 195 and 240 d). The results showed that the decomposition of the incorporated residues could be divided into two phases: an initial rapid phase followed by a slower phase of decomposition. The decomposition of straw residues was faster than root residues: with 73% of the straw residue being decomposed, compared with 56% of the root residue over 240-d incubation at 15 °C. The water-soluble organic C and microbial biomass C significantly increased after residue incorporation. The total SOC contents, however, slightly decreased, although significant amounts of straw C (14.2%) and root C (8.7%) were found in SOC at the end of incubation, suggesting that the degradation of native SOC occurred concomitantly. Similar to decomposition of the incorporated residues, the organic substances derived from rhizodeposition of the previous season were mineralized rapidly at first and then slowly. The decomposition of the intact root system, however, was extremely slow. This result suggested that the intact root system conserved more organic C in soils compared with the incorporation of fresh residues. Increase of temperature from 15 to 25 °C during the last 45-days of incubation significantly promoted the residue decomposition.     

Labile and recalcitrant plant fractions are utilised by distinct microbial communities in soil: Independent of the presence of roots and mycorrhizal fungi

Plant inputs of organic material to soil are thought to be key determinants of microbial activity, community composition and processes. However, the identity of organisms utilising these chemically diverse inputs is not well understood. In this study, we applied tracer amounts of highly enriched, ¹³C-labelled plant tissue fractions (whole, insoluble and soluble) to soil cores that either allowed or prevented access to roots and mycorrhizal fungi. For all tissue fractions, C derived from the additions was detected rapidly (<2 h after additions) in soil respiration. The additions did not alter microbial community structure, but their fate was strongly dependent on the addition type. The mineralisation of the soluble fraction was the most rapid and was recovered predominantly in bacterial PLFA biomarkers. In contrast, the insoluble addition was mineralised more slowly and recovered in fungal biomarkers to a greater extent. The presence of roots and/or mycorrhizas did not significantly affect rates of mineralisation or the biological fate of additions. A second harvest indicated that the distribution of substrate-derived C within the microbial community was still distinct (i.e. dependent on the form of addition) after 49 d, but with accumulation of ¹³C within the enchytraeid biomass. The results indicate that under the conditions of this experiment, transfer of C between microbial groups is relatively slow, suggesting that the range of chemical forms of plant inputs are likely important in maintaining microbial community structure in soils.     

Moisture modulates rhizosphere effects on C decomposition in two different soil types

While it is well known that soil moisture directly affects microbial activity and soil organic matter (SOM) decomposition, it is unclear if the presence of plants alters these effects through rhizosphere processes. We studied soil moisture effects on SOM decomposition with and without sunflower and soybean. Plants were grown in two different soil types with soil moisture contents of 45% and 85% of field capacity in a greenhouse experiment. We continuously labeled plants with depleted ¹³C, which allowed us to separate plant-derived CO₂-C from original soil-derived CO₂-C in soil respiration measurements. We observed an overall increase in soil-derived CO₂-C efflux in the presence of plants (priming effect) in both soils. On average a greater priming effect was found in the high soil moisture treatment (up to 76% increase in soil-derived CO₂-C compared to control) than in the low soil moisture treatment (up to 52% increase). Greater plant-derived CO₂-C and plant biomass in the high soil moisture treatment contributed to greater priming effects, but priming effects remained significantly higher in the high moisture treatment than in the low moisture treatment after correcting for the effects of plant-derived CO₂-C and plant biomass. The response to soil moisture particularly occurred in the sandy loam soil by the end of the experiment. Possibly, production of root exudates increased with increased soil moisture content. Root exudation of labile C may also have become more effective in stimulating microbial decomposition in the higher soil moisture treatment and sandy loam soil. Our results indicate that moisture conditions significantly modulate rhizosphere effects on SOM decomposition.     

Microbial immobilisation and turnover of N labelled substrates in two arable soils under field and laboratory conditions

Microbial biomass N dynamics were studied under field and laboratory conditions in soils of high yield (HY) and low yield (LY) areas in an agricultural field. The objective of the study was to determine the size and activity of soil microbial biomass in the soils of the different yield areas and to compare these data obtained under field and laboratory conditions. Soils were amended with ¹⁵N labelled mustard (Sinapis alba) residues (both experiments) and labelled nitrate (laboratory only) at 30 μg N g⁻¹ dry soil. Soil microbial biomass (SMB) N, mineral N (N_min) and total N content was monitored both in the field and in the laboratory. N₂O efflux was additionally measured in laboratory treatments. Isotope ratios were determined for SMB in both experiments, for all other parameters only in the laboratory treatments. In the laboratory less amounts of added substrate N were immobilised by the SMB in HY soils compared to LY soils, whereas in the field immobilisation of added N by SMB was higher in HY soils initially and slightly lower after 40 days of incubation. Calculated turnover times in the laboratory nitrate, laboratory mustard and field mustard amendments were 0.18, 0.27 and 0.74 years (HY) and 0.22, 0.61 and 1.01 years (LY), respectively. The turnover times of added substrate N always showed the trend to be faster in HY soils compared to LY soils. A faster turnover of nutrients in the HY soils may involve a better nutrient supply of the plants, which coincides with the higher agricultural yield observed in these areas.     

Microbial reaction in activity, biomass, and community structure after long-term continuous mixing of a grassland soil

Long-term continuous mixing at 40% water holding capacity (WHC) or as slurry at 400% WHC should result in increased soil organic matter decomposition rates in comparison to a control treatment at 40% WHC, but may have strong impacts on soil microbial indices for activity, biomass, and community structure. The amount of extractable inorganic N (NO₃-N+NH₄-N) accumulated in the soil solution after 40 weeks of incubation at 25 °C was 3% of total N in the control treatment and 4% in the two continuous mixing treatments. However, in the treatment mixing at 40% WHC, this 33% increase compared to the control treatment might be explained solely by the decrease in microbial biomass N. In the control treatment, microbial indices decreased in the order microbial biomass C (−10%), microbial biomass N (−40%), ergosterol (−45%) and ATP (−60%). In the treatment mixing at 40% WHC, all four microbial biomass indices were significantly lower than the respective index in the control treatment. This was especially true for microbial biomass N. In the treatment mixing as slurry, only the contents of microbial biomass C and ATP were significantly lower in comparison to the control treatment. The correspondence analysis ordination biplot of the phospholipid fatty acid (PLFA) profiles showed distinct clusters for the three treatments at the end of the incubation. The strongest relative decline of 64% was observed for the fungi-specific PLFA 18:3ω6 in the treatment mixing as slurry in comparison to the control treatment. The content of total bacterial PLFA decreased only by 23%. The differences between the control treatment and the treatment mixing at 40% WHC were less apparent. Fungi represent on average 21% of total microbial biomass C at the end of the incubation if the ergosterol content is recalculated into fungal biomass C. In accordance with this percentage, 22% of the group-specific PLFA could be attributed to fungi.     

Determination of the fate of C labelled maize and wheat rhizodeposit-C in two agricultural soils in a greenhouse experiment under C-CO2-enriched atmosphere

A deeper understanding of the contribution of carbon (C) released by plant roots (rhizodeposition) to soil organic matter (SOM) can help to increase our knowledge of global C-cycling. These insights can eventually lead to sustainable management of SOM especially in agricultural systems. This study was conducted to determine the fate of ¹³C labelled rhizodeposit-C of maize and wheat plants. They were grown in a greenhouse in permeable nylon bags filled with upper soil material from two agricultural soils of the same location, but with different crop yields. The bags were placed into pots, which were also filled with soil surrounding the bags. Soil inside the bags was considered as rhizosphere soil, wheras the one outside the bags represented bulk soil. The contributions of rhizodeposits to water extractable organic carbon (WEOC), microbial biomass-C (MB-C), CO₂-C evolution, and total organic carbon (C_org) were investigated during a 7-week growing period. The WEOC, MB-C, CO₂-C, C_org contents and the respective δ¹³C values were determined regularly, and a newly developed method for determining δ¹³C values in soil extracts was applied.In both soils, regardless of crop yield potential, significant incorporation of rhizodeposition-derived C was observed in the MB-C, CO₂-C, and C_org pool, but not in the WEOC. The pattern of C incorporation into the different pools was the same for both soils with both plants, and rhizodeposit-derived C was recovered in the order MB-C<C_org<CO₂-C. This showed that rhizodeposits were mainly respired, but since C_org was the second largest pool of the overall balances, they were also stabilized in the soils at least in the short term. It is suggested that the increased SOM mineralization observed in this study (positive priming effects) was probably induced by C exchange processes between the soil matrix and soluble rhizodeposits. Moreover, soluble rhizodeposit-C was detected in MB-C and CO₂-C evolved outside the direct root zone, showing the availability of these C-components in the bulk soil.     

Plant biomass influences rhizosphere priming effects on soil organic matter decomposition in two differently managed soils

We used a continuous labeling method of naturally ¹³C-depleted CO₂ in a growth chamber to test for rhizosphere effects on soil organic matter (SOM) decomposition. Two C3 plant species, soybean (Glycine max) and sunflower (Helianthus annus), were grown in two previously differently managed soils, an organically farmed soil and a soil from an annual grassland. We maintained a constant atmospheric CO₂ concentration at 400±5 ppm and δ¹³C signature at −24.4‰ by regulating the flow of naturally ¹³C-depleted CO₂ and CO₂-free air into the growth chamber, which allowed us to separate new plant-derived CO₂-C from original soil-derived CO₂-C in soil respiration. Rhizosphere priming effects on SOM decomposition, i.e., differences in soil-derived CO₂-C between planted and non-planted treatments, were significantly different between the two soils, but not between the two plant species. Soil-derived CO₂-C efflux in the organically farmed soil increased up to 61% compared to the no-plant control, while the annual grassland soil showed a negligible increase (up to 5% increase), despite an overall larger efflux of soil-derived CO₂-C and total soil C content. Differences in rhizosphere priming effects on SOM decomposition between the two soils could be largely explained by differences in plant biomass, and in particular leaf biomass, explaining 49% and 74% of the variation in primed soil C among soils and plant species, respectively. Nitrogen uptake rates by soybean and sunflower was relatively high compared to soil C respiration and associated N mineralization, while inorganic N pools were significantly depleted in the organic farm soil by the end of the experiment. Despite relatively large increases in SOM decomposition caused by rhizosphere effects in the organic farm soil, the fast-growing soybean and sunflower plants gained little extra N from the increase in SOM decomposition caused by rhizosphere effects. We conclude that rhizosphere priming effects of annual plants on SOM decomposition are largely driven by plant biomass, especially in soils of high fertility that can sustain high plant productivity.     

砂姜黑土有机碳与微生物群落特性之间的关系

砂姜黑土是我国典型的中低产土壤,提升土壤有机质是砂姜黑土改良的重要环节,然而砂姜黑土有机质与微生物之间的关联关系尚不明确。本文采集40个代表性苏北平原砂姜黑土样品,测定土壤有机碳、全氮、可溶性有机碳、微生物生物量碳氮,利用稳定性同位素13C标记的谷氨酸测定微生物碳利用率、16SrRNA基因高通量测序分析微生物群落结构,研究苏北平原砂姜黑土有机质及关键微生物的关联关系。结果显示:苏北平原砂姜黑土有机碳含量为15.82 g/kg,可溶性有机碳占土壤有机碳含量的2‰,表明有机质的碳有效性较低;砂姜黑土优势菌门为变形菌门,优势菌科以黄单胞菌科、Gaiellaceae、酸杆菌科细菌为主,但高效的碳转化菌群(如酸杆菌纲和放线菌)较少;微生物碳利用率普遍较低,在0.07～0.20之间,与有机碳、可溶性有机碳和微生物生物量碳含量显著相关。因此,较低的微生物碳利用率、较低的碳转化菌群组成比例制约了砂姜黑土有机质的形成与提升。     

New method to estimate root biomass in soil through root-derived carbon

Quantification of root biomass through the conventional root excavation and washing method is inefficient. A pot experiment was conducted to estimate root-derived carbon (C) in soil. Spring wheat (Triticum aestivum L. cv. ‘Quantum’) was grown in plastic containers (6 L) filled with sterilized sandy soil in a greenhouse. Plants were enriched with ¹³CO₂ in a glass chamber twice at growth stages GS-37 and GS-59 for 70 min at each time. In one treatment, roots were separated from soil at crop maturity, washed and dried for the determination of biomass. Isotope ratios were then separately analyzed for roots and soil. In a second treatment, roots were thoroughly mixed with the whole soil and representative samples were analyzed for ¹³C abundance at crop maturity. Control plants were untreated with ¹³C, in which roots were separated from soil. The root biomass was calculated based on the root-derived C, which was measured through ¹³C abundance in the soil and root mixed samples. A substantial amount of root-derived C (24%) was unaccounted while separating the roots from soil. Similarly, about 36% of the root biomass was underestimated if conventional root excavation and washing method is used. It has been shown that root biomass can be estimated more accurately from the root-derived C using ¹³C tracer method than the estimates made by the conventional excavation and washing method. We propose this as an alternative method for the estimation of root-derived C in soil, based on which root biomass can be estimated.     

Soil microbial biomass and activity under a potato crop fertilised with N with and without C

Summary A range of soil microbiological parameters were measured at intervals throughout the growing season of a potato crop. Treatments applied to the soil at sowing were zero N fertilisation of N fertilisation at 120 kg N ha^–1, either alone or supplemented with straw or sucrose at 1200 kg C ha^–1. C and N flushes determined by fumigation-incubation and fumigation-extraction, and substrate-induced respiration, were measured as indicators of microbial biomass. Microbial activity was measured as respiration (CO₂ production) and dehydrogenase activity (formazan production). The greatest effects were obtained from the addition of N plus sucrose. Both biomass size and activity were significantly stimulated for up to 25 days after incorporation, with the magnitude of the effects consistently diminishing over time. By 125 days after planting, there was no detectable legacy from any of the treatmentson any of the biomass parameters that were measured, and all values had reverted to those prevalent at planting. There was no consistent effect from adding N, either alone or supplemented with straw, on any of the biomass parameters. There was no evidence for crop-induced stimulation of the biomass. The experiment demonstrates that biomass is only influenced where the quantity, quality, and rate of incorporation of C into the soil is appropriate, in this case, only by adding C as a pulse of sucrose.