Rhizosphere priming effect: Its functional relationships with microbial turnover, evapotranspiration, and C–N budgets

Understanding soil organic matter (SOM) decomposition and its interaction with rhizosphere processes is a crucial topic in soil biology and ecology. Using a natural ¹³C tracer method to separately measure SOM-derived CO₂ from root-derived CO₂, this study aims to connect the level of rhizosphere-dependent SOM decomposition with the C and N balance of the whole plant–soil system, and to mechanistically link the rhizosphere priming effect to soil microbial turnover and evapotranspiration. Results indicated that the magnitude of the rhizosphere priming effect on SOM decomposition varied widely, from zero to more than 380% of the unplanted control, and was largely influenced by plant species and phenology. Balancing the extra soil C loss from the strong rhizosphere priming effect in the planted treatments with C inputs from rhizodeposits and root biomass, the whole plant–soil system remained with a net carbon gain at the end of the experiment. The increased soil microbial biomass turnover rate and the enhanced evapotranspiration rate in the planted treatments had clear positive relationships with the level of the rhizosphere priming effect. The rhizosphere enhancement of soil carbon mineralization in the planted treatments did not result in a proportional increase in net N mineralization, suggesting a possible de-coupling of C cycling with N cycling in the rhizosphere.     

Increased degradation of phenanthrene in soil by Pseudomonas sp. GF3 in the presence of wheat

A phenanthrene-degrading bacterial strain Pseudomonas sp. GF3 was examined for plant-growth promoting effects and phenanthrene removal in soil artificially contaminated with low and high levels of phenanthrene (0, 100 and 200 mg kg⁻¹) in pot experiments. Low and high phenanthrene treatments significantly decreased the growth of wheat. Inoculation with bacterial strain Pseudomonas sp. GF3 was found to increase root and shoot growth of wheat. Strain GF3 was able to degrade phenanthrene effectively in the unplanted and planted soils. Over a period of 80 days the concentration of phenanthrene in soil in which wheat was grown was significantly lower than in unplanted soil (p<0.05). At the end of the 80-d experiments, 62.2% and 42.3% of phenanthrene had disappeared from planted soils without Pseudomonas sp. GF3 when the phenanthrene was added at 100 and 200 mg kg⁻¹ soil, respectively, but 84.8% and 70.2% of phenanthrene had disappeared from planted soils with the bacterial inoculation. The presence of vegetation significantly enhances the dissipation of phenanthrene in the soil. There was no significant difference in soil polyphenol oxidase activities among the applications of 0, 100 and 200 mg kg⁻¹ of phenanthrene. However, the enzyme activities in planted and unplanted soils inoculated with the strain Pseudomonas sp. GF3 were significantly higher than those of non-inoculation controls. The bacterial isolate was also able to colonize and develop in the rhizosphere soil of wheat after inoculation.     

Nitrate removal from drained and reflooded fen soils affected by soil N transformation processes and plant uptake

Knowledge about nitrate transformation processes and how they are affected by different plants is essential in order to reduce the loss of valuable N fertiliser as well as to prevent environmental pollution due to nitrate leaching or N₂O emission after fertilisation or the reflooding of degraded fens with nitrate-containing municipal sewage. Therefore four microcosm ¹⁵N tracer experiments were performed to evaluate the effect of common wetland plants (Phalaris arundinacea, Phragmites australis) combined with different soil moisture conditions (from dry to reflooded) on nitrate turnover processes. At the end of experiment, the total formation of gaseous N compounds was calculated using the ¹⁵N balance method. In two experiments (wet and reflooded soil conditions) the N₂O and N₂ emissions were also directly determined.Our results show that in degraded fen soils, which process mainly takes place—denitrification or transformation into organic N compounds—is determined by the soil moisture conditions. Under dry soil moisture conditions (water filled pore space: 31%) up to 80% of the ¹⁵N nitrate added was transformed into organic N compounds. This transformation process is not affected by plant growth. Under reflooded conditions (water filled pore space: 100%), the total gaseous N losses were highest (77-95% of the ¹⁵N-nitrate added) and the transformation into organic N compounds was very low (1.8% of ¹⁵N nitrate added). Under almost all soil conditions plant growth reduced the N losses by 20-25% of the ¹⁵N nitrate added due to plant uptake. The N₂ emissions exceeded the N₂O emissions by a factor of 10-20 in planted soil, and as much as 30 in unplanted soil. In the treatments planted with Phragmites australis, N₂O emission was about two times higher than in the corresponding unplanted treatment. 15% of the N₂O and N₂ formed was transported via the Phragmites shoots from the soil into the atmosphere. By contrast, Phalaris arundinacea did not affect N₂O emissions and no emission via the shoots was observed.     

Plant roots and species moderate the salinity effect on microbial respiration,biomass, and enzyme activities in a sandy clay soil

The aim of this study was to determine the effects of plant absence or presence on microbial properties and enzyme activities at different levels of salinity in a sandy clay soil. The treatments involved five salinity levels—0.5 (control), 2.5, 5, 7.5, and 10 dS m^?1 which were prepared using a mixture of chloride salts—and three soil environments (unplanted soil, and soils planted with either wheat or clover) under greenhouse conditions. Each treatment was replicated three times. At the end of the experiment, soil microbial respiration, substrate-induced respiration (SIR), microbial biomass C (MBC), and enzyme activities were determined after plant harvest. Increasing salinity decreased soil microbial properties and enzyme activities, but increased the metabolic quotient (qCO₂) in both unplanted and planted soils. Most microbial properties of planted soils were greater than those of unplanted soils at low to moderate salinity levels, depending upon plant species. There was a small or no difference in soil properties between the unplanted and planted treatments at the highest salinity level, indicating that the indirect effects of plant presence might be less important due to significant reduction of plant growth. The lowered microbial activity and biomass, and enzyme activities were due to the reduction of root activity and biomass in salinized soils. The lower values of qCO₂ in planted than unplanted soils support the positive influence of plant root and its exudates on soil microbial activity and biomass in saline soils. Nonetheless, the role of plants in alleviating salinity influence on soil microbial activities decreases at high salinity levels and depends on plant type. In conclusion, cultivation and growing plant in abandoned saline environments with moderate salinity would improve soil microbial properties and functions by reducing salinity effect, in particular planting moderately tolerant crops. This helps to maintain or increase the fertility and quality of abandoned saline soils in arid regions.     

Scots pine (Pinus sylvestris L.) roots and soil moisture did not affect soil thermal sensitivity

Through their effects on microbial metabolism, temperature and moisture affect the rate of decomposition of soil organic matter. Plant roots play an important role in SOM mineralization and nutrient cycling. There are reports that rhizosphere soil exhibits higher sensitivity to temperature than root-free soil, and this can have implications for how soil CO₂ efflux may be affected in a warmer world. We tested the effects of 1-week incubation under different combinations of temperature (5, 15, 30 ^°C) and moisture (15, 50, 100% WHC) on the respiration rate of soil planted with Scots pine and of unplanted soil. Soil respiration in both soils was the highest at moderate moisture (p < 0.0001) and, increased with temperature (p < 0.0001). There was also marginally significant effect of soil kind on respiration rate (p < 0.055), but the significant interaction of temperature effect with soil kind effect, indicated, that soil respiration of planted soil was higher than unplanted soil only at 5 ^°C (p < 0.05). The soil kind effect was compared also as Q₁₀ coefficients for respiration rate, showing the relative change in microbial activity with increased temperature. However, there was no difference in the thermal sensitivity of soil respiration between planted and unplanted soils (p = 0.99), irrespective of the level of soil moisture. These findings were similar to the latest studies and confirmed, that in various models, being useful tools in studying of soil carbon cycling, there is no need to distinguish between planted and unplanted soil as different soil carbon pools.     

Comparative analysis of planted and unplanted controls for assessment of rhizosphere priming effect

The rhizosphere priming effect (RPE) is increasingly being considered to be an important regulator of soil organic matter (SOM) decomposition and nutrient turnover, with potential importance for the global CO₂ budget. As a result, studies on the RPE have rapidly increased in number over the last few years.Most of these experiments have been performed using unplanted soil as the control, which could potentially lead to incorrect assessment of the RPE. Therefore,we performed a greenhous...     

Effects of elevated CO2 concentration on rhizodeposition from Lolium perenne grown on soil exposed to 9 years of CO2 enrichment

The effects of enriched CO₂ atmosphere on partitioning of recently assimilated carbon were investigated in a plant-soil-microorganism system in which Lolium perenne seedlings were planted into cores inserted into the resident soil within a sward that had been treated with elevated CO₂ for 9 consecutive years, under two N fertilisation levels (Swiss FACE experiment). The planted cores were excavated from the ambient (35 Pa pCO₂) and enriched (60 Pa pCO₂) rings at two dates, in spring and autumn, during the growing season. The cores were brought back to the laboratory for ¹⁴C labelling of shoots in order to trace the transfer of recently assimilated C both within the plant and to the soil and microbial biomass. At the spring sampling, high N supply stimulated shoot and total dry matter production. Consistently, high N enhanced the allocation of recently fixed C to shoots, and reduced it to belowground compartments. Elevated CO₂ had no consequences for DM or the pattern of C allocation. At the autumn sampling, at high N plot, yield of L. perenne was stimulated by elevated CO₂. Consistently, ¹⁴C was preferentially allocated aboveground and, consequently belowground recent C allocation was depressed and rhizodeposition reduced. At both experimental periods, total soil C content was similar in all treatments, providing no evidence for soil carbon sequestration in the Swiss Free Air CO₂ Enrichment experiment (FACE) after 9 years of enrichment. Recently assimilated C and soil C were mineralised faster in soils from enriched rings, suggesting a CO₂-induced shift in the microbial biomass characteristics (structure, diversity, activity) and/or in the quality of the root-released organic compounds.     

13C标记磷脂脂肪酸分析在土壤微生物生态 研究中的应用

磷脂脂肪酸(PLFA)是微生物细胞膜的重要组成成分,不同微生物群落可通过不同生化途径合成不同的PLFA,因此可选择某些PLFA作为微生物群落结构变化的生物标志物。PLFA与稳定性同位素~(13)C标记(~(13)C-PLFA)技术结合,不仅能够确定原位土壤环境中微生物群落组成,而且能够定向发掘土壤生态系统中参与碳源代谢过程的微生物群落,提供复杂群落中土壤微生物相互作用的信息,具有广阔的应用前景。其基本原理为:将富集~(13)C稳定同位素的基质加入土壤中,土壤中的某些微生物群落利用基质~(13)C合成PLFA,提取并纯化土壤微生物的PLFA,利用气相色谱-燃烧-同位素比例质谱(GC-C-IRMS)测定其~(13)C丰度,通过对比分析,从而获取微生物群落组成与其功能的直接信息。本文在介绍了~(13)C-PLFA原理的基础上,综述了该技术在光合同化碳的根际微生物利用、土壤有机质分解的激发效应、甲烷氧化、有机污染物降解、外源简单碳源和外源复杂碳源的微生物利用等方面的应用,对此项技术的优缺点进行了分析并展望了其未来应用。     

Soil structural responses to alterations in soil microbiota induced by the dilution method and mycorrhizal fungal inoculation

This investigation examines the effect of manipulating soil microbial community composition and species richness on the development of soil structure over a seven month period in planted (with or without mycorrhizal fungi) and in unplanted macrocosms. The dilution method effectively resulted in soil communities with consistently contrasting levels of species (TRF) richness. In particular, the 10^?6 dilution of field soil resulted in less rich communities in bare unplanted soil than did the 10^?1 soil dilution. However, this was not the case in planted soils where root activity was a powerful influence on species richness. After seven months, principal components analysis (PCA) separated bacterial community composition primarily on planting regime; planted mycorrhizal, planted non-mycorrhizal and bare soil treatments all contained different bacterial community compositions. A consistent finding in planted and unplanted soils was that aggregate stability was positively correlated with small pore sizes. Mycorrhizal colonisation decreased plant biomass and also resulted in reduced soil bacterial species richness, lower percentage organic matter and smaller pore sizes relative to planted but non-mycorrhizal soils. However, soil aggregate stability and water repellency were increased in these (mycorrhizal) soils probably due to AMF hyphal activities including enmeshment and/or glomalin production. In contrast, bacterial TRF richness was positively correlated with aggregate stability in the bare and non-mycorrhizal planted soils. Soil organic carbon was an important factor in all treatments, but in the bare soil where there was no additional input of labile C from roots, the percentage C could be directly related to fungal TRF richness. The less species rich bare soil contained more organic C than the more species rich bare soil. This suggests a degree of redundancy with regard to mineralisation of organic matter when additional, more utilisable C sources are unavailable. Understanding the effects of microbial diversity on functional parameters is important for advancing sustainable soil management techniques, but it is clear that soil is a dynamic ecosystem.     

Carbon flow from C-labeled straw and root residues into the phospholipid fatty acids of a soil microbial community under field conditions

To better understand how residue quality and seasonal conditions influence the flow of C from both root and straw residues into the soil microbial community, we followed the incorporation of ¹³C-labeled crimson clover (Trifolium incarnatum) and ryegrass (Lolium multiflorum) root and straw residues into the phospholipid fatty acids (PLFA) of soil microbial biomass. After residue incorporation under field conditions in late summer (September), the ¹³C content of soil PLFA was measured in September, October, and November, 2002, and April and June, 2003. Multivariate non-metric multidimensional scaling techniques showed that the distribution of ¹³C among microbial PLFA differed among the four primary treatments (ryegrass straw and roots, clover straw and roots). Regardless of treatment, some PLFA remained poorly labeled with ¹³C throughout much of the study (16:1ω5, 10Me17:0; 0-5%), whereas other PLFA consistently contained a larger percentage of residue-derived C (16:0; 18:1ω9, 18:2ω6,9; 10-25%). The distribution of residue ¹³C among individual PLFA differed from the relative contributions of individual PLFA (mol%) to total PLFA-C, suggesting that a subset of the soil biomass was primarily responsible for assimilating residue-derived C. The distribution of ¹³C among soil PLFA differed between the sampling times, indicating that residue properties and soil conditions influenced which members of the community were assimilating residue-derived C. Our findings will provide the foundation for further studies to identify the nature of the community members responsible for residue decomposition at different times of the year, and what factors account for the dynamics of the community involved.     

Microbial communities of a native perennial bunchgrass do not respond consistently across a gradient of land-use intensification

To test if native perennial bunchgrasses cultivate the same microbial community composition across a gradient in land-use intensification, soils were sampled in fall, winter and spring in areas under bunchgrasses (‘plant’) and in bare soils (‘removal’) in which plots were cleared of living plants adjacent to native perennial bunchgrasses (Nassella pulchra). The gradient in land-use intensification was represented by a relict perennial grassland, a restored perennial grassland, and a perennial grass agriculture site on the same soil type. An exotic annual grassland site was also included because perennial bunchgrasses often exist within a matrix of annual grasses in California. Differences in soil resource pools between ‘plant’ and ‘removal’ soils were observed mainly in the relict perennial grassland and perennial grass agriculture site. Seasonal responses occurred in all sites. Microbial biomass carbon (C) and dissolved organic C were greater under perennial bunchgrasses in the relict perennial grassland and perennial grass agriculture site when comparing treatment means of ‘plant’ vs. ‘removal’ soil. In general, soil moisture, microbial respiration, and nitrate decreased from fall to spring in ‘plant’ and ‘removal’ soils, while soil ammonium and net mineralizable nitrogen (N) increased only in ‘plant’ soils. A canonical correspondence analysis (CCA) of phospholipid fatty acid (PLFA) profiles from all sites showed that land-use history limits the similarity of microbial community composition as do soil C and N dynamics among sites. When PLFA profiles from individual sites were analyzed by CCA, different microbial PLFA markers were associated with N. pulchra in each site, indicating that the same plant species does not retain a unique microbial fingerprint across the gradient of land-use intensification.     

Natural abundance δN and δC of DNA extracted from soil

We report the first simultaneous measurements of δ¹⁵N and δ¹³C of DNA extracted from surface soils. The isotopic composition of DNA differed significantly among nine different soils. The δ¹³C and δ¹⁵N of DNA was correlated with δ¹³C and δ¹⁵N of soil, respectively, suggesting that the isotopic composition of DNA is strongly influenced by the isotopic composition of soil organic matter. However, in all samples DNA was enriched in ¹³C relative to soil, indicating microorganisms fractionated C during assimilation or preferentially used ¹³C enriched substrates. Enrichment of DNA in ¹⁵N relative to soil was not consistently observed, but there were significant differences between δ¹⁵N of DNA and δ¹⁵N of soil for three different sites, suggesting microorganisms are fractionating N or preferentially using N substrates at different rates across these contrasting ecosystems. There was a strong linear correlation between δ¹⁵N of DNA and δ¹⁵N of the microbial biomass, which indicated DNA was depleted in ¹⁵N relative to the microbial biomass by approximately 3.4‰. Our results show that accurate and precise isotopic measurements of C and N in DNA extracted from the soil are feasible, and that these analyses may provide powerful tools for elucidating C and N cycling processes through soil microorganisms.     

Effects of multiple heavy metal contamination and repeated phytoextraction by Sedum plumbizincicola on soil microbial properties

The threat of heavy metal contamination to food and human health in south and east China has become a public concern as industrial development continues. The aims of this study were to investigate the influence of repeated phytoextraction over a two-year period by successive crops of the Zn and Cd hyperaccumulator Sedum plumbizincicola on multiple metal contaminated soils and to assess recovery of soil quality. Total and NH₄OAc-extractable Zn and Cd concentrations were significantly reduced in planted soils compared to unplanted soils. Microbial biomass C (C_mic), basal respiration and microbial quotient (qM) were significantly and positively correlated and soil metabolic quotient (qCO₂) was negatively correlated with heavy metal concentrations in unplanted soils (P < 0.05). However, C_mic, basal respiration and qM values increased significantly after phytoremediation by five crops over two years compared to unplanted soil. Urease, β-glucosidase, neutral phosphatase and arylsulfatase activities also increased significantly with decreasing heavy metal contents and hydrolase activity was enhanced in planted soil (P < 0.05) compared to the unplanted control. The data indicate the capacity of S. plumbizincicola to extract Zn and Cd from contaminated soil and also that phytoremediation had beneficial effects on soil microbial and hydrolase activities, with the metal phytoextraction procedure restoring soil quality.     

Carbon fluxes from plants to soil and dynamics of microbial immobilization under plastic film mulching and fertilizer application using 13C pulse-labeling

Plastic film mulching and fertilizer application have improved soil fertility and sustainable agricultural development in China. However, there is limited information on carbon (C) fluxes from plants to soil, and dynamics of microbial immobilization where these practices have been employed. The objectives of this study were to quantify the photosynthesized C transported from maize to soil, and to assess the effect of mulching (with or without plastic film mulching) and fertilizer (no fertilizer control, medium-level organic manure, and high-level organic manure) application on the dynamics of microbial C sequestration. We used in-situ ¹³C pulse-labeling of maize planted at an experimental site in Liaoning Province, China. We found, on average, from 12% to 15% of net fixed ¹³C was translocated belowground up to 15 days after labeling. More than 60% of ¹³C retained in soil was incorporated into microbial biomass C on the 1st day, but only less than 27% on the 15th day after labeling. Treatments that combined organic manure application with mulching showed the greatest proportion of photosynthesized ¹³C incorporated belowground and the largest amount of microbial biomass C derived from rhizodeposits. These results demonstrate that organic manure application coupled with plastic film mulching facilitates photosynthesized C sequestration belowground and enhances soil microbial activity during the maize seedling stage.     

Rhizosphere priming effect of Populus fremontii obscures the temperature sensitivity of soil organic carbon respiration

C efflux from soils is a large component of the global C exchange between the biosphere and the atmosphere. However, our understanding of soil C efflux is complicated by the “rhizosphere priming effect,” in which the presence of live roots may accelerate or suppress the decomposition of soil organic C. Due to technical obstacles, the rhizosphere priming effect is under-studied, and we know little about rhizosphere priming in tree species. We measured the rates of soil-derived C mineralization in root-free soil and in soil planted with cottonwood (Populus fremontii) trees. Live cottonwood roots greatly accelerated (a rhizosphere priming effect) or suppressed (a negative rhizosphere priming effect) the mineralization of soil organic C, depending upon the time of the year. At its maximum, soil organic C was mineralized nine times faster in the presence of cottonwood roots than in the unplanted controls. Over the course of the experiment, approximately twice as much soil organic C was mineralized in pots planted with cottonwoods compared to unplanted control pots. Soil organic C mineralization rates in the unplanted controls were temperature-sensitive. In contrast, soil organic C mineralization in the cottonwood rhizosphere was unresponsive to seasonal temperature changes, due to the strength of the rhizosphere priming effect. The rhizosphere priming effect is of key importance to our understanding of soil C mineralization, because it means that the total soil respiration is not a simple additive function of soil-derived and plant-derived respiration.     

Soil organic matter in soil depth profiles: Distinct carbon preferences of microbial groups during carbon transformation

This study investigates how carbon sources of soil microbial communities vary with soil depth. Microbial phospholipid fatty acids (PLFA) were extracted from 0–20, 20–40 and 40–60 cm depth intervals from agricultural soils and analysed for their stable carbon isotopes (δ¹³C values). The soils had been subjected to a vegetation change from C3 (δ¹³C≈?29.3‰) to C4 plants (δ¹³C≈?12.5‰) 40 years previously, which allowed us to trace the carbon flow from plant-derived input (litter, roots, and root exudates) into microbial PLFA. While bulk soil organic matter (SOM) reflected ≈12% of the C4-derived carbon in top soil (0–20 cm) and 3% in deeper soil (40–60 cm), the PLFA had a much higher contribution of C4 carbon of about 64% in 0–20 cm and 34% in 40–60 cm. This implies a much faster turnover time of carbon in the microbial biomass compared to bulk SOM. The isotopic signature of bulk SOM and PLFA from C4 cultivated soil decreases with increasing soil depth (?23.7‰ to ?25.0‰ for bulk SOM and ?18.3‰ to ?23.3‰ for PLFA), which demonstrates decreasing influence of the isotopic signature of the new C4 vegetation with soil depth. In terms of soil microbial carbon sources this clearly shows a high percentage of C4 labelled and thus young plant carbon as microbial carbon source in topsoils. With increasing soil depth this percentage decreases and SOM is increasingly used as microbial carbon source. Among all PLFA that were associated to different microbial groups it could be observed that (a) depended on availability, Gram-negative and Gram-positive bacteria prefer plant-derived carbon as carbon source, however, (b) Gram-positive bacteria use more SOM-derived carbon sources while Gram-negative bacteria use more plant biomass. This tendency was observed in all three-depth intervals. However, our results also show that microorganisms maintain their preferred carbon sources independent on soil depth with an isotopic shift of 3–4‰ from 0–20 to 40–60 cm soil depth.     

Fate of gram-negative bacterial biomass in soil—mineralization and contribution to SOM

Soil microorganisms contribute to the formation of non-living soil organic matter (SOM) by metabolic transformation of plant-derived material. After cell death, their biomass components with a specific molecular character become incorporated into SOM imprinting its chemical properties, although this process has not yet been quantified. In order to elucidate the contribution to SOM formation, we investigated the fate of gram-negative bacterial model biomass (Escherichia coli usually introduced into soil with manure or feces) during incubation of soil with isotopically (¹³C) and genetically (lux gene) labeled cells. The decline of living cells was monitored by the loss of bioluminescence. The carbon turnover and mineralization was balanced by bulk soil stable isotope analysis, and the persistence of nucleic acids was investigated by PCR amplification of the lux gene. During incubation, the number of viable E. coli cells decreased rapidly (99.9% within the first 42 d) serving as substrate for other microorganisms or for the formation of SOM, and bioluminescent cells could only be detected during the first 56 d. However, the lux gene was still detected after 224 d, which indicates stabilization of DNA in SOM. Although the survival of E. coli in soil is limited, only about 65% of the added labeled biomass carbon was mineralized to ¹³CO₂ and 51% remained in soil after 224 d with an average ¹³C recovery of 117%. The amount of ¹³C found in the PLFA representative of living cells had decreased to 25% of the initial value, suggesting a proportional decrease of the ¹³C in the soil microbial biomass. The extent of this decrease is higher than the mineralization of the bulk E. coli C and thus the difference of around 25% has to be stabilized as metabolites, or in non-living SOM. The data provide evidence that the genetic information and a considerable part of the carbon from dying bacterial biomass were retained in both the soil microbial food web and in non-living SOM.     

Soil carbon dioxide emission from intensively cultivated black soil in Northeast China: nitrogen fertilization effect

Purpose

The aim of this study was to understand the effect of nitrogen fertilization on soil respiration and native soil organic carbon (SOC) decomposition and to identify the key factor affecting soil respiration in a cultivated black soil.

Materials and methods

A field experiment was conducted at the Harbin State Key Agroecological Experimental Station, China. The study consisted of four treatments: unplanted and N-unfertilized soil (U0), unplanted soil treated with 225?kg?N?ha^?1 (UN), maize planted and N-unfertilized soil (P0), and planted soil fertilized with 225?kg?N?ha^?1 (PN). Soil CO₂ and N₂O fluxes were measured using the static closed chamber method.

Results and discussion

Cumulative CO₂ emissions during the maize growing season with the U0, UN, P0, and PN treatments were 1.29, 1.04, 2.30 and 2.27?Mg?C?ha^?1, respectively, indicating that N fertilization significantly reduced the decomposition of native SOC. However, no marked effect on soil respiration in planted soil was observed because the increase of rhizosphere respiration caused by N addition was counteracted by the reduction of native SOC decomposition. Soil CO₂ fluxes were significantly affected by soil temperature but not by soil moisture. The temperature sensitivity (Q ₁₀) of soil respiration was 2.16?C2.47 for unplanted soil but increased to 3.16?C3.44 in planted soil. N addition reduced the Q ₁₀ of native SOC decomposition possibly due to low labile organic C but increased the Q ₁₀ of soil respiration due to the stimulation of maize growth. The estimated annual CO₂ emission in N-fertilized soil was 1.28?Mg?C?ha^?1 and was replenished by the residual stubble, roots, and exudates. In contrast, the lost C (1.53?Mg?C?ha^?1) in N-unfertilized soil was not completely supplemented by maize residues, resulting in a reduction of SOC. Although N fertilization significantly increased N₂O emissions, the global warming potential of N₂O and CO₂ emissions in N-fertilized soil was significantly lower than in N-unfertilized soil.

Conclusions

The stimulatory or inhibitory effect of N fertilization on soil respiration and basal respiration may depend on labile organic C concentration in soil. The inhibitory effect of N fertilization on native SOC decomposition was mainly associated with low labile organic C in tested black soil. N application could reduce the global warming potential of CO₂ and N₂O emissions in black soil.     

Microbial community abundance and structure are determinants of soil organic matter mineralisation in the presence of labile carbon

Altered rates of native soil organic matter (SOM) mineralisation in the presence of labile C substrate (‘priming’), is increasingly recognised as central to the coupling of plant and soil-biological productivity and potentially as a key process mediating the C-balance of soils. However, the mechanisms and controls of SOM-priming are not well understood. In this study we manipulated microbial biomass size and composition (chloroform fumigation) and mineral nutrient availability to investigate controls of SOM-priming. Effects of applied substrate (¹³C-glucose) on mineralisation of native SOM were quantified by isotopic partitioning of soil respiration. In addition, the respective contributions of SOM-C and substrate-derived C to microbial biomass carbon (MBC) were quantified to account for pool-substitution effects (‘apparent priming’). Phospholipid fatty acid (PLFA) profiles of the soils were determined to establish treatment effects on microbial community structure, while the ¹³C-enrichment of PLFA biomarkers was used to establish pathways of substrate-derived C-flux through the microbial communities. The results indicated that glucose additions increased SOM-mineralisation in all treatments (positive priming). The magnitude of priming was reduced in fumigated soils, concurrent with reduced substrate-derived C-flux through putative SOM-mineralising organisms (fungi and actinomycetes). Nutrient additions reduced the magnitude of positive priming in non-fumigated soils, but did not affect the distribution of substrate-derived C in microbial communities. The results support the view that microbial community composition is a determinant of SOM-mineralisation, with evidence that utilisation of labile substrate by fungal and actinomycete (but not Gram-negative) populations promotes positive SOM-priming.     

Rhizosphere activity, grass species and N availability effects on the soil C and N cycles

We examined whether grass species and soil nitrogen (N) availability could enhance Carbon (C) and N turnover during root litter decay in grassland. Three species with increasing competitiveness (Festuca ovina, Dactylis glomerata and Lolium perenne) were grown at two N fertiliser levels in an undisturbed grassland soil, in which soil organic fractions derived for the last 9 years from Lolium root litter which was ¹³C-depleted. During the subsequent experimental year, the C turnover was calculated using the respective δ¹³C values of the old and new C in the root phytomass, in two Particulate Organic Matter (POM) fractions above 200 μm and in the lightest part of the aggregated soil fraction between 50 and 200 μm. Soil N availability was monitored during the regrowth periods with ion exchange resins (IER). The C decay rates of each particle size fraction were calculated with a simple mechanistic model of C dynamics. The N mineralisation immobilisation turnover (MIT) was characterised by dilution of ¹⁵N-labelled fertiliser in the N harvestThe C:N ratio and the residence time of C in the fractions decreased with particle size. The presence of a grass rhizosphere increased the decay rate of old C. Accumulation of new C in particle size fractions increased with species competitiveness and with N supply. Species competitiveness increased C turnover in the aggregated fraction, as a result of greater accumulation of new C and faster decay of old C. Fertiliser N increased N turnover and C mineralisation in the SOM. Species competitiveness decreased soil -N exchanged with the IER and increased dissolved organic C (DOC) content. The nature of the current rhizosphere is thus an important factor driving C and N transformations of the old root litter, in relation with grass species strategy. Plant competitiveness may stimulate the C and N turnover in the more evolved SOM fractions in a similar way to the mineral N supply.