Root trait plasticity and plant nutrient acquisition in phosphorus limited soil

To overcome soil nutrient limitation, many plants have developed complex nutrient acquisition strategies including altering root morphology, root hair formation or colonization by arbuscular mycorrhizal fungi (AMF). The interactions of these strategies and their plasticity are, however, affected by soil nutrient status throughout plant growth. Such plasticity is decisive for plant phosphorus (P) acquisition in P‐limited soils. We investigated the P acquisition strategies and their plasticity of two maize genotypes characterized by the presence or absence of root hairs. We hypothesized that in the absence of root hairs plant growth is facilitated by traits with complementary functions, e.g., by higher root mycorrhizal colonization. This dependence on complementary traits will decrease in P fertilized soils. At early growth stages, root hairs are of little benefit for nutrient uptake. Regardless of the presence or absence of root hairs, plants produced average root biomass of 0.14 g per plant and exhibited 23% root mycorrhizal colonization. At later growth stages of maize, contrasting mechanisms with functional complementarity explained similar plant biomass production under P limitation: the presence of root hairs versus higher root mycorrhizal colonization (67%) favored by increased fine root diameter in absence of root hairs. P fertilization decreased the dependence of plant on specific root traits for nutrient acquisition. Through root trait plasticity, plants can minimize trade‐offs for developing and maintaining functional traits, while increasing the benefit in terms of nutrient acquisition and plant growth. The present study highlights the plasticity of functional root traits for efficient nutrient acquisition strategies in agricultural systems with low nutrient availability.     

C fractionation at the root-microorganisms-soil interface: A review and outlook for partitioning studies

Natural variations of the ¹³C/¹²C ratio have been frequently used over the last three decades to trace C sources and fluxes between plants, microorganisms, and soil. Many of these studies have used the natural-¹³C-labelling approach, i.e. natural δ¹³C variation after C₃-C₄ vegetation changes. In this review, we focus on ¹³C fractionation in main processes at the interface between roots, microorganisms, and soil: root respiration, microbial respiration, formation of dissolved organic carbon, as well as microbial uptake and utilization of soil organic matter (SOM). Based on literature data and our own studies, we estimated that, on average, the roots of C₃ and C₄ plants are ¹³C enriched compared to shoots by +1.2 ± 0.6‰ and +0.3 ± 0.4‰, respectively. The CO₂ released by root respiration was ¹³C depleted by about −2.1 ± 2.2‰ for C₃ plants and −1.3 ± 2.4‰ for C₄ plants compared to root tissue. However, only a very few studies investigated ¹³C fractionation by root respiration. This urgently calls for further research. In soils developed under C₃ vegetation, the microbial biomass was ¹³C enriched by +1.2 ± 2.6‰ and microbial CO₂ was also ¹³C enriched by +0.7 ± 2.8‰ compared to SOM. This discrimination pattern suggests preferential utilization of ¹³C-enriched substances by microorganisms, but a respiration of lighter compounds from this fraction. The δ¹³C signature of the microbial pool is composed of metabolically active and dormant microorganisms; the respired CO₂, however, derives mainly from active organisms. This discrepancy and the preferential substrate utilization explain the δ¹³C differences between microorganisms and CO₂ by an ‘apparent’ ¹³C discrimination. Preferential consumption of easily decomposable substrates and less negative δ¹³C values were common for substances with low C/N ratios. Preferential substrate utilization was more important for C₃ soils because, in C₄ soils, microbial respiration strictly followed kinetics, i.e. microorganisms incorporated heavier C (? = +1.1‰) and respired lighter C (? = −1.1‰) than SOM. Temperature and precipitation had no significant effect on the ¹³C fractionation in these processes in C₃ soils. Increasing temperature and decreasing precipitation led, however, to increasing δ¹³C of soil C pools.Based on these ¹³C fractionations we developed a number of consequences for C partitioning studies using ¹³C natural abundance. In the framework of standard isotope mixing models, we calculated CO₂ partitioning using the natural-¹³C-labelling approach at a vegetation change from C₃ to C₄ plants assuming a root-derived fraction between 0% and 100% to total soil CO₂. Disregarding any ¹³C fractionation processes, the calculated results deviated by up to 10% from the assumed fractions. Accounting for ¹³C fractionation in the standard deviations of the C₄ source and the mixing pool did not improve the exactness of the partitioning results; rather, it doubled the standard errors of the CO₂ pools. Including ¹³C fractionations directly into the mass balance equations reproduced the assumed CO₂ partitioning exactly. At the end, we therefore give recommendations on how to consider ¹³C fractionations in research on carbon flows between plants, microorganisms, and soil.     

Effect of Clay Minerals on Immobilization of Heavy Metals and Microbial Activity in a Sewage Sludge-Contaminated Soil (8 pp)

Background   Aims, and Scope. Reducing heavy metal solubility and bioavailability in contaminated area without removing them from the soil is one of the common practices in decreasing the negative impacts on the environment and improving the soil quality. Therefore, our aim was to study the effect of clay minerals: Na-bentonite, Ca-bentonite, and zeolite applied to a contaminated soil on immobilization of heavy metals, as well as on some soil parameters related with microbial activity. Methods   A soil derived from sewage sludge was incubated with clay minerals of either Na-bentonite, Ca-bentonite, or zeolite for 111 days (d). During the incubation experiment, concentrations of water soluble Zn, Cd, Cu, and Ni were measured after extraction of 2 g air-dry soil with 50 ml of H2O for 2 h. After the water extraction, the soil sediment was extracted with 50 ml of 1 M NH4NO3 for 2 h to estimate the exchangeable amounts of heavy metals. Furthermore, soil microbial respiration, microbial biomass C, Corg mineralization, metabolic quotient (qCO2), and inorganic N were also investigated. Results and Discussion   Water extractable and exchangeable forms of heavy metals were changed by incubation and addition of clay minerals. Incubation of soil without addition of clay minerals (control) increased water extractable Cu by 12, 24 and 3.8% of initial content after 21, 62, and 111 d of incubation, respectively. The water extractable Zn decreased by 9% during 62 d of incubation and it tended to increase by 14% at the end of the incubation, as compared with the initial soil. Water extractable Cd decreased by 71, 66 and 33% of initial content, and Ni decreased by 54, 70, and 58%, after 21, 62, and 111 d of incubation, respectively. During the incubation experiment, the exchangeable form of all tested metals was decreased by incubation. The addition of clay minerals led to a significant decrease in water soluble and exchangeable forms of heavy metals during the incubation experiment, resulting in low metal extractability. The reduction in metal extractability was greater due to the addition of Na-bentonite or Ca-bentonite than that due to the addition of zeolite. During the first 3 weeks after addition of clay minerals, the studied biological parameters were not affected. However, as incubation progressed, the addition of Na- or Ca- bentonite led to a significant increase in soil respiration, microbial biomass C, Corg mineralization, and inorganic N; and a significant decrease in qCO2. This result is explained by sorption of heavy metals on Na-bentonite and Ca-bentonite and strong reduction of their toxicity. Conclusions   Our results clearly show that the addition of clay minerals, especially of Na-bentonite and Ca-bentonite, decreased the extractability of four metals during incubation. The decreased metal extractability was accompanied by an increase of soil respiration, Corg mineralization, microbial biomass C, and inorganic N and a decrease of metabolic quotient (qCO2), showing positive effect of clay mineral addition on soil biological parameters. Recommendations and Outlook   The use of Na-bentonite and Ca-bentonite is promising tool for reduction the extractability and possible toxicity of heavy metals in sewage sludge-contaminated soil. Therefore, the soils polluted with heavy metals may be ameliorated by addition of clay minerals, especially Na-bentonite and Ca-bentonite.     

Separation of root and microbial respiration: Comparison of three methods

In a laboratory experiment, the following methods of separating the soil CO₂ flux into the root respiration and the respiration of the rhizosphere and nonrhizosphere microorganisms were compared: (1) root exclusion, (2) component integration, and (3) ¹⁴C pulse labeling. Depending on the method used, the combined contribution of the rhizosphere microorganisms and roots varied from 18 to 40% of the total CO₂ emission; the contribution of the roots alone was 8–19%, and that of the nonrhizosphere microorganisms was 51–82%. The nonisotope methods (1 and 2) gave similar results of the separation. The pulse labeling of plants satisfactorily separated the root and microbial respiration, but it is unsuitable for determining the respiration of the nonrhizosphere microorganisms. Advantages and disadvantages of each method are discussed.     

Dynamics of Organic C Mineralization and the Mobile Fraction of Heavy Metals in a Calcareous Soil Incubated with Organic Wastes

Organic wastes such as sewage sludge and compost increase the input of carbon and nutrients to the soil. However, sewage sludge-applied heavy metals, and organic pollutants adversely affect soil biochemical properties. Therefore, an incubation experiment lasting 90 days was carried out to evaluate the effect of the addition of two sources of organic C: sewage sludge or composted turf and plant residues to a calcareous soil at three rates (15, 45, and 90 t of dry matter ha^–1) on pH, EC, dissolved organic C, humic substances C, organic matter mineralization, microbial biomass C, and metabolic quotient. The mobile fraction of heavy metals (Zn, Cd, Cu, Ni, and Pb) extracted by NH₄NO₃ was also investigated.The addition of sewage sludge decreased soil pH and increased soil salinity to a greater extent than the addition of compost. Both sewage sludge and compost increased significantly the values of the cumulative C mineralized, dissolved organic C, humic and fulvic acid C, microbial biomass C, and metabolic quotient (qCO₂), especially with increasing application rate. Compared to compost, the addition of sewage sludge caused higher increases in the values of these parameters. The values of dissolved organic C, fulvic acid C, microbial biomass C, metabolic quotient, and C/N ratio tended to decrease with time. The soil treated with sewage sludge showed a significant increase in the mobile fractions of Zn, Cd, Cu, and Ni and a significant decrease in the mobile fraction of Pb compared to control. The high application rate of compost resulted in the lowest mobility of Cu, Ni, and Pb. The results suggest that biochemical properties of calcareous soil can be enhanced by both organic wastes. But, the high salinity and extractability of heavy metals, due to the addition of sewage sludge, may limit the application of sewage sludge.     

Using natural 13C abundances to differentiate between three CO2 sources during incubation of a grassland soil amended with slurry and sugar

This study describes a novel approach to separate three soil carbon (C) sources by one tracer method (here ¹³C natural abundance). The approach uses the temporal dynamics of the CO₂ efflux from a C₃ grassland soil amended with added C₃ or C₄ slurry and/or C₃ or C₄ sugar to estimate contributions of three separate C sources (native soil, slurry, and sugar) to CO₂ efflux. Soil with slurry and/or sugar was incubated under controlled conditions, and concentration and δ¹³C values of evolved CO₂ were measured over a 2‐week period. The main assumption needed for separation of three C sources in CO₂ efflux, i.e., identical decomposition of applied C₃ and C₄ sugars in soil, was investigated and proven. The relative contribution to the CO₂ efflux was higher, but shorter with an increased (microbial) availability of the C source, i.e., sugar > slurry > SOM. The shortcomings and limitations as well as possible future applications of the suggested method are discussed.     

Changes in the carbon and nitrogen isotopic composition of organic matter in soils of different thermal stability after free-air CO2 enrichment for three years

The hypothesis that the biological availability of soil organic matter (SOM) pools is inversely proportional to their thermal stability was tested using the isotopic difference between the atmospheric CO₂ (δ¹³C = ?8.0‰) and ¹³C-enriched CO₂ (δ¹³C = ?47‰) fertilizers, as well as ¹⁵N-labeled fertilizers. The soil samples from spring wheat plots subjected to treatment with ambient (370 ppm) and elevated (540 ppm) CO₂ concentrations for three years were analyzed by the thermogravimetric method. Based on the weight loss, five SOM pools were distinguished where the total C and N contents and isotopic compositions (δ¹³C and (δ¹⁵N) were determined. The contents of new C and N and their mean residence times in pools were calculated. The incorporation of ¹³C and ¹⁵N and their turnover rates did not depend on the thermal stability of the SOM pools, which disproved the hypothesis being tested.     

The rates of organic matter renewal in gray forest soils and chernozems

The rates of soil carbon renewal were determined by the method of natural ¹³C abundance in a chernozem under a 40-year-long monoculture of corn and in a gray forest soil after application of corn residues. The mean rate of soil carbon renewal in the chernozem reached 1271–1498 years, whereas in the gray forest soil it depended on the amount of carbon introduced with corn residues and varied from 19 to 63 years. The rate of organic carbon renewal in the chernozem decreased from 697 years in the upper horizon to 2742 years in the layer of 40–60 cm. The mean residence time of organic carbon generally increased with a decrease in the size of particle-size fractions.     

Root uptake of N-containing and N-free low molecular weight organic substances by maize: A C/N tracer study

Most studies showing potential organic nitrogen uptake were conducted with amino acids. They conclude that, in some ecosystems, amino acids significantly contribute to the N demand of plants and that roots have special transporters to re-uptake amino acids released into the rhizosphere. However, the relevance of the uptake of organic N compounds can only be evaluated by comparing the uptake of N-containing and N-free organic substances. We compared the uptake of alanine, glucose and acetate labelled with ¹⁴C by maize. Additionally, the N uptake was estimated by ¹⁵N labelled alanine and KNO₃. We found a similar uptake of ¹⁴C from alanine, glucose and acetate, amounting for the whole plant less than 1% of ¹⁴C input. These results show that maize did not prefer N-containing to N-free organic substances. The uptake of ¹⁵N by maize exceeded that of ¹⁴C (10- to 50-fold), irrespective of the ¹⁵N source. However, plant uptake of nitrate (23.6–35.2% of ¹⁵N input) always exceeded the uptake of N from alanine (9.6–28.8%). The uptake of organically bound N by maize growing in soil occurred mainly by transpiration flow – as dissolved organics. The contribution of specific amino acid transporters was minor.