The methods used for estimating rhizodeposition of plants (carbon (C) deposition of living roots), and the results obtained for different plant species are reviewed. Three tracer techniques using C isotopes to quantify rhizodeposition are discussed: pulse labelling, continuous labelling, and natural 13C abundance. Only the tracer methods provided adequate results for the whole rhizodeposition. The differences in the below-ground C translocation pattern between cereals and grasses are discussed. Cereals (wheat and barley) transfer 20 – 30% of total assimilated C into the soil. Half of this amount is subsequently found in the roots and about one-third in CO2 evolved from the soil by root respiration and microbial utilization of root-borne organic substances. The remaining part of below-ground translocated C is incorporated into the soil microorganisms and soil organic matter (SOM). The portion of assimilated C allocated below the ground by cereals decreases during growth and by increasing N fertilization. Pasture plants translocated about 30 – 50% of assimilates below-ground, and their translocation patterns were similar to those of crop plants. On average, the total C amounts translocated into the soil by cereals and pasture plants are approximately the same (1.5 Mg C ha−1; calculated for the productivity of about 6 Mg grain yield), when the same growth period is considered. However, during one vegetation period the cereals and grasses allocated beneath the ground about 1.5 and 2.2 Mg C ha−1, respectively. Finally, a simple approach is suggested for a rough calculation of C input into the soil and for root-derived CO2 efflux from the soil. Contribution of C4 carbon (from maize and some C4 grasses) to turnover of SOM (C3 soils) estimated by natural 13C abundance is reviewed. In average for Ap horizons, the portion of maize derived carbon increases of about 0.98% of SOM content per year. Factors influencing the contribution of maize-derived carbon to soil organic carbon are discussed. The contribution of maize derived carbon decreases with soil depth, without fertilization, after removal of above ground biomass, and with soil tillage.
Rhizodeposition affects the microbial community in the rhizosphere, and microbial composition and activity may therefore differ in soil depending on plant species. We hypothesised that these differences increase over the plant growth period because roots occupy larger soil volumes and release more rhizodeposits. We tested how such medium-term responses of the microbial community can be explained by the short-term utilisation of root exudates. To test this we analysed 15N incorporation into microbial biomass, phospholipid fatty acid (PLFA) composition and 13C incorporation into the PLFAs of specific microbial groups in soil under white clover (Trifolium repens L.) and ryegrass (Lolium perenne L.) following leaf-labelling with 13C-bicarbonate and 15N-urea. In this way microbial N and 15N and the composition of PLFAs reflect the medium-term (two months) response of microorganisms to rhizodeposits, whereas the 13C-label of the PLFAs reflects the short-term (one week) utilisation of root exudates following labelling of shoots. In the medium term, microbial biomass N and 15N were greater under the ryegrass, whereas total PLFA was higher under white clover. The relative abundance of fungi and actinomycetes was unaffected by plant species, but pool of Gram-negative and Gram-positive bacteria was greater under white clover at the 10 percent significance level. In the short term, microorganisms more actively utilised fresh exudates (13C-labelled) of ryegrass than of white clover. We expected ryegrass exudates initially to be incorporated into bacterial PLFA and into fungi over time, but surprisingly fungi had the highest utilisation of ryegrass-derived C over the week. At 0–5 cm soil depth, white clover exudates were utilised only by bacteria, whereas fungi dominated at 5–15 cm. This reflects differences in the quality of white clover exudates or differences in the microbial community composition at the two depths. We conclude that despite clear short-term differences in microbial response to the exudates of white clover and ryegrass, this is only to a limited extent transferred into medium-term defects on the composition of the microbial communities under the two plant species. Hence, our study showed that different short-term C utilisation patterns may lead to similar medium-term responses of the microbial community.
Ensuring global food security and sustainable development remains a top priority at present [...]
PurposeSOMPACS is a project recommended by EJP SOIL for funding under the 1st External Call "Towards Healthy, Resilient and Sustainable Agricultural Soils". The purpose of SOMPACS is to disclose management practices enriching soils with the organic matter pools that are most resistant to microbial decomposition. The project started in 2022 and will be implemented by a consortium of 12 research institutions from Poland, Germany, Ireland, Lithuania, UK, Italy and the USA until 2025.MethodsSoil samples from eight long-term field experiments with different soil management and cultivation systems (conventional tillage vs. no-tillage; mineral vs. organic fertilization; management with and without catch crop; arable land vs. grassland; and cultivated vs. non-cultivated soils) will be investigated. Field experiments will include trials of increasing duration: 22-year (Lithuania); 26-year (Italy); 30-year (Poland, Ireland); 46-year (Poland); 54-year (Lithuania); 100-year (Poland), and 178-year Broadbalk experiment (UK). Experiments will also be carried out in production fields, where additives that stimulate root growth and provide very stable C (commercial humic products, biochar, and biogas digestate) will be applied. The effects of these additives on the content and properties of SOM will be investigated also in experimental plots accompanied by the incubation studies on the microbial decomposition of SOM and these additives. In parallel with soil sampling, plant productivity will be measured in all field experiments. Basic soil properties will be supplemented by the following investigations based on state-of-the-art approaches: SOM composition and stability by Py-GC-MS; aggregate size classes and C pools of increasing physicochemical protection; analysis of δ13C and δ15N of the separated SOM pools; microbiological properties (community-level physiological profiling, selected functional genes involved in C and N cycles, microbiome and mycobiome analyzes by next-generation sequencing, genetic diversity using terminal restriction fragment length polymorphism);  enzymatic activity; soil water retention and soil water repellency; mineral composition of clay fraction; soil structure stability. The most resistant SOM pool (humin) will be isolated by different methods (isolation vs. extraction) and examined for chemical composition and structure, using spectrometric and spectroscopic techniques (mass spectrometry, NMR, FTIR, EPR, UV-Vis-NIR, fluorescence). The C stocks in the soil profile will be evaluated and the extractable C in cold water will be determined to assess the potential leaching and microbial availability of C. Additionally, CO2 emissions from the soil of chosen experiments will be measured directly under field conditions.ResultsIn the first stage of the research, soil samples were collected from a depth up to 100 cm and the humin fraction from surface horizons was isolated for spectroscopic studies. Meantime, the impact of various types of cultivation on the yield was determined. ConclusionsA closer understanding of the persistence of SOC in top- and subsoil, as well as identifying management practices that contribute to minimizing greenhouse gas emissions, will show the possibilities of increasing the stable SOM pools, thus improving the potential of C sequestration. Understanding the impact of soil management on sustainable agricultural production and the environment, and in particular on climate change mitigation, should be widely promoted and put into practice.
Many previous studies on transformation of low molecular weight organic substances (LMWOS) in soil were based on applying 14C and/or 13C labeled substances. Nearly all these studies used uniformly labeled substances, i.e. all C atoms in the molecule were labeled. The underlying premise is that LMWOS transformation involves the whole molecule and it is not possible to distinguish between 1) the flux of the molecule as a whole between pools (i.e. microbial biomass, CO2, DOM, SOM, etc.) and 2) the splitting of the substance into metabolites and tracing those metabolites within the pools. Based on position-specific 14C labeling, we introduce a new approach for investigating LMWOS transformation in soil: using Na-acetate labeled with 14C either in the 1st position (carboxyl group, –COOH) or in the 2nd position (methyl group, –CH3), we evaluated sorption by the soil matrix, decomposition to CO2, and microbial uptake as related to both C atoms in the acetate. We showed that sorption of acetate occurred as a whole molecule. After microbial uptake, however, the acetate is split, and C from the –COOH group is converted to CO2 more completely and faster than C from the –CH3 group. Correspondingly, C from the –CH3 group of acetate is mainly incorporated into microbial cells, compared to C from the –COOH group. Thus, the rates of C utilization by microorganisms of C from both positions in the acetate were independently calculated. At concentrations of 10 μmol l−1, microbial uptake from soil solution was very fast (half-life time about 3 min) for both C atoms. At concentrations <100 μmol l−1 the oxidation to CO2 was similar for C atoms of both groups (about 55% of added substance). However, at acetate concentrations >100 μmol l−1, the decomposition to CO2 for C from –CH3 decreased more strongly than for C from –COOH. We conclude that the application of position-specifically labeled substances opens new ways to investigate not only the general fluxes, but also transformations of individual C atoms from molecules. This, in turn, allows conclusions to be drawn about the steps of individual transformation processes on the submolecular level and the rates of these processes.
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.
This study describes a novel approach to separate three soil carbon (C) sources by one tracer method (here 13 C natural abundance). The approach uses the temporal dynamics of the CO 2 efflux from a C 3 grassland soil amended with added C 3 or C 4 slurry and/or C 3 or C 4 sugar to estimate contributions of three separate C sources (native soil, slurry, and sugar) to CO 2 efflux. Soil with slurry and/or sugar was incubated under controlled conditions, and concentration and δ 13 C values of evolved CO 2 were measured over a 2‐week period. The main assumption needed for separation of three C sources in CO 2 efflux, i.e. , identical decomposition of applied C 3 and C 4 sugars in soil, was investigated and proven. The relative contribution to the CO 2 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.
