Separate estimation of sources of root-derived CO2 efflux from the soil into actual root respiration and microbial respiration of rhizodeposits is very important for determining the carbon (C) and energy balance of soils and plants, C sources for rhizosphere microorganisms, sources of soil organic matter (SOM), etc. Besides component integration, to date, only four adequate methods based on the pulse labeling of shoots in a 14CO2 atmosphere and subsequent monitoring of 14CO2 efflux from the soil have been suggested: 1) the isotope dilution, 2) the model rhizodeposition technique, 3) modeling of 14CO2 efflux dynamics, and 4) the exudate elution procedure. These methods are based on different assumptions and principles that are very difficult to check experimentally and have different results. Therefore, none of these methods can be accepted as a standard procedure allowing quantitative separate estimation of root respiration and rhizomicrobial respiration. This contribution provides an elaboration of the theoretical background of a procedure allowing quantitative separate estimation of root respiration and rhizomicrobial respiration in non-sterile soils. The method is based on 13C natural abundance by growing C4 plant on C3 soil or vice versa. Four δ13C values are necessary: of the SOM, of the roots, of soil microbial biomass, and of CO2 efflux from the soil. The advantages and assumptions of the new approach, as well as possible applications including FACE systems and continuous labeling experiments are discussed.
The interplay between plant genotype and nutrient management affects rhizodeposition, which in turn modulates the rhizosphere-microbiome and microbe-mediated functions. Substituting mineral nitrogen (N) with an N-fixing inoculant reduces reliance on N fertilizer while supplying N to crops. We evaluated the effectiveness of integrating maize near-isogenic lines (NIL 1 and NIL 2) with the biological nitrification inhibition (BNI) trait into management practices aimed at optimizing N provisioning. Management strategies included mineral N inputs (0 and 67 kg ha⁻¹) with and without an N-fixing inoculant. Our approach synthesized insights from amplicon sequencing data and evaluated nitrification rates, rhizosphere N content, maize N uptake, and N use efficiency (NUE). Genotypes and management structured prokaryotic communities, while the developmental stages of genotypes further refined both fungal and prokaryotic communities. The N-fixing inoculant increased N availability, triggering the BNI capacity without increasing the nitrification rate. This was reflected in lower NO₃⁻ and higher NH₄⁺ levels in BNI-NIL leachate compared to B73, suggesting improved N retention. NIL2, characterized by distinct fungal biomarkers, exhibited higher N content (72.3 kg ha⁻¹) and superior NUE compared to NIL1 (65.0 kg ha⁻¹). NIL2’s enhanced N uptake was associated with a robust microbial network, featuring Archangium (prokaryote) and Trichoderma (eukaryote) as keystone taxa. Notably, Archangium was linked to rhizosphere N dynamics Synergizing BNI with diazotroph inoculants reduces N fertilizer reliance and increases maize N supply for sustainable agroecosystems.
Carbonate-containing minerals comprise an additional form of soil carbon known as soil inorganic carbon (SIC). Though SIC stocks are large, they have been disregarded in most studies to carbon sequestration. After reviewing the main forms of SIC (geogenic, biogenic and pedogenic carbonates) and the chemical processes leading to formation of pedogenic carbonates, we review the importance of SIC in the global C cycle and ecosystem functions. Besides pH regulation, SIC and dissolved Ca2+ from carbonates dissolution: i) increase plant growth due to better root growth, nutrient availability and acquisition, as well as provide protection against pathogens; ii) increase activities of soil microorganisms mineralizing nutrients; and iii) bind organic compounds which, consequently, stabilize organic matter, produce larger and stable aggregates, and control water permeability and balance. Consequently, the SIC is crucial not only for pH regulation, but also strongly contributes to many other soil functions and health. Finally, we assess future SIC losses under anticipated global change processes such as increased N deposition and fertilization, elevated CO2, invasive plant distribution and climate change. These SIC losses damage soil functionality and make it more vulnerable to a broad range of degradation factors, including erosion, topsoil and subsoil compaction, acidification and nutrient depletion. Crucial is that in contrast to organic carbon, the SIC losses are irrecoverable. We conclude that SIC is an important soil constituent responsible for a broad range of physical, chemical and biological soil properties and processes as well as ecosystem services such as cycles of C, N and other elements.
The production of CO2 in soil strongly depends on the availability of organic carbon (C) for microorganisms. It is obvious, that C that entered the soil recently is more easily available for microorganisms in comparison to older C. However, only very few approaches allow for a quantitative estimation of the availability of C in relation to the time it is entering the soil. We hypothesized that δ13C values of CO2 and of soil organic matter (SOM) after a C3 to C4 vegetation change will enable to calculate the relative availability of younger (C4-derived) and older C (C3-derived) sources for microorganisms. Soil CO2 was sampled over one vegetation period at depths of 10, 40–50 and 60–70 cm at three treatments: a C3 reference (wheat), a C4/fallow (fallow after one year of maize cropping), and a C4/C4 (two years of maize cropping). Based on the δ13C of CO2 purified from the admixture of atmospheric CO2 by the Miller/Tans model and on the δ13C values of SOM, the contributions of younger and older C sources to CO2 and SOM were assessed. Depending on the soil depth and the presence of living roots, the contribution of younger C to soil CO2 ranged from 16 to 50%, but that to SOM was less than 5%. By comparing the contributions of older and younger C to CO2 and SOM, we found that the relative availability of organics recently introduced into the soil (C4-derived) was about 7 times higher than the availability of C stabilized in soil for longer than one year (C3-derived). We concluded that simultaneous analysis of the δ13C values of both SOM and of CO2 allows not only for the quantification of the CO2 sources, but also for the estimation of the availability of soil C pools of different age for microorganisms.
Despite its fundamental role for carbon (C) and nutrient cycling, rhizodeposition remains ‘the hidden half of the hidden half ’: it is highly dynamic and rhizodeposits are rapidly incorporated into microorganisms, soil organic matter, and decomposed to CO 2 . Therefore, rhizodeposition is rarely quantified and remains the most uncertain part of the soil C cycle and of C fluxes in terrestrial ecosystems. This review synthesizes and generalizes the literature on C inputs by rhizodeposition under crops and grasslands (281 data sets). The allocation dynamics of assimilated C (after 13 C‐ CO 2 or 14 C‐ CO 2 labeling of plants) were quantified within shoots, shoot respiration, roots, net rhizodeposition (i.e., C remaining in soil for longer periods), root‐derived CO 2 , and microorganisms. Partitioning of C pools and fluxes were used to extrapolate belowground C inputs via rhizodeposition to ecosystem level. Allocation from shoots to roots reaches a maximum within the first day after C assimilation. Annual crops retained more C (45% of assimilated 13 C or 14 C) in shoots than grasses (34%), mainly perennials, and allocated 1.5 times less C belowground. For crops, belowground C allocation was maximal during the first 1–2 months of growth and decreased very fast thereafter. For grasses, it peaked after 2–4 months and remained very high within the second year causing much longer allocation periods. Despite higher belowground C allocation by grasses (33%) than crops (21%), its distribution between various belowground pools remains very similar. Hence, the total C allocated belowground depends on the plant species, but its further fate is species independent. This review demonstrates that C partitioning can be used in various approaches, e.g., root sampling, CO 2 flux measurements, to assess rhizodeposits’ pools and fluxes at pot, plot, field and ecosystem scale and so, to close the most uncertain gap of the terrestrial C cycle.
Soil nitrogen (N) loss has been predicted to intensify with increased global precipitation changes. However, the relative contributions of leaching and gaseous N emissions to intensified N losses are largely unknown. Thus, we simulated intensified precipitation seasonality in a subtropical forest by extending the dry season via rainfall exclusion and increasing the wet‐season storms via irrigation without changing the total annual precipitation. Extending the dry season length increased the monthly mean soil NO 3 − content by 25%–64%, net N mineralization rate by 32%–40%, and net nitrification rate by 25%–28%. After adding water in the wet season, the monthly NO 3 − leaching was enhanced by 43% in the relatively dry year (2013, 2,094‐mm annual rainfall), but reduced by 51% in the relatively wet year (2014, 1,551 mm). In contrast, the monthly mean N 2 O emissions were reduced by 24% in 2013 but increased by 78% in 2014. Overall, the annual inorganic N content was decreased significantly by the precipitation changes. Decrease of soil inorganic N might be linked to the enhanced NO 3 − leaching in 2013, and be linked to the increased N 2 O emissions in 2014. However, in both years the annual total amount of N lost through leaching was significantly greater than that through N 2 O emissions. The enhanced N 2 O emissions driven by wet‐season storms were correlated with an increase in nirS abundance. Our results suggest that increased frequency of droughts and storms will decrease soil inorganic N content in warm and humid subtropical forests mainly through enhanced leaching losses.
One possibility to sequester carbon in soil could be the cultivation of renewable energy plants like Miscanthus x giganteus. In this preliminary investigation the contribution of Miscanthus derived carbon to formation of soil organic matter in a sandy and a loamy soil has been investigated using natural 13C abundance. In Ah of the loamy soil, 21% (0–10 cm) and 11% (20–30 cm) of soil organic carbon was derived from Miscanthus after 9 years of continuos cultivation, while portions of Miscanthus derived C exceeded 17% (0–10 cm) and 8% (10–20 cm) in the sandy soil. This significant contribution of Miscanthus derived carbon demonstrates the interesting possibility in using renewable energy plants for carbon sequestration in soils and using Miscanthus for further investigations of soil organic matter dynamics. This dynamic differ from that one of maize used in studies with natural 13C abundance method because of much higher below-ground biomass, absence of soil cultivation and deep root system of Miscanthus.
