Field 13C Pulse Labeling of Pea, Wheat, and Vetch Plants for Subsequent Root and Shoot Decomposition Studies

<div><p>ABSTRACT Isotopic labeling of plants is useful in tracking the fate of carbon (C) from different plant parts in a soil-plant system when these parts decompose simultaneously. Pulse labeling is a relatively simple technique and is amenable for use in the field. Therefore, we evaluated a 13CO2 pulse-labeling method to label crop plants under subtropical field conditions for simultaneous root and shoot decomposition studies. Wheat (Triticum aestivum L.), pea (Pisum sativum L.), and vetch (Vicia sativa L.) plants were grown inside polyvinyl chloride (PVC) cylinders and pulse labeled once a week for a total of 11 times. After harvest, “paired” treatments were designed by combining 13C-labeled shoots with unlabeled roots and unlabeled shoots with 13C-labeled roots, resulting in six treatments (2 combinations × 3 species), plus an unamended control treatment. The 13C enrichment of plant parts, chemical fractions, 13C recovery, and distribution in roots, shoots, and soil were determined. Soil CO2 emissions were measured continuously by the alkaline trap method for 180 days. Plant dry matter production and chemical composition were not modified by 13C labeling. The maximum level of 13C enrichment (δ13C) in plants was +495 %o in wheat, +426 %o in pea, and +378 ‰ in vetch plants. All three crops showed similar patterns of 13C distribution in the following order: shoots > roots > soil. On average, 81 to 89 % of the recovered 13C was in the shoots, 7 to 14 % was in the roots, and 2.7 to 4.3 % was in the soil. The rate of C mineralization and cumulative C mineralization were not different between “paired” treatments of the three crops, showing that the paired treatments were equally degradable. The pulse-labeling technique used under field conditions allowed for production of sufficiently labeled wheat, pea, and vetch plants. Therefore, it is a practical approach with respect to resource demand (tracer and labor costs), and it is suitable for in situ labeling.</p></div

Root and Shoot Contribution to Carbon and Nitrogen Inputs in the Topsoil Layer in No-Tillage Crop Systems under Subtropical Conditions

<div><p>ABSTRACT Recycling of carbon (C) and nitrogen (N) from plants into soils is decisive for maintaining soil organic matter and soil fertility. Therefore, we quantified plant biomass and C and N in the shoots and roots from the topsoil layer for a wide range of annual crops grown under subtropical conditions. We grew 26 species, 13 main crops, and 13 cover crops, in the field in standard sowing arrangements. Root biomass was recovered from the 0.00-0.20 m soil layer at flowering, and shoot biomass was measured at flowering for all crops and at maturity only for the main crops. Root dry matter (DM) exhibited an average of 14.9 ± 5.7 % of the total shoot biomass at flowering, and the mean shoot DM to root DM ratio was 6.9 (2.8-15.0) for the 26 crops considered. Leguminous species had less root DM (0.5 to 1.0 Mg ha-1) than grass species (1.1 to 2.3 Mg ha-1). The shoot C to root C ratio varied consistently with DM, while the root N to shoot N ratio varied considerably among species. Proportionally more biomass, C, and N was allocated to the root systems of grasses (Poaceae species) than non-grass species (especially Fabaceae species). The findings of this study contribute to designing rotations to include species that promote cycling of N and have high potential for adding C to the soil through roots. In this sense, the use of intercropped grasses and legumes is a promising strategy, especially for cover crops.</p></div

Nitrous Oxide Emissions in a Wheat/Corn Succession Combining Dairy Slurry and Urea as Nitrogen Sources

<div><p>ABSTRACT: The impact on nitrous oxide (N2O) emissions caused by combining dairy slurry (DS) and urea to supply crops with nitrogen (N) is still not well characterized. The main objective of this study was to compare the differences between N2O emissions in the cases of exclusive use of urea and the combined use of DS and urea as N sources to no-tillage wheat and corn. We also compared N2O emissions between two DS application methods (surface-broadcast vs. injection), as well as the addition of dicyandiamide (DCD) to DS. The experiment was conducted under no-tillage and six treatments were applied at sowing as follows: no fertilization (control), surface-broadcast urea (urea-N), surface-broadcast DS (DSs), surface-broadcast DS with DCD (DSs + DCD), shallow-injected DS (DSi), and shallow-injected DS with DCD (DSi + DCD). Urea was applied at side-dressing in all treatments with DS and urea-N. The N2O emissions were evaluated from the application of the treatments to wheat until 22 days after corn harvest, resulting in a total of 364 days. The partial supply of the N demand of wheat and corn, by DS application at sowing, and the posterior complement of N demand by side-dressing urea increased the annual N2O emissions only when DS was shallow-injected without DCD (DSi). Although the amount of N2O-N emitted from the DSi treatment was 1.04 kg ha-1 (91.2 %) higher than from DSs in corn, the annual N2O-N emissions did not differ between the two methods of DS application to the soil. The emission factors of N2O-N were low, ranging from 0.49 % (DSi + DCD) to 1.27 % (DSi). The results of this study suggest that the combined use of DS with urea in N fertilization of no-tillage wheat and corn (DS applied at sowing and urea at side-dressing) caused no changes in the annual N2O-N emissions compared to crops fertilized exclusively with urea (1/3 at sowing + 2/3 at side-dressing). However, DCD should be added if DS is shallow-injected.</p></div