Background and aims: Plant growth affects soil moisture, mineral N and organic C availability in soil, all of which influence denitrification. With increasing plant growth, root exudation may stimulate denitrification, while N uptake restricts nitrate availability. Methods: We conducted a double labeling pot experiment with either maize (Zea mays L.) or cup plant (Silphium perfoliatum L.) of the same age but differing in size of their shoot and root systems. The ¹⁵N gas flux method was applied to directly quantify N₂O and N₂ fluxes in situ. To link denitrification with available C in the rhizosphere, ¹³CO₂ pulse labeling was used to trace C translocation from shoots to roots and its release by roots into the soil. Results: Plant water and N uptake were the main factors controlling daily N₂O + N₂ fluxes, cumulative N emissions, and N₂O production pathways. Accordingly, pool-derived N₂O + N₂ emissions were 30–40 times higher in the treatment with highest soil NO₃⁻ content and highest soil moisture. CO₂ efflux from soil was positively correlated with root dry matter, but we could not detect any relationship between root-derived C and N₂O + N₂ emissions. Conclusions: Root-derived C may stimulate denitrification under small plants, while N and water uptake become the controlling factors with increasing plant and root growth.

Increased global production of animal-based protein results in high greenhouse gas (GHG) emissions and other adverse consequences for human and planetary health. Recently, commercial insect rearing has been claimed a more sustainable source of animal protein. However, this system also leaves residues called frass, which—depending on the insect diet—is rich in carbon (C) and nitrogen (N), and could thus be used as fertilizer in agriculture. The impact of this kind of fertilizer on soil GHG emissions is yet unknown. Therefore, we investigated the effect of black soldier fly (Hermetia illucens L.) frass derived from a carbohydrate (Carb-) or a protein (Prot-) based diet applied at two different application rates to an arable soil on C and N fluxes and microbial properties in a 40-day incubation experiment. CO₂, N₂O, NO, N₂, CH₄, water extractable organic C (WEOC), and inorganic N were continuously measured quantitatively. At the end of the incubation, microbial biomass (MB), stoichiometry, community composition, and abundance of functional genes were assessed. Along with a strong increase in WEOC and CO₂, Carb-frass caused strong initial N₂O emissions associated with high N and C availability. In contrast, Prot-frass showed lower CO₂ emissions and N₂O release, although soil nitrate levels were higher. At the end of incubation, MB was significantly increased, which was more pronounced following Carb-frass as compared to Prot-frass application, and at higher amendment rates. Fungal abundance increased most from both frass types with an even stronger response at higher application rates, whereas bacterial abundance rose following Carb-frass as compared to Prot-application. Abundance of functional genes related to ammonia-oxidizing bacteria and archaea were enhanced by high frass application but did not clearly differ between frass types. C use efficiency of microorganisms, as revealed by the metabolic quotient, was most strongly reduced in the high Prot-frass application rate. Overall, insect diet influenced available C and N in frass and thus affected mineralization dynamics, GHG emissions, and microbial growth. Overall, emissions were very high undermining the potential environmental benefit of insect based protein production and calling for more detailed analyses before frass is widely applied in agriculture.

Chemical composition of root and shoot litter controls decomposition and, subsequently, C availability for biological nitrogen transformation processes in soils. While aboveground plant residues have been proven to increase N₂O emissions, studies on root litter effects are scarce. This study aimed (1) to evaluate how fresh maize root litter affects N₂O emissions compared to fresh maize shoot litter, (2) to assess whether N₂O emissions are related to the interaction of C and N mineralization from soil and litter, and (3) to analyze changes in soil microbial community structures related to litter input and N₂O emissions. To obtain root and shoot litter, maize plants (Zea mays L.) were cultivated with two N fertilizer levels in a greenhouse and harvested. A two-factorial 22 d laboratory incubation experiment was set up with soil from both N levels (N1, N2) and three litter addition treatments (control, root, root + shoot). We measured CO₂ and N₂O fluxes, analyzed soil mineral N and water-extractable organic C (WEOC) concentrations, and determined quality parameters of maize litter. Bacterial community structures were analyzed using 16S rRNA gene sequencing. Maize litter quality controlled ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and WEOC availability and decomposition-related CO₂ emissions. Emissions induced by maize root litter remained low, while high bioavailability of maize shoot litter strongly increased CO₂ and N₂O emissions when both root and shoot litter were added. We identified a strong positive correlation between cumulative CO₂ and N₂O emissions, supporting our hypothesis that litter quality affects denitrification by creating plant-litter-associated anaerobic microsites. The interdependency of C and N availability was validated by analyses of regression. Moreover, there was a strong positive interaction between soil ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and WEOC concentration resulting in much higher N₂O emissions, when both ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and WEOC were available. A significant correlation was observed between total CO₂ and N₂O emissions, the soil bacterial community composition, and the litter level, showing a clear separation of root + shoot samples of all remaining samples. Bacterial diversity decreased with higher N level and higher input of easily available C. Altogether, changes in bacterial community structure reflected degradability of maize litter with easily degradable C from maize shoot litter favoring fast-growing C-cycling and N-reducing bacteria of the phyla Actinobacteria, Chloroflexi, Firmicutes, and Proteobacteria. In conclusion, litter quality is a major driver of N₂O and CO₂ emissions from crop residues, especially when soil mineral N is limited.

Returning crop residues to agricultural fields can accelerate nutrient turnover and increase N₂O and NO emissions. Increased microbial respiration may lead to formation of local hotspots with anoxic or microoxic conditions promoting denitrification. To investigate the effect of litter quality on CO₂, NO, N₂O, and N₂ emissions, we conducted a laboratory incubation study in a controlled atmosphere (He/O₂, or pure He) with different maize litter types (Zea mays L., young leaves and roots, straw). We applied the N₂O isotopocule mapping approach to distinguish between N₂O emitting processes and partitioned the CO₂ efflux into litter- and soil organic matter (SOM)-derived CO₂ based on the natural ¹³C isotope abundances. Maize litter increased total and SOM derived CO₂ emissions leading to a positive priming effect. Although C turnover was high, NO and N₂O fluxes were low under oxic conditions as high O₂ diffusivity limited denitrification. In the first week, nitrification contributed to NO emissions, which increased with increasing net N mineralization. Isotopocule mapping indicated that bacterial processes dominated N₂O formation in litter-amended soil in the beginning of the incubation experiment with a subsequent shift towards fungal denitrification. With onset of anoxic incubation conditions after 47 days, N fluxes strongly increased, and heterotrophic bacterial denitrification became the main source of N₂O. The N₂O/(N₂O+N₂) ratio decreased with increasing litter C:N ratio and C_org:NO₃⁻ ratio in soil, confirming that the ratio of available C:N is a major control of denitrification product stoichiometry.

The objectives of this field trial were to collect reliable measurement data on N₂ emissions and N₂O/(N₂O + N₂) ratios in typical German crops in relation to crop development and to provide a dataset to test and improve biogeochemical models. N₂O and N₂ emissions in winter wheat (WW, Triticum aestivum L.) and sugar beet (SB, Beta vulgaris subsp. vulgaris) were measured using the improved ¹⁵N gas flux method with helium–oxygen flushing (80:20) to reduce the atmospheric N₂ background to < 2%. To estimate total N₂O and N₂ production in soil, production-diffusion modelling was applied. Soil samples were taken in regular intervals and analyzed for mineral N (NO₃ ⁻ and NH₄ ⁺) and water-extractable Corg content. In addition, we monitored soil moisture, crop development, plant N uptake, N transformation processes in soil, and N translocation to deeper soil layers. Our best estimates for cumulative N₂O + N₂ losses were 860.4 ± 220.9 mg N m⁻² and 553.1 ± 96.3 mg N m⁻² over the experimental period of 189 and 161 days with total N₂O/(N₂O + N₂) ratios of 0.12 and 0.15 for WW and SB, respectively. Growing plants affected all controlling factors of denitrification, and dynamics clearly differed between crop species. Overall, N₂O and N₂ emissions were highest when plant N and water uptake were low, i.e., during early growth stages, ripening, and after harvest. We present the first dataset of a plot-scale field study employing the improved ¹⁵N gas flux method over a growing season showing that drivers for N₂O and N₂O + N₂ fluxes differ between crop species and change throughout the growing season.

Addition of organic material, such as plant litter, can strongly alter C and N turnover in soils and promote CO ₂ and N ₂ O formation. This study aimed to elucidate the role of C availability on N turnover processes leading to N ₂ O formation under moderately dry conditions. We measured CO ₂ , NO, and N ₂ O fluxes and contributing processes from an arable and a grassland soil with four different treatments: 50% water-filled pore space (WFPS), 50% WFPS + Maize litter, 60% WFPS, 60% WFPS + Maize litter. CO ₂ fluxes were partitioned into SOM- and litter-derived, and N ₂ O-forming processes were estimated applying a Bayesian mixing and fractionation model to infer source contributions and N ₂ O reduction rates. Further, we quantified bacterial and fungal population size, as well as genes involved in nitrification and denitrification at the end of the experiment. N ₂ O flux dynamics and contributing processes strongly differed between the C-limited arable soil and the C-rich grassland soil. In the arable soil, bacterial denitrification was consistently the main source of N ₂ O and strongly increased with addition of maize litter. In contrast, nitrification accounted for 51% of N ₂ O formation in the grassland soil without litter, while litter addition promoted fungal denitrification. We found higher N ₂ O emissions with higher total and SOM-derived CO ₂ emissions confirming that availability of organic C from SOM and litter are important controls of N ₂ O losses. However, neither soil organic C content, nor litter addition, nor soil moisture alone were appropriate to predict the main N ₂ O-forming process. Therefore, our study emphasizes the need to disentangle and quantify N ₂ O-forming processes to develop successful mitigation strategies.