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Under favorable growth conditions cover crops are efficient tools to scavenge available nitrogen in the soil after the main crop and reduce nitrate leaching. However, their ability to reduce nitrate leaching is influenced by the available growth conditions after sowing and their characteristics to assimilate biomass and nitrogen (N). A flexible sowing time of cover crops without compromising their benefits to the environment implies knowledge of how the growth conditions after sowing of the cover crop affect leaching. Fodder radish, phacelia, and oats have different growth and N uptake characteristics, and their ability to reduce nitrate leaching depending on growth conditions after sowing may therefore differ. In this study, these three cover crops were evaluated, and their ability for taking up soil N and reducing nitrate leaching was quantified based on four sowing times from early August to early September with intervals of 7–11 days in 2019, 2020, and 2021 in Denmark. N uptake was measured in autumn each year and nitrate leaching measurements took place during the drainage period until the following spring. The N uptake decreased with subsequent sowing times across the cover crops and years. However, the decrease was different in different cover crops and different years. Nitrate leaching reduction also decreased with subsequent sowing times. On average, fodder radish reduced nitrate leaching by 64 kg N ha-1, significantly more than phacelia and oats with 47 and 44 kg N ha-1, respectively, across all sowing times and drainage periods. With an average reduction of nitrate leaching of 0.8 kg N ha-1 per day and no interaction of N uptake and cover crops, the ability of fodder radish, phacelia, and oats to reduce nitrate leaching was not significantly different. Fodder radish and early sowing was the most effective combination to obtain the maximum benefits of nitrate leaching reduction by a cover crop.

The emission of nitrous oxide (N2O) from agricultural soils to the atmosphere is a significant contributor to anthropogenic greenhouse gas emissions. The recycling of organic nitrogen (N) in manure and crop residues may result in spatiotemporal variability of N2O production and soil efflux which is difficult to capture by process-based models. We propose a multi-species, reactive transport model to provide detailed insight into the spatiotemporal variability of nitrogen (N) transformations around such N2O hotspots, which consists of kinetic reactions of soil respiration, nitrification, nitrifier denitrification, and denitrification represented by a system of coupled partial differential equations. The model was tested with results from an incubation experiment at two different soil moisture levels (-30 and -100 hPa, respectively) and was shown to reasonably well reproduce the recorded N2O and dinitrogen (N2) emissions, and the dynamics of important carbon (C) and N components in soil. The simulation indicated that the four populations developed in closely connected, but separate layers, with denitrifying bacteria growing within the manure-dominated zone and nitrifying bacteria in the well-aerated soil outside the manure zone and with time also within the manure layer. The modeled N2O production within the manure zone was greatly enhanced by the combined effect of oxygen deficit, abundant carbon source and supply of nitrogenous substrates. In the wetter soil treatment with a water potential of -30 hPa, diffusive flux of nitrate (NO3-) across the manure-soil interface was the main source of NO3- for denitrification in the manure zone, while at a soil water potential of -100 hPa, diffusion became less dominant and overtaken by the co-occurrence of nitrification and denitrification in the manure zone. Scenarios were analyzed where diffusive transport of dissolved organic carbon or different mineral N species were switched off, and they showed that the simultaneous diffusion of NO3-, ammonium (NH4+), and nitrite (NO2-) were crucial to simulate the dynamics of N transformations and N2O emissions in the model. Without considering solute diffusion in process-based N2O models, the rapid turnover of C and N associated with organic hotspots can not be accounted for, and it may result in underestimation of N2O emissions from soil after manure application. The model and its parameters allow for new detailed insights into the interactions between transport and microbial transformations associated with N2O emissions in heterogeneous soil environments.

Soil structure plays a central role in many soil processes that are environmentally relevant. Intermittent freezing of the soil over winter is an important abiotic disturbance in temperate climates and its effects on soil structure depend on the soil's preexistent structural strength and cohesion. Management choices such as tillage and plant cover after harvest strongly influence soil structure, and therefore the soil's response to freeze-thaw. We examined the effects of 5 freeze-thaw cycles (FT) on the µCT-detectable structure of intact topsoil cores (Ø=100 mm, h=80 mm) from a long-term rotation and tillage experiment in Denmark. The cores were divided among two tillage treatments and two plant cover treatments, corresponding to a gradient of structural strength: CT-B1020 µm) and analyzed the macroporosity (Vt), mean macropore diameter (dm) and mean Euclidian distance to the nearest macropore (EDm). Additionally, we analyzed the effects of tillage and plant cover on several µCT-derived geometric parameters in Full Range. Overall, NT-B and NT-V resulted in lower macroporosity than in CT-B and CT-V. Similarly, we found fewer, less branched macropores with shorter mean branch length in NT compared to CT for both plant cover treatments. However, we propose that µCT-derived geometric parameters might be confounded by the overlapping influence of relatively few, complex and voluminous coarse macropores and the more abundant, less complex very fine macropores. Freeze-thaw, in turn, caused crumbling of soil around coarse macropores, reducing Vt and dm in Full Range and reducing Vt in the > 1020 µm range. Additionally, FT caused significant increases in Vt and reductions in dm and EDm in the < 300 µm range, indicating creation of new very fine macropores and expansion of previously indiscernible macropores. Overall, the effects of FT were reduced in NT (for equal plant cover treatments) and V (for equal tillage treatments), indicating greater resilience against FT in both cases.

Nitrous oxide (N2O) emissions from pastures can vary significantly depending on soil and environmental conditions, nitrogen (N) input, as well as the plant’s ability to take up the N. We tested the hypothesis that legume-based N sources are characterized by significantly lower emission factors than mineral N based dairy systems. Therefore, this study monitored N2O emissions for a minimum of 100 days and up to two growing seasons across a gradient of plant species diversity. Emissions were measured from both, grazed pastures and a controlled application of urine and dung using the static chamber method. About 90% of the accumulated N2O emissions occurred during the first 60–75 days. The average accumulated N2O emissions were 0.11, 0.87, 0.99, and 0.21 kg ha−1 for control, dung, urine patches, and grazed pastures, respectively. The N uptake efficiency at the excreta patch scale was about 70% for both dung and urine. The highest N2O-N emission factor was less than half compared with the IPCC default (0.3 vs. 0.77), suggesting an overestimation of N2O-N emissions from organically managed pastures in temperate climates. Plant diversity showed no significant effect on the N2O emissions. However, functional groups were significant (p &lt, 0.05). We concluded that legume-containing pasture systems without a fertilizer addition generally appear capable of utilizing nitrogen inputs from excreta patches efficiently, resulting in low N2O emissions.

Denitrification is a key but poorly quantified component of the Ncycle. Because it is difficult to measure the gaseous (NO$_{x}$, N$_{2}$O, N$_{2}$)and soluble (NO$_{3}$) components of denitrification with sufficientintensity, models of varying scope and complexity have beendeveloped and applied to estimate how vegetation cover, landmanagement and environmental factors such as soil type andweather interact to control these variables. In this paper we assessthe strengths and limitations of different modeling approaches,highlight major uncertainties, and suggest how differentobservational methods and process-based understanding can becombined to better quantify N cycling. Representation of howbiogeochemical (e.g. org. C., pH) and physical (e.g. soil structure)factors influence denitrification rates and product ratios combinedwith ensemble approaches may increase accuracy withoutrequiring additional site level model inputs.

The simultaneous retention of both cations and anions due to ion-pair adsorption (IPA) has been identifi ed in several soils. The mechanism controlling IPA is not yet understood, however, and the mathematical description of this phenomenon is only incipient. In this study, a parameterization analysis was performed using a recently proposed adsorption model that considers the occurrence of IPA. Th e model, which has two alternative approaches to describe IPA, has been shown in a previous study to describe experimental data reasonably well. Th at analysis was incomplete, however, as the parameter uncertainties were not estimated. In this study, with the parameterization procedure using the maximum-likelihood method, a robust estimation of the model parameters, including the analysis of their uncertainties, was developed. The model was fi tted to a data set obtained in batch experiments using the variable-charge Egmont soil, from New Zealand, and a comprehensive combination of SO4 and Ca concentrations. This procedure allowed the analysis of some of the underlying model characteristics and also some inferences about the IPA phenomenon. Overall, it was shown that the proposed adsorption model is a useful tool for the modeling of solutes in soils where IPA occurs. Refereed/Peer-reviewed