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Studies evaluating the complexity of models, which are suitable to simulate grass growth at regional scales in intensive grassland production systems are scarce. Therefore, two different grass growth models (GrasProg1.0 and APSIM) with different complexity and input requirements were compared against long-term observations from variety trials with perennial ryegrass (Lolium perenne) in Germany and Denmark. The trial sites covered a large range of environmental conditions, with annual average temperatures ranging from 5.9 to 10.3 °C, and annual rainfall from 536 to 1154 mm. The sites also varied regarding soil type, which were for modelling categorised into three different groups according to their plant available water (PAW) content: light soils with a PAW of 60 mm, medium soils with a PAW of 80 mm, and heavy soils with a PAW of 100 mm. The objective was to investigate whether the simple model performed equally well with the given low number of inputs, namely climate and PAW group. Evaluation statistics showed that both models provided satisfactory results, with root mean square errors for individual cuts ranging from 0.59 to 1.28 t dry matter ha−1. The model efficiency (Nash–Sutcliffe efficiency) for the separate cuts were also good for both models, with 81% of the sites having a positive Nash–Sutcliffe efficiency value with GrasProg1.0, and 72% with APSIM. These results reveal that without detailed site-specific descriptions, the less complex GrasProg1.0 model can be incorporated into a simple decision support tool for optimising grassland management in intensive livestock production systems. [ABSTRACT FROM AUTHOR]

Early sowing of winter cereals has been promoted as an alternative to catch crops for reducing nitrogen (N) leaching in intensively managed cropping systems. Underlying data for this are, however, scarce and it is unclear how year‐to‐year variations affect potential reductions in N leaching. Well‐tested cropping system models can be used to assess the impact of early sowing on crop yield and N leaching. We tested the ability of the Agricultural Production System Simulator (APSIM) to simulate grain yield, biomass, phenology, N uptake and N leaching from timely and early‐sown winter cereals. The evaluation was based on two extensive field experiments carried out in Denmark in continuous winter cereals systems with different N fertilization applications, and a farm experiment in Germany with a grass ley/winter wheat crop sequence and two different N fertilization rates. Additionally, observations from growth stages throughout Denmark were used. After changing a few crop model parameters, APSIM reproduced the general effects of N fertilization and crop sequence on yields and N uptake, as well as N leaching. The model was then used to simulate the effect of sowing time on grain yield over a period of 10 years, to capture year‐to‐year variability and enable a better evaluation of the efficacy of early sowing for reducing N leaching. In the continuous winter cereal systems, early sowing reduced N leaching at both sites in Denmark. In contrast, in the grass ley/winter wheat system in Germany early sowing increased N leaching, due to enhanced mineralization of grass residues. [ABSTRACT FROM AUTHOR]

Optimum nitrogen (N) fertilization, which aims for high productivity while minimizing N losses to the environment, requires a better understanding of the N leaching behaviour around the recommended N rate. A study was conducted with increasing N fertilization levels over 3 years (2016–2019) with early and timely sown winter cereals (rye and wheat) at two sites in Denmark with different climatic conditions. For winter rye N fertilizer application rates ranged from 0 to 258 kg N ha‒1, and for winter wheat from 0 to 300 kg ha‒1. Grain yields did not respond much to the N application above the recommended rate. Nitrate (NO3) leaching was very low in the first period, but increased in the second and third period. The effect of a potential N carry‐over on marginal N leaching (defined as the additional N leaching per unit of additional N fertilizer applied) is large. Under early sown winter rye, marginal N leaching is reduced from 36 to 14 kg N ha‒1 in the third period following N fertilization of the recommended rate of 170 kg N ha‒1 when carry‐over was accounted for, and under the timely sown winter rye from 68 to 24 kg N ha‒1. Our results show that marginal N leaching around the recommended rate is almost linear at the study site, when carry‐over is accounted for. Thus, the use of precision agriculture, with lower fertilization application rates to areas with high residual soil N, is less effective compared with sites, which have an exponential leaching behaviour. [ABSTRACT FROM AUTHOR]

The use of suctions cups is a common practice for estimating nitrate (NO3–N) leaching under agricultural systems despite the various uncertainties associated with the approach. One major uncertainty is water flux, which is required for calculating NO3–N leaching loads from measured concentrations. Another problem is the interpolation of NO3–N concentrations between measurement days. We investigated how differences in water flux, obtained from two different models (EVACROP and APSIM), affect NO3–N leaching loads. The effect of interpolation of NO3–N concentrations based on days or drainage was also addressed. The models were set up according to a 2‐yr field experiment with spring barley (Hordeum vulgare L. Quinch) with different levels of N fertilization rates on a loamy soil at Flakkebjerg, Denmark. Due to small differences in measured NO3–N concentrations between sequential samplings, the method of interpolation did not significantly affect NO3–N leaching in the two periods investigated. Although there is no standard against which leaching losses from different approaches can be tested, results highlight that the modeling of water uptake as affected by N supply influences the amount of drainage and thus calculated NO3–N leaching. Therefore, for experiments with varying N fertilization levels, the APSIM model, which accounts for N nutrition on crop water use, is likely more accurate. For common fertilization rates, the simpler EVACROP seems appropriate. Thus, when using suction cup data for testing models or for evaluating mitigation options for nitrate leaching, the use of an appropriate model for estimating water fluxes is important. [ABSTRACT FROM AUTHOR]

Abstract Catch crop (CC) mixtures of non-legumes (nL) and legumes (L) have been promoted as a strategy to achieve two different goals: to decrease the risk of nitrate leaching and to enhance the nitrogen supply to the subsequent crop. To investigate if two-component mixtures of nL + L have advantages over pure nL stands experiments were carried out over a two year period (2013–2015) at two contrasting field sites in Denmark. Nitrogen (N) uptake by the CCs was measured by aboveground biomass sampling, and N leaching by ceramic suction cups. When grown in pure stands, white clover (Trifolium repens) on coarse sand and common vetch (Vicia sativa) on sandy loam were less effective at reducing N leaching than perennial ryegrass (Lolium perenne) and fodder radish (Raphanus sativus). When the proportion of the nL + L in mixtures was similar or favored the nL, leaching was not significantly different from the nL in the pure stand. However, during one of the years on the sandy loam L (vetch) almost outperformed nL (fodder radish), resulting in N leaching from nL + L similar to L. The yield of the following spring barley was only significantly different from the yield in the plots with previously bare soil in one of the years on the coarse sandy soil. It is concluded that in nL + L mixtures L can take over and thereby lower the effect of the CCs on N leaching while not necessarily enhancing the N supply for the subsequent crop. Highlights • In monocultures legumes were less effective at reducing N leaching than non-legumes. • Non-legume/legume mixtures were as effective as non-legume catch crops in reducing N leaching. • When non-legume/legume mixtures are not balanced N leaching was similar to that from legumes. • Use a catch crop only increased the subsequent spring yield on the sandy loam in one of the years. • The catch crop had no effect on the yield of the subsequent crop on the clay soil. [ABSTRACT FROM AUTHOR]

Catch crops have been proven effective in reducing nitrogen leaching in cropping systems due to their uptake of post-harvest soil mineral N prior to the main percolation season in winter. Information on the residual effect of catch crops, and their nitrogen fertilizer replacement value is scarce. To determine nitrogen fertilizer replacement values for different catch crops a field experiment was conducted over a four-year period on a sandy soil in Denmark, involving different catch crops and catch crop mixtures with non-legume and legume species. The residual effect of the catch crops was measured in spring barley grown in the residual year. For comparison, winter rye was also included. Over the four-year period the spring barley/fodder radish showed significantly lower leaching compared with the other catch crops, and the winter rye had the highest leaching. Regarding the residual effect, fodder radish and ryegrass, were most valuable, with nitrogen fertilizer replacement values ranging from 13 to 41 kg ha-1, based on fertilizer recovery efficiency of 50%. The process based APSIM (Agricultural Production Systems sIMulator) model was used to simulate the spring barley/catch crop rotations and the winter rye, and showed good model performance regarding N leaching and, crop yield and N uptake. The model was then run over a period of 20 years, to determine variations in N leaching and residual effects due to environmental conditions. The simulations showed that catch crops reduced N leaching by 38–64% when grown every year, and by 21–39% when grown only every second year. Nitrogen fertilizer replacement values ranged on average from 28 to 44 kg N ha-1, but showed high inter-annual variability. The nitrogen fertilizer replacement values showed only a weak relationship with the N uptake of the catch crop prior to termination, likely due to long term effects of N mineralisation from catch crop residues and pre-emptive competition for N by the catch crop. • Catch crops (CCs) were effective at reducing N leaching to or below that of the reduced N fertilizer treatment. • APSIM modelling indicates nitrogen fertilizer replacement values (NFRV) of catch crops ranging from 28 to 44 kg N ha-1. • APSIM modelling indicates that volunteers can be used as a low-cost alternative to CCs. • NFRVs showed high inter-annual variability and were not related to the CCs' N uptake. [ABSTRACT FROM AUTHOR]

• Early sowing of winter cereals reduced N leaching compared with timely sowing. • Catch crops in spring barley reduced N leaching compared with bare soil. • Sowing time and catch crops did not affect grain yield or N concentration of grain. • In a wetter climate, early sown winter rye had similar N leaching as a spring barley/ryegrass. • In a drier climate, early sown winter wheat had higher leaching than spring barley/fodder radish. Sustainable management of intensively managed agricultural land requires high productivity with low environmental impacts. There is a lack of understanding concerning the effectiveness of early sowing of winter cereals compared with catch crops for reducing nitrogen (N) leaching losses. To test this, an experiment was conducted over three years (2016–2019) with different crop rotations and increasing rates of N fertilisation. The experiment was done at two contrasting sites in Denmark; one of the sites was at Foulum (FU) on a sandy loam in a wetter climate and the other at Flakkebjerg (FB) on a sandy loam soil in a drier climate. The main crops were either spring barley (SB), or winter rye (WR) at FU and winter wheat (WW) at FB. The SB received four different N fertilisation rates ranging from 0 to 200 kg N ha−1, and the winter cereals six different rates ranging from 0 to 255 for WR and from 0 to 300 kg N ha−1 for WW. In the SB treatments the soil was either left bare over winter, or sown with volunteer barley plants or catch crops. The catch crop used at FU was ryegrass, while fodder radish was used at FB. The WR and WW were either sown timely, according to common practice at the end of September or about three weeks earlier. Neither early sowing nor the use of a catch crop affected the grain yield or the grain N concentration. Nitrate leaching was reduced on both sites by early sowing and by the use of a catch crop. Early sowing of WR at FU reduced N leaching compared to timely sowing, and had almost the same N leaching as the spring barley/ryegrass rotation. At FB, early sowing of WW also reduced N leaching compared to timely sowing, but this was only significant in one of the three years, and leaching was much higher than in the spring barley/fodder radish rotation. Both early sowing of winter cereals and the use of a catch crop in spring cereals are mitigation options for reducing N leaching. [ABSTRACT FROM AUTHOR]

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. • Delayed sowing time decreased the ability of cover crops to reduce nitrate leaching. • Nitrogen uptake differed in cover crops based on sowing time. • Nitrate leaching reduction was similar in cover crops based on the sowing time. • Average reduction per day with sowing time was same in cover crops. • Fodder radish and early sowing was an effective combination to reduce most leaching. [ABSTRACT FROM AUTHOR]

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 (V t), mean macropore diameter (d m) and mean Euclidian dis t ance to the nearest macropore (ED m). 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 V t and d m in Full Range and reducing V t in the > 1020 µm range. Additionally, FT caused significant increases in V t and reductions in d m and ED m 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. • Soil structure was investigated via μCT analysis of macropores classified by size. • Freeze-thaw expands very fine macroporosity and reduces coarse macroporosity. • No-till reduces macroporosity with fewer, longer and less complex coarse macropores. • No-till and plant cover diminish structural changes after 5 freeze-thaw cycles. [ABSTRACT FROM AUTHOR]