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Growing cover crops promotes soil health as they retain nutrients during autumn/winter and provide organic matter to the soil biota, which in turn supplies nutrients to the main crop upon mineralisation in spring. Different cover crops have varying impacts on soil biology and nutrient dynamics due to the quantity and quality of plant material returned to the soil. To understand these effects, high-resolution data on crop responses is required. In this study, remote sensing was used to provide such data. The temporal dynamics of soil nitrogen (N) availability and N uptake in barley were studied in response to different cover crop monocultures and mixtures. This was achieved using high-resolution multispectral images of the main crop acquired from an unmanned aerial vehicle. Alongside this, in-situ collected plant and soil parameters were used in this 5-year cover crop field experiment. The results showed that cover crop legacies significantly affected barley N uptake, biomass, and canopy N content. In early June, at peak canopy N, the highest values were observed in barley grown after vetch-radish or oat-radish mixtures (84 kg N/ha) and the lowest in barley grown after fallow (63 kg N) or oat (53 kg N/ha on 23rd of June). At the start of the barley growing season, soil microbial biomass was not significantly affected by the cover crop legacies. However, differential N mineralisation between cover crop legacies can be attributed to differences in microbial activity associated with cover crop quantity and quality. This research demonstrates the potential of remote sensing to monitor and understand temporal and spatial variation of crop canopy N in response to cover crop N mineralisation by the soil biota which is an important component of soil health. This approach can contribute to more efficient N use by enabling fine-tuning of the type, quantity, timing, and location of fertilisation.

Soil health is a critical aspect of sustainable agriculture, with soil organic matter (SOM) serving as a key indicator. In arable fields, growing cover crops has been advocated as a prime practice to promote SOM accumulation. However, the effectiveness of cover crops to promote SOM accumulation can vary widely. Furthermore, accurate quantification of SOM at field scale is severely constrained by the labour intensity and destructive nature of traditional methods, which limits the ability to quantify and monitor cover crop impacts on SOM. We tested whether cover crop mixtures promote SOM accumulation more than cover crop monocultures in a 6-year field experiment with arable crop rotation on sandy soil. We found that the cover crops radish and oat-radish mixture significantly increased SOM levels compared to the fallow treatment. Next, on soil sampled in year 4, we explored the use of proximal (VIS-NIR, MIR) and remote sensing using Unmanned Aerial Vehicles (UAVs) to upscale SOM from wet lab-based point samples to the whole field and map its SOM status. Thereto, we used Random Forest (RF), Support Vector Regression (SVR), and Partial Least Squares (PLS) models and found that the best fitting model depended on the type of spectral sensor. With proximal sensing (MIR) the best SOM prediction was achieved using SVR (R2= 0.84, RMSE= 1.55 g/kg SOM). For UAV imagery with hyperspectral camera the best model was RF (R2 = 0.69, RMSE= 2.19 g/kg SOM) and enabled digital mapping of SOM distribution across the field. The accuracy of MIR enabled identifying radish cover crop treatments as having on average higher SOM levels compared to the fallow. However, infield spatial SOM variation can override cover crop effects on SOM levels. Therefore, UAV time series are required to remotely quantify cover crop impacts on SOM changes. Overall, our results show potential for combining proximal and UAV-based sensing SOM as a tool for more efficient and accurate spatiotemporal monitoring of SOM

Agricultural soils are at risk of nitrogen (N) leaching especially during the fallow period in autumn and winter. Cover crops are grown to capture soil mineral N that otherwise would leach to the groundwater. They can serve as green manure providing mineral N to the cash crop in spring. We investigated whether mixing species of cover crops can enhance N capture and therefore reduce N leaching more effectively than pure stands in autumn without increasing the risk of N leaching in spring. We hypothesised that mixed species with complementary traits will capture more N and accumulate more biomass. It was also expected that residues from cover crops with higher biomass and lower C:N ratio would mineralise faster and subsequently increase N leaching in spring. In a 4-year field experiment, cover crops were grown between August and February in a rotation with different cash crops. We used eight cover crop treatments, including three pure stands: radish (Raphanus sativus), vetch (Vicia sativa) and oats (Avena strigosa), all possible 2- and 3-species mixtures and a fallow (no cover crop). Treatment effects on leaching losses were estimated by analysing N concentrations in samples of leached pore water below the rooting zone and by modelling the volume of water leached per plot. Most N leaching occurred in autumn and winter while the amount of N leached in spring was negligible due to the lower precipitation. N leaching in autumn correlated negatively with cover crop biomass, N uptake and root length density. Radish and oats were the most productive species and dominated mixtures. Compared to the fallow, radish and mixtures that con- tained radish reduced N leaching by 49–73% and were characterized by quick soil cover, high N uptake and low to moderate C:N ratio. Subsequently, residues from radish and mixtures containing radish mineralized quickly, resulting in an increase in soil mineral N in spring by 70–

Winter cover crops are grown in temperate regions to capture nitrogen, build organic matter in soil, and suppress weeds during autumn and winter. This thesis aimed to identify whether growing species mixtures instead of pure stands strengthens the ecosystem services provided by cover crops. Emphasis was given to productivity, resource capture and yield stability. From 2015 to 2019, I conducted fourteen field experiments across three locations in the Netherlands and one location in Germany. Twenty-five cover crop species were characterized as to their productivity to produce a shortlist of promising species for further studies. I found large differences in productivity between species. Species with early ground cover, such as crucifers and black oats, had the highest productivity and they showed the greatest suppression of weeds. An analysis was made of complementarity and dominance effects in mixtures. Complementarity occurred more often when the mixed species were equally productive, e.g. when combining oilseed radish and black oats, while mixing species with different productivity resulted more often in dominance of the highly productive species in the mixture. Mixing cover crop species within the row or in alternate rows had a small and variable effect on the competitive relationship between mixed species. Across all experiments, mixtures produced 25 to 42% more biomass and captured 11 to 34% more nitrogen than pure stands. This large difference was reduced when the comparison was restricted to the most productive pure stands and species mixtures. In this case, mixtures and pure stands produced similar biomass in most experiments, though, on average, mixtures captured 9% more nitrogen from soil than the best pure stands. In an analysis of data collected in multiple treatments at multiple sites and years, I analysed how the variability of yield differed between mixtures and pure stands. The contributions to yield variability were estimated as variance components u

Context: Cover crops are sown in autumn after harvest of a main crop to capture residual nitrogen and to build biomass that will contribute to soil organic matter after being ploughed under. Mixtures are purportedly more productive than pure stands of single species. Research problem: Dry matter accumulation in field crops can be separated in the processes of resource capture and resource conversion. Here we apply this conceptual approach to analyse whether and how pure stands of single species and stands of species mixtures differ in radiation capture and radiation use efficiency. Methods: cover crops were sown as pure stands (12 treatments) or mixtures (11 treatments) at two years in four sites, three in the Netherlands and one in northern Germany. Ground cover was measured throughout a growing period of up to twelve weeks to quantify radiation capture while final biomass was determined at harvest. The ratio of biomass and cumulative radiation capture was used to calculate radiation use efficiency. Results: Oats and crucifers were the most productive species. Crucifers covered the soil quickly and their radiation capture was consequently high (517 MJ m−2) but their radiation use efficiency was low (0.80 g MJ-1). Oats intercepted less radiation (459 MJ m−2) than crucifers but had a higher radiation use efficiency (1.15 g MJ-1). Legumes had low radiation interception (332 MJ m−2) combined with low radiation use efficiency (0.64 MJ m−2) while the group of forb species belonging to other plant families (e.g. Linaceae, Boraginaceae and Asteraceae) had intermediate radiation capture (371 MJ m−2) and radiation use efficiency (0.84 g MJ-1). The radiation capture and radiation use efficiency of mixtures was similar to that of the dominant species in the mixtures, in all cases a crucifer or oats. Conclusions and implications: The analysis of radiation capture and radiation use efficiency in this study indicates that mixture performance was governed by species dominance with

Planting cover crops after harvest of the main crop has become a key practice in temperate agriculture to reduce N leaching and increase soil organic matter. However, the growth of cover crops can be affected by adverse weather. Growing mixtures is thought to increase yield and reduce variation in productivity, but quantitative information on this subject is limited. Moreover, uncertainty remains on the optimal choice of species and mixture composition for cover cropping to obtain high cover crop yields and resilient performance under different conditions. Here we tested a broad selection of pure stands and mixtures of cover crop species in two years (2017–2018) at four sites: three sites in the Netherlands (Wageningen, Neer and Scheemda) and one site in Germany (Grundhof). All pure stands and mixtures were grown for a period of 11–13 weeks between August and November in each year. Aboveground biomass and N yield were determined. Yields in different treatments (unique pure stands or mixture compositions) in each site-year were regressed on the mean yields in each site-year to assess differences in responsiveness between treatments. Mixed effects models were used to estimate and compare yield variability in pure stands and mixtures at three levels: 1) between site-years, 2) between treatments and 3) between plots. This analysis was performed for biomass and N yield. Across all pure stands and mixtures tested, average biomass was greater in mixtures than in pure stands, but average biomass was similar when this comparison was made between the five highest yielding pure stands and the five highest yielding mixtures across all site-years. Thus, the lower mean productivity in pure stands was mostly due to some low yielding species. The five best mixtures had 9% higher N yield than the five best pure stands. The response of treatment yields to mean site-year yield was similar for mixtures and pure stands. Variation in cover crop yield over site-years was large in both pur