To date, several studies have highlighted the environmental excellence of the perennial cup plant (Silphium perfoliatum L.) compared to the benchmark crop for anaerobic digestion: silage maize (Zea mays L.). Accumulation of soil organic carbon (Ruf and Emmerling 2026), improved soil structure (Rohlmann et al. 2025), reduced susceptibility to soil erosion (Auerswald et al. 2025), promotion of above and belowground agrobiodiversity and activity (Schorpp et al. 2016; Wöhl et al. 2024), low emissions of soil-borne greenhouse gases (Kemmann et al. 2021), and modest requirements concerning fertilization (Ruf and Emmerling 2022)—at the expense of about 10%–20% lower biogas yields per area (Von Cossel et al. 2020). Thus, I have read the study of Hollweg et al. with great interest and highly appreciate the outcomes of this sophisticated, four-year study. The comparative investigation of the nitrate retention under cup plant and silage maize adds another piece of the puzzle for a holistic evaluation of cup plant cultivation. In their lysimeter study, Hollweg et al. found significantly higher shoot and root biomass in cup plant, along with vanishingly small amounts of nitrate leaching compared to silage maize, starting from the second year of growth. Already in the abstract, they state “reduction of nitrate leaching of 88% in 2021 and by up to 99% in 2022 under cup plant compared to maize”. Values that appear astonishing already at first glance. We can see from Equation (2) that N-addition by fertilization (Nfert) and N-removal with crop biomass (Ncp-rem) are two factors that have a strong influence on N-leaching via seepage water (Nw-loss). The authors state that both crops were fertilized “to reach 190 kg N ha−1 as recommended for silage maize” and present the exact fertilization in table S1. Thus, the term “Nfert” in Equation (2) is equal for both crops. In all experimental years, except the establishment year of cup plant, silage maize produced significantly lower shoot biomass in both water regimes. Usually, the median shoot biomass of cup plant was, recalculated to a hectare basis, between about 20 Mg ha−1 and 35 Mg ha−1. Thus, it is approximately 2.5 to 4 times higher than that of silage maize. Based on the study of Ruf and Emmerling (2022) we can estimate that the N-demand of cup plant is roughly 50% of that of silage maize at the same yield level. As described by Hollweg et al., the cup plant produced really high shoot biomass and was able to develop an intense root system (figure 6 in the study of Hollweg et al.). The latter expresses in the significance of the factor Nroot-fix (nitrogen fixed by root development). In the first years, the development of the root system dominates over root turnover in the perennial system that prevents nitrogen from being leached. In older stands, it likely develops towards a balance between the new formation of roots and degradation of died off roots. Resulting from that it has to be assumed that the applied amount of N-fertilizer met its N-demand. In contrast, due to the significantly lower yield potential of silage maize, the applied amount of N-fertilizer exceeded by far the plants' demands. This finally led to the tremendous nitrate leaching from the lysimeters. There was also no subsequent crop that may have taken up remaining mineral nitrogen after the vegetation period of silage maize in each year. I fully understand that the experimental setup was not changed during the experiment. However, the presented data, in particular the difference in amounts of N leached, can be transferred to practice conditions to a very limited extent, only. Farmer and agricultural advisors would adjust the applied amount of fertilizer to the plants' demand which correlates with the expected yields. For the farmers, this is finally an economic question. Further, the huge shoot biomass of cup plant is methodologically defined to a large extent. One should be concise that the water content of the lysimeters was adjusted by an irrigation system. Under natural conditions, the soil storage would have been empty at a certain time. In the lysimeter approach, in contrast, the water resource in both water regimes was unlimited and cup plant could maintain its high respiration rate (Schoo et al. 2017) and photosynthetic activity without water restrictions. Summarizing, I haven't even the slightest doubt about the general statements, but I like to admonish a careful and reflective interpretation of the results. Under practice conditions, the difference in leaching will likely narrow down. Further, silage maize and cup plant should not be played off against each other. Both crops show unique advantages and disadvantages with respect to biomass and biogas yields, soil protection, groundwater protection, and different aspects of biodiversity. This constitutes the great opportunity for a site adapted cultivation of these crops in order to balance production and ecosystem protection. On a local scale, a higher share of perennial crops should be allocated to sensitive areas while higher yielding crops will maintain their legitimacy. This especially holds when they are cultivated under best practice conditions in long crop rotations incorporating cover crops that fix excess nitrogen and prevent leaching. Thorsten Ruf: conceptualization, writing – original draft. The author declares no conflicts of interest. The author has nothing to report.
Thorsten Ruf (Mon,) studied this question.