The results of this study provide valuable information on how different substrate treatments affect the growth (biomass) and nutrient allocation (macroelements) in and within oak and beech seedlings after production in the nursery (just before establishing them as a crop). The results showed significant variation in nutrient content across treatments, especially for nitrogen, phosphorus, and potassium, which are the most crucial nutrients required for the proper formation of root systems in seedlings. The dry mass and nutrient content analysis of different parts of the oak and beech seedlings under study suggest that the novel substrate mediums and fertilizers effectively promote root system development. In essence, the aboveground characteristics of the seedlings cultivated on the peat-free substrate, coupled with the liquid fertilizer developed by the University of Agriculture in Kraków, were in every respect close to those of the seedlings grown on the state forest substrate comprising peat and solid fertilizer (Rotowa et al. 2023b)
The novel substrate and fertilizers showed superior nutrient allocation in the belowground evaluation of the seedlings (Tables 5 and 6). The allocation pattern of major macronutrients with higher levels in the root could be attributed to the fact that the seedling samples were collected from the nursery at the end of the growing season. Rotowa et al. (2023), in a study carried out on the root systems of the same seedlings, reported treatment UR20 to be best in root morphology and characterization. In this study, however, the treatment UR22 showed the best performance in nutrient allocation in the roots of both species' seedlings. This therefore implies that peat-free substrates with the recommended dosage of fertilizer (Pająk, 2022) could be a viable alternative to traditional peat-based media, offering a more sustainable approach to seedling cultivation in forest nurseries.
The different substrates exhibited significant effects on the biomass characteristics of seedlings, especially in oak. This agrees with other studies that have highlighted the influence of compacted peat substrate on nutrient contents. For example, Pająk et al. (2022) reported higher macronutrient allocation in root of European beech seedlings grown on differently compacted peat substrates in a container nursery, and Rotowa et al. (2023) reported the influence of various substrates and fertilizers on the root systems of oak and beech. In addition, Banach et al. (2013) experiment conducted on F. sylvatica and Abies alba seedlings in a sawdust–peat substrate, finding that well-aerated substrates are essential for good growth in beech seedlings, affirmed our results. Additionally, Freschet et al. (2015) reported that nutrient allocation additionally relied on the quality of the medium on which they were grown.
Although the allocation of C, N and P in the studied species recorded higher values than those reported by Aoyagi and Kitayama (2016) However, comparison of nutrient allocation in the studied organs agrees with the optimal values reported by Wei et al. 2013; Pająk et al. 2022a&b; Marušić et al 2023. This showed that the allocation of nutrient fell within the optimal range. Furthermore, exceptions were observed for phosphorus in the UC treatment of oak, which fell below the lower threshold. Phosphorus and potassium remained at the lower limit to nitrogen, while magnesium in the SR treatments was recorded at its lower limit in oak. Although the studied samples were collected towards the ending of the growing season, However, This situation of observed decrease in the concentrations of nitrogen, phosphorus, potassium, and magnesium agrees with the study of Hidaka and Kitayama (2011) in tropical tree species, indicating a withdrawal into the stem and root, while the calcium content increased, especially towards the end of the growing season, as observed in this study. Calcium serves a structural role in the cell wall of plant organs and is immobile in leaves. Earlier studies (Peuke and Rennenberg 2011; Loewe-Muñoz et al. 2024) established a high requirement for calcium and a very high requirement for potassium. Beech seedlings have been reported to grow better in substrates rich in calcium, magnesium, and potassium (Pająk et al. 2022b). Moreover, Balcar et al. (2011) used dolomite (containing calcium and magnesium) to fertilize beech trees in plantations, and reported a positive effect on their survival and growth.
Variety in nutrient allocation was not just found to exist between the tree species, but also within different organs of each species. This concurs with various investigations on European beech and pedunculate oak (Poorter et al 2012; André 2010, Pretzsch 2014, Husmann et al 2018). Unlike beech, oak displayed essentially higher nutritional accumulation in the roots, shoots, and leaves. This study also re-established that nutrient allocation is generally higher in below ground organ than those that are above ground and this applied to the two species. Prominently, nutrient response efficiencies varied significantly among the studied species, with treatment on R22 of novel substrate and UAK fertilizer formulation accumulating more nutrients in the root. This corroborates previous studies (Freschet et al. 2015; Husmann et al. 2018; Klimešová, et al 2018, Rumpf et al. 2011; Meiwes et al. 2012; Pająk et al. 2022b; Rotowa et al. 2023b) that focused solely on the nutrient content of aboveground and belowground biomass. Plant roots serve as a primary storage organ for macronutrients, especially during periods when the plant is preparing for dormancy or reduced metabolic activity, as was the case for the sampled seedlings. After the developing season in the nursery, the transportation of nutrients to the roots for storage was focused on to guarantee that these nutrients were reserved for the next growing season (in the forest). This mobility and reallocation of these nutrients could be said to have brought about more nutrient allocation in the underground growth, as seen in this study. Also, the allocation of nutrient into plant-stable organs was seen to be quicker in oak than in beech, and this could justify why more supplements were assigned to the underground root growth of oak than to that of beech.
The distribution of plant biomass has been identified as a crucial factor that aids allocation of nutrients in plant organs (Rumpf 2011; Meiwes, 2012; Freschet et al., 2015; Husmann et al., 2018, Klimešová 2018; Yue, 2021). The importance of nitrogen and phosphorus partitioning between plant organs as a critical factor in regulating growth rate has been highlighted in previous studies (Laliberté et al 2012; Minden and Kleyer, 2014, Tang et al., 2018, Malhotra et al. 2018; Zhang et al. 2018, Yang et al. 2021) The results of this study show contrasting trends in nutrient allocation between nitrogen and phosphorus as both were influenced from negative side of distribution to positive as a result of the effect of fertilization. A similar divergence in nutrient availability was observed by Aoyagi and Kitayama (2016) in their investigation of nutrient allocation between plant organs in Bornean rainforests. The differences in nitrogen and phosphorus allocation among organs may be closely related to the different requirements for nitrogen and phosphorus in the construction of structural biomolecules.
In contrast to the findings of Zhao et al (2020) that nutrient allocation among organs of tree species is intimately related to it demand, the application of this novel fertilizer, as depicted in Fig. 5, appears to be markedly different. It is important to note that although the role of shoot phosphorus is less well established, the shoot could potentially act as a storage organ, as previously suggested by Sardans and Peñuelas (2015). The storage of carbon and nutrients in long-lived organs, such as shoots and roots, is considered essential to compensate for biomass losses due to fallen leaves and branches (Aoyagi and Kitayama, 2016), or to natural enemies (Fricke et al. 2014, Comita and stump, 2020)