Our results showed that the E. grandis plant growth was enhanced when provided with CRF or OMF. Thus, the benefit of using granulated OMF made from pig manure as an alternative source of nutrients for forest plantations is one of the main results of this study. Generally, E. grandis plants exhibited the best results in both morphological and physiological attributes when cultivated with CRF and OMF added to the soil at the time of planting.
Using CRF promoted an adequate supply of nutrients to E. grandis plants. Recently, other studies have highlighted the advantages of using CRF for the basal fertilization of forest species (Turchetto et al. 2019; Griebeler et al. 2023; Speetjens and Jacobs 2023). CRFs are water-soluble fertilizers enclosed in a semipermeable polymer coating that allows mineral ions to be available to plants at a controlled rate (Vejan et al. 2021). The regulated release of nutrients reduces leaching and improves the use efficiency compared with conventional non-encapsulated fertilizers.
This study demonstrated the high potential of OMFs based on pig manure for E. grandis cultivation. Plants cultivated with OMF had greater height and dry mass allocation than those cultivated with mineral fertilizer (MF). Other studies have shown similar or higher crop yields using organic fertilizers, such as pig manure or chicken litter, than with MF (Cassol et al. 2012; Corrêa et al. 2016). Adding pig manure compost enabled the better development of E. grandis seedlings grown in copper-contaminated soil (Negrini et al. 2022). Andriguetto et al. (2024)also found higher biomass allocation values in C. americana plants cultivated with OMF based on pig manure than the ones cultivated with MF.
Applying OMF to the soil had a similar effect as using CRF on the growth of E. grandis plants. According to Sánchez-Báscones et al. (2019), using organic solid waste reduces the leaching capacity of nitrate in the soil because it primarily contains nitrogen in its organic form. Furthermore, adding MF to organic compounds can improve the efficiency of nutrient utilization by plants (Morais and Gatiboni 2015; Borges et al. 2019).
The increased growth of E. grandis when cultivated with CRF or OMF was directly related to the improvement in the photosynthetic apparatus of the plants. CRF and OMF treatment resulted in higher values of the attributes generated by measuring chlorophyll a fluorescence. Chlorophyll fluorescence is sensitive to environmental stress, making it an indicator of the physiological state of plants owing to environmental or nutritional changes (Feng et al. 2015; Sim et al. 2021). In other words, lower chlorophyll a fluorescence emissions are related to lower nutrient restrictions by the plant.
Based on chlorophyll a fluorescence, the Fv/Fm ratio expresses the relative efficiency of light energy absorption by the PSII antenna complex, where higher values of Fv/Fm indicate greater efficiency in capturing energy from the PSII reaction center (Banks 2017). Y(II) represents the quantum yield of the photochemical energy conversion in PSII and is highly correlated with the photosynthetic rate (Baker 2008). The ETR, calculated as the product of PSII efficiency and absorbed light, is positively correlated with CO2 fixation (Krall and Edwards 1992; Wong et al. 2014), as the flow of electrons through PSII is primarily consumed by carbon assimilation (Oberhuber and Edwards 1993).
Therefore, the performance in capturing energy by the PSII reaction center, electron transfer efficiency, and light energy conversion by the PSII reaction center of E. grandis plants were significantly improved with the application of CRF and OMF, which was most evident in Fv/Fm and Y(II). Consequently, the enhanced photosystem performance provided more assimilation power (NADPH and ATP) for regenerating ribulose-1,5-bisphosphate, resulting in greater growth and biomass allocation in E. grandis plants.
Inoculation with T. harzianum also improved the E. grandis development, especially when associated with CRF or OMF. Griebeler et al. (2021) observed greater height values and improved photosynthetic apparatus in Cordia trichotoma (Vell.) Arráb. ex Steud., and Cedrela fissilis Vell. plants inoculated with T. harzianum and T. aspereloides compared to the control treatment. Chagas Jr et al. (2021) found that inoculation with T. longibrachiatum and T. harzianum promoted the highest initial growth of E. urophylla and E. brassiana seedlings.
Fungi of the Trichoderma genus can promote greater water and nutrient absorption, osmotic regulation, and the synthesis of metabolites involved in plant systemic resistance (Kafle et al. 2022; Corrales et al. 2022). Compounds produced by these fungi, including antibiotics, enhance root resistance to pathogens and their adverse effects (Zin and Badaluddin 2020). In a study on tomato plants, Cai et al. (2013) isolated and characterized a well-known butenolide metabolite, harzianolide, from the filtered fermentation broth of T. harzianum, which resulted in increased plant biomass.
Thus, secondary metabolites from Trichoderma can act as auxin-like compounds, positively influencing the growth of the colonized plants. Regarding the mechanism of action, RDM significantly increased in response to T. harzianum inoculation. Therefore, inoculation with T. harzianum influences the early stages of plant growth by improving root development. Greater root mass allows plants to explore a larger volume of soil owing to increased active absorption sites per unit area (Cai et al. 2013).
The present study demonstrated the benefits of using OMF based on pig manure and inoculating E. grandis plants with T. harzianum (Figs. 1 and 2). Due to necessity to feed a growing population while simultaneously reducing negative environmental impacts, pursuing sustainable and efficient agricultural practices has become increasingly relevant. In this context, using pig farming waste to formulate OMF, coupled with the use of bioinputs, such as T. harzianum inoculation, has become a promising alternative to a more sustainable forest production system. These practices offer substantial economic benefits, including reduced use of MF sources, increased forest growth and productivity, and income generation for producers. They also contribute to environmental conservation by promoting soil health and improving the quality of water resources.