Low rates of biochar and fertilizer addition show a synergetic effect, outperforming other treatments and the control on nearly all performance parameters studied, and for both species. Additionally, BC1 addition promotes the survival of both species when compared to all other treatments save for the control.
Our results showed that survivorship was highest in the biochar addition at the BC1 rate (1.1 t/ha), though these results are not significantly different from the control treatment (Figure 1
). A similar observation was made by Farias et al. [40
]. More strikingly, survival varied by a factor of two among the different treatments, with BC5 (5 t/ha), BC5 + F, and fertilizer alone, significantly reducing seedling survival as compared to the control and BC1 treatments. Indeed, the addition of NPK fertilizer had the lowest rate of survival of any treatment. This pattern contrasts with the pattern of growth response to the treatment (see below), which showed that biochar plus fertilizer treatments were superior to the control, fertilizer alone, or biochar alone. What this shows is that practitioners will need to assess the relative importance of post-planting survival and growth performance in project management objectives and manage this trade-off accordingly. Additionally, the biochar and fertilizer may be added in two phases, which has the potential to give the combination of the highest survival (BC1) and highest growth (BC1 + F) treatments, though at a small reduction to the overall savings in labor costs from the methodology outlined here.
A study on seedling germination and growth of temperate tree species on biochar enriched and control soil reports similar results [41
], with an increased growth in diameter and height but a lower survival rate for the seedlings in the biochar enriched treatment. This negative response to higher biochar application rate may be due to the nutrient immobilization effect of biochar, its indirect effect on the microbial community which could reduce plant growth (e.g., presence of volatile organic compounds in biochar that may affect microbial reactions or their interactions with plants), or to its high alkalinity, increasing the soil pH above optimal level of the native species tested [42
4.2. Plant Growth and Development
Considering both species, treatment BC1 + F had the most positive impact on seedlings’ developmental indicators in comparison to treatment with biochar or fertilizer alone and to the control, and in most cases exceeded or was equal to higher application rates of biochar + fertilizer (BC5 + F). Similarly, Ghosh et al. [44
] reported that the concomitant addition of biochar and compost had a stronger effect than biochar or compost alone on growth of two native tropical species in equivalent soil type (pH: 5.5 and density: 1.6 g/cm3
The slightly lower results of treatments containing BC5 compared to BC1 treatments suggest that a high application dose of biochar may impede seedling development. Meng et al. [45
] reported a significant increase of seedlings height and biomass in treatments containing activated microorganisms, fertilizers and low levels of biochar amendment (up to 20% in volume) compared to treatment with activated microorganisms, fertilizers and high levels of biochar amendment (20 to 40% in volume). These results coincide with ours, as best results are found at BC1 (8% volume) as compared to BC5 (41% volume). Similarly, Sarauer and Coleman [42
] observed that large doses (25% and 50% volume) of biochar in containerized growing media (peat) reduced the height and diameter growth rates of Douglas fir. According to them, this may have been due to an increase of soil pH to harmful levels for seedling development [42
Another significant result from this study is that biochar plus fertilizer showed a large and significant increase in plant performance, even though the fertilizer and biochar was only applied once at the beginning of the experiment. In their yearly assessment of native species growth in Peruvian abandoned artisanal gold mining areas, Román-Dañobeytia et al. [46
] found that the addition of biofertilizer (a macro- and micronutrient-rich solution resulting from the fermentation of organic material and minerals [46
]) at a rate of 55 and 555 L/ha significantly increased seedling monthly growth in height and diameter. However, biofertilizer application took place every 15 days for the first 6 months, while the growth increase witnessed in this study relied on a single application of fertilizers at the time of transplant. Over the lifetime of a project, the methodology from the present study would save substantial costs compared to traditional forestry techniques, as fertilizers and labor are regarded as significant expenses in reforestation projects [47
]. This may be due to the capacity of biochar to retain fertilizers, thus avoiding the leakage of nutrients [49
]. The nutrient retention effect of biochar could have large and positive repercussions for reforestation/restoration projects, substantially lowering the maintenance costs and increasing the success chance of a plantation [50
]. Additionally, this nutrient-capture effect has been suggested to increase as the biochar ages in soils [51
Below ground biomass results for Guazuma crinita
show an 85% and 83% increase for BC1 and BC1 + F treatments when compared to control and F treatments, respectively. Results for Terminalia amazonia
show a 22% and 88% increase for BC1 and BC1 + F treatments when compared to control and F treatments, respectively. These results are in accordance with the meta-analysis from Xiang et al. [52
], who observed an overall increase in root biomass of 32% after biochar application. Plants with a more robust root system are expected to perform better over time and to show higher resistance to drought in comparison to plants with a less robust root system. On the other hand, and for both species, BC5 and BC5 + F treatments showed a decrease or no response in root biomass when compared to control and F treatments, respectively, here again corroborating our hypothesis that BC5 application rate (i.e., 5 kg per seedling in field application) is excessive and impedes the growth of the species under study.
Our variable interaction assessment (see Appendix A
) indicates that the early successional species (Guazuma crinita
) showed a stronger positive response to BC1 + F and BC5 + F treatments for height growth and to BC5 + F for diameter growth compared to Terminalia amazonia
. This observation suggests that the synergetic beneficial effect of biochar and fertilizer addition may favor early successional species as compared to late ones, though this study does not have the variety of species needed to make this a robust conclusion. The current observation contrasts with Sovu et al. [53
], who noted a stronger positive effect of biochar on slow-growing tree species. The preferential response to biochar of either early or late successional species could have important impacts on the resource management plan of reforestation/restoration projects willing to use biochar and understanding the responses of a broader variety of tree species to biochar and fertilizer treatments should be a central goal of future research.