Experimental design

The experimental design was that of ^{León et al. (2016)}; the experiment gives continuity to a series of measurements carried out in the same trees established in 1971, to a mineral fertilization assay with a randomized block design consisting of four blocks and eight treatments; in these, NPK was applied fractionally with the formula 8-10-10, which indicates no changes in the proportion of NPK, but only in the amount administered to each tree (Table 1).

The fertilizer is made up of ammonium nitrate, mono ammonium phosphate and potassium sulfate, whose characteristics are the following: 35 % ammonium nitrate, with a solubility of 1 700 g L^-1, an electric conductivity of 850 µS cm^-1, and an acid reaction; mono ammonium phosphate, with 11 %, 48 % phosphorus, a solubility of 200 g L^-1, an electric conductivity of 455 µS cm^-1, and an acid reaction; potassium sulfate with 50 % richness, a solubility of 110 g L^-1, an electric conductivity of 880 µS cm^-1, and a neutral reaction.

The provenance of the seeds was the Marbajitas seed orchard, located in the municipality of La Palma, at a distance of 50 km from the place where the trees were planted. The planting frame was 3 × 3 m. The size of the plots was 225 m², with a total of 25 trees, and nine in the useful plot, whose individuals were labeled and followed up throughout the duration of the assay.

The experiment was established in 1971 at the Viñales Forest Experimental Station. The planting hole method was utilized; the fertilizer was applied to the furrows in a half-moon shape and covered with earth, at the start of the rainy season, in July, and during the first five years after the establishment of the plantation.

Mensuration data taken at the assay

In order to assess the response of the species to fertilization, the number of living trees ha^-1 was estimated, and the height and the diameter at 1.30 m above the ground of the trees of the useful plot were measured, using a Blume-Leiss-1969 hypsometer and a caliper (Varsi-1970), respectively).

The data were recorded from 1977 (at six years of age) to 2012 (at 41 years of age). The number of living trees per hectare was estimated considering only those present in the useful plot.

The content of retained carbon was determined, in (Mg ha^-1), based on the height, the number of living trees ha^-1, the diameter and the volume of the timber with bark at 6, 18, 15, 33, 35 and 41 years of age of the plantation.

Height (h) was measured using a calibrated ruler up to two years after the planting, and a Blume-Leiss hypsometer after five m.

The volumes of timber corresponding to each of the plots were estimated by adding the volumes of the surviving trees, so that the estimated values reflected the changes in the number of trees ha^-1 (Table 2).

Estimations were made from the age of six years, based on the number of living trees per hectare, using Hubert’s formula:

Where:

V = Volume of timber per tree, m³ .tree^-1

d ₌ Diameter at 1.30 m from the ground (cm)

h = Height (m)

C _f = Morphic coefficient (0.5 for Pinus caribaea)

Retained carbon

Carbon retention was calculated at 6, 8, 15, 33, 35 and 41 years of age of the plantation, using the methodology to establish the carbon retention baseline (^{Mercadet and Álvarez, 2009}), which considers the variables species, age (years, surface area (ha), height, m, diameter (cm), diameter (m), volume (m³), stem biomass (t) aerial biomass, root biomass, total biomass (t) and carbon biomass (t).

Economic analysis

The economic benefit was calculated considering carbon at the age of 41 years after panting (Table 2), the cost of the fertilizers expressed in UPC ($ 2 800.00 MXN), the cost of applying the fertilizer ($ 81.60 UPC), and the value of the ton of carbon (€ 10.01), annual mean for 2018 (https://www.sendeco2.com/es/precios-co2).

It was calculated using the formula:

Where:

EB = Economic benefit

T _c n = Tons of carbon for a treatment n

T ₀ = Tons of carbon for control

CT _c = Cost of the ton of carbon (UPC)

C _fer = Cost of fertilizers (UPC)

C _apfer = Cost of fertilizers application

The Value/Cost ratio (V/C) was calculated using the following expression:

The price of the ton of carbon in Euros was converted into UPC according to the current exchange rate of 1.23245 UPC, and this value was multiplied by $ 25.00 MXN at the current exchange rate for the Cuban peso, in order to unify the currency.

Statistical analysis

The amount of retained carbon was subjected to multivariate variance analyses and to Duncan’s mean comparison tests. The data were processed by means of a variance analysis, with a significance level of 0.05 %. Univariance analysis were carried out separately for the different ages. Compliance with the normality assumptions was verified, using the Shapiro-Wilk test, and the variance homogeneity, using Levene’s test.

Effect of fertilization on carbon retention

Controlled fertilization in forest plantations favors carbon storage in the trees. Table 2 shows that the treatments with the highest doses of fertilizers produced the largest carbon reserves and exceeded those of the control by an average of 41.8 Mg ha^-1 at the age of 41 years. Treatment T2, with the lowest dose, obtained lower values than the control, in agreement with the thesis of ^{Reyes et al. (2014)}, that the application of less than the appropriate dose of fertilizer produces nutritional imbalances and, therefore, less growth.

Statistical differences were registered in treatment T1 for the ages of 6, 8 and 15 years, as well as for higher ages; this was influenced by height, which exhibited significant differences among treatments only in the first years, after which stability was attained, and then lost at the age of 17 years (^{Jiménez and Herrero, 1994}); at the end of the period, only the treatment in which 300 g of NPK were applied per tree turned out to be statistically different, with a lower estimated value, which proves that height is an indicator of the quality of the site (^{Reyes, 2016}).

The results for the different treatments in which doses of mineral fertilizer equal to or above 600 g tree^-1 were applied exhibited larger volumes of timber, which resulted in a larger amount of carbon retained by the plant, as timber is one of the main variables that influence carbon retention in the species. There were no significant differences between treatment T1 at the age of 33 years and treatments T3 to T8. However, its numerical value is lower than the one attained by these other treatments, which ratifies the importance of applying mineral fertilizers, not only in order to increase the outputs in timber volume as the main economic benefit, but also as means to mitigate the effects of climate change through carbon sequestration and retention.

^{Bruce et al. (1996)}, cited by ^{Mogas and Riera (2004)}, suggested the possibility of using forest mass growth as a form of carbon storage for addressing the potential climate change and increasing the sequestration of this element by forest plantations.

Considering that the application of 800 to 1 000 g tree^-1 of chemical fertilizer contributes to increase carbon retention by an average of 41.8 Mt ha^-1, it may be said that the fractioned application of mineral fertilizer in the first five years after the establishment of the plantation favors carbon retention by the cultivated species.

The sustainability of forest plantations requires consideration of both economic and environmental factors. Field experiments prove that the use of large amounts of fertilizers increases tree growth (^{Muller da Silva et al., 2013}).

Fertilization with nitrogen has the ability to significantly alter the edaphic environment of forests and can increase carbon storage (^{Smaill et al., 2008}). According to ^{Shryock et al. (2014)}, fertilization with nitrogen significantly increased the sequestered carbon by each tree compared to unfertilized plots.

In studies conducted by ^{BBC WORLD (2003)} and ^{FAO (2011)}, in general, forests provide a unique environmental service by eliminating carbon dioxide from the atmosphere and storing it in the biomass, the soil, and the products; furthermore, they offer a sustainable alternative to fossile fuels. In particular, rapidly growing forest plantations (pine and eucalyptus) have a beneficial impact on the environment, as they contribute to the reduction of the greenhouse effect.

Eucalyptus plantations in Cuba exhibit no adverse results in terms of species displacement, as they are mainly used as low-size timber for assuring tobacco crops, as poles for the production of covered tobacco, and as sticks (“cujes”) for stringing the tobacco leaves.

There is evidence that soil conditions such as the porous space occupied by water, the temperature, and the availability of soluble carbon (C) have a dominant impact on N₂O emissions. Crop management and the source of fertilizer to be used are factors that can affect N₂O emissions, but, due to the interactions with soil factors, general conclusions are hard to reach.

Mismanagement of the correct dose, the source, the time, or the location of the fertilizer (N), and a lack of appropriate balance with other essential nutrients may increase the total loss of N, as well as the N₂O emissions. When N is applied in larger doses than the optimal economic dose, or if the N available in the soil (especially in forms of NO₃-) exceeds its absorption by the crop, the risk of increasing N₂O emissions increases (^{Leyva, 2015}).

Forest plantation can play an important role in carbon retention and in supporting global efforts to solve greenhouse effect and climate change issues (^{Salleh, 1997}).

The amount of carbon fixated per planted hectare varies according to the species, the design of the plantation, the management, and the soils, among other factors; however, the increase in the volume of accumulated biomass in the entire reforested area increases the number of tons of mitigated carbon (^{Arguedas-Marín, 2012},).

The graphic representation of the tons of carbon registered for the various treatments at the different ages reflects this (Figure 1). The treatments with doses of fertilizer of over 300 g tree^-1 are observed to produce the largest carbon retentions, significantly higher than the other treatments. Treatment T2, with the lowest dose, resulted in lower values than those of the control treatment; this ratifies the results obtained by ^{Vásquez (2001)} and ^{Reyes et al. (2014)}.

Figure 1 Retained carbon by age and by treatment. 

The orthogonal contrasts did not exhibit significant differences between the alternate and the continuous regimes as to the amount of carbon retained in any of the years, with values of P₁₉₇₇ =0.462; P₁₉₇₉= 0.881; P₁₉₈₆=0.693; P₂₀₀₄ = 0.550; P₂₀₀₆= 0.337; P₂₀₁₂= 0.362.

Economic analysis

The treatments T3, T4, T5, T6, T7 and T8 registered positive economic effects, except for treatment T2, in which a minimum dose of 300 g árbol^-1 was applied. Figure 2 shows the differences in economic benefit and the value/cost ratio of the treatments, compared with the control. The profits for treatments T3 to T8 are above $ 6 000 ha^-1; treatments T4 and T6 obtained the highest profits, above $ 12 000 ha^-1. The losses of treatment T2 were below $ 2 300 ha^-1, at 41 years of age of the plantation, and the V/C ratios for treatments T3, T4, T5, T6, T7 and T8 were above 2, an acceptable value for investment in fertilization, considering the volumes of timber obtained per hectare (^{FAO, 1986}).

Trataminetos = Treatments; Diferencias en beneficio económico con respecto al tratamiento testigo = Differences in economic benefit with respect to the control treatment

Figure 2 Economic analysis of the experiment on the application of mineral fertilizers in an Alitic soil with low clayey activity. 

In general, treatments T3, T4, T6, T7 and T8 had the best results, notably T4 and T6, with an economic benefit above $ 12 000 ha^-1 and a value/cost ratio above four. It is important to highlight that treatment T7 behaved as the most stable in all the assessed variables (^{Reyes, 2016}). In the economic analysis it obtained a value/cost ratio of 3.1, which is lower than that of other treatments; however, its economic benefit exceeds $ 10 000 ha^-1. Either of these variants represents an attractive option for forest management, in terms of the available fertilizer and the use given to the timber, in order to obtain the maximum carbon retention, the highest profits and the best value/cost ratio.