3.1. Diameter, fresh and dry weight of the roots
According to the analysis of variance, the results showed significant differences between the plots of different ages, with the 15-year-old plot showing the highest values in the variable as roots diameter (Fc = 263.98, df = 2, p = 0.0001, Fig. 3). Regarding the orientation (Fc = 0.69, df = 3, p = 0.560) and distance factor (Fc = 0.37, df = 2, p = 0.694), not significant differences were found. Other results of the analysis of variance that include interactions can see in Table 2. Although could seems it is logical that the ages of the plant can influence the diameter of the roots, our results showed that the distance of the palm trunk base to 1, 2, and 3 m did not affect the diameter of the primary roots. Jourdan and Rey (1997) found that the branching of horizontal and vertical roots in palms (between 3 and 20 years old) was constant, showed a relationship between the meters of branching and the root diameter in the adult phase. Jourdan and Rey (1997) point out that horizontal primary roots grow several meters with a diameter between 5.0 and 7.0 mm. In this work, we find similar data, roots between 7.0 mm and 7.3 mm for the ages of 3 and 5 years, respectively, while at age 15-years old, the average diameter was 1.17 cm. Other authors report similar data on the primary root diameters (Gerson et al., 2020). The variation in root diameter and growth is still under study, some reports mention that plantations worldwide have managed in different types of soil, so the size and diameter of the roots can be varied (Safitri et al., 2018). For instance, differences were demonstrated in horizontal and vertical distribution at two palm ages (4-year difference) on spodosol versus inceptisol soils. To the first soil type, roots spread at the same distance (6.5 m), but with differences in depth (30 cm difference). For inceptisol soils, the difference was higher as to depth (1 m difference). Therefore, the age of the crop and the type of soil determine the architecture and diameter of the roots (Safitri et al., 2018).
Table 2
Summary of the analysis of variance showing the effect of plant age, orientation, and sampling distance from the base of the trunk on the parameter’s diameter, dry, and fresh root biomass. The lowercase letter (a) indicates a significant difference (p ≤ 0.05). Values represent means (± standard error, n = 6).
Source | Df | Diameter | Fresh biomass | Dry biomass | |
Fc | P value | Fc | P value | Fc | P value | |
Age palm | 2 | 263.98 | 0.000a | 567.21 | 0.000a | 104.86 | 0.000a | |
Orientation | 3 | 0.69 | 0.560 | 0.49 | 0.690 | 0.35 | 0.790 | |
Distance | 2 | 0.37 | 0.694 | 2.39 | 0.094 | 1.05 | 0.352 | |
Age palm × Orientation | 6 | 0.45 | 0.847 | 1.00 | 0.428 | 0.47 | 0.827 | |
Age palm × Distance | 4 | 0.51 | 0.730 | 2.17 | 0.074 | 1.06 | 0.376 | |
Orientation × Distance | 6 | 0.80 | 0.574 | 1.32 | 0.250 | 0.63 | 0.704 | |
Error | 192 | | | | | | | |
Total | 215 | | | | | | | |
Regarding root biomass, the statistical analysis indicated significant differences between the ages of the palm, both for fresh biomass (Fc = 567.21, df = 2, p < 0.0001) and for dry biomass (Fc = 104.86, df = 2, p < 0.0001), highlighting that the biomass increases with the age of the plantation (Figs. 4 and 5). Concerning the distance from the trunk of the palm at 1, 2, and 3 m to the outside, not significant differences were found in fresh (Fc = 2.39 df = 2, p = 0.094) and dry biomass (Fc = 1.05, df = 2, p = 0.352), respectively. Likewise, not significant difference was found regarding the orientation in fresh (Fc = 0.49, df = 3, p = 0.690) and dry biomass (Fc = 0.35, df = 3, p = 0.790), respectively (Table 2). Therefore, our results suggest that the biomass presents a constant growth from the trunk base of the palm up to 3 m away. The results may have a logical response when we relate them to the reported by Reyes et al. (1997). The authors found a diameter and homogeneous horizontal growth in distant roots from the trunk base of the palm up to 50 cm in the first year, 1.5 m in the second year, and 2.0 m in the third year. In addition, other results show evidence that after a distance of three meters, the roots continue to grow. Intara et al. (2018) revealed that the growth of the roots can grow up to 6 m horizontally, highlighting that the primary roots serve to support the plant, which predominates at a depth of 40 cm. However, the soil conditions and texture are the main factors key to root growth. Interestingly, for our study in the three oil palm plots, the texture of soil was sandy loam. Therefore, we suggest that density, dry biomass, and root dispersal depend on the palm age (Jourdan and Rey, 1997). Despite the various studies that have estimated biomass and root distribution in oil palm plantations, authors suggest that this parameter remains difficult and slow to determine (Jourdan et al., 2015). They demonstrated that the OM and nutrients affect the biomass and the length of roots. They suggest that the reuse of leaves after pruning promotes the formation of root primary up to a depth of 20 cm (Jourdan et al., 2015).
The analysis of the correlation between the fresh biomass of the root, the dry biomass of the root, and the diameter of the root at three ages of the palms, showed that the fresh biomass of the roots and the dry biomass of the oil palms at 3 and 15 years old had a significant correlation (r2 = 0.68, p < 0.0001 and r2 = 0.61, p < 0.0001, respectively). Additionally, to further clarification of the relationship between the parameters studied, it was found that the first principal component explained 95.6% of variation, while the second component explained 4.41% of variation (Fig. 5). Also, on the scatter plot of the first principal component, the 15-year-old plots in the right quadrant had a positive relationship and, at the same time, was related to the fresh biomass and dry biomass of roots, while the rest plots lie in the lower left quadrant (Fig. 5).
Finally, to know the behavior of the root architecture (length, diameter, and biomass) in oil palm plantations, suggest the use of the minirhizotron, 2-D scanning, or tomography, which, compared with collecting soil samples, new technologies save time, cost, and labor.
3.2 Effect of the physicochemical properties of the soil by oil palm cultivated in plots at a different age.
Most of the parameters evaluated were significantly different between treatments (Table 3). Of soil physical parameters, the pH values appeared not to be affected by treatments p ≤ 0.05). The OM was higher in 3-year-old oil palm than in the 5- and 15-year-old and lightly different in grass plot (p ≤ 0.05). Soil Bd was significantly higher in 15- and 5-year-old palm plots than in the 3-year palm plot and the grass plot (p ≤ 0.05).
Table 3
Soil properties at different stages of oil palm (Elaeis guineensis Jacq.) in the municipality of Acapetahua, Chiapas, México.
Variables | Oil palm plantation ages | |
3 years old | 5 years old | 15 years old | Grass plot |
pH | 6.66 ± 0.08 a | 6.43 ± 0.20 a | 6.66 ± 0.13 a | 6.48 ± 0.03 a |
Bd (g/cm3) | 0.95 ± 0.01 c | 1.01 ± 0.01 b | 1.11 ± 0.00 a | 0.96 ± 0.00 c |
OM (%) | 2.96 ± 0.08 a | 1.70 ± 0.02 c | 0.99 ± 0.00 d | 2.73 ± 0.00 b |
CEC (meq/100g) | 98.80 ± 0.37 a | 26.50 ± 0.43 d | 65.76 ± 0.20 c | 75.57 ± 0.11 b |
Ntotal (%) | 0.17 ± 0.00 a | 0.14 ± 0.00 a | 0.40 ± 0.29 a | 0.16 ± 0.00 a |
S (mg/kg) | 21.86 ± 0.23 a | 18.91 ± 0.33 b | 11.25 ± 0.00 c | 18.91 ± 0.33 b |
P (mg/kg) | 24.13 ± 0.00 a | 21.75 ± 0.00 b | 10.36 ± 0.00 d | 16.75 ± 0.37 c |
Ca (meq/100g) | 31.80 ± 13.6 b | 43.20 ± 0.35 b | 98.50 ± 0.21 a | 34.73 ± 0.26 b |
Mg (meq/100g) | 1.85 ± 0.00 a | 1.44 ± 0.05 b | 1.27 ± 0.00 b | 1.87 ± 0.05 a |
K (meq/100g) | 0.26 ± 0.01 a | 0.28 ± 0.00 a | 0.13 ± 0.01 b | 0.27 ± 0.00 a |
Fe (mg/kg) | 68.88 ± 0.49 a | 55.00 ± 0.57ab | 18.06 ± 7.75 c | 37.74 ± 0.38 b |
Cu (mg/kg) | 3.96 ± 0.01 a | 3.22 ± 0.00 c | 3.55 ± 0.05 b | 3.64 ± 0.00 b |
Mn (mg/kg) | 3.56 ± 0.00 a | 2.63 ± 0.07 b | 1.86 ± 0.00 d | 3.13 ± 0.00 b |
Zn (mg/kg) | 3.24 ± 0.00 a | 1.66 ± 0.00 c | 0.87 ± 0.01 d | 2.56 ± 0.00 b |
B (mg/kg) | 3.13 ± 0.00 b | 1.80 ± 0.05 d | 4.35 ± 0.09 a | 2.55 ± 0.00 c |
Different letters on the same line indicate significant difference between plots (Tukey, p ≤ 0.05). Values represent means (± standard error, n = 3).
Regarding the soil's chemical properties, the Ntotal was statistically similar in all the treatments (including control treatment) (Table 3). However, the phosphorus was significantly higher in the 3- and 5-year oil palm than in the palm of 15-year and grass plot (p ≤ 0.05). The K and Mg were significantly higher grass plot, 3- and 5-year oil palm than in the 15-year palm (p ≤ 0.05). The Cu, S, Mn, and Zn were significantly higher in 3-year-old palm than in rest plots (p ≤ 0.05). Finally, only the Ca and B were higher in the 15-year-plot than in the rest of the plots (p ≤ 0.05). We observed that as the palm grows, the root system gains space in the soil and consequently increases the absorption of nutrients. This could be more critical since if the null practice or poor management of soil fertilization, whether chemical or organic, persists, the oil palm will continue to absorb nutrients to such a degree that the soil might become somewhat infertile. In addition to this, it knows as the palm plot becomes adult and influences the soil properties, as was stated by Basuki et al. (2014), found that the older the palm (3, 5, 7, 9, 14, and 16 years old) at four soil depths, observed un decreased in SOC, pH in water, pH in KCl. In our study the values between treatments ranged from pH 6.48 to 6.66, with a mean value of 6.56 (p ≤ 0.05; Table 3). Both palms and grass plots had a typically neutral to slightly acid pH (the soil texture for all the evaluated plots was sandy loam). Okon et al. (2017), compared plots from 1978, 1990, and 2005, and reported significant differences in porosity, pH, OM, SOC, Ntotal, available P and K, and moisture content. For our study, we suggest that the high OM values in a young 3-year-old plot are due to the low intensity of agronomic work compared to the rest of the plots. The contrast of the results of the previous studies made sense since the types of soils, sampling depth, and climatic conditions were not the same. For instance, Gandaseca et al. (2014) shown a contradictory result. The authors found that regardless of the age difference (2–3 compared with 15-years-old), the total carbon, OM, and EC were statistically similar, but the amount of N, P, K, C/N, and C/P ratios were significantly higher between three areas weather (2, 3 years old, and mature oil palm plantation). Also, Nelson et al. (2014) found that after 25 years of conversion from grasslands to oil palm, there was a decrease in the soil pH and exchangeable Mg, but without changes in C content. In addition, the results cause controversy because the sampled sites correspond to the same texture. These results differ from our study since the soil type in the three evaluated localities corresponds to the sandy loam class.
Regarding the N content, we found that the Ntotal was similar in all plots, including control treatment. The above suggests that the variations in indigenous soil N supply, N rates, application methods, organic or inorganic fertilizer, and other biotic and abiotic factors affect yield responses to Ntotal. (López-Valdez et al., 2011). Therefore, sometimes it is not possible to understand the contrasting results when are compare the palm of different ages and agricultural areas with excessive management. For instance, Behera et al. (2020) reported an increase in the parameters such as pH, Ca, exchangeable Mg, and available S down to a depth of 60 cm in oil palm plots compared with intensively managed land. Besides, in 6-, 12-, and 18-years-old palm oil trees, the available P increased with the age of the plantation. However, the concentrations of available N, exchangeable K, Ca, Mg, and available S and B did not change with the plantation age (Behera et al., 2020). Another study revealed that when was evaluated the N application and N uptake efficiency in oil palm crop versus a tropical forest, the root N content was one-third higher in the tropical forest compared with oil palm plantations. Nevertheless, the absorption efficiency was similar in both systems (Edy et al., 2020). Both in our results and in those discussed previously suggest that the decrease in the content of the elements in the soil is due to the need for absorption or use required by the oil palm for the formation of clusters of fresh fruits (Edy et al., 2020; Manorama et al., 2021). In this line, the present experiment showed that higher values of the elements K, Mg, Cu, S, Mn, and Zn are present in 3-year-old plots and the pastures compared to 15-year-old adult plants. It is logical to think that the null or poor management of monoculture systems results in soil degradation. In previous paragraphs, we mentioned that the plots evaluated in this work received little management (adult plantations), causing a decreased soil fertility. In fact, researchers argue that excess fertilization can increase leaching processes and modify soil reserves in oil palm (Dubos et al., 2021). In this sense, Salamat et al. (2021) highlight that excessive fertilizer applications result in high production costs and soil contamination. Likewise, the authors showed that when comparing the soils of adult palm plots with secondary forests, after 25 years of supplying chemical fertilizers in oil palm, reduced the content of N, organic carbon, and the healthy microbial community was altered, not well in the forest soil. The above explains the concern among researchers and environmentalists when they argue that the change of land use from forests and jungles to palm plots changes the soil quality and degrades even more due to oil palm poor management. It has been demonstrated in several investigations around the world that the oil palm causes compaction soil is practically in the absence of appropriate conservation practice (Ufot et al., 2016; Chikere-Njoku, 2019). Our results suggest that soil compaction increases as the palm plot reaches adulthood. These results can see in Table 3, with significant differences between crop systems (p ≤ 0.05). Bd was lower in the 3-year-old and the pasture plot. However, higher in 5 and 15 years. Besides, previously these results were reported, which indicate that root density and poor crop management affect compaction and, consequently, an increase in Bd (Bessou et al., 2017). In timber plantations such as Brazilian pine (Araucaria angustifolia [Bertol.] Kuntze, 1898) shown that soil Bd was related to thick and short roots (Mósena and Dillenburg, 2004). Furthermore, it makes sense with the similar results found by Enaruvbe et al. (2021) where revealed that the conversion of land use from rainforest to oil palm and rubber plantations showed that at a depth of 15 to 30 cm, reductions in SOC, Ntotal, and phosphorus compared with soil samples from tropical forests were observed. And also suggest that soil degradation is more severe in oil palm plantations than in rubber plantations (Enaruvbe et al., 2021). Recently, Prawito et al (2022) suggest that proper management and improving soil SOC with the maintenance of undergrowth vegetation can achieve soil sustainability and shown the understory vegetation biomass, weight, and density decreased with the increasing age of the plantations compared with young plants of 4 years (Prawito et al., 2022).
Palm plantations with organic management practices result in changes in soil properties. Indeed, investigations have shown positive effects (Rahman et al., 2021; Formaglio et al., 2020). For instance, Yeo et al. (2020) reported that in plantations older than 20 years, the amounts of C, N, and OM were higher compared with plantations of 13 years old, but similar to that of secondary forests. Therefore, it is necessary to understand that natural systems such as forests, jungles, and integrated systems increase ecosystem services, unlike poorly managed monocultures. Rahman et al. (2021) revealed the incorporation of cover crops after 15 years of palm establishment. Besides, the addition of EFB (26 t ha− 1) increased SOC and a higher yield per ha compared with unmanaged plots. Similarly, the reduced application of fertilization, mechanical weeding, and incorporation of OM caused an increase in the extractable organic carbon and higher the microbial activity in the soil (Formaglio et al., 2020). The results presented in Table 3, clearly show that the age of the crop causes changes in the properties of the soil to such an extent that fertility could decline, thus affecting the production of fresh fruit bunches (in our study, this parameter was not evaluated, but has been reported in other research) (Rahman et al., 2021).
The poor agronomic management of oil palm causes changes in the undergrowth, nutrient dynamics, and changes in biodiversity, among others (Matysek et al., 2018). In this sense, for the first time in the study region exist clear evidence of the effects that oil palm causes on the soil properties. Therefore, this study contributes to this field of knowledge that oil palm requires sustainable management, which implies the diversification of management practices that help increase soil quality, improve interactions between plant roots and microorganisms, as well as how to promote an increase in biodiversity of mesofauna and macrofauna of the soil (Ashston-Butt et al., 2018). In addition, the management of palm oil must be carried out holistically since only one type of management is not considered comprehensive (Jaroenkietkajorn and Gheewala, 2021b). These aspects are not considered by the governments that promote the increase of the area planted with oil palm, as in the case of Mexico (Cámara de Diputados, 2011). Studies have shown that the damage caused to the soil, the scarce availability of water, and the degradation of biodiversity, among others, it is due to inadequate management of palm plantations (Jaroenkietkajorn and Gheewala, 2021a; Jaroenkietkajorn and Gheewala, 2021b). Therefore, impacts can be reduced with best management practices and possibly maintain sustainable oil palm plantations.