4.2.2. Macronutrients
A significant increase in the content of N (43 g kg−1DW), P (9 g kg−1DW) and K (48 g kg−1DW) in the tubers with respect to the controls is inferred. The potato crop is particularly demanding in N, P and K, its quality will depend largely on the variety and the availability of nutrients.
Between 1998 and 2007, the MSWC has provided the highest N content in the tubers of the varieties Agria, Monalisa and Jaerla, only exceeded by clone A7677 (43 g kg
−1DW). In the 1998 to 2016 period, the ChMC is the second treatment to provide a high concentration of N, remaining constant along with the experiments (
Figure 4). The highest concentration of N obtained in clone A7677 by the application of MSWC from 2004 to 2007 (43 g kg
−1DW) largely exceeds those reported by Cabalceta et al., (2005) [
28], Alvarado et al., (2009) [
29], Correndo and Garcia (2012) [
30], Mahamudet al (2016) [
31] and Fernandes et al., 2017 (16.4–22.7 g kg
−1DW) [
32]. The yield obtained in all the varieties under this treatment (MSWC) may be due to the improvement in the availability of this element in the soil, as a consequence of the amount of nitrogen compounds added to it and the continued mineralization of N, favoring its absorption, higher plant biomass and as a consequence higher storage of this nutrient in its tubers [
33].
- 2.
Phosphorus.
The MSWC provides the highest P content in the tubers for the 1998 to 2004 period in the varieties Agria and Monalisa and for 1998 to 2007 in the variety Jaerla and in clone A7677. For 2007 to 2016, MF and ChMC lead to the highest P contents in the tubers (
Figure 5). These P contents exceed those reported in the literature by Cabalceta et al., 2005 [
28] and Alvarado et al., 2009 [
29], and Mahamud et al., 2016 (1.2 to 4.7 g kg
−1DW) [
31]. The addition of the considered amendments causes decomposition processes carried out by microorganisms, which would produce certain quantities and types of organic acids, siderophores, hydroxyl ions and other compounds [
34], which would facilitate the gradual conversion of phosphates and weak retention of these in the solid phase of the soil. Therefore, the increase in the P content due to the compost favored an adequate development of roots, increase in the aerial biomass, improvement in the quantity and quality of the tubers in all the varieties, which resulted in better use of this nutrient by crops, especially under the treatment with the MSWC and ChMC (2004 and 2007).
- 3.
Potassium.
For the 1998 to 2007 period, there was an increase in the K content with MSWC for the Agria, Monalisa and Jaerla varieties, only surpassed by the clone A7677 (48 g kg
−1DW) (
Figure 6). This K concentration exceeds those reported by Alvarado et al., 2009, Mahamud et al., 2016 [
31], Jahanzad et al., 2017 [
1] and Fernandes et al., 2017 (25.2–29.8 g kg
−1DW) [
32]. Potassium is essential for the synthesis of starch and simple sugars and for the translocation of carbohydrates, thus playing a pivotal role in maintaining the vigor and efficiency of the potato plant. Along with N, P is the most necessary mineral for the growth of plants [
35]. Furthermore, low K intake is associated with various non-communicable diseases, such as hypertension, cardiovascular diseases, chronic nephrolithiasis, osteopenia, etc. [
36]. With the rates observed in this study, the intake of K could be increased, which would help lower blood pressure and the risk of cardiovascular disease, improve bone mineral density and mitigate the negative consequences of consuming large amounts of sodium.
4.2.3. Micronutrients and Heavy Metals
The studied long-term treatments (1998 to 2016) show highly significant increases in micronutrient content with respect to the controls by consecutive applications of the studied composts (MSWC, ChMC, SMC, CMC and MF) when considering Fe, Mn, Zn, Cu. Additionally, relevant variations on heavy metal content (Pb, Cr and Ni) were inferred.
Iron deficiency represents one of the most serious problems in micronutrient nutrition in humans worldwide [
37] and there is a need to increase the amount of bioavailable iron in crops such as potatoes. Values of 15 to 20 mg kg
−1DW are considered as a baseline for Fe in potatoes [
12]. In this long-term study, for the 1998 to 2007 period, the MSWC provides the highest Fe content in the tubers of the varieties Jaerla, Monalisa and Agria, only exceeded by the clone A7677 (118 mg kg
−1DW) (
Figure 7). The normal potato content of Fe is in the 50–250 mg kg
−1 DW range [
38,
39], being largely dependent on the considered variety. The highest concentrations of Fe reported in this work (2007) with the MSWC in the Agria variety and the clone A7677 are higher than those reported by The Food Composition Table of The US Department of Agriculture (37.754 mg kg
−1DW) [
12]. We must bear in mind that the prevalence of anemia is higher in developing countries than in developed countries. Estimates have indicated that approximately half of this is attributed to Fe deficiency [
40]. Given that Fe availability in potatoes is high (10%) [
41], increasing the concentration of this micronutrient in our potato varieties (biofortification) could favor their intake in populations at risk of Fe deficiency anemia worldwide. In addition, our findings also exceed those reported by Burgos et al. [
42] and Brown et al. [
38] and are in the range of those by Khan et al. (2017a, 108.1 mg kgDW) [
39].
- 2.
Manganese
This element was the only one that shows an increasing trend during all the experiments, for all the treatments and varieties Agria, Jaerla and clone A7677. In the 1998 to 2016 period, MSWC provided the highest Mn content for the varieties Jaerla, Monalisa, Agria and clone A7677 (27 mg kg
−1DW). ChMC was the second treatment to achieve a higher Mn content in Jaerla, Agria, Monalisa and clone A7677 (19 mg kg
−1DW),
Figure 8. These values are within the normal range (20–300 mg kg
−1DW) and well below the phytotoxicity limits of 500 mg kg
−1DW [
43]. Likewise, Mn contents reported in this work exceed those by Baranowska [
44], Ali and Al-Qahtani [
45] and Fernandes et al. (2017, 6.7–11.5 mg kg
−1DW) [
32]. Mn is another essential nutrient required in very small amounts in the human body [
46]. However, the diet of the population based on cereals such as rice, wheat, cassava and corn contain insufficient amounts of this micronutrient [
47]. Increasing the Mn content in tubers (biofortification) can help improve this insufficient intake.
- 3.
Zinc
Another important micronutrient is this essential element because Zn deficit affects more than 30% of the world population [
10]. Hence the relevance of crops that can supply Zn in greater quantities [
48] such as potatoes, which contributes 2.6% and 3.2% of the daily human dietary requirements of Fe and Zn, respectively [
49]. Likewise, Zn regulates the formation of ribosomes, auxins, cellular components and increases the resistance of the plant against drought and diseases [
44], thus even mild deficiencies can have serious effects on the health and growth of plants. Even though the concentration of Fe and Zn in the potato is low compared to cereals and legumes, their bioavailability in potatoes may be higher due to the presence of high levels of ascorbic acid, which is a promoter of Fe absorption, and low levels of phytic acid (inhibitor) of the absorption of Fe [
41,
42]. In this study, for the 1998 to 2007 period there was a sustained increase in the Zn concentration in clone A7677 (34 mg kg
−1DW) due to MSWC treatment, leading to a high Zn concentration peak for Jaerla, Monalisa and Agria,
Figure 9. These data agree with those found in 2012 by Haynes et al., (13 to 35 mg kg
−1DW) [
50] and André et al. [
51] and are larger than those by Burgos et al. [
41,
42], Alvarado et al. [
29], Ali and Al-Qahtani [
45], and Baranowska et al. (2017, 19.79–21.34 mg kg
−1DW) [
44]. The increase in the concentration of Zn in the tubers obtained in our study (biofortification) would favor its intake in the population at risk of deficiency of this micronutrient [
52], since according to Burgos, the concentration of Zn in raw and cooked potatoes does not show significant differences due to their cooking [
41].
- 4.
Copper
A sustained increase in Cu content was observed in the 1998 to 2016 period for clone A7677 with MSWC (from 12 to 17 mg kg
−1DW), followed by CMC. For 1998 to 2007, MSWC treatment led to larger Cu contents for the varieties Agria, Monalisa and Jaerla. For the 2007 to 2016 period, the largest Cu content was inferred for CMC treatment with the varieties Agria, Monalisa and Jaerla,
Figure 10. The available literature shows very different Cu content in crops depending on the type of soil and crop, although the normal range is 2–7 mg kg
−1DW for the minimum value and 20–30 mg kg
−1DW for the maximum value [
43]. The highest concentration of Cu registered in our studies was in 2007 with MSWC for variety Agria and clone A7677. These values are higher than those reported by Correndo and Garcia in 2012 [
30], Brown et al., (2014) [
53], Pozzatti et al., (2017, 5.4 mg kg
−1DW) [
54], Alvarado et al. [
29], Ali and Al-Qahtani [
45], and Baranowska et al. (2017, 6.23–6.58 mg kg
−1DW) [
44]. However, our results are in the range of those obtained by Khan et al. (2017a, 11.83 to 18.03 mg kg
−1DW) [
27]. Cu is essential for the proper function of human organs and metabolic processes [
55], and Cu deficiency in humans can cause anemia that does not improve with daily intake of Fe from the diet [
56]. Increasing the Cu content (biofortification) in tubers can help reduce the high level of anemia worldwide.
- 5.
Lead
In the 1998 to 2016 period, MSWC provides the highest Pb content in the tubers of clone A7677 (6 mg kg
−1DW) and in the varieties Agria, Monalisa and Jaerla, observing a significant and sustained increase. However, in 2010 there was a reduction in the Pb content of 20% in the Agria variety and of 42% in the varieties Monalisa, Jaerla and clone A7677 with respect to 2007. This could be due to several factors, among which we have to take into account the absorption of this element in previous crops (legumes and corn); the organic matter that can increase or decrease the solubility of Pb, depending on its degree of polymerization and soil conditions, cation exchange capacity, pH, interaction with other bioavailable micronutrients and lead content in MSWCs. Pb concentrations compared to our controls increase with all treatments, leading to slightly larger values for SMC and ChMC than for CMC,
Figure 11. These values were maintained until the end of our study, being within the range considered normal (5–10 mg kg
−1DW) and well below phytotoxicity levels (30–300 mg kg
−1DW) [
43]. The reported results are comparable to those obtained by Ali and Al-Qahtani (2012, 1.51 to 6.19 mg kg
−1DW) [
45] in a heavy metal biomonitoring study and with those found in crops of potatoes exposed to irrigation by wastewater contaminated with Pb [
39]. On the contrary, they were below those obtained by Tadesse et al., in 2015 [
57], Jalali and Meyari in 2016 (19.5 mg kg
−1DW) [
58] and Angelova et al. (50 to 54 mg kg
−1DW [
59]. It should be remarked that Pb is not necessary for plants and can accumulate affecting different physiological and biochemical functions [
60]. Even though the Pb content in our study was not in phytotoxicity ranges, we must consider that the increase in the bioavailable content of Pb could be due to the continuous applications of the different amendments used throughout the study, which was giving us a product with a high content of lead, not suitable for human consumption (0.1 mg kg
−1) [
61].
- 6.
Chromium
There was a sustained increase in the Cr content for the 1998 to 2007 period for all the varieties, leading to the highest concentration with MSWC treatment for clone A7677 (20 mg kg
−1DW) and in the varieties Agria, Monalisa and Jaerla. For 2007 to 2016, CMC treatment led to the highest Cr contents in clone A7677 (16 mg kg
−1DW) and the varieties Monalisa, Agria and Jaerla (
Figure 12). These values exceed the level considered as normal (0.03–14 mg kg
−1DW), going into the phytotoxicity range (15 to 30 mg kg
−1DW) [
43]. Cr is an essential element for the normal metabolism of carbohydrates in animal and human nutrition [
62] but for plants, there is no conclusive evidence of their essentiality in the metabolism [
62,
63]. Additionally, Stasinos et al., (2014) [
64] point out that Cr falls into the category of heavy metal, which can be easily taken and bioaccumulated by tubers and food roots. However, it should be remarked that in 2004, the maximum content of Cr affect the yields obtained neither for clone A7677 (71 t ha
−1) nor for the varieties Agria, Monalisa and Jaerla. In 2007, there was only a decrease in the yields of the varieties Agria and Monalisa (40 t ha
−1) compared to the varieties Jaerla (54 t ha
−1) and the clone A7677 (76 t ha
−1), which showed a yield increase. These observations could make us suppose that the highest concentration of Cr in the MSWC in this year could have stimulated the growth and productivity of clone A7677 and the variety Jaerla, not showing any change in its morphological structure or in the development of the foliage, which was observed in the Agria and Monalisa varieties, such as the reduction in foliage growth, discoloration of leaves, decrease in plant height, which resulted in a decrease in yield. These results agree with Paiva et al. [
65], who proposed that Cr exposure in plants led to healthier conditions compared to control plants. Likewise, Guevara and Montes showed that increased exposure to Cr led to Cr concentration in the potato tubers [
66]. Several studies show that there is a clear correlation between the content of this metal in soils and the stimulation of growth and the absorption of underground organs [
64].
- 7.
Nickel
MSWC provides the highest Ni content for the 1998 to 2007 period for clone A7677 (20 mg kg
−1DW) and in the varieties, Agria and Monalisa and Jaerla, decreasing and then remaining constant for the 2007 to 2016 period (4–9 mg kg
−1DW). In contrast, for 2010 to 2016, SMC, ChMC and CMC provide similar Ni contents in the varieties Agria, Monalisa and Jaerla and with the clone A7677,
Figure 13. The highest Ni concentration exceeds normal values (0.02 to 5 mg kg
−1DW), going into phytotoxic levels (10–100 mg kg
−1DW) [
43]. The reported results agree with those by Mahmood and Malik [
67] but are larger than those by Khan et al., (6.84, 7.93 and 8.71mg kg
−1DW) [
39] in potato crops exposed to irrigation by Ni-contaminated wastewater. Ni is considered an essential micronutrient for the growth and development of plants with several metabolic roles [
68]. The maximum Ni contents (2007, into the phytotoxic range) were obtained for MSWC treatment in the clone A7677 and in the varieties Agria, Monalisa and Jaerla but they did not affect yields. However, in 2007, it is noted that only yields of the varieties Agria and Monalisa (40 t ha
−1) with contents of Ni in tubers of 13 mg kg
−1DW declined, compared with the variety Jaerla (54 t ha
−1) and the clone A7677 (76 t ha
−1) whose yields, on the contrary, increased, despite Ni content of 20 and 8 mg kg
−1DW, respectively. These results indicate that Ni has stimulated the growth and development of foliage, which agrees with the positive responses of plant growth in the presence of Ni [
48,
69]. Nevertheless, the possible Ni toxicity should be considered, with a negative impact on photosynthesis, on membranes permeability, and on the decrease in the micronutrient’s absorption [
70].