4.1. Inorganic P Available (iP-ava) and Remaining (iP-rem) Obtained in Routine Soil Fertility Analysis
This higher concentration of iP-avail can be attributed to the high phosphate fertilization used over 20 years in the CTS in the area, where cumulative amounts of P were applied during planting throughout the period. Additionally, the soil in the area has a low clay content (220 g kg
−1) as it is an oxisol with high levels of iron (Fe) and aluminum (Al), giving the soil a high P adsorption capacity (P), and consequently, lessening its available to the plants, as was also observed by Loss et al. [
7]. Conte et al. [
23] and Casali et al. [
28] have highlighted that the soil additions of P above the quantities required for plant development increase the fractions of inorganic P (iP), a process that may cause saturation of the adsorptive sites.
With the soil turning that occurs annually in the CTS area, the P comes into contact with the colloids, increasing their adsorption and resulting in weak binding energy, so it can be easily released into the soil solution and increase the P-avail. However, Guppy et al. [
20] and Souza [
21] have highlighted that, over time, this bond tends to become more stable, and the P is adsorbed with greater energy, making it less available to vegetables.
According to Santos et al. [
14], in cultivated soils where there are regular additions of phosphate fertilizers, management provides changes in the fractions and concentrations of P in the soil profile. According to Gatiboni et al. [
40], when P is applied in greater amounts compared to what is exported by the crops, this P accumulates in a moderately labile form and acts as a collector. However, when P is applied in small amounts, the P that is accumulated in a moderately labile form can act as a source, supplying the requirements of the culture.
Rheinheimer et al. [
41] stated that in areas under an NTS, the highest concentrations of P were observed in the surface layers of the soil, between 5 and 10 cm deep, which is explained by the location of fertilization (no tillage) and nutrient cycling, while in a CTS, the distribution of nutrients followed the depth of the soil plowing—behavior that was also observed in this study, however, the highest accumulation occurred in the CTS areas (
Table 3).
In their study, Maia et al. [
42] observed that the iP-avail was sensitive to the variation in soil moisture, which was lower in the CTS. This was associated with the lack of vegetation cover or its residues on the soil surface. The cycle of nutrients was the minimum, and thus, the availability of P in the soil decreased when the soil moisture was lower, since the diffusion process depends on water [
42]. However, this behavior was not observed in this study, since the CTS presented higher iP-avail and iP-rem values of 0–5 and 5–10 cm compared to the other management systems (
Table 3).
This pattern of P-rem at the most superficial depth (0–5 cm), equaling the levels in the NTS5 and NTS17 to the CTS, may have been due to the decomposition of the residues of Poaceae and Fabaceae used in the rotation of cultures in these areas, and the low molecular weight organic acids, which can block P adsorption sites, as highlighted by Bezerra et al. [
19].
In studying the maximum P adsorption capacity in an oxisol, Moura et al. [
16] observed that the great content of SOM in the topsoil contributed to reducing P retention. However, in the native area, a constant inflow of more lignified plant residues—slow decomposition materials—released small quantities of organic acids of low molecular weight. Based on the results obtained in this study, in analyzing the iP-rem, it is possible to affirm that the NC soil was responsible for the greater fixation action of the nutrient (8.45 mg kg
−1).
According to Conte et al. [
23] and Rheinheimer et al. [
41], the redistribution of P in various forms also occurs in NTS-cultivated soils since the adsorption of P occurs primarily in sites of low lability, and subsequently, the remnant P is redistributed in the forms retained with lower power and higher availability to the plants.
The cover crops used in the crop rotation system for the production of NTS straw effectively contributed to increasing the P-rem content at a depth of 0–5 cm in the soil, which corroborates the statement made by Fernandes et al. [
8] that the continuous supply of organic matter increases the P adsorption sites in the soil, which can reduce the adsorption and contributes to the increase in the P content in the soil.
4.2. Labile P
In analyzing the labile portion of P in the soil in this study, it was observed that the P contents (inorganic, organic, and total) were similar in the CTS, NTS5, and NTS17 at the evaluated depths (0–5 and 5–10 cm) and higher when compared to the NC (
Table 4), which is in line with the statement made by Santos et al. [
14] and Bravo et al. [
43] that successive phosphate fertilization promotes increased P lability because the adsorption sites are more avid for this element to be gradually filled, and new fertilizations increase the most labile fractions of P.
Results similar to those obtained in this study for the NTS5 and NTS17 areas were observed by Bezerra et al. [
19] and Rodrigues et al. [
2], who have also shown increases in the availability of P capable of being absorbed by plants in areas under an NTS when compared to a CTS in an oxisol in the Native Cerrado area.
Considering the initial concentration of P-avail in the areas under study (
Table 3), the increase in the levels of tP corresponded to the process of P cycling. This rise in the P content was more intense in areas associated with the use of P fertilizer since the fraction extracted with NaHCO
3 demonstrated that the increments in labile P were similar among the soil management systems, as was also observed by Beutler et al. [
25].
In the NTS areas, the P tended to accumulate on the soil surface due to fertilizer applications that were carried out at a depth of up to 10 cm, and with this, sorption, cycling, and nutrient recycling reactions occurred at these locations, as well as the mineralization of residues containing P, affecting the distribution of the P in the soil in different ways. Similar results were also observed in the study conducted by Bravo et al. [
43].
The labile P fraction shows the contribution of each management system to the P content in the soil, as the Pi-Bic contents in the 0–5 cm layer were similar in the CTS and NTS5, which were higher than in the NTS17 and NC. Meanwhile, from 5 to 10 cm, the CTS, NTS5, and NTS17 showed equal and higher values than the NC. This behavior at both depths allows us to assume that there was a relevant contribution of the phosphate fertilization carried out in areas with Pi-Bic accumulation in the management systems evaluated when compared to the NC.
Generally, only in the CTS, the inorganic forms of labile P were superior to the organic forms by more than 10%. In analyzing the discrepancy between the iP and the oP, the NTS5 presented the greatest similarity of values, having small iP predominance (51%) compared to oP (49%), which are fractions close to those found in the Cerrado biome.
The NTS17 presented 65% of the P in its organic form (0–5 cm), a proportion that was similar to the values found in the native area at the same soil layer (67% iP and 33% oP), which can be attributed to the input of the SOM in the system in the past 17 years, which presented values in the NC > NTS17 > NTS5 > CTS sequence at this same depth. However, these proportions were not maintained at the 5–10 cm soil depth since the levels of iP were superior to the levels of oP under the same conditions. In the native area, the contents of oP were greater than those found for iP, at an approximate ratio of 2:1.
This result reinforces the findings by Gatiboni et al. [
40] who, in analyzing the availability of P and its forms accumulated in an NTS, generalized that in the long term, the addition of fertilizers in sufficient quantities to supply plant development equilibrates the capacity of organic and inorganic forms of P to provide this nutrient to crops. Souza et al. [
1] and Rodrigues et al. [
2] have observed increments of organic and inorganic P in the soil’s superficial layer in an NTS when compared to a CTS, except in treatments where the crop residues were from maize, in which the labile inorganic P was superior in the NTS.
In 5-year NTS areas, Olibone and Rosolem [
44] evaluated the organic fractions of P after the application of phosphate fertilizers in the cultivation of soybeans. The authors found that there was an increase in oP after harvest, correlating this increase to the decomposition of the root systems of the cultivated plants.
4.3. Moderately Labile P
The moderately labile forms of P in soil (iP-H, oP-H, and tP-H) were extremely low in the NTS5 and NTS17 and abundant in the CTS and NC. Among the areas cultivated with annual crops, in the CTS, an expressive superiority of tP-H was observed, which may be explained by the heavy P fertilization used in the area at each cultivation cycle, because this area is cultivated only once a year and after is left fallow (native vegetation) until the next cycle.
According to Souza [
21] and Beutler et al. [
25], the application of annual heavy P fertilization increases the concentration of labile inorganic P fraction (iP). This iP phase is more strongly adsorbed with time with complexes of iron (Fe) and aluminum (Al) oxides, causing an increase in the moderately labile P fraction (iP-H).
In the NC area, where there is continuous deposition of organic material on the soil surface and annual crops are not cultivated, the maintenance of the soil moisture is greater, which favors P cycling, increasing its availability in the soil. According to Costa et al. [
45], when there is an increase in humidity in the area, the water film close to the solid soil particles becomes thicker, reducing the ion–colloid interaction.
The differences found in the Pi-H and Po-H values between the systems used are due to the accumulation of the inorganic form of P in the CTS, which reinforces the idea of ion–colloid contact.
Using the same soil chemical extractor, Beutler et al. [
25] observed great concentrations of tP-H (moderately labile) when compared to tP extracted with sodium bicarbonate (NaHCO
3) (labile fraction), in areas of pasture, integrated crop and livestock, and Cerrado biome (native area). In the present study, this pattern was restricted to the CTS and NC areas, and not observed in the NTS areas.
4.4. Moderately Resistant P
The contents of moderately resistant P (iP-OH, oP-OH, and tP-OH) extracted in alkaline solution (NaOH) were significantly higher in the NTS5, NTS17, and NC compared to the CTS, showing that among the cultivation systems, the forms that tended to have greater P lability were more present in the CTS, which can be explained by the high amounts of P applied to the soil after each cultivation cycle, which were adsorbed on the soil particles.
In this Cerrado region, where the study was conducted, the soils are highly weathered, and the highest proportions of P were found in the extractions with NaOH, probably due to the strong relationship of this fraction with the presence of Fe and Al oxides, kaolinite, and low organic matter content, which favor the adsorption of P in the soil [
19,
43].
Among the factors that potentiate the difficulty of P extraction, there is reduced P availability due to its fixation with Fe, Al, and Ca, which immobilize P [
24]. Corroborating this statement, Souza Júnior et al. [
46] observed that in most weathered soils with low pH and low contents of Ca, the majority of inorganic P forms occurred precisely in P-Fe and P-Al, while in alkaline soils, the predominating connection is P-Ca.
In natural conditions of strong P deficiency, Neufeldt et al. [
47] reported that more than 60% of the labile portion was derived from organic P, indicating that the primary contribution of organic P concentrates on the more labile fractions, which are also susceptible to rapid mineralization. In this study, even with the highest proportion of organic P in the soil, there was no greater availability for plants.
The NTS5 and CTS differed significantly for iP-OH at the 0–5 cm soil depth, with the area under the no-tillage system presenting 67.25 mg kg−1 of P and the CTS presenting 36.34 mg kg−1 of P. Meanwhile, the accumulation of oP-OH at the 5–10 cm soil depth was superior in both areas under the no-tillage system, even compared to the native area.
The greater accumulation of P in the superficial soil layers of the NTS5 can be explained by the application of phosphate fertilizers in the sowing line or application of spread. According to Redel et al. [
48], this accumulation is due to the limited mobility of this element in the soil profile. These results are in line with those presented in the studies by Rheinheimer et al. [
41,
49], where they highlighted that of the buffer fraction increases when P additions are greater than its production, which decreases in soils cultivated with a low P, and therefore, the accumulation depends entirely on what is returned and removed from the system.
The intense accumulation of P in the superficial soil layers of an NTS from the application of P fertilizers in the sowing line or spread application was reported by Redel et al. [
48]. This accumulation was due to the limited mobility of this element in the soil profile. Rheinheimer et al. [
41] showed that the magnitude of the buffering fraction increases when the addition of P is superior to its output, which decreases in soils cultivated with a low P reposition of the P exported, and thus, the accumulation depends entirely on what is returned and removed from the system.
According to Gatiboni et al. [
40], when the source of fertilizer is of organic origin and easily decomposed, the accumulation of P in the soil may initially be in the organic form, which is subsequently converted to the inorganic form due to microbial mineralization.
Considering the positive correlation between toP and organic C [
20], it is possible to observe the contribution generated by organic matter in the long-term cultivation area (NTS17). This proves the hypothesis tested in this study that there is an increase in organic and inorganic P levels, providing greater availability of the nutrient in the soil for crops. The same result was repeated at a 5–10 cm soil depth only in the NC.
In this way, by observing the conditions of the NTS17, it is worth noting that the majority of crops can only use P in its inorganic form, which makes the enzymatic activity of phosphatase crucial in the conversion of organic forms to its inorganic state, capable of absorption [
50,
51].