TOC, TN, Physical Fractionation and Carbon Management Index of Soil
The MP treatment provided higher levels of TN and TOC in the 0.10–0.20 m layer, similar to those obtained in NS (
Table 3). This system is implemented annually in the summer, and after the maize harvest, the forage remains in the area; thus, the soil remains covered throughout the year, favoring the development of forage roots and organic material accumulation in subsurface, evidenced by the high POC values (
Table 3), which are highly related to recent plant materials deposition [
20]. In systems with one or two years of
U. brizantha cv. Piatã, the forage remains in the area without receiving maintenance fertilization; thus, the nutrients supply via replacement is lower, compromising dry matter productivity. In the MP system, forage is desiccated annually for maize sowing, and these residues are important for increasing the N and C levels in the soil [
25,
26].
Considering the superficial layers average (0.0–0.20 m), where differences for TOC were significant, it is observed that in area with the P treatment there was a reduction of 22% in the TOC content in relation to the NS. However, in the MP, MP/S and MP/P systems, this average proportion is around 15%, suggesting that these treatments were more efficient in the maintenance and/or accumulation of TOC in the soil. This result may be due to the plant residues’ input and management with annual fertilization (MP and MP/S), or every two years in the case of MP/P. For the other treatments, the mean TOC was between 24 and 26%. The CPI establishes a relationship between the TOC of the cultivated areas and the TOC of the reference area (NS). The treatments MP/P, MP/S and S/MP/P/P in the 0–0.10 m layer and MP and MP/S in the 0.10 to 0.20 m layer showed CPIs closer to 1. In the 0.20–0.40 m layer, all treatments showed a CPI equal to or greater than 1. It can be inferred in these systems with higher CPI that there was an increase in, or at least maintenance of, C in the soil over the years with no-tillage [
27]. Thus, the contribution of residues from crop and forage straw over the years provided increasing increases in TOC, mainly in depth.
In treatments referring to monocultures (M and S), there is a lower proportion of labile C being added compared to other systems with the presence of forage, in which the highest POC levels were observed in the 0.20–0.40 m layer. This result may be related to the greater presence of residues from roots that are still in the initial decomposition stage [
28]. Forage roots have a well-developed root system and are able to reach greater depths in the soil, increasing POC levels [
29,
30]. Systems with the use of forages for straw production, compared to those exclusively with crops, showed higher C levels, which must be associated with the high plant material input commonly provided by pastures [
14].
For MOC contents, there was no significant difference (
p > 0.05) between treatments in the surface layer and at 0.20–0.40 m (
Table 3). Changes in this carbon fraction occur more slowly and gradually, and it represents the stable carbon in the soil [
31,
32,
33]. However, in the 0.10–0.20 m layer, high MOC levels in the MP and MP/S systems were similar to those observed in the NS, which can be explained by the constant C supply by cultivated species and also because they were treatments that received fertilization annually, or with N input via soybean in the case of the MP/S system. Increases in C levels in more stable SOM fractions depend on the balanced relationship between C and N inputs, in addition to the availability of other nutrients [
16]. Biomass-associated N availability has also been reported as another strategy to increase stabilized SOM content, as it increases microbial activity [
34]. It is understood that most stable SOM compounds have been transformed through N-accelerated biological activities [
35].
The lower lability (L) in the S system, in the 0.0–0.10 m layer, occurs due to the lower labile C (POC) content obtained in this treatment. Soybean crop provides less plant residues, and these residues have a low C/N ratio; therefore, it decomposes faster than Piatã residues. As the soil collection took place in March, that is, approximately one year after the previous year’s soybean harvest, it is likely that a large part of labile C was lost. Salton et al. [
36] studying SOM and soil aggregation in crop-pasture rotation, observed L lower than the values found in the present research, both for the area with native vegetation, ranging from 0.08 to 0.28, as in the area with NT farming, from about 0.08 to 0.26, in the 0.0–0.20 m layer. The divergence in the results is related to the lower TOC levels verified by Salton et al. [
36] and shorter conduct time in the experimental area.
However, as observed by Salton et al. [
36], there was greater L in the soil surface layer, which can be attributed to the greater amount of plant residues and POC concentration. The LI increased in depth, due to the reduction in L values in the reference area and, in general, in the treatments. This observation is also verified in studies by [
36,
37]. Although there is a difference in C levels between the treatments, through the LI, it is possible to observe whether the L of these is close to or below the natural condition without anthropic intervention, so that LI values close to or above 1.00 indicate that the production system employee is providing labile C addition similar to or greater than the original condition, respectively.
Therefore, the greatest contributions to the addition of labile C by production systems occurred at the deepest layer (0.20 to 0.40 m), where all cultivation systems that use Piatã grass presented LI > 1, with emphasis on the P, MP/P/S, MP/S and MP/P/P systems with ILs of 2.06, 2.01, 1.94 and 1.94, respectively, most likely due to the addition of labile C by grass roots. These values were lower than those obtained by [
37] with rotation systems involving
Urochloa ruziziensis, soybean and sorghum, in which the authors obtained LI between 1.1 and 4.9 in the 0.0–0.10 m layer and above 5.0 in the 0.10–0.20 m layer.
In general, the low CMI values in the 0.0 to 0.10 m layer indicate the need to improve cropping systems in order to match the original condition of the soil in a native forest condition. This is a difficult condition to be obtained in the region where the research was conducted due to the rapid degradation of labile organic matter in a tropical climate because of the higher decomposition rates favored by high temperatures characteristic of these regions [
38]. However, at greater depths in the profile, it is already possible to obtain CMI > 100 due to the constant contribution of root biomass, lower micro-organisms activity in deeper layers and the greater aggregates preservation due to lower anthropic action. Production systems with higher CMI values show the ability to promote the sustainability in tropical regions by maintaining C in the agricultural system [
39,
40].
Thus, higher CMI values mean a positive effect of soil use and management practices on organic matter content [
41,
42], while the lowest values indicate that the C compounds are being degraded [
21], reflecting the lower quantity and quality of residues added by the production systems, that is, lower labile C content [
43]. The CMI represents a measure of sustainability of different production systems or land uses and can be used to compare the changes that occur in labile C and TOC contents as a result of agricultural practices [
44]. Thus, low CMI values with monocultures S and M obtained in layers 0 to 0.10 and 0.10 to 0.20 m, respectively, indicate the lower capacity of these crops to promote sustainability to production systems.
Stable C presence in the soil is also an indication of the quality and sustainability of production systems, which generally suffer the greatest interference in the more superficial layers. In the layer from 0 to 0.10 m, there were the greatest differences in C
Hum between the production systems and the NS. Humin fraction makes up the most stable carbon and is very resistant to decomposition [
45], generally representing most of humidified C in tropical soils [
46], as verified in this research, since the C
Hum represented on average about 53% of humic substances in cultivated areas and 57% in the NS. The C reduction in this fraction of SOM indicates that soil management practices led to the loss of part of stable C over the cultivation years because of the native forest’s conversion into arable land.
The lower C
FA levels in soybean monoculture, mainly in the layer from 0 to 0.10 m, is related to the lower POC levels with this production system, due to the lower biomass contributions. The POC detects the portion of SOM that has been newly deposited and is less humified, justifying the lower C
FA levels in this treatment, since, among the humic substances evaluated, fuvic acid has a lower polymerization and humification degree [
47].
In this study, it was found that the C
HA/C
FA ratio varied between soil layers and cropping systems (
Table 5). Humic acid fractions were not always higher than those of fuvic acid, although, for the most part, the C
HA/C
FA values are close to or above 1.00, which indicate areas where there is a predominance of more stable and better-quality organic material [
38,
48]. However, it is noted that treatment S showed a C
HA/C
FA ratio greater than 1.00 in the entire profile, but this did not lead to higher C
HA and C
Hum levels compared to the other treatments, indicating that the highest C
HA/C
FA ratio in S is actually due to a lower C labile proportion (
Table 3), from which the humified SOM is formed, leading to lower C
FA levels in this treatment.
As for soil structure, the area without anthropic interference (NS) maintains its structure unchanged, and therefore, it has better soil aggregation indices. Production systems that involved the exclusive Piatã (P) growed and without soybean crop succession (M, MP, MP/P, MP/P/P) obtained, in general, better aggregation indices. Effective surface coverage by forages reduces or even prevents raindrop impact [
49], promotes hydraulic roughness and reduces surface runoff [
50,
51], favoring the preservation of soil moisture and contributing to a more favorable environment for aggregation [
52,
53].
In the research, treatments that presented higher CMI (
Table 4) also had better aggregation, except for the MP/S treatment. A possible explanation is the period with uncovered soil from the previous crop (2015/16), after the soybean harvest. It is observed that the cultivation time in the S treatment has a negative effect on the aggregation, and to a lesser extent, this effect occurs in other systems in which the area is left without cultivation in the fall–winter season (when there is soybean cultivation in the summer). This shows that the fact that soil remains part of year without vegetation implies soil destabilization over time because, in addition to soil protection, the vegetation acts in forming aggregates through the roots’ mechanical action and/or through the excretion of substances with cementing action [
54], which can serve as a substrate for microorganisms, stimulating their activity and leading to the production of new cementing agents [
55].
Larger aggregates can be formed around residues recently added to the soil, which make up the particulate organic matter [
56]. These residues allow the macroaggregates’ formation, as they are a source of labile C for microbial activity and for the production of binding agents [
57]. Therefore, particulate carbon probably favored aggregates formation and MWD in NS, P, MP/P and MP/P/P, given that NS and MP/P/P, followed by P and MP/ P, showed higher carbon lability (L) in relation to the other systems (
Table 4). In addition, in NS, MP/P and MP/P/P, there was a higher POC content in the soil surface layer (
Table 3).
In addition to effect of labile C (POC), the aggregation in treatments NS, MP/P and MP/P/P may be related to the higher C
HA values in these treatments (
Table 5) and to C
Hum values in the MP/P system in the surface layer. Humified organic matter has the ability to associate with soil mineral particles, forming clay–metal–humic bonds, which contributes to soil aggregation [
58].
The greater aggregation in NS and P areas can also be attributed, in part, to chemical processes, since these areas were not chemically corrected; thus, an acidic condition with high aluminum contents is expected, which may favor aggregation, as polyvalent cations have aggregating action in the soil [
59].