The PP, NT, and PNHR areas had similar values of dry mass litter deposited on the soil, around 1300.00 kg ha− 1, representing 58% of the material found in the NF area, which had 2218.00 kg ha− 1 (Fig. 3A).
The PP and NT areas presented similar and higher Bd values throughout all layers evaluated, around 1.40 Mg m− 3 at the surface, reaching 1.61 Mg m− 3 at 0.20–0.40 m. Lower Bd values were found in the PNHR and NF areas, in the surface layer, with values of 1.19 and 1.25 Mg m− 3, respectively. In the layers, 0.05–0.10 and 0.10–0.20 m, both areas obtained values close to 1.40 Mg m− 3, and in the last layer evaluated, the PNHR area obtained 1.54 Mg m− 3, while NF, 1.28 Mg m− 3, being different from each other. There was a tendency for higher Bd values to occur as the depth increased, while the NF area showed lower Bd values in the 0.20–0.40 m layer (Fig. 3B).
In the superficial layer, the PNHR area showed the highest TOC content, 19.94 g kg− 1, followed by NF, 12.65 g kg− 1 (Fig. 4A). The contents were significantly lower in PP and NT in the same layer, around 6.60 g kg− 1. In the 0.05–0.10 m layer, the area of PNHR still obtained the highest TOC content but showed a 60% decrease from the previous layer. The NF area had a content of 6.79 g kg− 1, similar to the NT area. The PP area showed the lowest content in this layer, 3.80 g kg− 1, with a significant drop in TOC with increasing depth (Fig. 4A).
In the 0.10–0.20 m layer, NT and NF maintained the contents near 6.00 g kg− 1, and PP and PNHR near 3.00 g kg− 1. In the 0.20–0.40 m layer, NT, PNHR, and NF had similar TOC contents. In all systems, TOC contents decreased with depth (Fig. 4A).
The Stock-C (Fig. 4B) generally followed the same patterns as the results of the TOC contents (Fig. 4A). In the superficial layer, the area of PNHR showed the highest Stock-C, 24.84 Mg ha− 1, while NF had 15.76 Mg ha− 1. In the 0.05–0.10 m layer, PNHR obtained 11.18 Mg ha− 1 and NF, 9.41 Mg ha− 1. The PP and NT areas in the same layers obtained low Stock-C, with the NT area stocking around 8.00 Mg ha− 1 in both layers and the PP area, 8.31 and 5.27 Mg ha− 1 in the 0-0.05 and 0.05–0.10 m layers, respectively (Fig. 4B).
In the 0.05–0.10 m layer, NT, PNHR, and NF areas did not obtain significant differences for Stock-C, while the PP area had the lowest Stock-C. In the 0.10–0.20 m layer, the PNHR area was similar to the PP area with lower Stock-C, and in the 0.20–0.40 m layer, NT obtained lower Stock-C compared to NF (Fig. 4B).
The highest STRATI observed was in the PNHR area, with a value of 5.28, while the lowest was in NT, 1.65, with the PP and NF areas having intermediate and similar values, 2.89 and 2.78, respectively (Fig. 5A). For ∆Stock-C, the PP and NT areas showed negative variation in almost all layers and the 0-0.40 m profile. The PNHR area showed positive variation in the 0.05–0.10 and 0.10–0.20 m layers and the 0-0.40 m profile (Fig. 5B).
Regarding the humic substances, in the 0-0.05 m layer, the highest levels of C-FA were found in the PNHR and NF areas, 2.80 and 2.54 g kg− 1, respectively. In the other layers evaluated, only the area of NF obtained higher contents, ranging from 2.42 to 1.91 g kg− 1 (Table 2). The C-HA contents in the surface layer were highest in the PP, NT, and PNHR areas, near 2.65 g kg− 1, and the lowest in PP, 1.20 g kg− 1. In the following layers, PP and NF were the areas with the highest C-HA contents, and in all layers, there was a similarity in the C-HA contents between these two areas. Overall, C-HA contents decreased with increasing depth (Table 2).
The most recalcitrant C fraction (C-HUM) was the one that predominated over the others in all layers. In the surface layer, the PNHR and NF areas had considerably higher levels of C-HUM, 18.12 and 11.36 g kg− 1, respectively, while PP and NT had levels close to 7.50 g kg− 1. Similar to the TOC and Stock-C results, the TOC-HUM contents of the PNHR and NF areas decreased with increasing depth, with contents close to 6.0 g kg− 1 in the 0.05–0.10 m layer, representing only 33% and 53% of the content found in the previous layer, respectively. The PP and NT areas obtained contents of 5.69 and 7.25 g kg− 1 in the same layer, respectively. In the 0.10–0.20 and 0.20–0.40 m layers, the PP, NT, and NF areas were similar concerning the C-HUM contents (Table 2).
Table 2
Carbon content of the fulvic acid (C-FA), humic acid (HA), and humin (HUM) fractions, carbon stock of the fulvic acid (StockC-FA), humic acid (StockC-HA), and humin (StockC-HUM) fractions, C-HA/C-FA and AE/C-HUM ratios of the different land use systems.
MS | C-FA | C-HA | | C-HUM | StockC-FA | StockC-HA | StockC-HUM | C-HA/C-FA | AE/C-HUM |
------------ g kg− 1 ----------- | ---------------- Mg ha− 1 ------------- | --- | --- |
0-0.05 m |
PP | 1.36b | 2.09a | | 7.56c | 1.70b | 2.61a | 9.42c | 1.54a | 0.53a |
NT | 1.22b | 1.20b | | 7.26c | 1.52b | 1.49b | 9.04c | 1.01b | 0.35a |
PNHR | 2.80a | 2.67a | | 18.12a | 3.49a | 3.33a | 22.57a | 0.94b | 0.33a |
NF | 2.54a | 2.39a | | 11.36b | 3.16a | 2.98a | 14.15b | 0.96b | 0.45a |
0.05–0.10 m |
PP | 1.07b | 1.47ab | | 5.69a | 1.49b | 2.04b | 7.88b | 1.39a | 0.54ab |
NT | 1.18b | 1.11b | | 7.25a | 1.64b | 1.54b | 10.05a | 0.98a | 0.34b |
PNHR | 1.32b | 1.06b | | 6.04a | 1.83b | 1.46b | 8.37ab | 0.78a | 0.44ab |
NF | 2.42a | 1.84a | | 6.11a | 3.35a | 2.55a | 8.47ab | 0.78a | 0.88a |
0.10–0.20 m |
PP | 0.90b | 1.43a | | 4.62a | 1.26bc | 2.00a | 6.46b | 1.68a | 0.58ab |
NT | 1.14b | 0.84b | | 6.77a | 1.60b | 1.17b | 9.45a | 0.75a | 0.36b |
PNHR | 0.86b | 0.70b | | 2.10b | 1.20c | 0.98b | 2.94c | 0.85a | 0.87a |
NF | 1.98a | 1.42a | | 5.16a | 2.77a | 1.98a | 7.21b | 0.72a | 0.68a |
0.20–0.40 m |
PP | 0.73c | 1.06ab | | 5.25a | 0.93c | 1.35a | 6.72ab | 1.45a | 0.37b |
NT | 1.03b | 0.83b | | 6.51a | 1.32b | 1.05ab | 8.27a | 0.83a | 0.36b |
PNHR | 0.77bc | 0.72b | | 2.29b | 0.99c | 0.91b | 2.89c | 0.97a | 0.70a |
NF | 1.91a | 1.32a | | 4.49a | 2.44a | 1.68a | 5.69b | 0.69a | 0.78a |
Means followed by equal letters in the column in each layer do not differ by the t student test (p ≤ 0.05). MS: Management system, PP: Permanent pasture, NT: no-tillage, PNHR: Private Natural Heritage Reserve, NF: Native forest.
The StockC-FA in the 0-0.05 m layer was higher in PNHR and NF, 3.49 and 3.16 Mg ha− 1, respectively, and lower in PP and NT. In the next layer, PP, NT, and PNHR areas were similar for Stock-CFA, ranging from 1.49 to 1.83 Mg ha− 1, while the NF area had the highest StockC-FA, 3.35 Mg ha− 1. Similar behavior of the previous layer, at 0.10–0.20 m, but the managed areas showed values ranging from 1.20 to 1.60 Mg ha− 1, while NF had StockC-FA of 2.77 Mg ha− 1. In the 0.20–0.40 m layer, the PP and PNHR areas obtained the lowest values of StockC-FA, 0.93 Mg ha− 1, and 0.99 Mg ha− 1, respectively, differently from that observed in NF, 2.44 Mg ha− 1 (Table 2).
The PP, PNHR, and NF areas obtained the highest StockC-HA in the surface layer, ranging from 2.61 to 3.33 Mg ha− 1, while NT had 1.69 Mg ha− 1. As with the results of StockC-FA, in the layer 0.05–0.10 m, the values of StockC-HA did not differ in PP, NT, and PNHR. In the 0.10–0.20 m layer, the PP and NF areas showed the highest StockC-HA, 2.00 and 1.98 Mg ha− 1, respectively, while NT and PNHR stored 1.17 and 0.98 Mg ha− 1, respectively. At 0.20–0.40 m, PP and NF had the highest StockC-HA, 1.68 and 1.35 Mg ha− 1, respectively, and the PNHR area had the lowest StockC-HA, 0.91 Mg ha− 1 (Table 2).
There was a high variation in the StockC-HUM according to the layer evaluated, especially in the PNHR area. In the 0-0.05 m layer, the PNHR obtained the highest StockC-HUM, with 22.57 Mg ha− 1, reaching 2.90 Mg ha− 1 in the 0.20–0.40 m layer, equivalent to only 12.84% of the value observed in the surface layer (Table 2). NF area had the second highest StockC-HUM in the surface layer, with 14.15 Mg ha− 1, while PP and NT obtained stocks of 9.04 and 9.42 Mg ha− 1, respectively.
For the HA/AF ratio, there was only a difference between the areas in the 0-0.05 m layer. PP showed the highest ratio, 1.54; PP was the only area with values higher than 1.00, with a predominance of the HA fraction concerning FA. Observing the AE/C-HUM ratio, the value in all areas was lower than 1.00, indicating a predominance of C-HUM concerning AE (Table 2).
For the 0-0.40 m section, the NT and PNHR areas showed positive variation in the StockC only for HUM. In the PP area, there was a negative variation in the Stock-C for all organic fractions (Fig. 6).
Generally, aggregates were more stable in the PP, PNHR, and NF areas. In the 0-0.05 m layer, we observed WMD values close to 4 mm and GMD values close to 3 mm, while the same indexes in the NT area were close to 2 and 1 mm, respectively (Fig. 7A). In the 0.05–0.10 m layer, the PP and NT areas showed similar WMD and GMD values compared to the 0-0.05 m layer, while in PNHR and NF, there was a decrease in values, but still similar to PP (Fig. 7B).
The PP and PNHR areas obtained SI close to 1.00 (NF reference value) in both layers, being similar, while NT obtained about half the value, 0.6 (Figs. 8A and B). In both layers, the area with the highest OLev was PNHR, 200 and 100, respectively, followed by NF, 120 and 100. The other areas, PP and NT, had considerably lower OLev, 60 and 40, respectively, related to the low levels of Stock-C in these areas (Figs. 8A and B).
The predominant aggregate class in the PP, PNHR, and NF areas were the macroaggregates (> 2.0mm), around 80%. In NT, the predominant aggregates were mesoaggregates (0.25 to 2.00mm), around 40%, followed by macroaggregates. For the microaggregates (< 0.25mm), there was a representation on the order of 20%, with NT having the highest percentage (Figs. 9A and 9B).