**3.5 Variability in the concentration of ions in areas with different land-use systems**

The concentrations of the ions studied presented heterogeneous results in the different land-use systems, with the highest concentration found in the slash-andburn agriculture, pasture, and riparian vegetation areas, as shown in **Table 5**.

With relation to depth, the ions concentration was variable between 30 and 60 cm. Pasture and chop-and-mulch stood out for having many high values at 60 cm (**Table 5**).

There were low correlations of DOC and DIC with Ca, R2 = 2.1% and 15%, respectively (Spearman correlation, p < 0.05) (**Figure 6**).

**Figure 6.**

*Spearman correlations between dissolved organic carbon (DOC) versus calcium (a) and dissolved inorganic carbon (DIC) versus calcium (b).*

**Figure 7.**

*Temporal distribution of the chemical constituents of the soil solution samples collected at depths of 30 and 60 cm in the 12 sampled areas.*

*Soil Solution Chemistry in Different Land-Use Systems in the Northeast Brazilian Amazon DOI: http://dx.doi.org/10.5772/intechopen.101856*

#### **3.6 Temporal variability of chemical compounds in areas with different land uses**

With respect to the temporal variability, the highest concentrations of ions occurred in the rainy and transition to dry periods in all six land-use systems (**Figure 7**).

**RV**: The concentrations of DOC and DON were highest in the riparian vegetation area in both microbasins. This was the only land-use class for which we managed to obtain soil solution samples in all four climate periods. In the Cumaru microbasin, we detected all ions in the rainy and TR1 periods. The highest concentrations of Cl (approximately 10 mg L<sup>1</sup> ) were found at 30 cm depth. In the São João microbasin, the highest concentrations of DOC, Cl, and NO3 occurred in the rainy, TR1, and dry periods.

**SF**: In the SFC, the concentrations of DOC and DON were highest in the rainy, TR1, and dry periods at both depths, while the inorganic ions stood out only in the rainy and TR1 periods. In SFSJ, the concentrations of these compounds were highest at depth of 30 cm, except for the phosphate levels in the secondary forest areas, which were below the limit of detection.

**P**: In the pastures, the concentrations of DOC and DON were highest at the 30 cm depth, principally in the São João microbasin. SO4 <sup>2</sup>, K<sup>+</sup> , and Ca2+ were the predominant ions in the land-use areas in both watersheds.

**SB**: In the slash-and-burn agriculture areas, the concentrations of DOC were higher than 10 mg L<sup>1</sup> , with the greatest values finding at a depth of 60 cm in the rainy period and the transition from rainy to dry period in Cumaru. At the 30 cm depth, in the rainy period, the highest levels were measured of NO3 ≈ 16 mg L<sup>1</sup> , PO4 ≈ 5 mg L<sup>1</sup> , and K<sup>+</sup> ≈ 8 mg L<sup>1</sup> ). The same pattern occurred for these parameters in the São João microbasin, but with lower values in the rainy, TR1, and dry periods.

**CM**: In the chop-and-mulch system, phosphate stood out with the highest concentration (≈ 35 mg L<sup>1</sup> ) at depth of 60 cm in the rainy period in the Cumaru microbasin. The other ions in this system had lower concentrations in the rainy, TR1, and dry periods, with values below 15 mg L<sup>1</sup> in both basins at the two depths.

**AFS**: In the agroforestry system, the ions had low concentrations, except for DOC, which had the highest concentrations in Cumaru at a depth of 60 cm and in São João at 30 cm, with approximate values of 9 mg L<sup>1</sup> . DOC and DON in the AFS sampling areas presented the highest concentrations in the dry period in Cumaru and the rainy period in São João.

## **4. Discussion**

#### **4.1 Temporal variability of C and N in the areas with different land uses**

The concentrations of dissolved organic carbon at a depth of 30 cm were higher than at 60 cm; however, Marques et al. [13] did not observe differences between those two depths in a study in the Central Amazon near the city of Manaus, Brazil.

We found the concentrations of DOC in the slash-and-burn agriculture areas, mainly in the Cumaru microbasin, to be highest in the transition from the rainy to dry period (16.23–24.02 mg L<sup>1</sup> ) and also in the dry season (15.39–33.48 mg L<sup>1</sup> ). These are the periods when farmers most often practice burning.

In the São João microbasin, the maximum levels of DOC were also found in the slash-and-burn agriculture area at depth of 30 cm (57.86 mg L<sup>1</sup> ). In this area, the

soils are predominantly sandy rather than clayey, resulting in little retention of DOC and consequently a substantial presence in the soil solution, as also observed by Sommer [11] and Wickel [5].

The secondary forest and riparian vegetation areas presented the highest concentrations of dissolved organic carbon and nitrogen, probably originating from the decomposition of leaves and roots and the activity of the microbial biomass in the soil, among other factors, such as higher temperature and flow of the soil solution in the transition period TR1 (DOC: 1.37–19.98 mg L<sup>1</sup> ; DON: 0.57–1.14 mg L<sup>1</sup> ) and rainy period (DOC: 2.70–17.65 mg L<sup>1</sup> ; DON: 0.67–1.39 mg L<sup>1</sup> ). This is similar to the findings of Marques et al. [13], ≈ 10 mg L<sup>1</sup> for DOC. The spatial variations, that is, according to land use, can be observed in the DOC/DON ratio, where low ratios indicate higher concentrations of DON, which stand out in the agroforestry systems. The flows of DOC and DON are connected by belonging to the same organic matter chain in the soil. Part of the DOC is used in the soil as substrate by microorganisms, causing an increase in the mineralization of DON and consequently the nitrification process. The concentration of DOC in the soil solution can also affect the speed of the denitrification process and the concentration of DON, as well as the flows of nutrients and metalloids in the soil, also depending on the pH, redox potential, and cationexchange capacity [2, 14].

Riparian vegetation, AFS, and secondary forest systems are known to retain nutrients in the soil, with only small losses due to leaching [15] because of the quality and quantity of residues produced by the plant cover. With respect to the nitrogen compounds, the concentrations of nitrate were higher than those of ammonium and dissolved organic nitrogen in the six systems (DON: 0.6–1.7 mg L<sup>1</sup> ; NH4 + : 0.035 mg L<sup>1</sup> ; NO3 : 0.1–12.0 mg L<sup>1</sup> ). This result was expected, since NO3 is more soluble than DON and is not retained in clay minerals such as NH4 <sup>+</sup> [2, 16], where the conversion of ammonium into nitrate happens rapidly. However, the highest concentration of NH4 <sup>+</sup> occurred in the riparian vegetation, where the nitrification process is slow. Alfaia [17] found the presence of NH4 <sup>+</sup> in floodplain areas rich in clays. Gruditz and Dalhammar [18] reported that the nitrification process (the transformation of ammonium into nitrite and nitrate) occurs at a pH of approximately 8.0, a level higher than in our study (in acidic soils). In the pastures, we observed the greatest variations of DOC and DON in the dry period at both depths. This response is likely due to the concentration of aggregates transported in the soil profile because in this period there is less water available [13].

#### **4.2 Chemical variability in soil solution extracts based on different land uses**

At both depths, ion exchange probably occurred, depending on the soil chemical composition and the presence of organic matter, in which the ions were carried through the unsaturated zones where plant roots are located, reaching the groundwater.

The different land uses influenced the nutrient cycling, depending on the management and complexity of the landscapes of the areas studied. In these areas, the soil composition is more sandy than clayey, except in the AFSs and pastures. In the unmanaged pasture and slash-and-burn agriculture areas, the adsorption of ions by the soil is hampered, mainly at the depth of 60 cm. This is characteristic of weathered soils with low cation-exchange capacity and high acidity. As described by other authors, the variations of pH and ionic strength are factors that influence the release of ions from the soil to the solution, and hence influence the processes of adsorption of

#### *Soil Solution Chemistry in Different Land-Use Systems in the Northeast Brazilian Amazon DOI: http://dx.doi.org/10.5772/intechopen.101856*

cations and anions. In other words, acidic pH and low ion-exchange strength favored their release in the areas studied [5]. Besides this, the processes of nitrogen mineralization and subsequent oxidation (formation of nitrite and/or nitrate), along with the solubilization of aluminum, create conditions for the soil not to retain these ions [2]. The presence of iron and aluminum oxides in the soil can influence the electrochemical processes, resulting in increased exchange of anions and reduced exchange of cations between the soil and soil solution [16].

Competition exists between sulfate and nitrate ions, evidenced by the ionic strength and reduction of pH. This correlation was observed in the chop-and-mulch agriculture and AFS areas, with variations between these two parameters (**Table 2**). High concentrations of sulfate were observed in these areas, indicating the low competition of other anions such as phosphate and nitrate, since in these areas low concentrations of iron and aluminum predominated because the soils are sandier [7]. According to Borba et al. [16], sulfate ions are preferentially adsorbed by the soil in comparison with nitrate ions. The presence of sulfate in the soil solution in the AFS and chop-and-mulch agriculture systems can be attributed to soils with greater clay content than in areas of pasture and slash-and-burn agriculture, probably due to leaching of nutrients in soils impacted by external factors (physical–chemical destructuring) [15]. In this study, we observed a greater variation of Cl ions in the soil solution with lower retention, probably due to the competition for the adsorption of other negatively charged ions in the soil, such as NO3 and SO4.

With respect to cations, Ca2+, Na+ , K<sup>+</sup> , Mg2+, and NH4 <sup>+</sup> had the highest concentrations in the chop-and-mulch agriculture and secondary forest systems, where the biomass left in the soil probably contributed to the incorporation of organic acids in the soil. This is an important factor for the enrichment of nutrients in the soil. With respect to the mechanism for the exchange of macronutrients and micronutrients, according to Borba et al. [16], this occurs mainly in shallower soil layers, where the greater presence of organic matter increases the cation-exchange capacity (CEC). According to Denich [19] and Cattanio [20], the level of organic material is affected by the composition of the leaf litter in secondary forest areas, in turn, determined by the plant species and their contribution of nutrients, as well as the pattern of decomposition and mesofauna in the soil. This same process probably occurred in the areas of riparian vegetation and agroforestry systems studied by us.

In the slash-and-burn agriculture areas, the concentration of nitrate in the soil solution was high, in some cases above the limits of potability (> 10 mg L<sup>1</sup> ), indicating potential leaching into the soil solution, eventually reaching groundwater reservoirs. Williams et al. [21] conducted studies of the hydrochemical changes caused by land clearance through burning for agriculture in small plots in the central Amazon Basin on the northern side of the Solimões River and concluded that high leaching of nutrients occurs after slash-and-burn preparation, which gradually diminishes, mainly in the riparian zone. The authors also observed high concentrations and N and P in surface runoff, confirming the importance of riparian forests as nutrient buffers in the aquatic and terrestrial systems of the basin. In our study, in turn, we noted this same loss of micro-and macronutrients in the slash-and-burn agriculture and pasture areas.

The nitrogen in its inorganic form left after burning of vegetation is known to accelerate the decomposition of forest biomass [21], so that percolation of the solution through the soil profile in nitrate-rich soils occurs mainly in disturbed soils because this disturbance diminishes the nitrogen uptake of plants and enhance its availability. We found increased NH4 <sup>+</sup> in the soil solution to be common after burning and in areas with riparian vegetation of the Cumaru microbasin. This was probably due to

microbiological processes, where ammonium is released from anoxic sediments to be oxidized into nitrate by nitrifying organisms, as well as caused by competition for N by plants [21].

Borba et al. [2] reported that the combination of dissolved inorganic carbon and Ca+ ions, where carbon is converted into carbonates, caused an increase in soil pH. This linear correlation (**Figure 6**) between dissolved carbon and calcium can be related to the availability of this nutrient by the decomposition of biomass in agricultural systems, which, depending on the soil conditions (clay percent, acidity, and water availability) can increase or decrease the productivity of crops [22].

We observed higher concentrations of calcium, magnesium, potassium, and phosphate ions in the chop-and-mulch agriculture areas, probably resulting from the mineralization of the chopped plant residues, as reported by Figueiredo et al. [23] in a study of similar areas.

The concentration of ions in the soil increased rapidly after the burning of vegetation to plant crops, with various ionic compounds remaining for brief periods [6]. Changes in soil management are clearly related to loss and mobilization of nutrients affecting soil fertility, which is also affected by factors such as soil class, degree of disturbance, and effects of burning on the biological and physical aspects of the soil. The results of this study clearly indicated the importance of research regarding nutrient flows in areas with different soil management types in tropical areas, to evaluate the efficiency of different crops to absorb these nutrients without loss or excess of these compounds in the soil and water.
