**4.3 Temporal variability of nutrients in different land uses**

We observed differences in the concentrations of carbon and nutrients in the soil solution in areas with different land uses and different climate periods (rainy, TR1, dry, and TR2) during the year in the two microbasins. To facilitate the discussion, we have divided the elements analyzed into three groups (**Figure 7**), which are Group 1 – dissolved organic carbon, dissolved organic nitrogen, and dissolved inorganic carbon; Group 2 – ammonium, potassium, and magnesium; and Group 3 – chloride, nitrate, phosphate, sulfate, sodium, and calcium.

In all land-use areas except pasture and AFSs, where the soils had a more clayey texture, the absence or presence of biomass (influenced by leaf concentration) along with rainfall affected the ions and organic compounds in the soil solution extracts, as also observed by Markewitz et al. [1], who reported greater nutrient flows in natural and secondary forests. In the riparian vegetation in the rainy season and the transition to the dry season (TR1), mainly in the latter case, there were greater movements of DOC, DON, and DIC at 30 cm depth compared to 60 cm depth. Additionally, chloride, sodium, calcium, and sulfate were present in higher concentrations than ammonium, potassium, and magnesium. Rainwater and its interaction with the canopy and litter affect the chemical composition of the soil solution, which is enhanced by the fact that the soil is sandy, causing little influence given its lesser capacity to retain nutrients. The greater presence of calcium and DIC in secondary forest areas can be related to the different compositions of trees (fewer species), probably due to the increased pH, favoring cation concentration in the soil. These factors were also observed by Markewitz [24] and Figueiredo [23], who described a substantial increase of cationic components in secondary vegetation. Despite the losses by leaching in this type of vegetation, the roots remain in the soil, where they form a protective network, hindering the passage of nutrients to watercourses [5].

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

The soils in the pastures at the two depths (30 and 60 cm) stood out for high concentrations of potassium and calcium in the rainy and TR1 periods. The mobility of these ions in the soil profile is intense after heavy rainfall, so their concentration in the soil solution increases. This is common in tropical soils in unmanaged pasture areas [25]. The soils in these areas, with low CEC, sparse organic matter, and high acidity, are prone to leaching due to high water percolation [26]. This process influenced the soil solution, mainly in the pasture area of the São João microbasin, where the DOC concentration reached 30 mg L<sup>1</sup> in the rainy season. The slash-and-burn areas of the two microbasins stood out with the loss of carbon from the soil solution via DOC, and of nitrogens via nitrate (NO3 ) and DON, at the depth of 60 cm. This probably favored the movement of these compounds to the groundwater, as also observed by other authors [23, 24]. The amounts of DON and DOC lost to the soil solution in the process of burning vegetation and the decomposition of organic matter cause an increase in the flow of nitrate. These processes in the rainy season and transition to the dry season in the burned areas resulted in the loss of these compounds, which were intensely released from the soil surface by rainwater, rather than resulting from processes in the deeper soil levels, where they are transported to the groundwater.

In both microbasins, there were greater concentrations of phosphate at a depth of 60 cm in the chop-and-mulch agriculture areas in the rainy season, as well as of DOC with origin in the biomass on the soil surface, as also reported by Kato et al. [6]. According to Markewitz et al. [24], phosphorus can be converted to its inorganic state during the decomposition of organic matter. In our case, this nutrient in the soil came from the chopped branches and leaves. This process was also reported by Neill et al. [27] as the result of burning. This did not occur in this study, where it only happened in the chop-and-mulch area when the biomass supplied phosphorus to the surface soil and consequently to the soil solution. Kato et al. [6] noted a positive balance of nutrients, mainly phosphorus, through the cutting and mulching of secondary vegetation, which served as a source of organic matter to the soil. We also observed intense retention of water by the soil in this system, which hampered the collection of samples by the soil extractors. This was expected of the chop-and-mulch system as a factor of water conservation.

In the agroforestry systems, the concentrations of all ions were lower in the rainy, TR1, and dry seasons in relation to the other land uses. Ca2+, Cl, Na<sup>+</sup> , NO3 , and SO4 were present, along with DOC, DON, and DIC. The diversity of leaves from different plant species in the soil promotes the entry of nutrients because the presence of roots and microorganisms favors their absorption with less water availability [28]. Because of the presence of many arboreal species, studies of nutrient cycling in this complex composition are still scarce, but AFS models, implemented mainly by family farmers, have proven to be well-adapted, helping to improve local socioeconomic conditions [29].
