Soil phosphorus (P) tests for flooded rice (Oryza sativa L.) generally present uncertainties for estimating P availability. Bray 1, 1% citric acid, Mehlich 3, Olsen extractants (dry samples), and Bray 1 extractant after 3 days (BI3) and 7 days (BI7) of anaerobic incubations were evaluated to estimate P availability for rice in 43 Uruguayan soils. Field trials were conduced at each site (0, 13, 26, and 39 kg P applied ha⁻¹). Relative yield and absolute and relative yield increases were determined. Extracted P was variable for the different tests. For silty soils, P availability was better estimated by citric acid, Mehlich 3, and Bray 1, with similar soil P critical concentrations (6−8 mg P kg⁻¹). The BI3 and BI7 tests showed greater soil P critical concentration but poorer correlations with yield indexes. This study contributes to the scientific basis of P fertilization for flooded rice, promoting more effective fertilizer use and minimizing environmental P losses.

The replacement of native pasture by exotic commercial forest species is an infrequent situation worldwide. In these systems, a new component is introduced, forest litter, which constitutes one of the main ways of incorporating carbon into the soil–plant system. The present work seeks to establish a methodological approach to study the dynamics of litter production and decomposition in an integrated way. The general objective was to characterize and compare the litter production dynamics in 14-year-old Eucalyptus grandis Hill ex Maiden and Pinus taeda L. commercial plantations. During two years, seasonal evaluations of fall, decomposition and accumulation of litter were carried out in stands of both species. In turn, the contribution of carbon from forest species to the soil through isotopic analysis techniques was quantified. Litterfall in E. grandis showed maximums during the spring of the first year and in the spring and summer of the second. In P. taeda, the maximums occurred in summer of the first year and in autumn of the second. In relation to the decomposition rate, the results based on short periods of evaluation between 15 and 21 months did not show differences between species, nor for the different moments of beginning of the evaluation, obtaining average values of 0.0369 month⁻¹ for E. grandis and 0.0357 month⁻¹ for P. taeda. In turn, both the decomposition rate of the material as a whole and the estimates of accumulated biomass in equilibrium state did not show significant differences between the species. Additionally, there was a relevant incorporation of carbon into the soil by forest species, fundamentally in the first few centimeters, substituting an important proportion of the carbon inherited by the original cover of native pastures. Finally, it is necessary to specify that the scope of the findings obtained is greatly limited by the sample size used in this study.

The installation of commercial stands with exotic forest species on low fertility soils originally covered by native pastures is an unusual situation worldwide. In recent years, the area occupied by forest systems designed for pulp or wood production with immediate replanting has increased strongly in the Pampean region of South America. In this context, the study of nutrient recycling from forest litter decomposition acquires particular relevance. This work seeks to evaluate and compare the nutrient release from the decomposition of forest litter produced by 14-year-old Eucalyptus grandis Hill ex Maiden and Pinus taeda L. stands and test the applicability of a new sampling methodology in the nutrient recycling assessment. For two years, the evaluation of N, P, K, Ca, Mg, Fe, Mn, Cu and Zn dynamics during litter decomposition was carried out. In general, K concentration decreased through decomposition, meanwhile, all other nutrients showed some degree of immobilization, but this was counteracted by biomass loss for most of them. This mainly resulted in net nutrient release from litter. A higher release rate of all nutrients from P. taeda forest litter compared to E. grandis, with the exception of Mn, was verified. Fe immobilization was observed in both species showing a higher immobilization rate in E. grandis compared to P. taeda. Finally, Zn exhibited immobilization processes in E. grandis and releases in P. taeda. This might suggest higher temporal and quantitative availability of nutrients in P. taeda, due their faster return to the soil. These findings could be relevant in the development of models for sustainable management, adapting the demand for nutrients to the supply during forest rotations.

The sustainable management of forest plantations by keeping the harvest residues on site improves the soil’s chemical, physical and biological properties while constituting an important nutrient reserve. Our objectives were: (a) to identify and quantify the characteristics of Eucalyptus dunnii, Eucalyptus grandis and Eucalyptus globulus that affect the decomposition rates of harvest residues, as well as indicators that can explain the process and (b) to quantify the potential recycling of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) to the soil from residue decomposition and the quantitative and qualitative differences between the species. We analyzed the information of five commercial plantations of Uruguay. At the harvest, the biomass of leaves, thin and thick branches, bark and their respective N, P, K, Ca and Mg contents were quantified. At each site, bags with samples of the different residues were left to decompose and were periodically collected throughout 24 months. Eucalyptus dunnii presented the largest amounts of residues of all parts. The decomposition rates of the different residues depended on their chemical constitution, part size and the species. Eucalyptus dunnii leaves showed the shortest half-life (0.94 years), while the bark of the same species presented the longest (5.62 years). Total nitrogen and carbon (total and soluble) contents, which can be easily determined, emerged as good predictors for half-life estimation. The release patterns of nutrients depended more on their dynamics in the plant and their parts than on the species itself. The results highlight the importance of nutrient recycling to ensure the sustainability of the productive system in the medium and long term.

The sustainability of forest production is based on an adequate balance between soil nutrient supply and its demand by forest plantations, which may vary depending on the species and the site. In Uruguay, the low to medium fertility of the soils under forest production makes this aspect relevant. The objectives of the study were: (a) to quantify the concentration and distribution of nutrients in the harvested biomass of Eucalyptus dunnii, Eucalyptus grandis and Eucalyptus globulus in the predominant soils of the zones of greater forestry aptitude of Uruguay; and (b) to compare the nutrient use efficiencies of the three mentioned species in different soil types, as a criterion to evaluate the sustainability of the productive system in the long term. In 29 sites located in the three main forest areas of the country, nine trees with average diameter at breast height (DBH) and height were harvested at eight to eleven years of age. The aerial biomass and nutrient uptake (nitrogen, phosphorus, potassium, calcium and magnesium) in commercial wood, bark, leaves and branches were quantified, and two nutrient use efficiency indexes (nutrient use efficiency and biological utilisation coefficient) were calculated. Although the zones offered different conditions for growth and nutrient uptake, E. dunnii presented the highest extraction of nutrients in commercial wood, considering all sites, these indexes being lower for E. dunnii. Therefore, when planting E. dunnii, its greater use of the soil’s resources must be considered, particularly in soils that are naturally less fertile. Likewise, E. grandis presented a greater extraction of cations than E. globulus because of more extensive growth.