In native campos of Uruguay, overseeding legumes coupled with phosphorus (P) fertilization is a technology used to increase animal production. Short-term improvements in both forage productivity and quality are repeatedly reported. However, some evidence suggests that this management may at times lead to the collapse of the native community and invasions by exotic species. Indeed, it is yet unclear to what extent overseeding legumes into native grasslands affects its long-term integrity. This study uses data from a long-term experiment to assess whether increased P fertilizer rates—typically used to encourage legume establishment and growth—are associated with reduced species diversity. In 1996 a grazed native grassland in eastern Uruguay was either left untouched (control) or overseeded with a mix of Trifolium repens and Lotus corniculatus and then fertilized at either a moderate or high rate of P (197 or 394 kg · ha−1 over 13 years, respectively). The three treatments were arranged in a randomized block design with four replicates of 2 hectares each. In 2005 the experiment was exhaustively sampled: 11 georeferenced sampling points per replicate, each encompassing ~20 m2. Extractable P was measured in the 0−5- and 5−15-cm soil layers. In 2009, species presence and cover were measured at the same points. Across treatments, wherever legumes were introduced, extractable soil P was negatively related to species richness and diversity (P < 0.01) and native grass cover was reduced. This effect became asymptotic once soil P exceeded 27 and 36 mg · kg−1 of P (0–5 cm), respectively. Therefore the documented reduction in species richness and diversity suggests a trade-off between increased pasture production and decreased vegetation stability may be operating in response to P fertilization of overseeded grasslands. The underlying ecophysiological mechanisms, as well as grazing management options to mitigate species diversity decline, should be further studied.

Livestock production has been challenged as a large contributor to climate change, and carbon footprint has become a widely used measure of cattle environmental impact. This analysis of fifteen beef grazing systems in Uruguay quantifies the range of variation of carbon footprint, and the trade-offs with other relevant environmental variables, using a partial life cycle assessment (LCA) methodology. Using carbon footprint as the primary environmental indicator has several limitations: different metrics (GWP vs. GTP) may lead to different conclusions, carbon sequestration from soils may drastically affect the results, and systems with lower carbon footprint may have higher energy use, soil erosion, nutrient imbalance, pesticide ecotoxicity, and impact on biodiversity. A multidimensional assessment of sustainability of meat production is therefore needed to inform decision makers. There is great potential to improve grazing livestock systems productivity while reducing carbon footprint and other environmental impacts, and conserving biodiversity.

Physical fractionation and C-13 determinations are useful techniques for soil organic carbon (SOC) dynamics studies. Changes in SOC content, distribution and origin were assessed after 9.5-year crop-perennial (C3 species) rotation on a Uruguayan Mollisol under conventional tillage (CT) and no-tillage (NT). Soil samples were collected at depths of 0-6, 6-12 and 12-18 cm in 1994 and 2003. Determinations were made of total SOC, particulate organic matter C (POM-C) and mineral-associated organic matter C (MAOM-C). In addition, C-13 determinations were made on the total sample and the different particle size fractions. None of the studied variables were affected significantly by the tillage system. SOC levels in 2003 did not differ significantly from those of 1994 at any of the studied depths. However, changes were found in fraction distribution. Within 0-18 cm of the soil surface, POM-C decreased by 63%, whereas MAOM-C did not vary significantly. After 9.5 years, only 14.5% of SOC within 0-18 cm of the soil surface was young SOC. The largest proportion was incorporated within 0-6 cm of the soil surface and in the coarsest physical fractions of organic matter. Only 17% of the estimated C input from crops for the study period was retained by the topsoil. The estimated half-life of SOC within the upper 18 cm of soil was 28 years. Within this layer, the C half-life varied from less than 5 years for POM-C to more than 400 years for MAOM-C. These results suggest that agricultural rotation systems including perennial pastures are capable of maintaining SOC levels even under CT. However, C cycling and other ecosystem processes may be altered due to the significant loss of labile organic matter. The use of C-13 analysis enabled the estimation of parameters relevant to the modeling of SOC dynamics. (C) 2013 Elsevier B.V. All rights reserved.

Soil organic carbon (SOC) is one of the principal indicators of soil quality. Its size fractions have been proposed as high sensitivity indicators in order to detect changes generated by different soil use and management intensities. The objective was to compare the impact of different soil management practices after 10 years on SOC distribution and its size fractions. Treatments consisted in two rotation systems (rotations of continuous annual crops and rotations of 3 years of crops and 3 years of pastures), performed with conventional tillage (CT) and no-till (NT). In 2000, NT treatments were additionally split into C3 or C4 summer crops. In 2003, soil was sampled at 0–3, 3–6, 6–12, 12–18, 18–40, 40–60 and 60–80cm depths and SOC was determined. At the first four depths, SOC associated with particulate organic matter (POM-C) and with the soil mineral fraction (MAOM-C) were determined. Changes in carbon indicators (SOC and its size fractions) occurred mainly in the first 3cm of soil, and with the exception of POM-C, were diluted when considering the 0–18cm depth. Inclusion of pastures in the rotation was a better alternative to continuous cropping in CT systems, since it had better C indicator values. However, NT improved indicator values compared with LC, especially when C4 species were included in the rotation; no differences were found between continuous cropping or crop-pasture rotations. These results allowed discriminate different combinations of crops and tillage systems that contribute to maintain or increase SOC, suggesting a sustainable management of the soil resource.

The potential export of nutrients from Eucalyptus plantations harvested for pulp production may be high. However, depending on the harvest method, the nutrients from the residue can be recycled. The aims of this study were (i) to quantify the content and distribution of nutrients in different residue components at harvest for a Eucalyptus dunnii Maiden plantation; and (ii) to quantify the decomposition rates of the harvest residues, and the return of nutrients to the soil in the temperate climate conditions of Uruguay. Six trees of a 9-year-old E. dunnii plantation with average diameter at breast-height (DBH) were harvested, and the biomass produced and the N, P, K, Ca and Mg contents in commercial and non-commercial logs, leaves, branches, bark and litter were estimated. Decomposition of the remains of leaves, branches, non-commercial logs, bark and litter was studied in the field for 2 years. Although commercial logs accounted for 61% of the biomass produced, only 27% of the N, 35% of the P, 18% of the K, 16% of the Ca and 41% of the Mg present in the forest were exported with the product. When logs are exported without de-barking in the site, the nutrient export would increase to 41%, 55%, 46%, 68% and 66% of the total extraction of N, P, K, Ca and Mg, respectively. Residue decomposition showed that the leaves lost the highest proportion of biomass (half life 0.86 years), and bark was most resistant to decomposition in the field (half life 5.36 years). As regards the nutrients, K was lost most rapidly and Ca showed the slowest loss, while N, P and Mg losses were generally more gradual, and proportional to the decomposition rate.

Soil organic carbon (SOC) is important for animal production systems sustainability under grazing. We evaluated. soil use intensity effects on SOC and particulate organic carbon (C-POM; 53-2000 mu m) in a 72 ha no-till crop-pasture rotation experiment in Uruguay (Typic Argiudols). Treatments included: 1) continuous cropping (CC) of Lolium multiflorum Lam. or Avena sp. in winter and Sorghum bicolor L. or Setaria italica in summer; 2) short rotation (SR): two years idem CC and two years pasture of Trifolium pretense L. and Lolium multiflorum; 3) long rotation (LR): two years idem CC and four years pasture of Dactylis glomerata L., Trifolium repens L. and Lotus corniculatus L.; 4) permanent pasture (PP): natural pasture overseeded with legumes used in LR. All rotation phases were present each year. After 8 years, LR increased SOC (0-15 cm depth) by 19% compared to CC (31.8 Mg ha(-1)); no SOC differences were found between LR, SR and PP. Plots containing pastures had 14% greater SOC than plots containing crops (33.7 Mg ha(-1)). Crops rotated with pastures (LR, SR) had 5% greater SOC than crops in CC (31.8 Mg ha(-1)). There were no differences on SOC between pastures in LR and PP, but 3-4 years pastures had 33% greater SOC than 1-2 years pastures (34.4 Mg ha-1). The lowest and greatest C-POM was found in CC and in 3-4 years pastures of LR (8.2 and 12.6 Mg C ha(-1), respectively). No till crop-pasture rotation systems including long-term pastures preserved SOC, even in high biomass extractive grazing systems.