Crotalaria spectabilis and Crotalaria juncea are cover crops (CC) that are used in many different regions. Among the main attributes of these species are their high potential for biomass production and biological fixation of nitrogen (BNF). Attempting to maximize these attributes, while minimizing water consumption through high transpiration efficiency (TE), is a challenge in the design of sustainable agricultural rotations. In this study, the relationship between biomass productivity, BNF, and TE in C. spectabilis and C. juncea was evaluated. For this purpose, an experiment was carried out under controlled conditions without water limitations and using non-inoculated soil. BNF was determined by the natural abundance of ¹⁵N, while TE was estimated by several different methods, such as gravimetric or isotopic method (¹³C). C. juncea produced 42% less dry matter, fixed 28% less nitrogen from the air, and had 20% less TE than C. spectabilis. TE results in both species were consistent across methodologies. Under simulated environmental conditions of high temperature and non-limiting soil water content, C. spectabilis was a relatively more promising species than C. juncea to be used as CC.

In this work, we evaluated the possibility of predicting the geographic origin of Uruguayan honeys using discriminant analysis (DA) on mineral concentration. Although the DA results appeared to discriminate between honeys from the south, central and north, the subsequent cross-validation analysis did not confirm this result. We also compared honeys from Uruguay and the Buenos Aires province (Argentina) using DA on mineral composition data. In this case, a clear difference between these two origins was observed. It seems possible to differentiate between Uruguayan honeys and those produced in a neighbouring country based on multivariate statistical methods.

Cover crops can increase agricultural sustainability by protecting soil from erosion, increasing biodiversity, and symbiotically incorporating fixed nitrogen (N) into the soil. Nowadays, however, in Uruguay mostly grasses are planted in autumn to protect the soil from erosion. Another option is to study tropical legumes’ performance as cover crops, which can fix substantial amounts of nitrogen in short growing periods, thereby bridging the knowledge gap in Uruguayan agriculture. The main objective was to evaluate and compare the performance of six tropical legumes (Crotalaria juncea, Crotalaria spectabilis, Crotalaria ochroleuca, Cajanus cajan, Dolichos lablab, Mucuna pruriens) and the temperate legume Glycine max. The evaluation focused on aboveground biomass and the N mass derived from fixation (NmdFix), as well as other attributes; three field experiments were conducted on a southern Uruguay farm during the summers of 2017, 2018, and 2019. The growing cycle lengths for the cover crops in 2017, 2018, and 2019 were 117, 130, and 90 days, respectively. The results showed that when planting was done at late December (2017 and 2018 growing cycles), the species with the highest mean biomass yield were Crotalaria juncea (two year average 12.0 Mg ha^-1) and Cajanus cajan (11.0 Mg ha^-1), but Cajanus cajan (149 kg ha^-1) more than doubled the NmdFix mass of Crotalaria juncea (57 kg ha^-1). In 2018 biomass yields were much higher than in 2017, with Glycine max (20.0 Mg ha^-1) yielding at a similar level to Crotalaria juncea and Cajanus cajan (20.5 and 18.7 Mg ha^-1, respectively). Amounts of NmdFix, however, were much higher in Glycine max and Cajanus cajan (263 and 253 kg N ha^-1, respectively), than in Crotalaria juncea (91 kg N ha^-1). In 2019 planting had to be delayed until early February and only Glycine max maintained acceptable biomass and NmdFix levels. In conclusion, based on its fixing N potential, for late December sowings Cajanus cajan and Glycine max would be the most promising species for cover crop use, while for late January or early February sowings, only Glycine max would an option because the tropical species seriously impaired their productivity when grew longer into the cooler autumn temperatures.

Summer legume cover crops (CC) such as Crotalaria juncea, Crotalaria spectabilis, Crotalaria ochroleuca, and Cajanus cajan could offer diverse advantages for the environment and productive cropping systems. A low transpiration efficiency (TE) of CC can induce soil water content to levels that present a challenge for the subsequent crop. In a 75-day growth chamber experiment, using the natural abundance of ¹³C, ¹⁸O, and ¹⁵N we evaluated the TE and BNF under two soil water conditions. Our results showed that the four species tested are good candidates for their use as CC because they showed good results in terms of productivity parameters, TE, and BFN. Cajanus cajan had the highest TE, a high shoot dry matter production, and accumulated more N from BFN in the shoot than C. spectabilis, C. juncea, and C. ochroleuca. ∆¹⁸O increased under moderate water deficit and showed an inversely proportional relationship with the amount of transpired water, supporting the use of this isotopic indicator as a proxy for transpiration and stomatal conductance. For the isotopic parameters no interaction between the factors water regimen and species were found. We propose the mass ratio of nitrogen fixed by the volume of transpired water and the isotopic discrimination of ¹³C as useful indicators of drought fixing legumes tolerance.