Predictive theory on how plant diversity promotes herbivore suppression through movement patterns, host associations, and predation promises a potential alternative to pesticide-intensive monoculture crop production. We used meta-analysis on 552 experiments in 45 articles published over the last 10 years to test if plant diversification schemes reduce herbivores and/or increase the natural enemies of herbivores as predicted by associational resistance hypotheses, the enemies hypothesis, and attraction and repellency model applications in agriculture. We found extensive support for these models with intercropping schemes, inclusion of flowering plants, and use of plants that repel herbivores or attract them away from the crop. Overall, herbivore suppression, enemy enhancement, and crop damage suppression effects were significantly stronger on diversified crops than on crops with none or fewer associated plant species. However, a relatively small, but significantly negative, mean effect size for crop yield indicated that pest-suppressive diversification schemes interfered with production, in part because of reducing densities of the main crop by replacing it with intercrops or non-crop plants. This first use of meta-analysis to evaluate the effects of diversification schemes, a potentially more powerful tool than tallies of significant positive and negative outcomes (vote-counting), revealed stronger overall effects on all parameters measured compared to previous reviews. Our analysis of the same articles used in a recent review facilitates comparisons of vote-counting and meta-analysis, and shows that pronounced results of the meta-analysis are not well explained by a reduction in articles that met its stricter criteria. Rather, compared to outcome counts, effect sizes were rarely neutral (equal to zero), and a mean effect size value for mixed outcomes could be calculated. Problematic statistical properties of vote-counting were avoided with meta-analysis, thus providing a more precise test of the hypotheses. The unambiguous and encouraging results from this meta-analysis of previous research should motivate ecologists to conduct more mechanistic experiments to improve the odds of designing effective crop diversification schemes for improved pest regulation and enhanced crop yield.

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Plantulas de tomate.Foto: Álvarez-Herrera. Efecto de abonos de liberación lenta en la lixiviación de nitratos y nutrición nitrogenada en esteviaEffect of slow-release fertilizers on nitrate leaching and nitrogen nutrition in stevia Lisímetros de drenaje para la evaluación de lixiviación de nitratos en estevia. Foto: E. Aguirre RESUMENLa fertilización nitrogenada debe considerar las visiones agronómica y ambiental como el rendimiento del cultivo y menor contaminación para contribuir a la sostenibilidad de los sistemas agrícolas; los abonos de liberación lenta (ALL) integran estas dos visiones. El objetivo del estudio fue evaluar la aplicación de ALL en la lixiviación de nitratos y en características agronómicas de Stevia rebaudiana Bertoni. Se realizó un experimento completamente al azar con tres repeticiones haciendo uso de lisímetros de drenaje, evaluando dos abonos de síntesis química: nitrato de calcio y sulfato de amonio en tres dosis (100, 150 y 200 kg ha -1 de N), tres ALL: urea con inhibidores de la enzima ureasa en tres dosis (100, 150 y 200 kg ha -1 de N) y dos abonos orgánicos (lombricompost y gallinaza) en tres dosis (200, 250 y 300 kg ha -1 de N). Se evaluó el nitrógeno (N) foliar, el N lixiviado y N total del suelo a los 45 y 90 días después del trasplante. También se evaluaron altura de planta, diámetro de tallo, masa seca y rendimiento, determinando índices de eficiencia del N, en donde los abonos orgánicos obtuvieron las mayores concentraciones de N foliar y el menor volumen de N lixiviado; no se encontraron diferencias significativas en las características agronómicas de las plantas y se presentaron eficiencias en el uso de N por debajo del 50%, donde los abonos de síntesis química fueron más eficientes (40%) al suministrar el N a las plantas de forma oportuna.

Biowastes (BW), the main raw materials for the composting installations in developing countries, are characterized for containing uncooked food wastes (FW), high moisture content, low porosity, acidic pH, and low C/N ratios which affects the overall composting process (CP). In this study, we evaluated the effect of adding sugarcane bagasse (SCB) and star grass (SG) (Cynodon plectostachyus (K. Schum.) Pilg.) as bulking materials (BM) over the quality of the substrate, progress of the process, and quality of the obtained product. In this sense, two pilot-scale experiments were performed. The first one contained a substrate formed by 78% BW and 22% SCB (pile A). The second experiment contained a substrate formed by 66% BW and 34% SG (pile B). For each experiment, control treatments (piles A´ and B´ respectively) were performed by using 100% BW without BM. The results showed that in both cases the adding of BM improved substrate quality (pH, moisture, and total organic C content [TOC]), speeding up the starting step (2-3 d) and reducing the duration of the thermophilic phase of CP (3 d). However, the physico-chemical properties of both BM increased cooling and maturation phases duration (between 15 and 20 d). Obtained products quality was improved in terms of higher TOC, cation-exchange capacity, bulk density, and higher water holding capacity. Application of obtained products A and B could improve some soil properties like major nutrient, water retention, and increasing the organic matter.