Crop fertilization affects pollination service provision - Common bean as a case study

Abstract

The demand for insect-pollinated crops is increasing. Conventional agricultural intensification heavily relies on increased input of fertilizers, which can have negative effects on local biodiversity. Such effects may be particularly accentuated in biodiversity hotspots that are naturally nutrient-poor. Ecological intensification of farming, i.e. practices that increase production through the increase of ecosystem services, emerges as an alternative to conventional intensification. For example, practices that boost abundance and diversity of crop pollinators can lead to substantial increases in cropland productivity. However, little is known about the synergisms and trade-offs between fertilizer input and such ecological intensification practices. Here we investigate interactive effects between fertilization practices and the provision of ecosystem services in a biodiversity hotspot where conventional agriculture is rapidly expanding (Brazilian savannas). We focus on a highly nitrogen-demanding crop species that benefits from pollinators (the common bean, Phaseolus vulgaris L.), for which nitrogen input greatly varies in the study region. Our findings show that positive effects of native pollinators on crop yield are most accentuated under low inputs of nitrogen (e.g. equal to or below 72kg ha-1). This interactive effect could be due to changes in flower visitor community composition or behaviour. Our study also suggests that landscape management practices that minimize isolation from patches of natural vegetation and maximize its cover nearby (within 500 meters) of production areas can increase pollinator and biocontrol agent abundance and richness. Overall, these results suggest that ecological intensification is a valuable alternative for common bean production in Brazil, and potentially other regions of the world. Land productivity can be enhanced if an adequate balance of chemical inputs and landscape management is achieved.

Fig 1. Effect of the density of exotic pollinators (i.e. Apis mellifera) on common bean productivity.

Points represent partial residuals (i.e. variability not explained by the other variables included in the model). Model estimates are based on Model 2 from Table 1. Shaded area represents 95% confidence interval.

Fig 2. Estimated effect of density of native pollinators on common bean productivity under different levels of nitrogen (N) input.

(N) Nitrogen application (varied from 36.0 and 130.5 kg ha-1 between fields). Model estimates are based on Model 4 from Table 1. Shaded area represents 95% confidence interval.

Fig 3. Effects of local management practices (nitrogen input and maintenance of native vegetation within agricultural landscape) on pollinators, biocontrol agents and species richness.

The points represent partial residual. The graphs are based on the variables selected on the best models (ΔAICc < 2), having used always the best model that includes most environmental variables (see Table G in S1 File). For the analyses of abundance of native pollinators the vegetation cover scale used was 2000 meters. For the remaining variables, the vegetation cover scale used was 500 meters (see details of vegetation cover scale selection in Table D in S1 File). Shaded area represents 95% confidence interval.