Ecosystem service benefits in a perennial fruit crop: how do insect pollinators, arbuscular mycorrhizal fungi and soil organic matter shape raspberry production?

Chen, Ke


Future agriculture faces a twofold challenge: increasing yields to safeguard food security while simultaneously reducing its environmental impact. Ecological intensification has been proposed as a solution to this challenge. It entails the integration of ecosystem service management into farming practices to enhance or maintain yields while minimizing artificial inputs and environmental externalities. An increasing number of empirical studies have shown beneficial effects of ecosystem services on crop production, but the majority of these studies have focused on the effects of only a single ecosystem service in isolation. In contrast, in real-world agricultural systems, crop yield is shaped by several ecosystem services simultaneously. These different ecosystem services may interact (positively or negatively) and their interactions may furthermore depend on agricultural management practices. To assess the potential of ecological intensification in promoting yields and reducing external inputs, it is essential to understand the combined effects of different ecosystem services on agricultural production. The aim of this thesis was to examine whether and to what extent different ecosystem services interactively shape crop yield, and whether and how this is affected by agricultural management. Focusing on raspberry (Rubus idaeus L.), a globally important perennial fruit crop, I examined the potential interactive effects of the ecosystem services provided by insect pollinators, arbuscular mycorrhizal fungi (AMF) and soil organic matter (SOM) on the quantity and quality of raspberry production, and whether these effects were influenced by artificial fertilizer inputs.

Natural relatives of commercial raspberry are widespread in temperate forests. These natural relatives depend entirely on several ecosystem services for survival and reproduction. Therefore, investigating the effects of ecosystem services on natural raspberry production would be informative to fully understand their importance, and could act as a benchmark for commercial raspberry production. In chapter 2, I set out to test the relationships between SOM content and AMF colonization rate on natural raspberry production. I found that under the examined natural conditions, effects of AMF colonization rate and SOM content did not interact but instead independently affected wild raspberry production. AMF colonization rate was negatively related to berry weight and yield, possibly because of the unusually hot and dry weather during the study period, which may have resulted in unbalanced cost-benefit relationships between AMF and the host plants. Conversely, SOM content was positively related to single berry weight and branch yield of wild raspberry, possibly due to the positive effects of SOM on the water holding capacity of the soils. These results suggest that under low input conditions, maintaining or enhancing SOM content can make a significant functional contribution to raspberry production.

In chapter 3, I performed a pot-field experiment to unravel the combined effects of pollinators, SOM content and fertilizer inputs on raspberry production. The experiment followed a randomized complete block design, with the experimental plants being exposed to two levels of SOM content (low vs high), two levels of insect pollination (open pollination vs pollinators excluded) and four levels of fertilizer applications. Insect pollination and fertilizer application independently increased single berry weight and yield. SOM content increased single berry weight, and also affected soluble solids content of fruits. The latter effect depended on the effects of insect pollination and fertilizer inputs though (i.e. three-way interaction), with positive effects of SOM being most pronounced at intermediate fertilizer levels and when pollinators were excluded. Total yield was not affected by SOM content. Although SOM content enhanced the visitation rate of pollinators, no interaction was found between the effects of insect pollination and SOM content on production. The positive effects of pollination and SOM content on the quality and/or quantity of raspberry production suggest that these ecosystem services can be key components in the implementation of ecological intensification in this cropping system.

In the next chapter, I explored the combined effects of insect pollinators, AMF inoculation and fertilizer inputs on raspberry production (chapter 4). A randomized block design was adopted with three crossed factors: insect pollination (open pollination vs pollinators excluded), AMF inoculation (AMF inoculated vs non-inoculated) and four levels of fertilizer applications. I found synergistic interactions between insect pollination and fertilizer inputs on raspberry fruit set and fruit number, and consequently on fruit yield. AMF inoculation significantly increased flower number, fruit number and yield, independently from insect pollination and fertilizer application. AMF inoculation furthermore had indirect effects on insect pollination through increasing pollinator visitation rate under intermediate fertilizer levels. Single berry weight was influenced interactively by AMF inoculation and fertilizer application, with a much more pronounced positive response of AMF inoculation under high fertilizer inputs. Results in this chapter indicate that the effects of insect pollination and AMF inoculation on raspberry production can be synergistic or additive to the effects of conventional management practices (i.e. fertilizer application).

In chapter 5, following the same experimental design as the previous two chapters, I conducted an experiment to explore the interactive effects of AMF inoculation, SOM content and fertilizer inputs on raspberry production. I found that only fertilizer inputs significantly increased flower number, fruit number and yield. However, AMF inoculation, SOM content and fertilizer inputs interactively influenced fruit set and single berry weight (three-way interactions). In low SOM soils, fruit set increased with fertilizer inputs for both AMF inoculated and non-inoculated plants, but in high SOM soils, the fruit set of AMF inoculated plants decreased with fertilizer inputs. In low SOM soils, the single berry weight of AMF inoculated plants increased more pronouncedly than the non-inoculated plants when the fertilizer inputs were high. In high SOM soils, the net benefits of AMF inoculation on single berry weight decreased with increasing fertilizer inputs. These results indicate that the beneficial effects of AMF and SOM can potentially be offset by each other, particularly under high fertilizer input levels.

The results of these studies show that, although some yield relevant parameters depended on complex three-way or non-linear two-way interactions between ecosystem services (provided by insect pollinators, AMF and SOM) and fertilizer application, effects of ecosystem services on raspberry yield are mainly additive, and generally complement the effects of agricultural inputs. Results of this thesis also demonstrated the individual benefits of the examined ecosystem services. Comprehensively, insect pollination should be the most important ecosystem service for raspberry production to be maintained in this system, followed by AMF inoculation and then the SOM content. Additionally, this thesis highlighted the importance of fertilizer inputs in maximizing production, as in the low inputs of fertilizer, the benefits of ecosystem services disappear. A general conclusion would be that insect pollination, AMF inoculation and SOM content have potential to be managed as components of ecological intensification, and fertilizer inputs are essential to sustain their benefits. However, before implementing management to enhance these ecosystem services in real-world farming systems, more studies are needed to test their effects on other crops in the broader context, to explore the underlying mechanisms as well as to evaluate the net economic benefits.