Impact of arbuscular mycorrhizal fungi on nutrient cycling in agroecosystems

Abstract

The intensification of agricultural production to meet global food demands has led to excessive nutrient leaching from agricultural areas. These losses have negative environmental impacts and pose a waste of valuable fertilizer. Soil biota are essential for nutrient cycling in soil and thus could be considered as a management tool to reduce nutrient input and losses. Arbuscular mycorrhizal fungi (AMF) are a group of soil fungi that form symbiotic associations with the majority of land plants. The fungus forages efficiently for nutrients that are delivered to their host plants in exchange for carbon. AMF have recently been reported to reduce nutrient leaching losses from soil. Because few experiments have been conducted on this aspect of the mycorrhizal symbiosis, many questions remain unanswered. For this reason, we performed several greenhouse experiments with grassland microcosms with sterile or unsterile substrate comparing mycorrhizal and non-mycorrhizal treatments. After a growth period, the microcosms were fertilized and exposed to artificial rain. The resulting leachate was collected and analyzed. Several questions concerning AMF symbiosis and nutrient leaching were addressed: Do AMF species differ in their effect on nutrient leaching? Can enhanced AMF abundance in natural field soil reduce nutrient leaching and is the effect dependent on soil type? How does the sand content affect the mycorrhizal effect on nutrient leaching? Do AMF communities shaped by different soil management practices differ in their ecosystem services like reduction of leaching? This thesis demonstrates that AMF can have both positive and negative effects on nitrate losses. While the reduction of nitrate leaching by AMF was partly strong, a critical assessment of the ecological relevance under natural conditions is still necessary: no effects were observed with different unsterile field soils. Nevertheless, these results provide further evidence for a role of AMF in N-cycling. Our results on P and ammonium leaching do not suggest any ecological relevance of AMF for reducing leaching losses in agriculture. Absolute leaching amounts of P where relatively small and in most cases not significant. Therefore, the impact of these soil fungi for P leaching should be assessed more critically. Mycorrhizal effects on leaching losses were dependent on soil type and sand content showing that conclusions should be made within the context of experimental factors. Furthermore, we observed that AMF species identity or AMF community structure (e.g. determined by different soil management practices) will influence the amount of nutrients leached. In terms of biomass production, host plants can benefit from the mycorrhizal symbiosis through enhanced competitive ability and increased P uptake under a variety of growth conditions. These effects are dependent on host plant identity, AMF identity, soil type and nutrient availability. Farmers can capitalize on these benefits by adding a potent inoculum or supporting the inherent mycorrhizal community via management practices. We have shown that AMF communities shaped by no-till systems are more efficient in nutrient uptake than fungi from tilled fields suggesting that targeted farming practices can be used to harness the power of beneficial soil biota to reduce fertilizer input and increase sustainability.