Soil fungi benefit both plants and biocrusts in arid lands: a test of the fungal loop hypothesis
Species interactions may couple the resource dynamics of primary producers that are disconnected in space and time. Arid ecosystems are characterized by low-density plant communities and soil surfaces colonized by biological crusts. Metabolic activity in biocrusts is rapidly stimulated by rainfall events of many sizes, but plants require precipitation events large enough to infiltrate root systems. Many biocrusts fix nitrogen and intercept atmospheric N, but this bioavailable N may be inaccessible to plants. The fungal loop hypothesis proposes that fungi take up and transport resources between plants and biocrusts, increasing production. However, this mechanism has not been tested experimentally.
We studied how fungi affect the interactions between plants and biocrusts. We transplanted bunchgrasses and biocrusts into pots in the field then manipulated the connections between biocrusts and roots and/or soil fungi using root- or fungal-excluding mesh placed horizontally below biocrusts. We compared the performance of each producer grown alone and with the other producer. To test for context dependency on the precipitation regime, pots were spray-irrigated in a single monthly pulse or in small weekly events. We hypothesized that when fungal connections were intact, plants and biocrusts would exchange resources resulting in higher production for both, even in poor conditions.
When plants and biocrusts established together, fungal connections improved productivity after one year. With fungal connections intact, plant biomass increased 23% more (P=0.08) and biocrust chlorophyll content increased 138% more (P = 0.04) relative to pots without biotic connections. Unexpectedly, the producers may compete for light or soil resources because each had significantly higher performance when growing alone than with the other producer. Fungal connections had stronger effects when the other producer was present (23%, 138%), than when either was grown alone (7% increase in plant and 44% in crust). These results support the fungal loop hypothesis because the presence of fungal connections enhanced the performance of both producers, and soil fungi were more beneficial when producers were grown together than when alone.
Water regime did not affect the interactions between plants, fungi, and biocrusts. Both producers performed best under large infrequent waterings. As climate change alters precipitation patterns and human activity disturbs arid ecosystems, understanding the interactions between primary producers and belowground biota will allow better predictions of shifts in productivity in arid lands.