Abstract
Summary Root exudates play an important role in ecosystem response to climate change, but the functional consequences of drought‐induced changes in the quality of root exudates are unknown. Here, we addressed this knowledge gap in a unique experimental approach.We subjected two common grassland species that differ widely in their growth strategies and root systems, the grass Holcus lanatus and the forb Rumex acetosa, to 2 wk of drought. We collected root exudates and soils at the end of the drought and after 2 wk of recovery and readded all root exudates to all soils in a fully reciprocal set‐up to measure root‐exudate‐induced respiration.We found that soil treatment was unimportant for determining root‐exudate‐induced respiration. By contrast, root exudates collected from plants that had experienced drought clearly triggered more soil respiration than exudates from undroughted plants. Importantly, this increased respiration compensated for the lower rates of root exudation in droughted plants.Our findings reveal a novel mechanism through which drought can continue to affect ecosystem carbon cycling, and a potential plant strategy to facilitate regrowth through stimulating microbial activity. These findings have important implications for understanding plant and ecosystem response to drought.
Highlights
Ecologists increasingly recognize the role of plant root exudates in ecosystem functioning
For drought pots, watering was ceased until a soil moisture content of 20% of the water-holding capacity (WHC) was reached, after which the pots were kept at that weight until the end of the 2 wk drought period, whereas control pots were watered every 2–3 d to a moisture content of 60% WHC (Supporting Information Fig. S1)
Drought reduced the aboveground biomass of Holcus more than that of Rumex, and the reduction in root biomass of Rumex was stronger at the 2 wk recovery harvest than at the end-of-drought harvest
Summary
Ecologists increasingly recognize the role of plant root exudates in ecosystem functioning. Root exudates – complex, soluble mixtures of carbon (C) compounds, such as sugars, amino acids, organic acids, and secondary metabolites – can comprise up to 10% of plant photosynthate and are a highly dynamic source of C input into the soil (Jones et al, 2004, 2009). Despite early acknowledgement of the role of root exudates in, for example, phosphorus (P) solubilization (Hoffland et al, 1989) and the priming of soil organic matter decomposition (Kuzyakov et al, 1999), root exudation has long been considered to be a passive process (Jones et al, 2009). From an evolutionary perspective, it is hard to explain why plants would leak substantial amounts of a key resource into the soil. It has been hypothesized that plants and heterotrophic soil microbes have coevolved and that root exudates are one of the main pathways of providing C for microbial growth and, play a vital role in plant–microbe communication (Mendes et al, 2013; Mommer et al, 2016)
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