Interactive effects of soil water content and nutrients on root exudation in two Mediterranean tree species

Abstract

Root exudation is affected by the soil water content (SWC) and availability of nutrients, but the impact of the interaction between these abiotic factors is unclear. Understanding the combined effect of these factors is vital for all ecosystems, including Mediterranean forests, where droughts are increasing. A greenhouse experiment was established with a drought treatment (control versus drought) and the subsequent recovery, factorially combined with four nutrient treatments (control, nitrogen (N) addition, phosphorus (P) addition, and both N and P addition). N was added as ammonium nitrate at a rate of 50 kg N ha−1 and P was added as calcium phosphate at a rate of 25 kg P ha−1. Exudation of total organic carbon (TOC) was measured from the roots of 64 saplings of each of two important tree species that coexist in montane Mediterranean regions, Pinus sylvestris (Scots pine) and Quercus ilex (holm oak). The amount of TOC released from individual roots of the saplings was measured to estimate root exudation per gram of root mass. Chlorophyll fluorescence (Fv/Fm) was also monitored to check for treatment effects on leaf performance. Chlorophyll fluorescence was not affected by SWC or nutrient addition in either species, but root exudation was, and varied between the two species. SWC was positively correlated with the amount of TOC released during the drought period for Q. ilex but had no effect on P. sylvestris. The addition of P increased root exudation in Q. ilex. The amount of TOC released in the recovery period for both species with no added nutrients was weakly negatively correlated with SWC during the drought period, potentially indicating a delayed positive effect of the earlier water stress. During recovery, SWC positively affected the amount of TOC collected from the roots when P was added for both species and when N was added for P. sylvestris. Our results indicate that the impacts of drought on root exudation will likely vary between species and will depend on the availability of soil nutrients.