Abstract
While a severe decrease in phosphorus (P) availability is already taking place in a large number of ecosystems, drought and nitrogen (N) deposition will likely further decrease the availability of P under global change. Plants have developed physiological strategies to cope with decreasing P resources, but it is unclear how these strategies respond to elevated N deposition and summer droughts. We investigated the influence of N and P availability and soil drought on P uptake (H333PO4 feeding experiment) and use efficiencies in young Quercus calliprinos Webb. trees. We hypothesized that (H1) the expected increases in soil N:P ratios will increase the efficiencies of P uptake and use of oak saplings but will decrease the efficiencies of N uptake and use, whereas (H2) drought will affect P uptake efficiency more than N uptake efficiency. In confirmation of (H1) we found that a sharp increase of the soil N:P ratio from 4 to 42 g g-1 significantly increased the instantaneous 33P uptake efficiency (33PUptakeE) by five-fold and long-term P uptake efficiency (PUptakeE) by six-fold, while it decreased N uptake efficiency (NUptakeE) and N use efficiency (NUE). In contradiction to (H1), P use efficiency (PUE) did not respond to the simulated extended gradient of soil N:P ratios but remained relatively constant. (H2) was only partially confirmed as soil drought reduced PUptakeE by up to a fourth at high soil N:P ratios but had no significant effect on NUptakeE. As a consequence, increasing summer droughts may decrease the response of PUptakeE to increasing P limitation, which – in the absence of adjustments of the efficiency of P use – can aggravate growth reductions in this eastern Mediterranean tree species under global change.
Highlights
Increasing N deposition and summer droughts under global change will induce nutritional imbalances and large-scale undersupply of essential plant nutrients such as P in a large number of natural forest ecosystems (Talkner et al, 2015; Sardans et al, 2016), but especially in Eastern Mediterranean forests where extreme climate conditions are already common and the effect of global change will be disproportional (Lelieveld et al, 2012)
We analyzed the effects of the soil N:P ratio and soil moisture on the analyzed parameters in two-way analyses of variance (ANOVA), where both effects were treated as fixed effect
Elevated N deposition and summer droughts in the course of global change are assumed to deteriorate the availability of P for important forest trees, but it remains unclear to what extent plants may adapt their uptake and use efficiencies for P and N under these conditions
Summary
Increasing N deposition and summer droughts under global change will induce nutritional imbalances and large-scale undersupply of essential plant nutrients such as P in a large number of natural forest ecosystems (Talkner et al, 2015; Sardans et al, 2016), but especially in Eastern Mediterranean forests where extreme climate conditions are already common and the effect of global change will be disproportional (Lelieveld et al, 2012). Southern Europe and elsewhere (Elser et al, 2007; Peñuelas et al, 2012) This trend of increasing P deficiency of forest trees is even more pronounced in regions where P availability had already been low before (Jonard et al, 2015): Mediterranean ecosystems are commonly characterized by geological substrates poor in mineral P content and by accumulation of occluded P with longer soil development (Singer, 2007; Cramer and Hoffman, 2015) and, may shift from latent to apparent P deficiency under global change. Plants can respond to P deficiency by increasing their P uptake and use efficiencies. In contrast to PUptakeE, increases in P use efficiency (PUE) are mostly achieved by internal reallocation of plant P to photosynthetic and growth functions (Lambers et al, 2012; Veneklaas et al, 2012)
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