The effect of drought and interspecific interactions on depth of water uptake in deep- and shallow-rooting grassland species as determined by δsup18/supO natural abundance

摘要

Increased incidence of drought, as predicted under climate change,has the potential to negatively affect grassland production. Compared tomonocultures, vertical belowground niche complementarity between shallow-and deep-rooting species may be an important mechanism resulting in higheryields and higher resistance to drought in grassland mixtures. However, verylittle is known about the belowground responses in grassland systems andincreased insight into these processes may yield important information bothto predict the effect of future climate change and better designagricultural systems to cope with this.This study assessed the effect of a 9-week experimental summer drought onthe depth of water uptake of two shallow-rooting species (Lolium perenne L. andTrifolium repens L.) and two deep-rooting species (Cichorium intybus L. and Trifolium pratense L.) in grassland monocultures andfour-species mixtures by using the natural abundance δ18Oisotope method. We tested the following three hypotheses: (1) drought results in ashift of water uptake to deeper soil layers, (2) deep-rooting species take upa higher proportion of water from deeper soil layers relative toshallow-rooting species, and (3) as a result of interspecific interactions inmixtures, the water uptake of shallow-rooting species becomes shallower whengrown together with deep-rooting species and vice versa, resulting inreduced niche overlap.The natural abundance δ18O technique provided novel insightsinto the depth of water uptake of deep- and shallow- rooting grasslandspecies and revealed large shifts in depth of water uptake in response todrought and interspecific interactions.Compared to control conditions, drought reduced the proportional wateruptake from 0–10 cm soil depth (PCWU_0–10) of L. perenne, T. repens and C. intybus in monocultures byon average 54%. In contrast, the PCWU_0–10 of T. pratense in monocultureincreased by 44%, and only when grown in mixture did the PCWU_0–10of T. pratense decrease under drought conditions. In line with hypothesis (2), inmonoculture, the PCWU_0–10 of shallow-rooting species L. perenne and T. repens was 0.53averaged over the two drought treatments, compared to 0.16 for thedeep-rooting C. intybus. Surprisingly, in monoculture, water uptake by T. pratense was shallowerthan for the shallow-rooting species (PCWU_0–10 = 0.68).Interspecific interactions in mixtures resulted in a shift in the depth ofwater uptake by the different species. As hypothesised, the shallow-rootingspecies L. perenne and T. repens tended to become shallower, and the deep-rooting T. pratense made adramatic shift to deeper soil layers (reduction in PCWU_0–10 of 58%on average) in mixture compared to monoculture. However, these shifts didnot result in a reduction in the proportional similarity of the proportionalwater uptake from different soil depth intervals (niche overlap) in mixturescompared to monocultures.There was no clear link between interspecific differences in depth of wateruptake and the reduction of biomass production under drought compared tocontrol conditions (drought resistance). Cichorium intybus, the species with water uptakefrom the deepest soil layers was one of the species most affected bydrought. Interestingly, T. pratense, which was least affected by drought, also had thegreatest plasticity in depth of water uptake. This suggests that there maybe an indirect effect of rooting depth on drought resistance, as itdetermines the potential plasticity in the depth of water uptake.