Identity of Nutrient Added (N or P) and Timing of Application Affects Root System Architecture Responses of Hordeum vulgare

Although different plant foraging responses to the two elements nitrogen (N) and phosphorus (P) are well researched, the effect of the timing of fertilizer on root system architecture (RSA) remains largely unknown. With the goal of understanding whether RSA responds significantly to timing of N and P addition, we set up a rhizobox experiment with spring barley (Hordeum vulgare cv. Barke) growing over 38 days in poor soil. All plants were watered with nutrient solution (Hoagland), lacking either N or P, with the total amount of the absent element added once after either 2, 3, or 4 weeks. Controls were either N and P applied continuously or a false control receiving both N and P only after 3 weeks. We tracked root growth over time by taking high resolution photos and measured plant biomass. Late addition of N and P both had significant effects on above-and belowground biomass and root length, with late N resulting in lowered biomass and less total root length than the control or in late P levels. We found no significant effects within N application times, but a significant decrease of plant biomass when P was added after 3 or 4 weeks compared to 2 weeks. Interestingly the root:shoot ratio remained the same over all treatments, but late N had relatively higher root biomass in lower layers. The visible root length (VRL) correlated well with the total root length of a subsample (r=0.95, P<0.001). When plant still lacked N after three weeks, we detected a foraging response (increase in total VRL) followed by an alignment of the different application timings after the fifth week. Overall, although the adaptation of RSA to N and P application in our study is very dissimilar, we did not observe fast foraging responses to timing of either N or P application.