*3.2. PRS® Probes*

Overall, the addition of plant-based amendments significantly decreased the N and P supply rates and significantly increased the S supply rate (Table 3). Significant differences were observed between the sampling zones, with N, Ca, Mg, and S showing significant decreases and Fe, Mn, Cu, and Zn showing significant increases in the root zone as compared to the bulk soil.

The following sections present more detailed results for each of the aforementioned nutrients.

#### 3.2.1. Nitrogen

N supply rates differed significantly with amendments, sampling zone, time, and interaction between sampling zone and time (Table 3). The N supply rate decreased significantly in both the miscanthus- (−76%) and willow- (−75%) amended treatments as compared to the control (Figures 4 and 5). Overall, the N supply rate decreased by 90% from the beginning to the end of lettuce growth and was 34% higher in the bulk soil than in the root zone. The significant interaction between amendment and sampling zone showed that the N supply rates were significantly lower in the root zone as compared to the bulk soil in the control and miscanthus-amended soil but not in the willow-amended soil (Figure 5). Finally, the plant N uptake index, calculated as the difference between the N supply rate in the bulk soil and that in the root zone (Figure 6), revealed a clear difference between the control treatment and the amended soils. While the plant N uptake index for the control treatment increased and then decreased markedly during lettuce growth, the index showed a consistent decrease in the miscanthus treatment for the same period. In the willow treatment, where the difference between the root zone and the bulk soil was not significant, the plant N uptake index was negative at the beginning of lettuce growth and increased slowly over time during lettuce growth.

**Figure 4.** Total N supply rates measured in the bulk soil and the root zone as a function of time and amendment.

**Figure 5.** Statistical differences for total N supply rates between sampling zones and treatments. Different letters indicate significance at *p* < 0.05 using the *post hoc* Tukey HSD test.

**Figure 6.** Plant N uptake index as a function of time and amendment. The colored area represents the error envelope as plotted with the geom\_ribbon function in the ggplot2 R package.
