Plant uptake of N released from animal manure gradually increases with progressing regrowth of perennial grasses [
3,
18,
23]. In the present study, at the final regrowth (56 d), we found a significant increase in herbage N content in the HQ+DCD plot, and NdfSU in herbage of all plots applied with the inhibitors (
Figure 1B). However, the soil total N content was not affected by the inhibitors throughout the experimental period (
Figure 2A). This indicates that enhanced N uptake and herbage growth in the HQ and/or DCD applied plots are due to inorganic N released from organic N rather than the N pool size in soil [
3,
23]. The NdfSU in the soil total N gradually decreased from 134.6 (at 7 d) to 89.2 kg N/ha (at 56 d) (based on average values of 4 treatments), corresponding to a decrease of
15N recovery in soil from 67.0% to 44.8% (
Figure 2). This implies that N released from the applied urea in pig slurry dilutes the soil inorganic N pool, which is available for herbage regrowth. However, the NdfSU in herbage was not significantly affected by HQ and/or DCD application during the first 14 d of regrowth, although the amount of N derived from the pig slurry-urea in the soil NH
4+ (NdfSU-NH
4+) or NO
3− fractions (NdfSU-NO
3−) decreased in the HQ and/or DCD treatments from 7 d (
Figure 3). This may reflect a common N utilization pattern during the early regrowth characterized by low exogenous N uptake because shoot regrowth during this period depends on a large portion of endogenous N rather than exogenous N uptake [
21]. In addition, during the first 7 d of regrowth, urea
15N in pig slurry was mineralized mainly to NH
4+-N, which accounted for 63.6% to 88.6% of total NdfSU in the soil mineral N (sum of NdfSU-NH
4+ and NdfSU-NO
3−) (
Figure 3). The NdfSU-NH
4+ was lower in the plots applied with the inhibitors, especially in the presence of HQ (e.g., HQ and HQ+ DCD treatments) during the first 14 d, suggesting that HQ delayed the hydrolysis of urea in pig slurry [
7]. The NdfSU-NH
4+ in soil slowed down with progressing regrowth with an opposite increase in the NdfSU-NO
3− (
Figure 3B, D), reflecting nitrification of the NH
4+ released from pig slurry-urea. The NdfSU-NO
3− in the soil at 56 d of regrowth was significantly higher in the presence of the inhibitors, especially in the presence of DCD (e.g., DCD and HQ+DCD treatments), compared with the control (
Figure 3). At the final regrowth (56 d), the N content converted to soil inorganic N from pig slurry-urea (NdfSU-NH
4+ + NdfSU-NO
3−) was higher in the presence of DCD (70.4 to 77.3 kg N/ha) compared to that of control (60.0 kg N/ha) (
Figure 3). Retention of higher NdfSU-NH
4+ and NdfSU-NO
3− in the soils amended the inhibitors may reflect the active onset of hydrolysis of urea and subsequent nitrification during the latter regrowth period when the uptake of exogenous N strongly occurs as a primary N source for the herbage regrowth [
21]. Thus, enhanced final regrowth yield (
Figure 1A) and higher NdfSU in herbage at 56 d (
Figure 1C) in the HQ and/or DCD plots are certainly attributed to the higher availability of N released from pig slurry, as evidenced by higher percentages of urea
15N recovered in the soil inorganic N, i.e., 38.6%, 33.6%, and 31.5% of the
15N applied in the DCD, HQ, and HQ+DCD plots, respectively, compared with the control (22.4%). Many studies have shown positive effects of urease and/or nitrification inhibitors on plant nutrient availability in soil, enhancing yields of annual crops [
24,
25] and herbage in perennial grasslands [
26].