Scanning electron microscopy (SEM) of FTHSB and their capacity to adsorb different forms of N
The structural material of FTHSB was analyzed by SEM-electron microscopy to determine its surface morphology and elemental composition (Fig. 2a-t). The morphology was flocculent, homogeneous and microporous, which was more pronounced at 2 µm and 1 µm. Fluorescent labeling results showed that N, P, and K elements were uniformly distributed on FTHSB. FTHSB exhibited significant differences in N content after drip application of different forms of N treatments, showing TU > TN > TA, TU was significantly increased by 63.66% and 16.27% over TA and TN, respectively(P < 0.05).
The mass of dry matter in grape seedlings
Application of FTHSB significantly increases the mass of dry matter in the leaves, new sprouts and roots of grape seedlings (P < 0.05) (Fig. 3). In the treatments of TA, TN and TU, the mass of dry matter in leaves is significantly increased by 31.92%, 31.95% and 47.40% respectively compared with that in leaves in the treatments of CKA, CKN and CKU. In addition, the mass of dry matter in new vine shoots is significantly increased by 34.70%, 31.99% and 41.81% respectively and the mass of dry matter in roots is significantly increased by 47.94%, 34.56% and 64.51% respectively. In the treatment of CK, the mass of dry matter in the leaves, new vine shoots and roots of grape seedlings shows the sequence of CKN > CKU > CKA. The mass of dry matter in the new vine shoots and stems of grape seedlings under the condition with FTHSB show the sequence of TN > TU > TA. However, the mass of dry matter in roots show the sequence of TU > TN > TA. The mass of dry matter in the seedlings under TU is significantly increased by 20.76% and 21.91% respectively compared with that under TA and TN (P < 0.05).
Fine root morphology of grape seedlings
Total length, total surface area, total root volume and total number of root tips of fine roots in grape seedlings show a trend of significant increase in the soil layer of 15–30 cm after the application of FTHSB(Fig. 4). The application of FTHSB has the most significant effect on the total length and total area of fine roots (P < 0.05). Total length and total surface area of fine roots in the soil layer of 15–30 cm under the treatment of TA, TN and TU are significantly increased by 16.34%, 20.15%, 7.47%, 20.01% and 38.03%, 34.45% respectively than those under the treatment of CKA, CKN and CKU (P < 0.05). Total length, total surface area, total volume and total tips of fine roots in the soil layer of 15–30 cm under the treatment of TU are significantly higher than those under the treatment of TA and TN (P < 0.05). In comparison with the treatment of TA and TN, the treatment of TU has the most significant effect on promoting the total surface area and total volume of fine roots. Total length of fine roots and total volume of fine roots in the soil layer of 15–30 cm under the treatment of TU are significantly increased by 24.75%, 22.02% and 12.53%, 11.53% respectively compared with those under the treatments of TA and TN (P < 0.05).
Ndff value and 15N distribution rate in each organ of grape seedlings
The application of FTHSB has significantly increased the Ndff value of grape seedling leaves and stems (Fig. 5a) (P < 0.05). Ndff value of the leaves in the treatment of TA and TN is significantly increased by 51.65% and 51.70% respectively compared with that of the leaves in the treatment of CKA and CKN; Ndff value of stems is significantly increased by 86.38% and 66.87% respectively (P < 0.05). Different forms of N have a significant effect on the Ndff value for different organs of grape seedlings. In the treatment of TA, the Ndff value for each organ shows the sequence of stem > root > leaf. In the treatment of TU, it shows the sequence of leaf > root > stem. In the treatment of TU, the Ndff value of leaf is significantly increased by 48.12% and 64.25% compared with that of root and stem (P < 0.05).
The application of FTHSB has significantly improved the distribution rate of 15N in roots and leaves (P < 0.05). The distribution rate of 15N in roots reaches up to 47.81% in the treatment of TA but is only 38.81% in the treatment of CKA (Fig. 5b). In all the treatments, the distribution of 15N in different organs of grape seedlings shows the sequence of leaf > root > stem. Moreover, the application of N in different forms has a significant effect on the 15N distribution in different organs of grape seedlings. Without such brick, the distribution rate of 15N in leaves reaches the maximum in the treatment of CKA. However, with the application of such brick, the distribution rate of 15N in leaves reaches the maximum in the treatment of TU, 17.42% and 20.69% higher than that in the treatment of TA and TN.
15N use efficiency of grape seedlings
As can be seen from Fig. 6, the NUE of seedlings varies greatly in different treatments (P < 0.05). The application of FTHSB has significantly increased the 15N use efficiency of grape seedlings (P < 0.05). The 15N use efficiency in the treatment of TA, TN and TU is significantly increased by 23.31%, 21.56% and 42.18% (P < 0.05) compared with that in the treatment of CKA, CKN and CKU. Without such brick, the effect of N in different forms on 15N use efficiency of grape seedlings is not significant (P > 0.05). With the application of such brick, the 15N use efficiency of grape seedlings in the treatment of TU is significantly increased by 15.61% and 10.33% compared with that in the treatment of TA and TN (P < 0.05).
N migration and accumulation in soil
Ndff value of the soil layer of 15–30 cm and 30–45 cm with such brick is significantly increased in comparison with that without such brick (P < 0.05) (Fig. 7a). In the soil layer of 30–45 cm, Ndff value in the treatment of TA, TN and TU is significantly increased by 13.69%, 14.48% and 30.52% in comparison with that in the treatment of CKA, CKN and CKU (P < 0.05). Without such brick, all three forms of N have the largest Ndff value for the soil layer of 15–30 cm and 30–45 cm in the treatment CKN. However, with the application of such brick, Ndff value for the soil layer of 15–30 cm in the treatment of TU is significantly increased by 35.15% and 17.01% in comparison with that in the treatment of TA and TN (P < 0.05).
Without such brick, 15N content in the treatments by three different forms of N shows the trend of increasing gradually with the increase of soil depth and reaches the maximum at the soil depth of 45 cm (Fig. 7b). With the application of such brick, 15N content in the soil treated by three different forms of N shows the trend of increasing first and then decreasing with the increase of soil depth. In the soil layer of 15–30 cm, 15N content in the treatment of TA, TN and TU is significantly increased by 61.30%, 69.23% and 66.08% in comparison with that in the treatment of CKA, CKN and CKU (P < 0.05). Without such brick, 15N content in the soil layer of 30 cm treated by different forms of N shows the sequence of CKA > CKU > CKN. With the application of such brick, 15N content in the soil layer of 15–30 cm treated by different forms of N shows the sequence of TU > TN > TA.
Enzymes related to N metabolism
GS and NR are essential enzymes in the process of plant N metabolism. Application of FTHSB has significantly increased the GS activity of grape seedling leaves by 89.3%, 90.7% and 108.9% compared with that in the treatment of CKA, CKN and CKU (P < 0.05) (Fig. 8a). The application of such brick has the most significant promotive effect on the GS activity in the treatment of TU and the GS activity is significantly increased by 47.75% and 46.47% (P < 0.05) compared with that in the treatment of TA and TN. Application of FTHSB has significantly increased the NR activity of grape seedling leaves by 43.84%, 61.11% and 144.94% compared with that in the treatment of CKA, CKN and CKU (P < 0.05) (Fig. 8b).
As shown in Fig. 8c, with the application of such brick and the treatment of three different forms of N, the urease activity in the soil layer of 15–30 cm is significantly increased (P < 0.05). In detail, the urease activity in the soil layer of 15–30 cm in the treatment of TA, TN and TU is significantly increased by 63.98%, 27.08% and 65.69% compared with that in the treatment of CKA, CKN and CKU (P < 0.05). Urease activity reaches the maximum in the soil layer of 0–15 cm and 30–45 cm under CKU of CK series treatments. Urease activity reaches the maximum in the soil layer of 0–15 cm and 15–30 cm under TU of T series treatments.
Relations between N utilization, N partitioning rate, root morphology of plant fine roots, soil N content and N utilization related enzymes
As shown in Fig. 9, 15N use efficiency of plants shows a significant positive correlation with total root length and moisture content in the soil layer of 30–45 cm (P < 0.05), shows a positive correlation with 15N distribution rate in leaves and stems, 15N content in the soil layer of 0–15 cm and 15–30 cm, root surface area and root volume and shows a negative correlation with the 15N content in the soil layer of 0–15 cm and 30–45 cm. The above correlations indicate that increased 15N content in the soil layer of 15–30 cm has enlarged plant root surface area and root volume and improved 15N distribution rate of leaves, thus promoting the 15N use efficiency of plants. The increased 15N content in the soil layer of 30–45 cm accelerated the leaching loss of 15N and the 15N use efficiency is reduced accordingly.