The trees S. superba and P. massoniana are both well known for their ability to grow and thrive under adverse environmental conditions, especially low-P soil. Research has shown that S. superba forms a fibrous root system and P. massoniana forms a tap root system [27–28]. Our previous study demonstrated that P. massoniana was more suited to low-P soil than S. superba [28]. In the present study, we found the root phenotypic and physiological characteristics of P. massoniana had significantly lower values than those of S. superba; however, the PAE and NAE of P. massoniana significantly exceeded that of S. superba (Tables 2 and 3). This may be due to the ectomycorrhizal fungi and molecular mechanism of higher P efficiency in vivo of P. massoniana [−]. Work by Zhang et al. revealed four members of the Pht1 phosphate transporter protein family encoding phosphate transporters in Masson pine (PmPTs), whose up-regulated genes could improve P nutrition [31]. By contrast, S. superba survives in barren soil by increasing its root quantity and exudates to promote both its PAE and NAE [11].
Nitrogen addition led to the coarsening of roots and a greater root surface area (Table 2). For example, compared with N0, the average AD and SA increased 4.4% and 19.6% in S. superba and 5.4% and 58.2% in P. massoniana, respectively, when N was added. The thickening and increasing surface area of roots was more pronounced in P. massoniana, driven by roots with a diameter over 1.0 mm (Fig. 3). With an increasing N level, the D = 0–0.5 mm roots grew fast in P. massoniana, and their N absorption was correspondingly higher (growth ratio of PAE, GRPP. massoniana = 80.8%, GRNS. superba = 101.8%; growth ratio of NAE, GRNP. massoniana = 71.1%, GRNS. superba = 50.7%) when compared with N0; however, the D > 0.5 mm roots grew faster in S. superba and their P absorption was higher. This pattern in the results indicated that N addition promotes fine root growth and their absorption of elements depends on tree species identity. Although the rate of root growth was not faster in S. superba than in P. massoniana, the former’s nutrient absorption was increased and the N addition enhanced its P utilization [−]. The fine root fraction differed between the two species, in that for the 0–0.5 mm root, the proportion was 74.9–80.9%, with an average of 77.9% in S. superba, whereas it was 68.6–83.7%, with an average of 73.4% in P. massoniana (Table 2, Fig. 3). By contrast, for the 0–1.5 mm range roots, their proportions were stable overall (S. superba: 96.4–98.7%, 97.6%; P. massoniana: 96.6–99.3%, 97.8%). This was mainly affected by different N forms and we describe this situation using a root growth model for the two tree species under low-P soil conditions when N is added (Fig. 4). Our results suggested NH4+ could affect the relationship between the > 1.5 mm roots and nutrient absorption in S. superba, whereas the relationship between > 0.5 mm roots and nutrient absorption was affected by NH4+ in P. massoniana, whose height growth aboveground was much more apparent (Table 4, Fig. 4). Compared with NH4+, we find that NO3− had an opposite effect on the relationship between fine roots and absorption, and the stem diameter growth aboveground was more notable [28]. We speculate that NH4+ could promote thicker root activity and that NO3− may have stimulated the thinner root activity in the fibrous root plants with a larger quantity of roots, whereas NO3− and NH4+ have the opposite function in a tap root system with slender roots.
Table 4
Pearson correlation coefficients between the fine root ratio and nutrient absorption in Schima superba and Pinus massoniana.
| | 0.0–0.5 mm | 0.5–1.0 mm | 1.0–1.5 mm | > 1.5 mm | < 1.5 mm |
S. superba | | | | | | |
NH4+ | PAE | –0.379 | 0.004 | 0.138 | 0.702 | –0.811* |
| NAE | –0.373 | –0.003 | 0.139 | 0.699 | –0.810* |
NO3− | PAE | –0.775 | 0.601 | 0.670 | 0.810* | –0.824* |
| NAE | –0.703 | 0.497 | 0.613 | 0.847* | –0.868* |
P. massoniana | | | | | | |
NH4+ | PAE | –0.703 | 0.620 | 0.574 | 0.854* | –0.875* |
| NAE | –0.671 | 0.574 | 0.581 | 0.866* | –0.866* |
NO3− | PAE | –0.369 | 0.060 | 0.324 | 0.845* | –0.741 |
| NAE | –0.372 | 0.056 | 0.333 | 0.856* | –0.754 |
There was a negative relationship between the growth of roots 0–0.5 mm in diameter and both PAE and NAE after the N addition in both species. The roots consume over 50% carbon fixation on root consumption, including new root growth, ion uptake and transport, and maintenance processes, for which the maintenance of respiration is linearly related to root fresh weight []. When S. superba is growing in its preferred P and N environment, it decreases its proportion of 0–0.5 mm roots in the root system and remains in a low−consumption mode [8,11,34−]. For P. massoniana, this negative relationship was likely caused by abiotic stress and the death of the 0–0.5 mm roots (Table 2).
The root-secreted acid phosphatase and other organic acids may promote P absorption, and they may also influence the roots’ physiological activity. Since oxalic acid is the most abundant organic acid in the two tree species, we chose to study this acid herein. N addition increased the acid phosphatase and oxalic acid secretion in P. massoniana, and this may be an important reason for why this plant’s PAE was enhanced. The S. superba plants use oxalic acid for main auxiliary absorption (data not published), and only the highest N level addition was able to augment its secretion (Table 3). This result suggested that S. superba was mainly through the active absorption combined with root morphological changes.