C, N and P concentrations and C:N:P ratios in leaf and litter layers
The green leaf nutrient concentrations and their stoichiometric ratios differed among seasons. A seasonal research method was used to study the nutrient contents of the leaves and litter of cypress and Chinese toon, and four gap sizes were investigated in the same year. Due to the characteristics of deciduous trees, green leaves were not collected in winter, and litter was not collected in April and July for the four different gap sizes with Chinese toon. In the same season, the leaf C and litter C of cypress were significantly higher than those of the mixed gaps of Chinese toon (P < 0.05, Fig. 1a, b). There were no significant differences in leaf C among the different sizes of Chinese toon mixed gaps, and the only significant difference in litter C occurred in December, with a significantly lower value in the 50 m2 gap than in the 200 m2 gap (P < 0.05). The leaf C and litter C of cypress showed the same seasonal changes: spring > autumn > winter > summer. The leaf C of Chinese toon did not show a significant change with the seasons, and the winter litter C was lower than the autumn litter C. The seasonal variation in N and P in the leaf and litter of cypress and Chinese toon was as follows: spring < summer < autumn < winter.
The C:N and C:P of cypress leaves were significantly higher than those of Chinese toon leaves (Fig. 1g,i). There was no significant difference among the different Chinese toon mixed gap areas. The leaf C:N was higher for the 50 m2 gap than for the 200 m2 gap in spring and summer, and the leaf C:P was higher for the 50 m2 gap than for the 200 m2 gap in spring. The N:P of the cypress leaves was significantly lower than that of the Chinese toon mixed gap leaves in spring and summer, but the N:P of cypress leaves was significantly higher than that of Chinese toon leaves in autumn and winter (P < 0.05, Fig. 1k). The C:N:P stoichiometric ratio of leaves and litter varied with the seasons as follows: spring < summer < autumn < winter.
The leaf C: P in different seasons was ordered from high to low as follows: winter, autumn, summer, spring (Table 4). The N:P in autumn and winter increased from 10:1 to 12:1 in spring and summer, respectively. C:P decreased with increasing gap size in the mixed gaps with Chinese toon, and the C:P was the lowest in the 200 m2 gap in the three seasons. The N:P ratio was the highest in summer (14:1), the N:P in the 150 m2 and 200 m2 gaps was higher than that in the 50 m2 and 100 m2 in autumn, and the N:P of the 200 m2 gap in spring (11:1) was lower than that in other seasons.
The CRE in the pure cypress forest was the lowest, and the CRE of the four sizes of gaps with Chinese toon was significantly higher than that of the pure cypress forest, and the CRE of the 50 m2 gap was the largest (Table 2). The NRE and PRE of the cypress forest were also significantly lower than those of the mixed gaps with Chinese toon, and there were no significant differences in these values among different gap sizes (Table 2).
Patterns of C, N, and P concentrations and ratios in the soil
The 0–5 cm soil C of the cypress forest was significantly lower than that of the mixed gaps with Chinese toon, and the 0–5 cm soil C of the 200 m2 gap was significantly higher than that of smaller gap sizes (Fig. 2a). The above rules were consistent in all four seasons. The 5–20 cm soil C values of the cypress forest and the 50 cm2 gap were the same, and the 5–20 cm soil C of 150 cm2 and 200 cm2 gaps were still significantly higher than those of the smaller gap sizes (Fig. 2b). With the deepening of the soil layer, the differences among the soil C values of different gap sizes gradually disappeared, and the difference in soil C between the cypress forest and the mixed gaps of different sizes also disappeared (Fig. 2c). Furthermore, with increasing soil layer depth, the soil C showed a decreasing trend.
The 0–5 cm soil N of the cypress forest was significantly higher than that of the mixed gaps with Chinese toon, and the 0–5 cm soil C of 200 m2 gap was significantly higher than that of the smaller gap sizes. In the 5–20 cm soil layer, the soil N decreased in the following order: 150 m2 gap, cypress forest, 100 m2 gap, 200 m2 gap, and 50 m2 gap (Fig. 2d, e, f). The 0–5 cm soil P of the cypress forest was lower than that of the mixed gap with Chinese toon, and the soil P of the larger gap sizes (150 m2, 200 m2) was higher than that of the smaller gap sizes (50 m2, 100 m2) (Fig. 2g).
The soil C:N of the cypress forest was less than that of the 4 gap sizes in the three soil layers 0–5 cm, 5–20 cm, and 20–40 cm. The soil C:P of the cypress forest was higher than that of the 4 gap sizes in 5–20 cm and 20–40 cm soil layers. The soil N:P of the cypress forest was higher than that of the 4 gap sizes in the three soil layers, i.e., 0–5, 5–20, and 20–40 cm (Fig. 2p, q, r).
In the cypress forest, the soil C:P ratio was higher in the 20–40 cm layer than in the 0–5 cm layer and was lowest in the 5–20 cm layer. This ratio was the highest in the 0–5 cm layer in the mixed gap with Chinese toon (Table 4). The N:P of the cypress forest was higher in the 0–5 cm layer than in the 5–20 layer and was the lowest in the 20–40 cm layer, and trend also occurred in the mixed gaps with Chinese toon, except for in the 150 m2 gap. The mean value of C:P in the soil (0–40 cm) of different gap sizes decreased as follows: 0 > 100 m2 > 200 m2 > 50 m2 > 150 m2. In addition, the soil N:P of the cypress forest was higher than that of the mixed gaps with Chinese toon.
Relationships between leaf and litter stoichiometry
There is a correlation between the element contents and stoichiometry ratios in leaves and litter. In the cypress forest, there was a significant positive correlation between litter C and leaf C, litter N, litter P, leaf N and leaf P were significantly positively correlated with each other, and there was a significant negative correlation between litter N and P and leaf C:N:P (Table 3). The leaf N and leaf P showed a significant negative correlation with litter C:N and litter C:P, while the leaf C only showed a significant negative correlation with litter N/P (P < 0.05). There was a significant or very significant positive correlation between the stoichiometric characteristics of leaves and litter. The leaf N and leaf P showed a significant negative correlation with litter C:N and litter C:P, while the leaf C only showed a significant negative correlation with litter N/P (P < 0.05). There was a significant or very significant positive correlation between the stoichiometric characteristics of leaves and litter. This correlation changed under the mixed gaps with Chinese toon. The positive correlation between leaf N and P and litter nutrients became a significant negative correlation, and the only positive correlation remaining was between leaf C and litter nutrients. At the same time, the significant negative correlation between leaf N and P and the litter stoichiometry ratios also changed to a positive correlation.
As for the two-way ANOVAs conducted for the leaves, seasonal factors significantly affected the N and P and C:N:P ratios, while the interaction effects for seasons and gap size were only significant for the C, N, and P contents and did were not significant for the C:N:P ratio (Table 5). Two-way ANOVAs indicated that season significantly affected the leaf N and P and leaf C:N:P ratios. In the two-way ANOVAs of litter, the gap showed an insignificant effect on N:P that was inconsistent with the fresh leaves, and the significant interaction between the season and the gap disappeared. The soil layer was introduced in the analysis of variance for soil, and the soil nutrients and nutrient ratios showed significant responses for the soil layer, season, and gap size (Table 5). The interaction between the soil layer and gap also responded significantly to soil nutrients and nutrient ratios, and this significant response disappeared during the interaction between the soil layer and season, and in the interaction of the soil layer, season and gap only responded significantly to soil N and P. A two-way ANOVA indicated that both season and soil depth affected the soil stoichiometric characteristics. The interaction between season and soil depth was not significant.