Nutrient cycling characteristics along a chronosequence of forest primary succession in the Hailuogou Glacier retreat area, eastern Tibetan Plateau

Study site

Gongga Mountain (29°20′–30°20′N, 101°30′–102°15′E) is located at the junction of the Sichuan Basin and the Qinghai-Tibet Plateau, in the transitional zone between the alpine zone of the Qinghai-Tibet Plateau and the warm and humid subtropical monsoon region. The Hailuogou Glacier retreat area (29°34′N, 102°59′E), located on the eastern slope of Gongga Mountain, belongs to the alpine temperate climate type, with an annual average temperature of 3.8 °C, more than 260 rainy days per year, and an annual average precipitation of approximately 1,960 mm. Since the Hailuogou Glacier began to retreat after the Little Ice Age, it has been less disturbed by humans, and the glacier retreat is significant without any glacial advance process. This has led to the formation of a complete forest primary succession sequence of approximately 2 km, accompanied by a continuous soil formation process (Bai et al., 2019).

The field surveys has been described in detail elsewhere (Yang et al., 2021b). Briefly, six sampling sites were selected for vegetation and soil samples collection at various glacier retreat time (Table 1). Specifically, Site 1 (S1) represents 15 y of glacial retreat and was invaded by pioneer herbs and trees, including Hippophae rhamnoides Linn., Salix spp., Populus purdomii Rehd., and several other leguminous herbs. Site 2 (S2), Site 3 (S3) and Site 4 (S4) represent glacier retreat after 35 y, 45 y and 57 y, respectively. During the broad-leaved forest stage (S2–S4), P.purdomii became the predominant species owing to its rapid growth rate and high photosynthetic rate, which meant that it was able to outcompete H. rhamnoides and Salix spp. Site 5 (S5) and Site 6 (S6) experienced glacial retreat after 85 y and 125 y, respectively. During the coniferous forest stage (S5–S6), P. purdomii was gradually replaced by Abies fabri (Mast.) Craib.

Vegetation sample collection

Our research is in the Gongga Mountain Nature Reserve, and the current study was carried out on the fixed sample plot set up by the research station. We work in close coordination with the management department of the Reserve. Our work is also part of the monitoring and managment of the Reserve. The general observation experiment did not need to be approved. Therefore, all field sampling mentioned in this study was permitted.

The sample collection method as previously described in Yang et al. (2021b). Specifically, we established three quadrats of 10 m × 10 m at each site, and recorded the species name, DBH, height and geographical coordinates of the tree with a diameter at breast height (DBH; 1.3 m height aboveground) >2 cm. The vegetation biomass estimated by harvesting the dominant tree species in each quadrat to obtain the species-specific allometric equations (Yang et al., 2015). Collection of vegetation samples (foliage, twig, stem, bark, and root) of all tree species at six sampling sites were conducted once in each year from 2015 through 2016 during the peak of deciduous foliage mass. Meanwhile, we established a quadrat of 5 m × 5 m to harvest the shrub biomass and a quadrat of 1 m × 1 m to harvest the herb and moss biomass in each 10 m × 10 m quadrat. The tree materials and understory vegetation materials were oven-dried at 60 °C to a constant weight. The dried samples were ground to a fine powder and used to measure the nutrient concentration. The N concentration was measured by an element analyzer (Vario Macro Cube C, Elementar, Wetzlar, Germany). The P, K, Ca and Mg concentrations were analyzed using an American Leeman Labs Profile Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES).

Litter and soil sample collection

According to the terrain, slope, and vegetation distribution, we established a 0.5 m × 0.5 m soil pit in each 10 m × 10 m quadrat. The sample collection method as previously described in Yang et al. (2021b). Specifically, we collected the litter (undecomposed) from the surface of the soil pit, and its biomass and nutrient concentration were determined in the same way as those of the vegetation material. For the soil sampling, no obvious A layer were found because of the short period of soil development, we established organic soil and C layer by using the protocol described elsewhere (Deyn, Raaijmakers & Putten, 2004). We used a steel auger drill (5 cm diameter) to obtain the soil-core sample in each pit. Then, all core samples were cut into horizontal slices of Oe (intermediate decomposed organic layer), Oa (highly decomposed organic layer) and C layer. We collected soil samples and measured their thickness, and a separate set of samples for soil bulk density was collected with a cutting ring in each soil layer. Whereafter, all soil samples were air dried at room temperature (–15 °C) by placing the samples on kraft paper and then measuring the moisture and calculating the bulk density. Each soil sample was ground in an agate mortar and sieved through a 200 mesh sieve. The N concentration of the soil was measured using the semimicro Kjeldahl method. The P, K, Ca and Mg concentrations were also analyzed as method used for the vegetation material.

Statistical analysis

The specific pool size of N, P, K, Ca, and Mg at each site are estimated based on the measured concentration of of N, P, K, Ca, and Mg in samples (tree, shrub, herb, and moss) and bulk soil and biomass densities:

(1) $$\mathrm{P}\mathrm{o}\mathrm{o}{\mathrm{l}}_{\mathrm{v}\mathrm{e}\mathrm{g}}=\sum (\mathrm{C}\mathrm{o}{\mathrm{n}}_{\mathrm{v}\mathrm{e}\mathrm{g}}\times \mathrm{B}\mathrm{i}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{s}{\mathrm{s}}_{\mathrm{v}\mathrm{e}\mathrm{g}})$$

(2) $$\mathrm{P}\mathrm{o}\mathrm{o}{\mathrm{l}}_{\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}}=\sum (\mathrm{C}\mathrm{o}{\mathrm{n}}_{\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}}\times \mathrm{B}\mathrm{u}\mathrm{l}\mathrm{k}\mathrm{D}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}{\mathrm{y}}_{\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}}\times \mathrm{D}\mathrm{e}\mathrm{p}\mathrm{t}\mathrm{h}\times {10}^{-1}).$$

The characteristics of the nutrient cycle were analyzed using biological cycle parameters such as retention, return, annual absorption, utilization coefficient, circulation coefficient and turnaround time. The retention is the annual accumulation or net accumulation of nutrient elements, calculated by multiplying the annual growth of the biomass of vegetation (Yang et al., 2015) by its nutrient concentration. The return was calculated by measuring the annual litter amount and nutrient concentration. Other biological cycle parameters as follows:

(3) $$\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=\mathrm{R}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}+\mathrm{R}\mathrm{e}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{n}$$

(4) $$\mathrm{U}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{t}=\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}/\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{v}\mathrm{e}\mathrm{g}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{p}\mathrm{o}\mathrm{o}\mathrm{l}$$

(5) $$\mathrm{C}\mathrm{i}\mathrm{r}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{t}=\mathrm{R}\mathrm{e}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{n}/\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}$$

(6) $$\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{t}=\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}/\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{o}\mathrm{r}\mathrm{g}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c}\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}\mathrm{p}\mathrm{o}\mathrm{o}\mathrm{l}$$

(7) $$\mathrm{T}\mathrm{u}\mathrm{r}\mathrm{n}\mathrm{a}\mathrm{r}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}=\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{v}\mathrm{e}\mathrm{g}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{p}\mathrm{o}\mathrm{o}\mathrm{l}/\mathrm{R}\mathrm{e}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{n}$$where absorption is the total amount of nutrients absorbed by plants from the environment; utilization coefficient is used to reflect the magnitude of the storage rate of elements in the ecosystem, the greater the coefficient is, the lower the utilization efficiency of plants for elements; circulation coefficient reflects the residual amount of elements in the cycle process, the larger the coefficient is, the faster the element circulation rate and the higher the fluidity; absorption coefficient reflects the vegetation absorption of nutrients from the soil; the turnaround time reflects the time it takes for nutrients to go through a cycle (Pang et al., 2002; Yu et al., 2015).

The element pools in the soil

The nutrient pools of N and P in the Oe layer gradually increased with soil development (Fig. 1). At site S6, the pools reached 1,395.34 kg hm⁻² N, 124.37 kg hm⁻² P, 1,469.73 kg hm⁻² K, 3,349.68 kg hm⁻² Ca, and 1,188.86 kg hm⁻² Mg. In the Oa layer, the N and Ca pools gradually accumulated and reached maximum values of 3,040.94 kg hm⁻² and 3,244.83 kg hm⁻² at site S6, respectively; however, the K and Mg pools fluctuated with soil development and reached a maximum at site S3, and the P pool reached a maximum at site S4. The pools of N, P, K, Ca, and Mg in the C layer were the highest during the initial soil development stage and then decreased slightly afterwards. The pools of P, K, Ca, and Mg in all soil layers were greater in the C layer, while for N, the pool was greater in the organic soil. Additionally, in the organic soil, the pools of elements followed the order of Ca > N > K > Mg > P, while in the C layer, they followed the order of Ca > K > Mg > P > N.

The element pools in vegetation

Tree layer

The pools of N, P, and K in the tree layer gradually increased with forest primary succession, reaching 1,128.99 kg hm⁻² N, 175.64 kg hm⁻² P, and 1,315.61 kg hm⁻² K at site S6 (Fig. 2). However, the pools of Ca and Mg reached the maximum 1,966.63 kg hm⁻² and 175.91 kg hm⁻² at site S4 , respectively. Based on the proportion of nutrient elements pools of dominant species in the tree layer at each sample site, N, P, K, and Mg were the most abundant in H. rhamnoides at site S1, accounting for 66.21% N, 59.43% P, 58.82% K and 43.83% Mg. From sites S2 to S4, the proportion of nutrient elements pools in P. purdomii were the highest, accounting for 86.11% N, 88.06% P, 92.48% K, 95.96% Ca, and 92.97% Mg at site S4. While at sites S5 and S6, the proportion of nutrient elements stored in A. fabri was significantly higher than that in P. purdomii, and almost all A. fabri occupied at site S6.

Understory vegetation

The pools of N, P, K, Ca, and Mg in the understory vegetation gradually increased with forest primary succession, reaching 429.75 kg hm⁻² N, 31.94 kg hm⁻² P, 204.46 kg hm⁻² K, 282.79 kg hm⁻² Ca, and 42.02 kg hm⁻² Mg at site S6, and the order of the pools were N > Ca > K > Mg > P (Fig. 3). At site S1, the proportions of nutrient elements pools in the herb were relatively high, accounting for 74.15% N, 59.06% P, 79.16% K, 55.47% Ca and 48.62% Mg. At site S2, the proportions of P, Ca and Mg were highest in the moss, accounting for 50.02% P, 52.30% Ca, and 54.82% Mg. From sites S3 to S6, most of the nutrients in understory vegetation were mainly stored in shrub, accounting for 74.72% N, 70.02% P, 72.12% K, 73.07% Ca and 60.62% Mg at site S6. It was observed that the proportions of these elements in the shrub were less than 5% during the early stage of this succession sequence. However, with the increase in shrub biomass (Yang et al., 2015), particularly at site S6, the proportions of the N, P, K, Ca, and Mg pools within the shrub exceeded 50 %. This finding indicated that as the understory vegetation continuously developed, the shrub became the primary storage units for nutrient elements in the understory vegetation.

Proportion of the element pool in the vegetation and organic soil

Due to the extremely low N concentration in the C layer and its primary function in nutrient cycling within organic soil, this study focused exclusively on the organic soil (Oe and Oa layers) and vegetation when addressing element distribution within the ecosystem (Fig. 4). As primary succession progresses, notable differences in the distribution of N, P, K, Ca, and Mg were observed between organic soil and vegetation. These differences were primarily manifested as follows: (1) At site S1, the limited duration of soil development and the low N concentration (only 0.44 g kg⁻¹) resulted in only 4.64% of N being stored in organic soil. In contrast, at site S6, a significant increase was observed, with 74.03% of N being retained in organic soil, while vegetation contributed merely 25.97%. Consequently, soil has emerged as the predominant N pool within the ecosystem. (2) More than 60% of the P and K were predominantly stored in organic soil at site S1, with a relatively small proportion found in vegetation. However, as the pools of P and K within the vegetation continuously increased, their proportions surpassed those in the organic soil, reaching 60.71% for P and 56.86% for K at site S5. We proposed that a substantial amount of soil P and K has been transformed into biomass P and K pools. (3) At each site, the proportions of Ca and Mg pools in organic soil exceeded those observed in vegetation, establishing organic soil as the primary storage unit for Ca and Mg within the ecosystem.

The cycling characteristics of N, P, K, Ca, and Mg

The accumulation of N, P, and K in both vegetation and organic soil were observed to reach their peak at site S6 (Figs. 5–9). However, the accumulation of Ca and Mg reached its peak at site S4, whereas their accumulation in organic soil was highest at sites S6 and S1, respectively. Furthermore, the retention and absorption of N and K in vegetation were found to reach their maxima at site S6. In contrast, the retention and absorption of P, Ca, and Mg peaked at site S4. In addition, the absorption coefficient exhibited a gradual decrease throughout the process of forest succession. The annual returns of N and P were highest at site S4, whereas the maximum returns for K, Ca, and Mg occurred at site S6. Based on the utilization characteristics of elements, a lower utilization coefficient signifies a higher efficiency of element use by vegetation. The utilization coefficients of N, P, K, and Mg were found to be lowest at site S6. In contrast, the utilization coefficient of Ca exhibited its lowest value at site S4. A higher cycling coefficient of elements suggests a more rapid cycling rate of the element in vegetation. The cycling coefficients of N and K reached their highest levels at site S3. Conversely, P exhibited the highest coefficient at site S2, Ca was maximal at site S1, and Mg peaked at site S6. In addition, the turnaround times of N and K were observed to be the longest at site S5, P exhibited the longest turnaround time at site S6, while Ca and Mg showed their longest turnaround times at site S4.

Element dynamics in this succession sequence

At the early stage of this succession sequence (S1), N was primarily stored in vegetation. Several studies indicate that biological N fixation plays a crucial role in ecosystem N supply (Clevaland et al., 2022; Liang et al., 2022). The N concentration in the fine sand of Hailuogou Glacier sediment is extremely low, around 0.05 g kg⁻¹ (Yang et al., 2021b). But the initial N-fixing colonizers serve to facilitate the establishment of late-successional dominants and promote soil N accumulation (Chapin et al., 1994; Menge & Hedin, 2009; Pérez et al., 2016), such as A.adsurgens and H.rhamnoides in this succession sequence (Sun et al., 2022). The N fixation rates for the roots of leguminous plants, such as A.adsurgens, ranged from 0.15 to 6.21 mg N g⁻¹ d⁻¹, while the roots of elaegnaseae species like H.rhamnoides exhibited rates between 0.13 and 5.36 mg N g⁻¹ d⁻¹. Notably, during the early stages of this forest succession sequence, the N fixation rate from root nodules accounted for approximately 94.8% of the overall ecosystem N fixation rate (Zhang, 2020). Thus, the N storage in the herb layer constituted 74% of that found in the understory layer, while the H.rhamnoides accounted for 66% of the N storage in the tree layer at site S1. Additionally, atmospheric N deposition serves as a significant source of N within ecosystems. The total rate of atmospheric N deposition in the study area was approximately 8 kg hm⁻² y (Chang et al., 2018). Nevertheless, previous studies have shown that the N accumulation rate in organic soils (41 kg hm⁻² y) exceeds the atmospheric N deposition rate (Yang et al., 2021b). Thus, the rate of N deposition was insufficient to account for the substantial increase in organic soil N. Consequently, biological N fixation may one important source of N accumulation within ecosystems during this succession sequence. Notably, over 125 years, organic soil stored more than twice the amount of N as vegetation, indicating soil as the primary N reservoir.

The P and K pools also increased with succession. Initially, the organic soil held larger P and K pools than vegetation at S1, but by S6, vegetation pools were equivalent to those in the soil. Wu reported that the bioavailable P in the soil increased significantly after 30 years of glacier retreat within this sequence. Moreover, the development of A. fabri in this context led to a decrease in soil pH, with values declining from 8.5 at the end of glaciation to 4.4 at site S6 (Wu et al., 2014b). This process can enhance the dissolution of certain silicate minerals, subsequently facilitating the production of bioavailable phosphorus P to satisfy the nutritional requirements of various types of vegetation (Zhou et al., 2013). The antagonism between Ca and K can impede K absorption when Ca levels are high, but as Ca in the soil steadily declines with succession, plants absorb more K from the soil. Consequently, K gradually accumulates in vegetation, surpassing soil levels at later stages. All of these studies corroborate our finding that a significant portion of the organic soil P and K at older sites has been converted into biomass P and K pools. Particularly after 85 years of recession, the P and K pools in vegetation exceed those in the organic soil, especially in coniferous-dominated forests. Furthermore,Ca and Mg pools in organic soil remained higher than those in vegetation throughout the succession, indicating that soil is the primary storage reservoir for Ca and Mg.

In the understory vegetation, the accumulation of elements occurred most rapidly during the broad-leaved forest stage (sites S4–S5). This phenomenon can be primarily attributed to the broad-leaved forest stage, during which P. purdomii emerged as the dominant species within the tree layer. The expansion of its ecological niche resulted in heightened intra- and interspecific competition, thereby reinforcing self-thinning and other thinning effects within the population (Yang et al., 2015). The emergence of numerous forest gaps within the community provides favorable conditions for the growth of shrubs and hers plants beneath the trees. This phenomenon leads to a significant increase in both the diversity and abundance of these plant species in that specific environment (Yang et al., 2021a). This phenomenon also resulted in an accelerated accumulation of nutrients within the understory vegetation during this period. The nutrient accumulation in understory vegetation is intricately linked to habitat changes prompted by the succession of dominant species.

The mechanism of element cycling in forest primary succession