Stoichiometric traits (N:P) of understory plants contribute to reductions in plant diversity following long-term nitrogen addition in subtropical forest

Nitrogen budgets of a lower subtropical forest as affected by 6 years of over-canopy and understory nitrogen additions

Although tropical and subtropical regions have replaced temperate regions as the global-change hotspots for increased atmosphere nitrogen (N) deposition, whether the regional forests reach N saturation is still unclear. Understory or floor N addition has been commonly used in N-deposition studies, but the results of such studies have recently been challenged because they fail to account for canopy interception, assimilation, and leaching processes. Here, we conducted a field experiment to quantify the effects of over-canopy and understory N addition on N budgets in a lower subtropical monsoon evergreen broadleaved (LSMEB) forest. We found that the LSMEB forest was not N saturated after receiving additional N at 25 and 50 kg ha^-1 yr^-1 for 6 years. Plants were able to absorb the added N by increasing the N concentrations in their organs, with 120-412 % increasing trend of plant N pools under N-addition treatments. Canopy absorption of N resulting from over-canopy N addition led to increases in N concentrations in tree organs but not to increases in tree biomass. Understory N addition could underestimate the effects of N deposition in forests due to neglecting canopy N interception and canopy effects on N redistribution. Additional experiments using over-canopy N addition are needed to assess the true effects of N deposition on different forest ecosystems in different climate zones.

High N Storage but Low N Recovery After Long-Term N-Fertilization in a Subtropical Cunninghamia lanceolata Plantation Ecosystem: A 14-Year Case Study

Forests are among the most important N pools of all terrestrial ecosystems. Elevated atmospheric N deposition in recent decades has led to increased interest in the influences of N application on forest N cycles. However, accurate assessments of N storage in forest ecosystems remain elusive. We used a 14-year experiment of a Chinese fir [Cunninghamia lanceolata (Lamb.) Hook] plantation to explore how long-term N fertilization affected N storage and recovery rates. Our study plots were located in a field that had been continuously fertilized over 14 years (2004-2017) with urea at rates of 0 (N0, control), 60 (N60, low-N), 120 (N120, medium-N), and 240 (N240, high-N) kg N hm^-2a^-1. Data were collected that included N content and biomass in the understory, litter, and various plant organs (i.e., leaves, branches, stems, roots, and bark), as well as soil N content and density at different depths. Results showed that the total ecosystem N storage in the N-fertilized plots was 1.1-1.4 times higher than that in the control plots. About 12.36% of the total ecosystem N was stored in vegetation (plant organs, litter, and understory) and 87.64% was stored in soil (0-60 cm). Plant organs, litter, and soil had higher N storage than the understory layer. Significantly higher plant N uptake was found in the medium-N (1.2 times) and high-N (1.4 times) treatments relative to the control. The N recovery rate of the understory layer in the N-fertilized treatments was negative and less than that in the control. Application of long-term N fertilizer to this stand led to a low N recovery rate (average 11.39%) and high loss of N (average 91.86%), which indicate low N use efficiency in the Chinese fir plantation ecosystem. Our findings further clarify the distribution of N in an important terrestrial ecosystem and improve our understanding of regional N cycles.

C:N:P Stoichiometry of Plant, Litter and Soil along an Elevational Gradient in Subtropical Forests of China

The internal correlation of plant, litter and soil stoichiometric characteristics and their responses to the environment are helpful for revealing nutrient cycling mechanisms. However, few studies have assessed the nutrient relationship between plant, litter and soil and nutrient stock along elevational gradients, which limit the understanding of nutrient relationships in the ecosystem. To gain insight into the forces of nutrient stock and its stoichiometric ecological characteristics along the elevational gradients in forest ecosystem, we investigated the carbon (C), nitrogen (N) phosphorus (P) contents and stoichiometric ratios of dominant plants, litter and soil layers at different elevations (900–1600 m) in Daiyun Mountain. The results showed the following: (1) C, N and P contents showed an increasing order as plant > litter > soil in each elevation of Daiyun Mountain. Dominant plants were limited by N each elevation. C, N and P contents of plants at high elevation were higher than those at low elevation and significant correlations were found between plant and litter TN, TP and air and soil temperature (negative), which conforms to the Temperature-Plant Physiological Hypothesis (TPPH). (2) Significant correlations were found between plant C:N and litter C:N (positive); between litter C:P and soil N:P (positive); and between litter C:P and soil C:N (negative). (3) Elevation and slope were essential environmental factors to the stoichiometric ratio of plant and litter, and pH was the main factor that correlated negatively to soil stoichiometry ratio. Litter provided a link between plant and soil, and there was a coupling among plant, litter and soil nutrients. The results could provide a theoretical basis for understanding the nutrient cycling for the subtropical forest ecosystem of China.

Key factors influencing on vegetation restoration in the gullies of the Mollisols

Natural vegetation restoration (NVR) highly relates to the development of gully erosion, and is mainly determined by both the soil properties and species competition in the gullies. However, it is still not clear what are the key factors influencing on the vegetation restoration in the gullies with the poor soil properties (e.g. low soil organic matter and nutrients) under the special hydrological process (e.g. high runoff intensity and long flow duration). In this study, soil total organic carbon (SOC), total nitrogen (TN), available nitrogen (AN), ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), total phosphorus (TP), available phosphorus (AP), pH, soil moisture (SM) were investigated, and both regression and structural equation model analysis were used for detecting how soil properties and species competition influence the herbaceous plants restoration in the poor quality of Mollisols in gullies of Northeast China. The results show that, (1) influence of NH₄⁺-N, AN, TN, pH on biomass was stronger in 0-10 cm than that in 10-20 cm soil depth, opposite was stronger in 10-20 cm than that in 0-10 cm soil depth for NO₃^--N, SOC and SM (P < 0.05). (2) NH₄⁺-N, NO₃^--N, AN, TN, SOC, pH, C:N were all negative, while SM was positive to plant biodiversity in soil layers (P < 0.05). (3) SOM mainly mediates the TN and NH₄⁺-N and then directly or indirectly influences on biodiversity and biomass, and P changed the species richness when AP >20 mg kg^-1 in 10-20 cm soil depth. (4) Vegetation restoration was mainly determined by the dynamics of Elymus dahuricus Turcz. firstly, and then by Leymus chinensis(Trin.) Tzvel. at the early of vegetation restoration. Generally, the heterogeneity of SOC and SM in soil layers and AP in deep soil layer was the key factors determining NVR in the gullies of Mollisols watershed. At the end of paper, the NVR process in Moillosols in gullies was classified as four stages, and each stage was depicted in detail.

Effect of Environmental Stress on the Nutrient Stoichiometry of the Clonal Plant Phragmites australis in Inland Riparian Wetlands of Northwest China

Clonal plants play an important role in determining ecosystem properties such as community stability, species diversity and nutrient cycling. However, relatively little information is available about the stoichiometric characteristics of clonal plants and their drivers in inland riparian wetlands under strong environmental stress. In this manuscript, we studied the clonal plant Phragmites australis in an inland riparian wetland of Northwest China and compared its nutrient distribution and stoichiometry trade-offs as well as its responses to soil environmental factors in three different environments, namely, a wetland, a salt marsh, and a desert. We found that (1) P. australis could adapt to heterogeneous environments by changing its nutrient allocation strategies, as evidenced by the significant decrease in N and P concentrations, and significant increase in whole-plant C:P and N:P ratios from the wetland to the desert habitats. (2) P. australis adapted to stressful environments by changing its nutrient allocation patterns among different modules, showing a greater tendency to invest N and P in underground modules (rhizomes and roots) and an increase in the utilization efficiency of N and P in the leaves, and stems as environmental stress increased. (3) The C-N, C-P, and N:P-C in the whole plant and in each module showed significant anisotropic growth relationships in the three habitats (P < 0.05). (4) Soil water, pH and salt were the main factors limiting nutrient stoichiometry. The results of this study clarified the ecological adaptation mechanism of the clonal plant P. australis to heterogeneous environments and provided targeted protection strategies for inland riparian wetlands in Northwest China.