Stoichiometric Variation in Soil Carbon, Nitrogen, and Phosphorus Following Cropland Conversion to Forest in Southwest China

Grazing exclusion is more beneficial for restoring soil organic carbon and nutrient balance than afforestation on degraded sandy land

Introduction

Vegetation restoration is an effective measure to improve the ecosystem service of degraded sandy land ecosystem. However, it is unclear how vegetation restoration on severely desertified land affect soil organic carbon (SOC) sequestration and nutrients balance. Therefore, this study was designed to clarify the response of SOC, total nitrogen (TN), total phosphorus (TP), and the resulting stoichiometric ratios (C:N:P) to afforestation and grazing exclusion, and to quantify their dynamics over time.

Methods

We conducted vegetation community investigation and soil sampling in natural sparse-forest grassland (the climax community stage), afforestation (Pinus sylvestris var. mongolica (40-year, 48-year), Caragana microphylla (20-year, 40-year)), and grazing exclusion (20-year, 40-year) in China's Horqin Sandy Land. Soil C:N:P stoichiometry and its driving factors under different restoration measures were then studied.

Results

Afforestation and grazing exclusion significantly (p < 0.05) increased SOC, TN, and TP concentrations. Vegetation restoration significantly increased C:N, C:P, and N:P ratios, indicating that nutrient limitations may occur in the later stages of restoration. The C:N, C:P, and N:P ratios after a 40-year grazing exclusion were closest to those of natural sparse-forest grassland. The N:P under grazing exclusion increased from 3.1 to 4.1 with increasing restoration age (from 20 to 40 years), which was close to the national mean values (4.2). Moreover, afforestation may lead to water deficit in the surface soil. Vegetation restoration is the main factor leading to changes in soil C:N:P stoichiometry, and indirectly affects soil C:N:P stoichiometry by altering soil structure and chemical properties.

Conclusion

In terms of ecological stoichiometry, grazing exclusion was more conducive to restore SOC and nutrient balance than afforestation on severely desertified land. Due to the poor soil nutrients, attentions should be paid to the soil nutrients and water conditions in the later stages of vegetation restoration. Those findings can provide valuable information for the restoration of degraded sandy land in semi-arid areas.

Promoting effects of soil C and N and limiting effect of soil P jointly determine the plant diversity during the aerial seeding restoration process in Mu Us sandy land, China

Introduction

Exploring the change and maintaining mechanism of plant diversity is of great significance for guiding the restoration of degraded ecosystems. However, how plant taxonomic, functional, and phylogenetic diversity change during long-term ecosystem restoration process and their driving factors remain unclear.

Methods

Based on the 35-year time gradient of aerial seeding restoration in Mu Us sandy land, this study explored the changes in plant taxonomic, functional, and phylogenetic diversity and the driving factors.

Results

The results showed that plant taxonomic, functional, and phylogenetic diversity showed consistent response with the aerial seeding restoration, all of which increased first and then tended to a saturation state in the middle of restoration (14 years). TN, TOC, and NO3 --N increased with aerial seeding restoration and showed a significant positive correlation with plant diversity of the three dimensions, while AP showed a negative correlation. Soil nitrogen and carbon promoted the increase of diversity of three dimensions in the early restoration period, while phosphorus limited the increase of diversity of three dimensions in the middle and late restoration periods. The diversity of three dimensions was mainly affected by restoration time, soil nutrients, and climate factors, and the coupling effect of restoration time and soil nutrients was dominant.

Discussion

These findings indicate that the plant diversity in different dimensions and soil nutrients are improved by aerial seeding restoration. Our study highlights that aerial seeding restoration mainly improves plant diversity by increasing soil nutrients, and the relative effects of different soil nutrients on plant diversity during restoration are inconsistent.

Responses of carbon, nitrogen, and phosphorus contents and stoichiometry in soil and fine roots to natural vegetation restoration in a tropical mountainous area, Southern China

The stoichiometry of key elements such as C, N, and P is an important indicator of ecosystem nutrient status and biogeochemical cycling. Nevertheless, the responses of soil and plant C:N:P stoichiometric characteristics to natural vegetation restoration remain poorly understood. In this study, we investigated C, N, and P contents and stoichiometry in soil and fine roots along vegetation restoration stages (grassland, shrubland, secondary forest, and primary forest) in a tropical mountainous area in southern China. We found that soil organic carbon, total N, C:P ratio, and N:P ratio significantly increased with vegetation restoration and significantly decreased with increasing soil depth, whereas there was no significant effect on soil total P and C:N ratio. Furthermore, vegetation restoration significantly increased the fine root N and P content and N:P ratio, whereas soil depth significantly decreased the fine root N content and increased the C:N ratio. The increasing average N:P ratio in fine roots from 17.59 to 21.45 suggested that P limitation increased with vegetation restoration. There were many significant correlations between C, N, and P contents and their ratios in soil and fine roots, indicating a reciprocal control of nutrient stoichiometric characteristics between them. These results contribute to our understanding of changes in soil and plant nutrient status and biogeochemical cycling during vegetation restoration and provide valuable information for restoration and management of tropical ecosystems.

Hyperspectral Inversion of Soil Carbon and Nutrient Contents in the Yellow River Delta Wetland

Hyperspectral inversion techniques can facilitate soil quality monitoring and evaluation. In this study, the Yellow River Delta Wetland Nature Reserve was used as the study area. By measuring and analyzing soil samples under different vegetation types and collecting soil reflectance spectra, the relationships between vegetation types, soil depth, and the changes in soil total carbon (TC), total nitrogen (TN), and total phosphorus (TP) contents were assessed. The spectral data set was changed by spectral first derivative processing and division of the sample set according to vegetation type. The correlation between soil carbon, nitrogen, and phosphorus contents, and soil spectra was also analyzed, sensitive bands were selected, and the partial least-squares (PLS) method, support vector machine (SVM) method, and random forest (RF) model were used to establish the inversion model based on the characteristic bands. The optimal combination of spectral transformation, sample set partitioning, and inversion model was explored. The results showed significant differences (p < 0.05) in soil TC, TN, and TP contents under reed and saline alkali poncho vegetation, but not between soil element contents under different stratifications of the same plant species. The first derivative reflectance had higher correlation coefficients with soil TC, TN, and TP contents compared with the original reflectance, while the sensitive bands and quantities of the three elements differed. The division of the sample sets according to vegetation type and the first derivative treatment can improve the prediction accuracy of the model. The best combination of sample set plus FD plus RF for TC, TN, and TP in reed soil and sample set plus FD plus SVM for TC, TN, and TP in saline alkali pine soil provides technical support to further improve the prediction accuracy of TC, TN, and TP in wetland soil.