Patterns and controlling factors of soil carbon sequestration in nitrogen-limited and -rich forests in China-a meta-analysis.

Soil organic carbon (SOC) management has the potential to contribute to climate change mitigation by reducing atmospheric carbon dioxide (CO2). Understanding the changes in forest nitrogen (N) deposition rates has important implications for C sequestration. We explored the effects of N enrichment on soil carbon sequestration in nitrogen-limited and nitrogen-rich Chinese forests and their controlling factors. Our findings reveal that N inputs enhanced net soil C sequestration by 5.52-18.46 kg C kg-1 N, with greater impacts in temperate forests (8.37-13.68 kg C kg-1 N), the use of NH4NO3 fertilizer (7.78 kg Ckg-1 N) at low N levels (<30 kg Ckg-1 N; 9.14 kg Ckg-1 N), and in a short period (<3 years; 12.95 kg C kg-1 N). The nitrogen use efficiency (NUE) varied between 0.24 and 13.3 (kg C kg-1 N) depending on the forest type and was significantly controlled by rainfall, fertilizer, and carbon-nitrogen ratio rates. Besides, N enrichment increased SOC concentration by an average of 7% and 2% for tropical and subtropical forests, respectively. Although soil carbon sequestration was higher in the topsoil compared to the subsoil, the relative influence indicated that nitrogen availability strongly impacts the SOC, followed by dissolved organic carbon concentration and mean annual precipitation. This study highlights the critical role of soil NUE processes in promoting soil C accumulation in a forest ecosystem.

Global cross-biome patterns of soil respiration responses to individual and interactive effects of nitrogen addition, altered precipitation, and warming.

Anthropogenic activities have increased atmospheric N, precipitation, and temperature events in terrestrial ecosystems globally, with N deposition increasing by 3- to 5-fold during the previous century. Despite decades of scientific research, no consensus has been achieved on the impact of climate conditions on soil respiration (Rs). Here, we reconstructed 110 published studies across diverse biomes, magnitudes, and driving variables to evaluate how Rs responds to N addition, altered precipitation (both enhanced and reduced precipitation or precipitation changes), and warming. Our findings show that N addition significantly increased Rs by 44 % in forests and decreased it by 19 % and 26 % in croplands and grasslands, respectively (P < 0.05). In forests and croplands, altered precipitation significantly increased Rs by 51 % and 17 % (all, P < 0.05), respectively, whereas impacts on grassland were insignificant. In comparison, warming stimulated Rs by 62 % in forests but inhibited it by 10 % in croplands (all, P < 0.05), whereas impacts on grassland were again insignificant. In addition, across all biomes, the responses of Rs to altered precipitation and warming followed a Gaussian response, increasing up to a threshold of 1800 mm and 25 °C, respectively, above which respiration rates decreased with further increases in precipitation and temperature. Our work suggests that the dual interaction of warming × altered precipitation promotes belowground CO2 emission, thus enhancing global warming. In general, the interactive effect of N addition × altered precipitation decreases Rs. Soil moisture was identified as a primary driver of Rs. Given these findings, we recommend future research on warming vs. changed precipitation to better forecast and understand the interaction between Rs and climate change.

Correction: Ecological restoration stimulates environmental outcomes but exacerbates water shortage in the Loess Plateau.

[This corrects the article DOI: 10.7717/peerj.13658.].

Ecological restoration stimulates environmental outcomes but exacerbates water shortage in the Loess Plateau.

Restoration is the natural and intervention-assisted set of processes designed to promote and facilitate the recovery of an ecosystem that has been degraded, damaged, or destroyed. However, it can also have an adverse effect on the environment. Thus, assessing an ecological restoration project's impact is crucial to determining its success and optimum management strategies. We performed a meta-analysis concerning the environmental outcomes during the years 2000-2015 resulting from the "Grain for Green" Project (GFGP) implementation in the Loess Plateau (LP). Data were gathered from 40 peer-reviewed English-language articles chosen from a pool of 332 articles. The results showed that, on average, GFGP increased forest coverage by 35.7% (95% CI [24.15-47.52%]), and grassland by 1.05% (95% CI [0.8-1.28%]). At the same time, GFGP has a positive impact on soil carbon (C) sequestration, net ecosystem production (NEP), and net primary production (NPP), from the years 2000 to 2015 by an average of 36% (95% CI [28.96-43.18%]), 22.7% (95% CI [9.10-36.79%]), and 13.5% (95% CI [9.44-17.354%]), respectively. Soil erosion, sediment load, runoff coefficient, and water yield were reduced by 13.3% (95% CI [0.27-25.76%]), 21.5% (95% CI [1.50-39.99%]), 22.4% (95% CI [5.28-40.45%]) and 43.3% (95% CI [27.03-82.86%]), respectively, from the years 2000 to 2015. Our results indicate that water supply decreased with the increase of vegetation coverage. Therefore, to balance the needs for green space, GFGP policies and strategies should recover, enhance, and sustain more resilient ecosystems.

Variability of soil carbon and nitrogen stocks after conversion of natural forest to plantations in Eastern China.

Forest plantation, either through afforestation or reforestation, has been suggested to reverse and mitigate the process of deforestation. However, uncertainties remain in the potential of plantation forest (PF) to sequestrate carbon (C) and nitrogen (N) compared to natural forest (NF). Soil C and N stocks require a critical and updated look at what is happening especially in the context of increasing rate of land use change and climate change. The current study was conducted in China's Eastern forest to estimate soil C and N stocks in six depth layers (0-10, 10-20, 20-40, 40-60, 60-80 and 80-100 cm) and two forest types (NF and PF) at four sites along climate factors gradient. The results showed that the overall mean soil C and N amounts to a depth of 20 cm ranged from 2.6 ± 1.1 Mg ha-1 to 38.6 ± 23.1 Mg ha-1, and soil nitrogen stock ranged from 0.2 ± 0.1 Mg ha-1 to 3.3 ± 1.5 Mg ha-1. Moreover, a loss of C stock was observed at Qingyuan (QY) by -7%, Dinghushan (DH) by -26%, Jianfengling (JF) by -13% while that of N stock was observed at QY (-8%), DH (-19%) and JF (-12%) at both depth layers. These results indicate that NFs have a better capacity to accumulate soil C and N. The soil C and N decreased from the southeast to the northeast and increased from tropical to temperate mixed forests zone in the eastern part of the study area. The C and N stock mainly occurred in the topsoil and decreased significantly with depth. Moreover, soil C and N stocks increased with age of plantation. This study provides an overview of the current spatial distribution and soil stocks of C and N, as well as the effects of environmental factors on soil C and N stocks. It also indicated that, although mean annual temperature and mean annual precipitation are the key factors affecting the variations in soil C and N, their vertical and horizontal distribution differed in various aspects.

Effects of land use change from natural forest to plantation on C, N and natural abundance of 13C and 15N along a climate gradient in eastern China.

Soil C and N turnover rates and contents are strongly influenced by climates (e.g., mean annual temperature MAT, and mean annual precipitation MAP) as well as human activities. However, the effects of converting natural forests to intensively human-managed plantations on soil carbon (C), nitrogen (N) dynamics across various climatic zones are not well known. In this study, we evaluated C, N pool and natural abundances of δ13C and δ15N in forest floor layer and 1-meter depth mineral soils under natural forests (NF) and plantation forest (PF) at six sites in eastern China. Our results showed that forest floor had higher C contents and lower N contents in PF compared to NF, resulting in high forest floor C/N ratios and a decrease in the quality of organic materials in forest floor under plantations. In general, soil C, N contents and their isotope changed significantly in the forest floor and mineral soil after land use change (LUC). Soil δ13C was significantly enriched in forest floor after LUC while both δ13C and δ15N values were enriched in mineral soils. Linear and non-linear regressions were observed for MAP and MAT in soil C/N ratios and soil δ13C, in their changes with NF conversion to PF while soil δ15N values were positively correlated with MAT. Our findings implied that LUC alters soil C turnover and contents and MAP drive soil δ13C dynamic.