Initial Soil Organic Matter Content Influences the Storage and Turnover of Litter, Root and Soil Carbon in Grasslands

Fire frequency and type regulate the response of soil carbon cycling and storage to fire across soil depths and ecosystems: A meta-analysis

Fire is a very common disturbance in terrestrial ecosystems and can give rise to significant effects on soil carbon (C) cycling and storage. Here, we conducted a global meta-analysis on the response of soil C cycling and storage across soil profiles (organic layer, 0-5 cm, 0-10 cm, 0-20 cm, and 20-100 cm) to fire reported in 308 studies across 383 sites and examined the role of fire frequency, fire type, soil type, ecosystem type, and post-fire time in regulating the response of soil C dynamics to fire. Overall, we found soil C cycling and storage were more responsive to one fire and wildfire as compared to frequent fire and prescribed fire, respectively. Soil respiration significantly decreased by 22 ± 9% by one fire, but was not significantly affected by frequent fire across ecosystems. One fire significantly reduced soil C content in the organic, 0-10 cm, and 20-100 cm layers by 27 ± 16%, 10 ± 9%, and 33 ± 18%, respectively, while frequent fire significantly reduced soil C content at a depth of 0-5 cm and 0-20 cm by 29 ± 8% and 16 ± 12%, respectively. Soil C cycling and storage showed little response to frequent prescribed fire. In addition, the response of soil C cycling and storage varied among different soil and ecosystem types, with a stronger response being observed in forest than in grassland. Within 20 years post-fire, soil C cycling and storage tended to recover only after one fire but not after frequent fire. We also found that soil physicochemical properties and microbial communities were more responsive to one fire than frequent fire, which could indirectly affect the effects of fire on soil C cycling and storage. The results of our study have filled some critical gaps in previous meta-analyses in fire ecology.

Distinct storage mechanisms of soil organic carbon in coniferous forest and evergreen broadleaf forest in tropical China

The impact of human activities on soil carbon (C) storage in tropical forests has aroused wide concern during the past decades, because these ecosystems play a key role in ameliorating global climate change. However, there remain uncertainties about how land-use history alters soil organic carbon (SOC) stability and storage in different forests. In this study, we measured the C content and mass distributions of soil aggregates, density fractions, mineral-bound C and microbial biomass C in the organic horizon, 0-10 cm and 10-20 cm soil layers in coniferous forest and evergreen broadleaf forest at Dinghushan Biosphere Reserve in tropical China. The broadleaf forest had larger SOC stocks than the coniferous forest, but the proportion of SOC stored in different density fractions at 0-10 cm soils was similar between forest types, while a greater proportion of SOC was stored in microaggregates in the coniferous forest. Most of the SOC was held as light fraction C in the organic horizon in the coniferous forest, whereas the concentrations of mineral-bound C were higher in the broadleaf forest. These findings indicate clear differences in the protection of SOC between broadleaf and coniferous forests growing on the same soil type. We propose that historic conversion of broadleaf forest to coniferous forest has reduced soil C sequestration capacity by altering the diversity and quality of plant inputs to the soil, which in turn affected macroaggregate formation, soil chemical properties and microbial biomass. Our results thus demonstrate that changes in forest tree species composition could have long-lasting effects on soil structure and carbon storage, providing crucial evidence for policy decisions on forest carbon sink management.

Influence of Mining and Vegetation Restoration on Soil Properties in the Eastern Margin of the Qinghai-Tibet Plateau

Mining causes serious destruction of the surface morphology and soil structure of lands, and vegetation restoration on post-mining lands provides an effective way for soil and water conservation. To determine the influence of mining and vegetation restoration on soil properties in the eastern margin of the Qinghai-Tibet Plateau, four land sites, including two vegetation restoration sites (restorated by Elymus nutans and Picea crassifolia, respectively), one non-vegetated mining site and one native grassland site, were selected. Fifty-two topsoil (0–10) samples were collected from these four sites, and then soil properties, trace metals and soil enzyme activities were analyzed. The results showed that there was an increase in soil pH (>8.0) after mining, while vegetation restoration decreased the soil pH compared with native grassland; the soil organic matter and total nitrogen in the site restored with E. nutans increased by 48.8% and 25.17%, respectively, compared with the site restored with P. crassifolia. The soil enzyme activities decreased after mining, and there were no significant increases in urease, phosphatase, β-glucosidase and β-1,4-N-acetylglucosaminidase activities after five years of restoration. In addition, the contents of soil trace metals (cadmium, chromium, mercury, lead and zinc) after mining were lower than the Chinese threshold (GB 15618/2018), but the content of arsenic in non-vegetated soil and P. crassifolia-restored soil exceeded the threshold by 22.61 times and 22.86 times, respectively. Therefore, As-contaminated land areas should be accurately determined and treated in a timely way to prevent arsenic from spreading, and plant species with tolerance to alkaline soil should be selected for vegetation restoration on post-mining lands.

High biomass production with abundant leaf litterfall is critical to ameliorating soil quality and productivity in reclaimed sandy desertification land

Long-term data regarding soil properties and crop growth are powerful resources substantially contributing to our knowledge of soil-forming processes of reclaimed sandy desertification land. Generalized ecological principles derived from long-term observations that help to maintain or improve soil quality and productivity is critical for guiding field management practices while suitable for newly reclaimed sandy desertification land still need to be evaluated. Here, a 14-yr old experiment showed that soil quality index (SQI) had an "increase-decline-recovery" tendency in irrigation and fertilizer addition desertification lands while it remained at constantly low levels in desertification land with only irrigation. Stably decent yield and net incomes were obtained after 3-4 years' consecutive irrigation and fertilizer addition management. Correlation between crop productivity and SQI followed a saturation characteristic curve with threshold at 0.5, corresponding to soil organic carbon (SOC) ~5.0 g kg^-1, below which crop productivity was linearly declined. 60% of observed inter-annual variations in SQI were explained by quantity of leaf litter, which was three times higher than explanatory power of root residue. No substantial changes occurred in soil mechanical components while the soil microbial biomass carbon, water-stable aggregate and heavy carbon pool in SOC were significantly improved by 2-9 folds in reclaimed desertification lands. Results revealed that increased biomass production with abundant residue retention is crucial for ameliorating soil quality, stabilizing high yield and economic gains, supporting the "High Biomass Cropping System" ecological hypothesis. Ecological limitations and opportunities to sustainable utilization of sandy desertification land were discussed.