The response of soil-atmosphere greenhouse gas exchange to changing plant litter inputs in terrestrial forest ecosystems.

Various global change factors (e.g. elevated CO2 concentrations, nitrogen deposition, etc.) can alter the amount of litterfall in terrestrial forests, which could subsequently lead to changes in the physical, chemical, and biological properties of forest soils. Yet, there is hitherto a lack of consensus on the role of litter in governing the soil-atmosphere exchange of greenhouse gases (GHGs) in forest ecosystems, which can significantly affect the overall climatic cooling impacts of forests as a net carbon sink. In this study, we carried out a meta-analysis of over 250 field observations to determine the response of soil GHG fluxes to in situ litter manipulation in global forests. Our results showed that overall, litter addition enhanced soil CO2 emissions from terrestrial forests by 26%, while litter removal reduced soil CO2 emissions from these forests by 26%. The negative response of soil CO2 emissions to litter removal was stronger in the tropical forests (-33%) than in the subtropical (-27%) and temperate (-21%) forests, and was significantly correlated with mean annual temperature and precipitation. Moreover, litter removal was observed to enhance soil CH4 uptake in tropical (+24%) and temperate (+9%) forests, but not in subtropical forests. Litter removal reduced N2O emissions from forest soils by 20% on average, with this negative effect increasing with mean annual precipitation. The duration of litter removal experiment was negatively correlated with the response of soil CO2 emissions but had no influence on the response of soil CH4 and N2O fluxes. We found that plant litter supply could alter soil GHG fluxes in forests by modulating the microclimate as well as the labile and recalcitrant soil carbon pools. Our findings highlighted the importance of considering the effects of changing plant litter inputs on soil-atmosphere GHG fluxes in terrestrial forests and their spatio-temporal variability in biogeochemical models.

Revealing the millipede and other soil-macrofaunal biodiversity in Hong Kong using a citizen science approach.

Background: Soil biodiversity plays important roles in nutrient recycling in both the environment and agriculture. However, they are generally understudied worldwide. To reveal the diversity of soil macrofauna in Hong Kong, here we initiated a citizen science project involving university, non-governmental organisations and secondary school students and teachers. It is envisioned that the citizen science approach used in this study could be used as a demonstration to future biodiversity sampling and monitoring studies. New information: Throughout a year of monitoring and species sampling across different localities in Hong Kong, 150 soil macrofaunal morphospecies were collected. Eighty five of them were further identified by morphology and DNA barcoding was assigned to each identified morphospecies, yielding a total of 646 DNA barcodes, with new millipede sequences deposited to the GenBank. The soil macrofauna morphospecies in Hong Kong found in this study are mainly dominated by millipedes (23 out of 150) and oligochaetes (15 out of 150). Amongst the twenty three identified millipedes, two polyxenid millipedes, Monographisqueenslandica Huynh & Veenstra, 2013 and Alloproctoidesremyi Marquet and Condé, 1950 are first recorded in Hong Kong. Information has been curated on an online platform and database (http://biodiversity.sls.cuhk.edu.hk/millipedes). A postcard summarising the findings of millipedes in Hong Kong has also been made as a souvenir and distributed to citizen participants. The identified macrofauna morphospecies and their 646 DNA barcodes in this study established a solid foundation for further research in soil biodiversity.

Variability and controls of soil CO2 fluxes under different tillage and crop residue managements in a wheat-maize double-cropping system.

The spatial and temporal variability of soil CO2 emissions from agricultural soils is inherently high. While tillage and crop residue practices play vital roles in governing soil CO2 emission, their effects on the variability of soil CO2 fluxes across depths and seasons are still poorly understood. To address this, an experiment consisting of four treatments, namely conventional tillage with (CT+) and without crop residue application (CT-), as well as no tillage with (NT+) and without crop residue application (NT-), was conducted to investigate soil CO2 fluxes at top 40 cm soils with 10-cm depth intervals in a winter wheat-summer maize rotation system in the North China Plain. Our results showed soil CO2 fluxes increased with depth in both the wheat- and maize-growing seasons. However, the dominant factors in regulating soil CO2 fluxes changed with soil depth and seasons. In the wheat-growing season, increase in soil CO2 fluxes with depth was attributed to the increase of dissolved organic carbon-to-nitrogen ratio (DOC/DON) and a decline in soil DON concentration along the soil profile. These factors explained about 55-96% of the total variation in soil CO2 fluxes at different soil depths. In the maize-growing season, the dominant factors were soil DOC/DON ratio, soil DON concentrations, and soil moisture. These factors explained approximately 79-96% of the total variation in soil CO2 fluxes along the soil depth. Greater soil CO2 fluxes (except at 30-40 cm depth) were observed in NT- than CT- treatments. Furthermore, crop residue application enhanced soil CO2 fluxes across different depths, but the enhancement was more prominent in CT+ than NT+. Moreover, soil CO2 fluxes in the maize-growing season were greater than those in the wheat-growing season. Our results demonstrate that the effects of tillage regimes and crop residue management practices on soil CO2 emissions are not confined only to the plough layer but can extend to soils of over 30 cm depths. We also need to revisit the general conventional view that no tillage can significantly reduce soil CO2 emissions compared with conventional tillage for better climate change mitigation.

Phosphorus retention and release by sediments in the eutrophic Mai Po Marshes, Hong Kong.

This study examined the phosphorus retention and release characteristics of sediments in the eutrophic Mai Po Marshes in Hong Kong. Results of chemical fractionation show that the sum of inorganic P pools exceeded 50% of the total sediment P content, with the redox-sensitive iron-bound P (Fe(OOH) approximately P) being the dominant P fraction. Given the considerable average Fe(OOH) approximately P concentration of 912 microg g(-1), Mai Po sediments demonstrated a great potential to release bioavailable P under low sediment redox potentials. This was further supported by the high mean anaerobic P flux of 31.8 mg m(-2)d(-1) recorded in Mai Po sediment cores, indicating the role of bottom sediments as a net P source. Although sediments in Mai Po had appreciable Langmuir adsorption maxima (1642-3582 mg kg(-1)), the high zero equilibrium P concentrations (0.02-0.51 mg L(-1)) obtained suggest that sediment sorption processes would contribute to sustaining the eutrophic conditions in overlying water column even with a further reduction in external P load. Concerted efforts should be made to reduce internal loading of P, especially under reducing conditions, to complement the implementation of zero discharge policy for Deep Bay for effective eutrophication abatement and long-term water quality improvement in the Mai Po Marshes.