Editorial: Carbon and nitrogen cycling in grassland ecosystems.

The recent international ‘4 per mille’ initiative for ‘Soils for Food Security and Climate’ launched at the COP21 meeting aims to increase global soil organic carbon (SOC) stocks by 4 parts per 1000 (or 0.4 %) per year to help negate global greenhouse gases emissions by anthropogenic sources. Grasslands account for about 40% of the world’s terrestrial surface with a high-potential capacity for soil carbon (C) sequestration. Grasslands have been a particular area of interest for C capture and storage, and their soil C sequestration has thus been proposed as a plausible partial climate mitigation strategy. Nevertheless, significant uncertainties exist in grassland C cycling that determines soil C sequestration. For example, current estimates of soil C stocks in grasslands range from a significant source to a small sink. These uncertainties and controversies reflect a lack of a comprehensive assessment of C cycling under different grassland management practices and climate regimes. Moreover, C and nitrogen (N) cycling is highly coupled in terrestrial ecosystems, and N is an essential nutrient and determines the ability of soils to sequester and store more C. Quantifying how C and N cycle and how their stocks respond to various environmental factors and management practices is needed to better inform decisions on land-based mitigation strategies.