Publications

Abstract. The long-term carbon balance (NBP) of grasslands is estimated by combining scarce multi-year eddy-covariance observations at ecosystem observation sites where information on carbon inputs and harvesting removals is available. Following accounting for carbon leached to rivers, we estimated grasslands to be net carbon sinks of 74±10 g C m−2 yr−1. Uncertainties arise from the small number of sites and the short measurement period. Only 11 sites, out of a total of 20 grassland sites in Europe where eddy covariance systems are installed, were set-up for estimating NBP. These 11 selected sites are representative of intensive management practice and we lack information on disturbance history, such as plowing. This suggests that the grassland NBP estimate is likely biased towards overestimating the sink, compared to the European average. Direct measurements of Net Primary Productivity (NPP) are not possible in grasslands given permanent biomass removal by grazing and mowing, uncertainties in rhizodeposition and production of volatile organic carbon compounds lost to the atmosphere. Therefore, the grassland process-based ecosystem model PASIM was used to estimate the spatial-temporal distribution of NPP, providing a European average value of 750±150 g C across extensively grazed, intensively grazed pastures, and forage production systems. In Europe the NPP of grasslands seems higher than that of croplands and forests. The carbon sequestration efficiency of grasslands, defined as the ratio of NBP to NPP, amounts to 0.09±0.10. Therefore, per unit of carbon input, grasslands sequester 3–4 times more carbon in the soil than forests do, making them a good candidate for managing onsite carbon sinks. When using the 100 yr greenhouse warming potential for CH4 and N2O, their emissions due to management of grasslands together offset roughly 70–80% of the carbon sink. Uncertainties on the European grassland greenhouse gas balance, including CO2, CH4 and N2O fluxes are likely to be reduced in the near future, with data being collected from more sites, and improved up-scaling methods.

Abstract Green manure (GM) enhances the ecological services in agricultural ecosystems, including soil health and carbon sequestration. However, its effect on regional methane (CH 4 ) emissions from paddy fields is unclear. Here we clarify the impacts of GM rotation by combining process‐based modeling with microbial gene abundance information and coordinated distributed observations at 14 sites in southern China. We found that GM management, including application rate and rotation year, mainly affects CH 4 emissions in GM‐rice systems by impacting soil biotic factors, which explain 78.4% of the variation ( p < 0.001). The most influential factor is the ratio of soil CH 4 production to oxidation gene abundances ( R 2 = 0.510; p < 0.001), which decreases with GM rotation year due to increased activity of methane‐oxidizing soil microbes ( p < 0.001), indicating that CH 4 emissions from GM‐rice systems decrease with increased GM rotation year. By incorporating these microbial mechanisms as quantitative parameters in process‐based model, we project that approximately 76% of the paddy rice areas in southern China, which have relatively low GM biomass and baseline CH 4 emissions, can achieve reductions in CH 4 emissions through nearly 15 years of GM crop rotation. This study indicates that CH 4 emissions from GM‐rice rotations with appropriate GM application rate over the long term will not significantly increase, resolving the contradictions in previous research.

The greenhouse gas removal (GGR) potential of soil carbon sequestration (SCS) and biochar is achieved via a deliberate increase in the soil organic carbon (SOC) stocks. This can be realised by improving management practices in agriculture and grassland systems by increasing the organic carbon (C) input or reducing soil disturbances. Soil C sequestration potential differs depending on climate, soil properties and management, with the highest potential for poor soils (SOC stock farthest from saturation). The main downside of using SCS for CO2 removal is the limited time C can be sequestrated until a new C equilibrium is reached. SCS management needs to continue even after C saturation in the soil occurs, otherwise, the newly gained SOC stock might lose the C again, reversing the SCS impact. However, agricultural and grassland soils have the feasible technical potential of 2–5 Gt CO2eq per year globally and an economic potential of 1.5–2.6 Gt CO2eq per year (at prices up to 100 $ per t CO2eq). They can act as a C sink over the next 20 to 30 years following SCS management implementation. Biochar is more recalcitrant, and a new C equilibrium is expected to take much longer, so, biochar will probably be a long-term option for CO2 removal. Biochar's feasible technical potential is 2.6 Gt CO2eq per year. Both SCS and biochar can be implemented on existing land and bring additional benefits for soil quality and health, which provides further advantages like an increase in productivity. Thus, SCS and biochar can contribute greatly to sustainable productivity and food security.

Abstract Agricultural soils play a very important role in regulating the carbon dioxide (CO 2 ) content of the atmosphere, and can behave either as carbon sources or sinks. We have simulated the dynamics of carbon in the soil under different land uses and soil‐management systems in a Mediterranean olive grove with the Rothamsted carbon (RothC) model. To this end we chose patches of native vegetation (NV) and two different olive grove soils (chromic calcisols and calcic vertisols) under different soil‐management systems: conventional tillage (T), and mulching with shredded olive‐pruning debris and residues from olive‐fruit cleaning (PD + CR). We measured the clay content, bulk density, soil organic carbon (SOC) and total nitrogen (N) in each patch. The SOC and N values decreased by more than 30% as a result of a change in soil use from NV to T olive grove. After adding PD + CR these values rose once more, even to levels above NV. The RothC model performed well for covered soils (NV and PD + CR) but overestimated the SOC values after the soil use was changed from NV to T olive grove, probably due to high carbon losses caused by erosion, common to T soils in the Mediterranean basin. As a result of mulching the soil with only pruning debris, CO 2 emitted to the atmosphere was reduced by >55% for both soils. Associated with this decrease in the emission rate, RothC estimated a potential carbon sequestration of 0.5 and 0.6 t C/ha/yr for chromic calcisols and calcic vertisols, respectively. The reuse of organic debris generated in the olive grove, such as pruning debris and residues from olive‐fruit cleaning, is an efficient way of improving soil properties, diminishing CO 2 emissions and increasing the soil’s capacity to store carbon.

International audience