Full accounting of the greenhouse gas (CO2, N2O, CH4) budget of nine European grassland sites

Soil carbon sequestration (enhanced sinks) is the mechanism responsible for most of the greenhouse gas (GHG) mitigation potential in the agriculture sector. Carbon sequestration in grasslands can be determined directly by measuring changes in soil organic carbon (SOC) stocks and indirectly by measuring the net balance of C fluxes. A literature search shows that grassland C sequestration reaches on average 5 ± 30 g C/m2 per year according to inventories of SOC stocks and −231 and 77 g C/m2 per year for drained organic and mineral soils, respectively, according to C flux balance. Off-site C sequestration occurs whenever more manure C is produced by than returned to a grassland plot. The sum of on- and off-site C sequestration reaches 129, 98 and 71 g C/m2 per year for grazed, cut and mixed European grasslands on mineral soils, respectively, however with high uncertainty. A range of management practices reduce C losses and increase C sequestration: (i) avoiding soil tillage and the conversion of grasslands to arable use, (ii) moderately intensifying nutrient-poor permanent grasslands, (iii) using light grazing instead of heavy grazing, (iv) increasing the duration of grass leys; (v) converting grass leys to grass-legume mixtures or to permanent grasslands. With nine European sites, direct emissions of N2O from soil and of CH4 from enteric fermentation at grazing, expressed in CO2 equivalents, compensated 10% and 34% of the on-site grassland C sequestration, respectively. Digestion inside the barn of the harvested herbage leads to further emissions of CH4 and N2O by the production systems, which were estimated at 130 g CO2 equivalents/m2 per year. The net balance of on- and off-site C sequestration, CH4 and N2O emissions reached 38 g CO2 equivalents/m2 per year, indicating a non-significant net sink activity. This net balance was, however, negative for intensively managed cut sites indicating a source to the atmosphere. In conclusion, this review confirms that grassland C sequestration has a strong potential to partly mitigate the GHG balance of ruminant production systems. However, as soil C sequestration is both reversible and vulnerable to disturbance, biodiversity loss and climate change, CH4 and N2O emissions from the livestock sector need to be reduced and current SOC stocks preserved.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/75C37264D0DBC0B6175DFA61574E3A6D/S1751731109990784a.pdf/div-class-title-mitigating-the-greenhouse-gas-balance-of-ruminant-production-systems-through-carbon-sequestration-in-grasslands-div.pdf

Mitigating the greenhouse gas balance of ruminant production systems through carbon sequestration in grasslands

A data-driven analysis of energy balance closure across FLUXNET research sites: The role of landscape-scale heterogeneity

. The rewetting of dry soils and the thawing of frozen soils are short-term, transitional phenomena in terms of hydrology and the thermodynamics of soil systems. The impact of these short-term phenomena on larger scale ecosystem fluxes is increasingly recognized, and a growing number of studies show that these events affect fluxes of soil gases such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ammonia (NH3) and nitric oxide (NO). Global climate models predict that future climatic change is likely to alter the frequency and intensity of drying-rewetting events and thawing of frozen soils. These future scenarios highlight the importance of understanding how rewetting and thawing will influence dynamics of these soil gases. This study summarizes findings using a new database containing 338 studies conducted from 1956 to 2011, and highlights open research questions. The database revealed conflicting results following rewetting and thawing in various terrestrial ecosystems and among soil gases, ranging from large increases in fluxes to non-significant changes. Studies reporting lower gas fluxes before rewetting tended to find higher post-rewetting fluxes for CO2, N2O and NO; in addition, increases in N2O flux following thawing were greater in warmer climate regions. We discuss possible mechanisms and controls that regulate flux responses, and recommend that a high temporal resolution of flux measurements is critical to capture rapid changes in gas fluxes after these soil perturbations. Finally, we propose that future studies should investigate the interactions between biological (i.e., microbial community and gas production) and physical (i.e., porosity, diffusivity, dissolution) changes in soil gas fluxes, apply techniques to capture rapid changes (i.e., automated measurements), and explore synergistic experimental and modelling approaches.

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Effects of soil rewetting and thawing on soil gas fluxes: a review of current literature and suggestions for future research

In order to estimate potential greenhouse gas flux rates from soils under different land use and climate, and to particularly assess the influence of soil temperature and soil moisture, we measured fluxes of nitrous oxide (N2O), nitric oxide (NO), carbon dioxide (CO2) and methane (CH4) from intact soil cores obtained from 13 European sites under controlled laboratory conditions. The soils covered the different climates of Europe and included four different land-use types: croplands, forests, grasslands and wetlands. In a two-factorial experimental design, the soil cores were incubated under four temperatures (5–20°C) and water contents (20–80% water-filled pore space). We found a non-linear increase of N2O, NO and CO2 emissions with increasing temperature. Nitrous oxide emissions were positively correlated with soil moisture, while NO emission and CH4 oxidation rates were negatively correlated with soil moisture. Maximum CO2 emissions occurred at intermediate soil moisture.

Different land-use types strongly affected greenhouse gas fluxes. Nitrous oxide and CO2 emissions were highest in grassland soils, while NO emissions were highest in forest soils. In grasslands, high soil microbial activity stimulated by high carbon (C) and nitrogen (N) contents, dense root systems and high C input from above-ground decaying biomass was the most likely cause for high N2O and CO2 emissions. High NO emissions from forest soils were mainly attributed to low pH and high soil porosity. Northern soils showed the greatest capacity to take up CH4 under warmer and dryer soil conditions. Nitric oxide emissions were positively correlated with N input.

A. van del Pol-van Dasselaar

Papers

Effects of climate and management intensity on nitrous oxide emissions in grassland systems across Europe