Lack of increased availability of root-derived C may explain the low N2O emission from low N-urine patches

Urine deposition on grassland causes significant N2O losses, which in some cases may result from increased denitrification stimulated by labile compounds released from scorched plant roots. Two 12-day experiments were conducted in 13C-labelled grassland monoliths to investigate the link between N2O production and carbon mineralization following application of low rates of urine-N. Measurements of N2O and CO2 emissions from the monoliths as well as δ13C signal of evolved CO2 were done on day -4, -1, 0, 1, 2, 4, 5, 6 and 7 after application of urine corresponding to 3.1 and 5.5 g N m-2 in the first and second experiment, respectively. The δ13C signal was also determined for soil organic matter, dissolved organic C and CO2 evolved by microbial respiration. In addition, denitrifying enzyme activity (DEA) and nitrifying enzyme activity (NEA) were measured on day -1, 2 and 7 after the first urine application event. Urine did not affect DEA, whereas NEA was enhanced 2 days after urine application. In the first experiment, urine had no significant effect on the N2O flux, which was generally low (-8 to 14 μg N2O-N m-2 h-1). After the second application event, the N2O emission increased significantly to 87 μg N2O-N m-2 h-1 and the N2O emission factor for the added urine-N was 0.18 %. However, the associated 13C signal of soil respiration was unaffected by urine. Consequently, the increased N2O emission from the simulated low N-urine patches was not caused by enhanced denitrification stimulated by labile compounds released from scorched plant roots

Compte-rendu de la séance du sous-groupe "Perceptions et pratiques des agriculteurs" de l'atelier de réflexion scientifique "La recherche française autour du programme de recherche 4 pour 1000" à Sète le 8 novembre 2018

Le sous-groupe "Perceptions et pratiques des agriculteurs" de l'atelier de Sète a réuni 22 personnes qui se sont attachées à répondre individuellement puis collectivement aux questions posées pour faire le point sur la façon dont les travaux sur le 4p1000 tiennent compte des agriculteurs, de leurs perceptions, leurs pratiques et ce qui les déterminent. Le sous-groupe reflétait une diversité de points de vue, pourtant les réponses ont convergé en grande partie. Il a été constaté que la prise en compte des perceptions et pratiques des agriculteurs passe généralement par une traduction du 4p1000 en termes de pratiques de gestion des matières organiques et de fertilité des sols et par des approches participatives encore insuffisantes et trop unilatérales. Le besoin de co-construction des travaux et propositions sur le 4p1000 a été souligné pour que les objectifs et les contraintes des agriculteurs soient appréhendés par les chercheurs dans une optique d'adaptation non seulement des pratiques agricoles mais aussi des approches et méthodes d'évaluation de ces pratiques. Enfin il a été reconnu que l'initiative 4p1000 est porteuse à la fois d'incertitudes et de nouveaux risques mais aussi d'opportunités pour mieux faire face aux risques et incertitudes dans l'agriculture. Deux principaux fronts de science ont ainsi été esquissés qui devront être confirmés et précisés par un état de l'art plus systématique : sur la transdisciplinarité nécessaire à une compréhension partagée du 4p1000 ; sur la caractérisation de l'hétérogénéité des agriculteurs vis-à-vis de leur contribution à un objectif commun

Higher soil respiration under mowing than under grazing explained by biomass differences

Different management practices may change the rate of soil respiration, thus affecting the carbon balance of grasslands. Therefore, we investigated the effect of grazing and mowing on soil respiration along with its driving variables (soil water content, soil temperature, above and below ground biomass, vegetation indices and soil carbon) in adjacent treatments (grazed and mowed) at a semi-arid grassland in Hungary (2011-2013). The average soil respiration over three years was higher in the mown (6.03±4.07 µmol CO2 m-2 s-1) than in the grazed treatment (5.29±3.50 µmol CO2 m-2 s-1). While soil water content and soil temperature did not differ between treatments, mowing resulted in 20 % higher soil respiration than grazing, possibly due to 17% higher average above ground biomass in the mowed than in the grazed treatment. Inclusions of vegetation index VIGreen in the soil respiration model in addition to abiotic drivers improved the explained Rs variance by 16% in the mowed and by 5% in the grazed site, respectively. VIGreen alone proved to be a simple and fast indicator of soil respiration (r2=0.31 at grazed, r2=0.44 at mowed site). We conclude that soil respiration is responsive to the combined effect soil water content, soil temperature, biomass and soil carbon content as affected by the management (grazing vs. mowing) practice

Strategic Management of Grazing Grassland Systems to Maintain and Increase Organic Carbon in Soils

Understanding management-induced C sequestration potential in soils under agriculture, forestry, and other land use systems and their quantification to offset increasing greenhouse gases are of global concern. This chapter reviews management-induced changes in C storage in soils of grazing grassland systems, their impacts on ecosystem functions, and their adaptability and needs of protection across socio-economic and cultural settings. In general, improved management of grassland/pasture such as manuring/slurry application, liming and rotational grazing, and low to medium livestock units could sequester C more than under high intensity grazing conditions. Converting cultivated land to pasture, restoration of degraded land, and maximizing pasture phases in mixed-cropping, pasture with mixed-livestock, integrated forestry-pasturage of livestock (silvopastoral) and crop-forestry-pasturage of livestock (agro-silvopastoral) systems could also maintain and enhance soil organic C density (SOCρ). In areas receiving low precipitation and having high erodibility, grazing exclusion might restore degraded grasslands and increase SOCρ. Yet, optimizing C sequestration rates, sowing of more productive grass varieties, judicial inorganic and organic fertilization, rotational grazing, and other climate-resilient approaches could improve overall farm productivity and profitability and attain sustainability in livestock farming systems

Priming effect and microbial diversity in ecosystem functioning and response to global change: a modeling approach using the SYMPHONY model

International audienceIntegration of the priming effect (PE) in ecosystem models is crucial to better predict the consequences of global change on ecosystem carbon (C) dynamics and its feedbacks on climate. Over the last decade, many attempts have been made to model PE in soil and its underlying mechanisms. However, PE has not yet been incorporated into any ecosystem models. Here we build plant-soil models to explore how PE and microbial diversity influence soil-plant interactions and ecosystem C and nitrogen (N) dynamics in response to global change (elevated CO2 and increased atmospheric N deposition). Our results show that plant persistence, soil organic matter (SOM) accumulation and low N leaching in undisturbed ecosystems relies on a fine adjustment of microbial N mineralization to plant N uptake. This adjustment can be modeled in the SYMPHONY model by considering the destruction of SOM through PE and the interactions between two distinct microbial functional groups: SOM-decomposers and SOM-builders. After estimation of parameters, SYMPHONYprovided realistic predictions on forage production, soil C storage and N leaching for a permanent grassland. Consistent with recent observations, SYMPHONY predicted a CO2-induced modification of soil microbial communities leading to an intensification of SOM mineralization and a decrease in the soil C stock. SYMPHONY also indicated that atmospheric N deposition may stimulate SOM accumulation via changes in the structure and metabolic activities of microbial communities. Collectively, these results suggest that PE and the functional role of microbial diversity may be incorporated in current ecosystem models with a few additional parameters, improving accuracy of predictions

Long-term impacts of season of grazing on soil carbon sequestration and selected soil properties in the arid Eastern Cape, South Africa

BACKGROUND AND AIMS : The karoo biomes of South Africa are major feed resources, yet soil nutrient depletion and degradation is a major problem. The objective of this study was to assess impacts of long-term (>75 years) grazing during spring (SPG), summer (SUG), winter (WG) and exclosure (non-grazed control) treatments on soil nutrients, penetration resistance and infiltration tests. METHODS : A soil sampling campaign was carried out to collect soil to a depth of 60 cm to analyse bulk density, soil physical and chemical parameters as well as soil compaction and infiltration. RESULTS : Generally, grazing treatments reduced soil organic C (SOC) stocks and C:N ratios, and modified soil properties. There was higher SOC stock (0.128 Mg ha-1 yr-1) in the exclosure than in the SPG (0.096 Mg ha-1 yr- 1), SUG (0.099 Mg ha-1 yr-1) and WG (0.105 Mg ha-1 yr-1). The C:N ratios exhibited similar pattern to that of C. From the grazing treatments, the WG demonstrated 7 to 10% additional SOC stock over the SPG and SUG, respectively. CONCLUSIONS : Short period animal exclusion could be an option to be considered to improve plant nutrients in sandy soils of South Africa. However, this may require a policy environment which supports stock exclusion from such areas vulnerable to land degradation, nutrient and C losses by grazing-induced vegetation and landscape changes.Department of Science and Technology University of Pretoria) and the European Communities, 7th framework program under the grant agreement No. 266018, ANIMALCHANGE project.http://link.springer.com/journal/111042016-12-31hb201

Interactions of mean climate change and climate variability on food security extremes. [P-2223-18]

The Coordinated Climate-Crop Modeling Project (C3MP) has conducted a common set of sensitivity tests on more than 1100 simulation sets representing different farm systems in more than 50 countries, with carbon dioxide, temperature, and precipitation change sensitivities gauged for ~20 crop species and ~20 crop models. Here we present an analysis of C3MP results indicating how mean climate changes are likely to affect variability and extreme events within future time periods. Recognizing that climate change will affect agricultural systems both through mean changes and through shifts in climate variability and associated extreme events, C3MP can elucidate several aspects of these changes. First, mean climate changes can affect yields across an entire time period. Second, extreme events (when they do occur) may be more sensitive to climate changes than a year with normal climate. Third, mean climate changes can alter the likelihood of climate extremes exceeding critical biophysical thresholds, leading to more food security extremes. Finally, shifts in climate variability can result in an increase or reduction of mean yield, as extreme climate events tend to have lower yield than years with normal climate.This presentation will demonstrate each of these effects and illustrate the potential implications for future food production and associated agricultural economies under climate change