Organic Farming and Soil Carbon Sequestration: What Do We Really Know About the Benefits?

Organic farming is believed to improve soil fertility by enhancing soil organic matter (SOM) contents. An important co-benefit would be the sequestration of carbon from atmospheric CO₂. Such a positive effect has been suggested based on data from field experiments though many studies were not designed to address the issue of carbon sequestration. The aim of our study was to examine published data in order to identify possible flaws such as missing a proper baseline, carbon mass measurements, or lack of a clear distinction between conventional and organic farming practices, thereby attributing effects of specific practices to organic farming, which are not uniquely organic. A total of 68 data sets were analyzed from 32 peer-reviewed publications aiming to compare conventional with organic farming. The analysis revealed that after conversion, soil C content (SOC) in organic systems increased annually by 2.2% on average, whereas in conventional systems SOC did not change significantly. The majority of publications reported SOC concentrations rather than amounts thus neglecting possible changes in soil bulk density. 34 out of 68 data sets missed a true control with well-defined starting conditions. In 37 out of 50 cases, the amount of organic fertilizer in the organic system exceeded that applied in the compared conventional system, and in half of the cases crop rotations differed between systems. In the few studies where crop rotation and organic fertilization were comparable in both systems no consistent difference in SOC was found. From this data analysis, we conclude that the claim for beneficial effects of organic farming on SOC is premature and that reported advantages of organic farming for SOC are largely determined by higher and often disproportionate application of organic fertilizer compared to conventional farming.     

Predicting community sensitivity to ozone, using Ellenberg Indicator values

This paper develops a regression-based model for predicting changes in biomass of individual species exposed to ozone (RS(p)), based on their Ellenberg Indicator values. The equation (RS(p)=1.805-0.118Light-0.135 square root Salinity) underpredicts observed sensitivity but has the advantage of widespread applicability to almost 3000 European species. The model was applied to grassland communities to develop two further predictive tools. The first tool, percentage change in biomass (ORI%) was tested on data from a field-based ozone exposure experiment and predicted a 27% decrease in biomass over 5 years compared with an observed decrease of 23%. The second tool, an index of community sensitivity to ozone (CORI), was applied to 48 grassland communities and suggests that community sensitivity to ozone is primarily species-driven. A repeat-sampling routine showed that nine species were the minimum requirement to estimate CORI within 5%.     

Critical levels for ozone effects on vegetation in Europe

The evidence of detrimental effects of ozone on vegetation in Europe, and the need to develop international control policies to reduce ozone exposures which are based on the effects of the pollutant, has led to attempts to define so-called critical levels of ozone above which adverse effects on trees, crops and natural vegetation may occur. This review is a critical assessment of the scientific basis of the concepts used to define critical levels for ozone and identifies the key limitations and uncertainties involved. The review focuses on the Level I critical level approach, which provides an environmental standard or threshold to minimise the effects of ozone on sensitive receptors, but does not seek to quantify the impacts of exceeding the critical level under field conditions. The concept of using the AOT (accumulated exposure over a threshold) to define long-term ozone exposure is demonstrated to be appropriate for several economically important species. The use of 40 ppb (giving the AOT40 index) as a threshold concentration gives a good linear fit to experimental data from open-top chambers for arable crops, but it is less certain that it provides the best fit to data for trees or semi-natural communities. Major uncertainties in defining critical level values relate to the choice of response parameter and species; the absence of data for many receptors, especially those of Mediterranean areas; and extrapolation to field conditions from relatively short-term open-top chamber experiments. The derivation of critical levels for long-lived organisms, such as forest trees, may require the use of modelling techniques based on physiological data from experimental studies. The exposure-response data which have been applied to derive critical levels should not be used to estimate the impacts of ozone over large areas, because of the uncertainties associated with extrapolation from the open-top chamber method, especially for forest trees, and because of spatial variation in atmospheric and environmental conditions, which may alter ozone uptake.     

Effects of ozone on managed pasture: I. Effects of open-top chambers on microclimate, ozone flux, and plant growth

Open-top chambers (OTC) were established in a field of managed pasture, and environmental parameters were recorded inside and outside to study the influence of OTCs on radiation, air temperature (T(air)), saturation vapour pressure deficit (svpd), and soil water content in relationship to plant growth and yield. Canopy development in OTCs supplied with non-filtered air (NF) and in ambient (AA) plots was followed by measuring leaf area index (LAI). The dry matter yield was determined after three growth periods in each of two consecutive seasons. Boundary layer conductance (g(bw)) and wind speed (u) were measured along a vertical profile, and day-time flux were measured along a vertical profile, and day-time flux of O(3) was estimated throughout the experiment on the basis of a mass balance. The vertical profile of u showed values in the range 1-1.2 m s(-1) at the top of the canopy, and maximum g(bw) was 20-25 mm s(-1). Average reduction in global radiation in OTCs was 25%, and volumetric soil water content was reduced by about 5%. Daily mean T(air) was increased by 1.3 degrees C, mean daily maximum svpd by 0.08 kPa, and the temperature sum (degree days with base temperature of +5 degrees C) by 12%. Fluctuations in the difference in daily mean T(air) and svpd during the daytime between OTCs and ambient air were related to canopy structure. Differences were largest after each cut and declined with increasing LAI. A small effect of changes in LAI on T(air) and svpd occurred during periods with low soil water content. The flux of O(3) in OTCs was largest (>100 microg m(-2) min(-1)) before and smallest (<20 microg m(-2) min(-1)) after each cut. Calculated deposition velocities for O(3) (nu(d)) in the range 0-3 cm s(-1) were generally higher than those measured under most field conditions. Overall, in OTCs the deficit in soil and atmospheric moisture was larger than in the open field, and the increase in daily mean T(air) was strongly influenced by the stage of canopy development. Changes in microclimate and incoming radiation affected pasture development. LAI was slightly reduced in OTCs as compared to AA plots. The total accumulated dry matter yield for all six growth periods was only about 7% lower in OTCs, but the contribution of clover to total forage mass declined during the experiment. OTCs had no significant effect on weeds. The results indicate that OTCs reduced the competitiveness of clover, and that the increase in growth of grasses compensates for the loss in clover yield. The shift in species composition caused by OTCs must be considered when studying the effect of pollutants on pasture.     

Regional assessment of climate change impacts on maize productivity and associated production risk in Switzerland

A simple model of yield was used along with climate scenarios to assess the impact of climate change on grain maize productivity and associated economic risk in Switzerland. In a first application, changes in the precipitation regime alone were shown to affect the distribution of yield considerably, with shifts not only in the mean but also in the standard deviation and the skewness. Production risk was found to respond more markedly to changes in the long-term mean than in the inter-annual variability of seasonal precipitation amounts. In a further application, yield projections were generated with respect to a full climate scenario, with the emission pathway as specified in the IPCC A2 scenario. Anticipation of the sowing date was found to reduce the negative impact of climate change on yield stability, but was not sufficient to ensure average productivity levels comparable to those observed at present. We argued that this was caused by the reduction in the duration of the growing season, which had a stronger impact than suggested by previous studies. Assuming no change in price relations, the results also revealed a strong increase in production risk with climate change, with more than a doubling in the probability of yield falling short of a critical threshold as compared to today’s situation.     

Nitrous Oxide Concentrations in the Soil of a Mown Grassland: Comparison of Model Results with Soil Profile Measurements

Nitrous oxide (N₂O) concentrations in the soilprofile of a fertilized grassland on the Swiss plateauwere measured at irregular intervals during one year.Air samples were taken from air-permeable tubesinstalled at depths between 2 and 100 cm belowthe surface. Highest concentrations in thetopsoil were observed following precipitationafter fertilization. In the subsoil,concentrations were highest when the soil watercontent fell below about 92% after a wet periodand mineral nitrogen was available from priorfertilization. N₂O concentrations in thetopsoil were simulated with the process-basedgrassland ecosystem model PaSim 2.5 (PastureSimulation Model) and compared with measured soilN₂O concentrations. The model simulated wellthe concentration peaks after fertilizeradditions and the concentrations in winter. Butthe simulated baseline concentrations during thegrowing season were overestimated. Possiblecauses for this discrepancy are discussed andsuggestions are made to improve the model.     

Distribution of the Lamellibrachia spp. (Siboglinidae,Annelida) and their trophosome endosymbiont phylotypes in the Mediterranean Sea

During the 2010/2011 Exploration vessel Nautilus expedition to the Mediterranean Sea, samples of Lamellibrachia (Siboglinidae, Annelida) were imaged in situ and collected from hydrothermal vent and methane “cold seeps.” An analysis of these Lamellibrachia and their endosymbiotic thioautotrophic gammaproteobacteria reveals two distinct endosymbiont phylotypes. Phylotype 1 was present in Lamellibrachia specimens from 947 m at the Eratosthenes seamount seep (a seep off Cyprus in the Eastern Mediterranean), and Phylotype 2 was found in siboglinids from 618 m at a hydrothermal vent within Palinuro volcanic complex in Tyrrhenian Sea. Both phylotypes coexist in siboglinids at 1,036 m from the Palmachim disturbance, a cold seep in the Eastern Mediterranean’s Levantine basin. Our results, combined with existing knowledge of siboglinid host and endosymbiotic bacteria biogeography, reveal that two major groups of endosymbionts coexist within lamellibranchids and escarpids. The phylogenetic clustering of these bacteria is primarily influenced by geographic location, rather than selection by the siboglinid host.     

Paternal inheritance in mealybugs (Hemiptera: Coccoidea: Pseudococcidae)

Mealybugs have a haplodiploid reproduction system, with paternal genome elimination (PGE); the males are diploid soon after fertilization, but during embryogenesis, the male paternal set of chromosomes becomes heterochromatic (HC) and therefore inactive. Previous studies have suggested that paternal genes can be passed on from mealybug males to their sons, but not necessarily by any son, to the next generation. We employed crosses between two mealybug species—Planococcus ficus (Signoret) and Planococcus citri (Risso)—and between two populations of P. ficus, which differ in their mode of pheromone attraction, in order to demonstrate paternal inheritance from males to F2 through F1 male hybrids. Two traits were monitored through three generations: mode of male pheromone attraction (pherotype) and sequences of the internal transcribed spacer 2 (ITS2) gene segment (genotype). Our results demonstrate that paternal inheritance in mealybugs can occur from males to their F2 offspring, through F1 males (paternal line). F2 backcrossed hybrid males expressed paternal pherotypes and ITS2 genotypes although their mother originated through a maternal population. Further results revealed other, hitherto unknown, aspects of inheritance in mealybugs, such as that hybridization between the two species caused absence of paternal traits in F2 hybrid females produced by F1 hybrid females. Furthermore, hybridization between the two species raised the question of whether unattracted males have any role in the interactions between P. ficus and P. citri.