Long-term Soil Carbon Sequestration Research Articles

Organic matter stabilization in soil aggregates: Understanding the biogeochemical mechanisms that determine the fate of carbon inputs in soils

We studied the biochemical and biophysical processes of carbon sequestration in an intensive agroforestry system on two soils (Feralsol – Luero; Arenosol – Teso) in W. Kenya to elucidate the mechanisms associated with long-term carbon storage. Specifically, we looked at a top-down model (macro-aggregates form around organic matter particles and micro-aggregates form within the macro-aggregates) and a bottom-up model (micro-aggregates form independently and are incorporated into macro-aggregates) of soil aggregate formation. Soil samples were collected from experiments on improved tree fallows using different species and two tillage treatments; water-stable aggregates were extracted and sorted into three size classes: macro-aggregates (> 212 μm), meso-aggregates (53–212 μm) and micro-aggregates (20–53 μm). Organic matter characterization of each fraction was based on 13C isotope abundance, Fourier transform infrared (FTIR) spectroscopy and the abundance of polysaccharides. Improved fallows increased soil C by 0.28 and 0.26 kg m ­2 in the top 20 cm of the soil profile in Luero and Teso, respectively. Tillage altered the distribution of aggregates among size classes. Changes in the δ 13C signature in each fraction indicated that more of the new carbon was found in the macro-aggregates (35–70%) and meso-aggregates (18–49%) in Luero and less (9–17%) was found in the micro-aggregates. In Teso, about 40–80% of the new aggregate C was found in the meso-aggregates, 14–45% was found in the micro-aggregates and only 4–26% was found in the macro-aggregates. The meso-aggregates and macro-aggregates to a lesser extent, in both sites, were enriched in carboxylic-C and aromatic-C, indicating the importance of OM decomposition and plant-derived C in the stabilization of larger aggregates, supporting the top-down model of aggregate formation. Microbially derived polysaccharides play a leading role in the formation of stable micro-aggregates and carboxylic-C promotes stabilization through surface occlusion. This bottom-up process is essential to promote long-term carbon sequestration in soils. Additionally, the micro-aggregates at both sites were enriched in polysaccharides and had elevated ratios of galactose + mannose:arabinose + xylose than the other aggregate fractions, indicating the importance of microbial processes in the formation of stable micro-aggregates and supporting the bottom-up model.

Impact of site management on changes in soil carbon after afforestation: A review

Afforestation and forest management can increase carbon stocks and account for emission reduction according to the Kyoto Protocol. Site management has important effects on the accumulation of soil carbon after afforestation. This review examines the effects of site management, including soil disturbance, fertilization, thinning, weed control, harvesting and controlled burning, on soil carbon dynamics in plantations, based on recent published results. Soil disturbance can enhance soil carbon losses, with whole ploughing causing the most and disking the least loss of soil carbon. The effects of fertilization and thinning on soil carbon are inconclusive. Weed control can prevent the carbon input from above-ground residue and root turnover and increase soil temperature and soil erosion, which in turn reduces the amount of carbon on the soil surface. Soil carbon decreases with the increase of harvesting intensity and the retention of harvest residue can significantly enhance the accumulation of carbon in the soil. Controlled burning before afforestation has a short-term benefit for soil carbon, but it is not beneficial to the sequestration of soil carbon over the long term. Future studies should focus on investigating the long-term impact of site management practices, especially soil fertilization and thinning, on carbon, identifying the response of major functional pools of soil carbon to management practices, understanding the dynamics of soil nitrogen pools and their role in long-term soil carbon sequestration, as well as quantifying soil carbon processes under different climate conditions and site management scenarios using models.

Potential mechanisms for achieving agricultural benefits from biochar application to temperate soils: a review

Natural organic biomass burning creates black carbon which forms a considerable proportion of the soil’s organic carbon. Due to black carbon’s aromatic structure it is recalcitrant and has the potential for long-term carbon sequestration in soil. Soils within the Amazon-basin contain numerous sites where the ‘dark earth of the Indians’ (Terra preta de Indio, or Amazonian Dark Earths (ADE)) exist and are composed of variable quantities of highly stable organic black carbon waste (‘biochar’). The apparent high agronomic fertility of these sites, relative to tropical soils in general, has attracted interest. Biochars can be produced by ‘baking’ organic matter under low oxygen (‘pyrolysis’). The quantities of key mineral elements within these biochars can be directly related to the levels of these components in the feedstock prior to burning. Their incorporation in soils influences soil structure, texture, porosity, particle size distribution and density. The molecular structure of biochars shows a high degree of chemical and microbial stability. A key physical feature of most biochars is their highly porous structure and large surface area. This structure can provide refugia for beneficial soil micro-organisms such as mycorrhizae and bacteria, and influences the binding of important nutritive cations and anions. This binding can enhance the availability of macro-nutrients such as N and P. Other biochar soil changes include alkalisation of soil pH and increases in electrical conductivity (EC) and cation exchange capacity (CEC). Ammonium leaching has been shown to be reduced, along with N2O soil emissions. There may also be reductions in soil mechanical impedance. Terra preta soils contain a higher number of ‘operational taxonomic units’ and have highly distinctive microbial communities relative to neighbouring soils. The potential importance of biochar soil incorporation on mycorrhizal fungi has also been noted with biochar providing a physical niche devoid of fungal grazers. Improvements in soil field capacity have been recorded upon biochar additions. Evidence shows that bioavailability and plant uptake of key nutrients increases in response to biochar application, particularly when in the presence of added nutrients. Depending on the quantity of biochar added to soil significant improvements in plant productivity have been achieved, but these reports derive predominantly from studies in the tropics. As yet there is limited critical analysis of possible agricultural impacts of biochar application in temperate regions, nor on the likelihood of utilising such soils as long-term sites for carbon sequestration. This review aims to determine the extent to which inferences of experience mostly from tropical regions could be extrapolated to temperate soils and to suggest areas requiring study.

The Biochar Option to Improve Plant Yields: First Results From Some Field and Pot Experiments in Italy

The pyrolysis conversion of agricultural residues into biochar and its incorporation in agricultural soil, avoids CO2 emissions providing a safe long-term soil carbon sequestration. Furthermore, biochar application to soil seems to increase nutrient stocks in the rooting zone, to reduce nutrient leaching and to improve crop yields. This study reports some preliminary results obtained using biochar in two typical Italian agricultural crops. Two field experiments were made on durum wheat (Triticum durum L.) in Central Italy and maize (Zea mays L.) in Northern Italy. In both the field experiments, an increase in yields (+ 10% and + 6% in terms of grain production, respectively) was detected after a biochar application of 10 t ha-1. A further increase in grain production (+24%) was detected when biochar was added with maize residues. The biochar dose-effect curve was studied on perennial ryegrass (Lolium perenne L.) in a pot experiment. The highest increase of dry matter (+120%) was obtained at a biochar rate of 60 t ha-1 and above this threshold, a general reduction of biomass was observed. Results demonstrate the potential of biochar applications to improve in terms of dry matter production, while pointing out the needs for long-term field studies to better understand the effects of biochar on soil.

Isotopic evidence for condensed aromatics from non‐pyrogenic sources in soils – implications for current methods for quantifying soil black carbon

Black carbon (BC) is a complex continuum of partly charred organic matter predominantly consisting of condensed aromatic and graphitic moieties and it has high potential for long-term carbon sequestration in soils and sediments. There has been common agreement that BC is exclusively formed by incomplete combustion of organic matter, while non-pyrogenic sources are negligible. In this study, we investigated the stable carbon isotope signature of benzenepolycarboxylic acids (BPCAs) as molecular markers for BC to test if there is also a significant contribution of non-pyrogenic carbon to this fraction in soils. BPCAs were formed by hot nitric acid oxidation of different soils and analyzed by three different procedures: (i) elemental analysis - isotope ratio mass spectrometry (EA-IRMS) of bulk BPCAs and gas chromatography - combustion - isotope ratio mass spectrometry (GC-C-IRMS) of (ii) BPCA trimethylsilyl (TMS) derivatives, and (iii) BPCA methyl derivatives. Best accuracy and precision of isotope measurements were obtained by EA-IRMS of bulk BPCAs although this method has a risk of contamination by non-BC-derived compounds. The accuracy and precision of GC-C-IRMS measurements were superior for methyl derivatives (+/-0.1 per thousand and 0.5 per thousand, respectively) to those for TMS derivatives (+3.5 per thousand and 2.2 per thousand, respectively). Comparison of BPCA delta(13)C values of soil samples prior to and after laboratory and field incubations with both positive and negative (13)C labels at natural and artificial abundances revealed that up to 25% of the isolated BC fraction in soils had been produced in situ, without fire or charring. Commonly applied methods to quantify BC exclusively formed by pyrogenic processes may thus be biased by a significant non-pyrogenic fraction. Further research is encouraged to better define isolated BC fractions and/or understand mechanisms for non-pyrogenic BC production in soils.

Detecting Publication Bias in Meta-analyses: A Case Study of Fluctuating Asymmetry and Sexual Selection.

Those familiar with human nature and the publication process acknowledge that biases due to selective reporting of results are likely widespread in all fields of academic inquiry that depend on tools of statistical inference (e.g., see Begg and Berlin 1988; Iyengar and Greenhouse 1988; and the extensive discussion following each). However, although qualitative and quantitative methods exist for assessing the prevalence of selective reporting, and selective reporting has been studied in the medical and social sciences (Begg 1994, and references therein), the issue has received little attention in recent meta-analyses of ecological and evolutionary patterns. Clearly, “if publication bias is present, and if it operates in the same direction for all studies (as is likely), then [meta-analysis] is likely not only to produce biased summary estimates but also to produce estimates which are apparently precise and accurate leading to conclusions which may not only be wrong but appear convincing” (Begg and Berlin 1988, p. 437). To biologists unacquainted with the formal study of publication patterns, the terms “selective reporting” (statistical significance of an outcome influences its likelihood of being reported or published) and “publication bias” (the inflation of average effect size due to selective reporting) may imply a conscious intent to deceive, but this

The effects of elevated [CO2] on plant-soil carbon below-ground: A summary and synthesis

We undertake a synthesis of the most relevant results from the presentations at the meeting “Plant-Soil Carbon Below-Ground: The Effects of Elevated CO2” (Oxford-UK, September 1995), many of which are published in this Special Issue. Below-ground responses to elevated [CO2] are important because the capacity of soils for long-term carbon sequestration. We draw the following conclusions: (i) several ecosystems exposed to elevated [CO2] showed sustained increased CO2 uptake at the plot level for many years. A few systems, however, showed complete down-regulation of net CO2 uptake after several years of elevated [CO2] exposure; (ii) under elevated [CO2], a greater proportion of fixed carbon is generally allocated below-ground, potentially increasing the capacity of below-ground sinks; and (iii) some of the increased capacity of these sinks may lead to increased long-term soil carbon sequestration, although strong evidence is still lacking. We highlight the need for more soil studies to be undertaken within ongoing ecosystem-level experiments, and suggest that while some key experiments already established should be maintained to allow long term effects and feedbacks to take place, more research effort should be directed to mechanisms of soil organic matter stabilization.