European Journal of Soil Science

The soil is important in sequestering atmospheric CO₂ and in emitting trace gases (e.g. CO₂, CH₄ and N₂O) that are radiatively active and enhance the ‘greenhouse’ effect. Land use changes and predicted global warming, through their effects on net primary productivity, the plant community and soil conditions, may have important effects on the size of the organic matter pool in the soil and directly affect the atmospheric concentration of these trace gases.

A discrepancy of approximately 350 × 10¹⁵ g (or Pg) of C in two recent estimates of soil carbon reserves worldwide is evaluated using the geo‐referenced database developed for the World Inventory of Soil Emission Potentials (WISE) project. This database holds 4353 soil profiles distributed globally which are considered to represent the soil units shown on a ¹/₂° latitude by ¹/₂° longitude version of the corrected and digitized 1:5 M FAO–UNESCO Soil Map of the World.

Total soil carbon pools for the entire land area of the world, excluding carbon held in the litter layer and charcoal, amounts to 2157–2293 Pg of C in the upper 100 cm. Soil organic carbon is estimated to be 684–724 Pg of C in the upper 30 cm, 1462–1548 Pg of C in the upper 100 cm, and 2376–2456 Pg of C in the upper 200 cm. Although deforestation, changes in land use and predicted climate change can alter the amount of organic carbon held in the superficial soil layers rapidly, this is less so for the soil carbonate carbon. An estimated 695–748 Pg of carbonate‐C is held in the upper 100 cm of the world's soils. Mean C: N ratios of soil organic matter range from 9.9 for arid Yermosols to 25.8 for Histosols. Global amounts of soil nitrogen are estimated to be 133–140 Pg of N for the upper 100 cm. Possible changes in soil organic carbon and nitrogen dynamics caused by increased concentrations of atmospheric CO₂ and the predicted associated rise in temperature are discussed.

Mechanisms for C stabilization in soils have received much interest recently due to their relevance in the global C cycle. Here we review the mechanisms that are currently, but often contradictorily or inconsistently, considered to contribute to organic matter (OM) protection against decomposition in temperate soils: (i) selective preservation due to recalcitrance of OM, including plant litter, rhizodeposits, microbial products, humic polymers, and charred OM; (ii) spatial inaccessibility of OM against decomposer organisms due to occlusion, intercalation, hydrophobicity and encapsulation; and (iii) stabilization by interaction with mineral surfaces (Fe‐, Al‐, Mn‐oxides, phyllosilicates) and metal ions. Our goal is to assess the relevance of these mechanisms to the formation of soil OM during different stages of decomposition and under different soil conditions. The view that OM stabilization is dominated by the selective preservation of recalcitrant organic components that accumulate in proportion to their chemical properties can no longer be accepted. In contrast, our analysis of mechanisms shows that: (i) the soil biotic community is able to disintegrate any OM of natural origin; (ii) molecular recalcitrance of OM is relative, rather than absolute; (iii) recalcitrance is only important during early decomposition and in active surface soils; while (iv) during late decomposition and in the subsoil, the relevance of spatial inaccessibility and organo‐mineral interactions for SOM stabilization increases. We conclude that major difficulties in the understanding and prediction of SOM dynamics originate from the simultaneous operation of several mechanisms. We discuss knowledge gaps and promising directions of future research.

This review discusses the causes and consequences of ‘non‐equilibrium’ water flow and solute transport in large structural pores or macropores (root and earthworm channels, fissures and interaggregate voids). The experimental evidence suggests that pores larger thanc. 0.3 mm in equivalent cylindrical diameter allow rapid non‐equilibrium flow. Apart from their large size and continuity, this is also due to the presence of impermeable linings and coatings that restrict lateral mass exchange. Macropores also represent microsites in soil that are more biologically active, and often more chemically reactive than the bulk soil. However, sorption retardation during transport through such pores is weaker than in the bulk soil, due to their small surface areas and significant kinetic effects, especially in larger macropores. The potential for non‐equilibrium water flow and solute transport at any site depends on the nature of the macropore network, which is determined by the factors of structure formation and degradation, including the abundance and activity of soil biota such as earthworms, soil properties (e.g. clay content), site factors (e.g. slope position, drying intensity, vegetation) and management (e.g. cropping, tillage, traffic). A conceptual model is proposed that summarizes these effects of site factors on the inherent potential for non‐equilibrium water flow and solute transport in macropores. Initial and boundary conditions determine the extent to which this potential is realized. High rain intensities clearly increase the strength of non‐equilibrium flow in macropores, but the effects of initial water content seem complex, due to the confounding effects of soil shrinkage and water repellency. The impacts of macropore flow on water quality are most significant for relatively immobile solutes that are foreign to the soil and whose effects on ecosystem and human health are pronounced even at small leached fractions (e.g. pesticides). The review concludes with a discussion of topics where process understanding is still lacking, and also suggests some potential applications of the considerable knowledge that has accumulated in recent decades.

The term ‘carbon sequestration’ is commonly used to describe any increase in soil organic carbon (SOC) content caused by a change in land management, with the implication that increased soil carbon (C) storage mitigates climate change. However, this is only true if the management practice causes an additional net transfer of C from the atmosphere to land. Limitations of C sequestration for climate change mitigation include the following constraints: (i) the quantity of C stored in soil is finite, (ii) the process is reversible and (iii) even if SOC is increased there may be changes in the fluxes of other greenhouse gases, especially nitrous oxide (N₂O) and methane. Removing land from annual cropping and converting to forest, grassland or perennial crops will remove C from atmospheric CO₂ and genuinely contribute to climate change mitigation. However, indirect effects such as conversion of land elsewhere under native vegetation to agriculture could negate the benefit through increased CO₂ emission. Re‐vegetating degraded land, of limited value for food production, avoids this problem. Adding organic materials such as crop residues or animal manure to soil, whilst increasing SOC, generally does not constitute an additional transfer of C from the atmosphere to land, depending on the alternative fate of the residue. Increases in SOC from reduced tillage now appear to be much smaller than previously claimed, at least in temperate regions, and in some situations increased N₂O emission may negate any increase in stored C. The climate change benefit of increased SOC from enhanced crop growth (for example from the use of fertilizers) must be balanced against greenhouse gas emissions associated with manufacture and use of fertilizer. An over‐emphasis on the benefits of soil C sequestration may detract from other measures that are at least as effective in combating climate change, including slowing deforestation and increasing efficiency of N use in order to decrease N₂O emissions.

Agricultural soils are the main anthropogenic source of nitrous oxide (N₂O), largely because of nitrogen (N) fertilizer use. Commonly, N₂O emissions are expressed as a function of N application rate. This suggests that smaller fertilizer applications always lead to smaller N₂O emissions. Here we argue that, because of global demand for agricultural products, agronomic conditions should be included when assessing N₂O emissions. Expressing N₂O emissions in relation to crop productivity (expressed as above‐ground N uptake: ‘yield‐scaled N₂O emissions') can express the N₂O efficiency of a cropping system. We show how conventional relationships between N application rate, N uptake and N₂O emissions can result in minimal yield‐scaled N₂O emissions at intermediate fertilizer‐N rates. Key findings of a meta‐analysis on yield‐scaled N₂O emissions by non‐leguminous annual crops (19 independent studies and 147 data points) revealed that yield‐scaled N₂O emissions were smallest (8.4 g N₂O‐N kg⁻¹N uptake) at application rates of approximately 180–190 kg N ha⁻¹ and increased sharply after that (26.8 g N₂O‐N kg⁻¹ N uptake at 301 kg N ha⁻¹). If the above‐ground N surplus was equal to or smaller than zero, yield‐scaled N₂O emissions remained stable and relatively small. At an N surplus of 90 kg N ha⁻¹ yield‐scaled emissions increased threefold. Furthermore, a negative relation between N use efficiency and yield‐scaled N₂O emissions was found. Therefore, we argue that agricultural management practices to reduce N₂O emissions should focus on optimizing fertilizer‐N use efficiency under median rates of N input, rather than on minimizing N application rates.

The stability of biochar in soils is the cornerstone of the burgeoning worldwide interest in the potential of the pyrolysis/biochar platform for carbon (C) sequestration. While biochar is more recalcitrant in soil than the original organic feedstock, an increasing number of studies report greater C‐mineralization in soils amended with biochar than in unamended soils. Soil organisms are believed to play a central role in this process. In this review, the variety of interactions that occur between soil micro‐, meso‐ and macroorganisms and biochar stability are assessed. In addition, different factors reported to influence biochar stability, such as biochar physico‐chemical characteristics, soil type, soil organic carbon (SOC) content and agricultural management practices are evaluated. A meta‐analysis of data in the literature revealed that biochar‐C mineralization rates decreased with increasing pyrolysis temperature, biochar‐C content and time. Enhanced release of CO₂ after biochar addition to soil may result from (i) priming of native SOC pools, (ii) biodegradation of biochar components from direct or indirect stimulation of soil organisms by biochar or (iii) abiotic release of biochar‐C (from carbonates or chemi‐sorbed CO₂). Observed biphasic mineralization rates suggest rapid mineralization of labile biochar compounds by microorganisms, with stable aromatic components decomposed at a slower rate. Comparatively little information is available on the impact of soil fauna on biochar stability in soil, although they may decrease biochar particle size and enhance its dispersion in the soil. Elucidating the impacts of soil fauna directly and indirectly on biochar stability is a top research priority.

Plant roots influence the biological, chemical and physical properties of rhizosphere soil. These effects are a consequence of their growth, their activity and the exudation of organic compounds from them. In natural ecosystems, the linkages between inputs of carbon from plants and microbial activity driven by these inputs are central to our understanding of nutrient cycling in soil and the productivity of these systems. This coupling of plant and microbial productivity is also of increasing importance in agriculture, where the shift towards low‐input systems increases the dependence of plant production on nutrient cycling, as opposed to fertilizers. This review considers the processes by which plants can influence the cycling of nutrients in soil, and in particular the importance of organic inputs from roots in driving microbially mediated transformations of N. This coupling of plant inputs to the functioning of the microbial community is beneficial for acquisition of N by plants, particularly in low‐input systems. This occurs through stimulation of microbes that produce exoenzymes that degrade organic matter, and by promoting cycling of N immobilized in the microbial biomass via predation by protozoa. Also, plants increase the cycling of N by changes in exudation in response to nitrogen supply around roots, and in response to browsing by herbivores. Plants can release compounds in exudates that directly affect the expression of genes in microbes, and this may be an important way of controlling their function to the benefit of the plant.

The small organic matter content of mineral soils makes it difficult to obtain ¹³C and ¹⁵N nuclear magnetic resonance (NMR) spectra with acceptable signal‐to‐noise ratios. Subjecting such samples to hydrofluoric acid removes mineral matter and leads to a relative increase in organic material. The effect of treatment with 10% hydrofluoric acid on bulk chemical composition and resolution of solid‐state ¹³C NMR spectra was investigated with six soils, some associated particle size fractions, plant litter and compost. The treatment enhanced the signal‐to‐noise ratio of the solid‐state ¹³C NMR spectra. The improvement in spectrum quality was greatest in the clay fraction of soil contaminated with coal ash. The removal of paramagnetic compounds associated with the ash may be the main reason for the improvement. Based on total C, total N, C/N ratio and intensity distribution of the solid‐state ¹³C NMR spectra, no changes in organic matter composition could be detected, except for a possible loss of carbohydrates. After treatment with HF, solid‐state ¹⁵N NMR spectra of particle size fractions were obtained and indicated that the observable nitrogen is present mostly as peptides and free amino groups. Extraction with hydrofluoric acid is recommended as a routine treatment prior to solid‐state ¹³C and ¹⁵N NMR on soil containing little C or N and soil samples containing paramagnetic compounds from natural or anthropogenic sources.

Loss on ignition (LOI) is one of the most widely used methods for measuring organic matter content in soils but does not have a universal standard protocol. A large number of factors may influence its accuracy, such as furnace type, sample mass, duration and temperature of ignition and clay content of samples. We conducted a series of experiments to quantify these effects, which enabled us to derive (i) guidelines for ignition conditions (sample mass, duration and temperature), (ii) temperature‐specific soil organic matter (SOM) to soil organic carbon (SOC) conversion factors and (iii) clay content‐dependent correction factors for structural water loss (SWL). Bulk samples of a sandy soil (4% clay) and a silt loam soil (25% clay) were used to evaluate the effects of ignition conditions. Samples with a range of clay contents (0–50%) were used to quantify conversion and correction factors. Two furnaces, one without and one with pre‐heated air, did not show significant differences in terms of within‐batch LOI variability. In both furnaces less combustion occurred close to the door, which necessitated tray turning at half‐time as this reduced the standard deviation per batch significantly. Variation in mass loss declined exponentially with sample mass (range, 0.15–20 g). The LOI increased with duration at lower temperatures (≤ 550°C) for the sandy soil. At greater temperatures (600 and 650°C), no effect of duration was found. For the silt loam soil, LOI values increased with duration for each temperature, which was attributed to SWL. The SOM to SOC conversion factor decreased strongly with temperature at an ignition duration of 3 hours from 0.70 (350°C) to 0.57 (500°C) and stabilized around 0.55 between 550 and 650°C, indicating that at temperatures ≥ 550°C all SOM had been removed. The clay correction factor for SWL increased from 0.01 to 0.09 as the temperature of ignition increased from 350 to 650°C. To minimize within‐batch LOI variation we recommend a standard ignition duration of 3 hours, tray turning at half‐time, a sample mass ≥ 20 g and temperatures equal to or greater than 550 °C. To avoid over‐estimates of SOM through structural water loss, the presented SWL correction procedure should always be applied.