Soil amendment with biochar is evaluated globally as a means to improve soil fertility and to mitigate climate change. However, the effects of biochar on soil biota have received much less attention than its effects on soil chemical properties. A review of the literature reveals a significant number of early studies on biochar-type materials as soil amendments either for managing pathogens, as inoculant carriers or for manipulative experiments to sorb signaling compounds or toxins. However, no studies exist in the soil biologyliterature that recognize the observed largevariations ofbiochar physico-chemical properties. This shortcoming has hampered insight into mechanisms by which biochar influences soil microorganisms, fauna and plant roots. Additional factors limiting meaningful interpretation of many datasets are the clearly demonstrated sorption properties that interfere with standard extraction procedures for soil microbial biomass or enzyme assays, and the confounding effects of varying amounts of minerals. In most studies, microbial biomass has been found to increase as a result of biochar additions, with significant changes in microbial community composition and enzyme activities that may explain biogeochemical effects of biochar on element cycles, plant pathogens, and crop growth. Yet, very little is known about the mechanisms through which biochar affects microbial abundance and community composition. The effects of biochar on soil fauna are even less understood than its effects on microorganisms, apart from several notable studies on earthworms. It is clear, however, that sorption phenomena, pH and physical properties of biochars such as pore structure, surface area and mineral matter play important roles in determining how different biochars affect soil biota. Observations on microbial dynamics lead to the conclusion of a possible improved resource use due to co-location of various resources in and around biochars. Sorption and therebyinactivation of growth-inhibiting substances likelyplaysa rolefor increased abundance of soil biota. No evidence exists so far for direct negative effects of biochars on plant roots. Occasionally observed decreases in abundance of mycorrhizal fungi are likely caused by concomitant increases in nutrient availability,reducing theneedfor symbionts.Inthe shortterm,therelease ofavarietyoforganic molecules from fresh biochar may in some cases be responsible for increases or decreases in abundance and activity of soil biota. A road map for future biochar research must include a systematic appreciation of different biochar-types and basic manipulative experiments that unambiguously identify the interactions between biochar and soil biota.

http://www.css.cornell.edu/faculty/lehmann/pictures/publ/SoilBiolBiochem%2043,%201812–1836,%202011%20Lehmann.pdf

Biochar effects on soil biota – A review

The relationship between soil structure and the ability of soil to stabilize soil organic matter (SOM) is a key element in soil C dynamics that has either been overlooked or treated in a cursory fashion when developing SOM models. The purpose of this paper is to review current knowledge of SOM dynamics within the framework of a newly proposed soil C saturation concept. Initially, we distinguish SOM that is protected against decomposition by various mechanisms from that which is not protected from decomposition. Methods of quantification and characteristics of three SOM pools defined as protected are discussed. Soil organic matter can be: (1) physically stabilized, or protected from decomposition, through microaggregation, or (2) intimate association with silt and clay particles, and (3) can be biochemically stabilized through the formation of recalcitrant SOM compounds. In addition to behavior of each SOM pool, we discuss implications of changes in land management on processes by which SOM compounds undergo protection and release. The characteristics and responses to changes in land use or land management are described for the light fraction (LF) and particulate organic matter (POM). We defined the LF and POM not occluded within microaggregates (53–250 μm sized aggregates as unprotected. Our conclusions are illustrated in a new conceptual SOM model that differs from most SOM models in that the model state variables are measurable SOM pools. We suggest that physicochemical characteristics inherent to soils define the maximum protective capacity of these pools, which limits increases in SOM (i.e. C sequestration) with increased organic residue inputs.

Stabilization mechanisms of soil organic matter: Implications for C-saturation of soils

Summary
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.

Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions – a review

Instead of containing stable and chemically unique ‘humic substances’, as has been widely accepted, soil organic matter is a mixture of progressively decomposing organic compounds; this has broad implications for soil science and its applications. The exchange of nutrients, energy and carbon between soil organic matter, the soil environment, aquatic systems and the atmosphere is important for agricultural productivity, water quality and climate. Long-standing theory suggests that soil organic matter is composed of inherently stable and chemically unique compounds. Here we argue that the available evidence does not support the formation of large-molecular-size and persistent ‘humic substances’ in soils. Instead, soil organic matter is a continuum of progressively decomposing organic compounds. We discuss implications of this view of the nature of soil organic matter for aquatic health, soil carbon–climate interactions and land management. Soil organic matter contains a large portion of the world's carbon and plays an important role in maintaining productive soils and water quality. Nevertheless, a consensus on the nature of soil organic matter is lacking. Johannes Lehmann and Markus Kleber argue that soil organic matter should no longer be seen as large and persistent, chemically unique substances, but as a continuum of progressively decomposing organic compounds.

The contentious nature of soil organic matter

Black Carbon (BC) may significantly affect nutrient retention and play a key role in a wide range of biogeochemical processes in soils, especially for nutrient cycling. Anthrosols from the Brazilian Amazon (ages between 600 and 8700 yr BP) with high contents of biomassderived BC had greater potential cation exchange capacity (CEC measured at pH 7) per unit organic C than adjacent soils with low BC contents.Synchrotron-based near edge X-ray absorption fine structure (NEXAFS) spectroscopy coupled with scanning transmission X-ray microscopy (STXM) techniques explained the source of the higher surface charge of BC compared with non-BC by mapping crosssectional areas of BC particles with diameters of 10 to 50 mm for C forms. The largest cross-sectional areas consisted of highly aromatic or only slightly oxidized organic C most likely originating from the BC itself with a characteristic peak at 286.1 eV, which could not be found in humic substance extracts, bacteria or fungi. Oxidation significantly increased from the core of BC particles to their surfaces as shown by the ratio of carboxyl-C/aromatic-C. Spotted and non-continuous distribution patterns of highly oxidized C functional groups with distinctly different chemical signatures on BC particle surfaces (peak shift at 286.1 eV to a higher energy of 286.7 eV) indicated that non-BC may be adsorbed on the surfaces of BC particles creating highly oxidized surface. As a consequence of both oxidation of the BC particles themselves and adsorption of organic matter to BC surfaces, the charge density (potential CEC per unit surface area) was greater in BC-rich Anthrosols than adjacent soils. Additionally, a high specific surface area was attributable to the presence of BC, which may contribute to the high CEC found in soils that are rich in BC.

/pdf/black-carbon-increases-cation-exchange-capacity-in-soils-3fak9ns4e1.pdf

Black Carbon Increases Cation Exchange Capacity in Soils

The nature of organic carbon in the < 2, 2–20, 20–53, 53–200, and 200–2000 mu m fractions of four surface soils was determined using solid state 13C nuclear magnetic resonance (n.m.r.) spectroscopy with cross polarisation and magic angle spinning (CP/MAS). Analyses were repeated after high energy ultraviolet photo-oxidation was performed on the three finest fractions. All four soils, studied contained appreciable amounts of physically protected carbon while three of the soils contained even higher amounts of charcoal. It was not possible to measure the charcoal content of soils directly, however, after photo-oxidation, charcoal remained and was identified by its wood-like morphology revealed by scanning electron microscopy (SEM) together with a highly aromatic chemistry determined by solid state 13C n.m.r. Charcoal appears to be the major contributor to the 130 ppm band seen in the n.m.r. spectra of many Australian soils. By using the aromatic region in the n.m.r. spectra, an approximate assessment of the charcoal distribution through the size fractions demonstrated that more than 88% of the charcoal present in two of the soils occurred in the < 53 µm fractions. These soils contained up to 0.8 g C as charcoal per 100 g of soil and up to 30% of the soil carbon as charcoal. Humic acid extractions performed on soil fractions before and after photo-oxidation suggest that charcoal or charcoal-derived material may also contribute significantly to the aromatic signals found in the n.m.r. spectra of humic acids. Finely divided charcoal appears to be a major constituent of many Australian soils and probably contributes significantly to the inert or passive organic carbon pool recognised in carbon turnover models.

The chemistry and nature of protected carbon in soil

Five surface soils of differing chemical and mineralogical compositions were subjected to either a sequence of dithionite/citrate extractions in which the soil: citrate ratio was varied or to a sequence of 1% or 2% HF extractions. The 2% HF treatment resulted in the removal of the highest Fe concentrations (79-95%) while the dithionite/citrate extractions were less effective in removing Fe from the same soils (18-75%). The Fe remaining after HF treatment appeared to be mostly associated with ilmenite crystals which were only slowly attacked by the dilute acid. During the 2% HF treatments, some organic carbon was lost (8-17%), but this loss had no significant effect on the organic chemistry of the samples as determined by CP/MAS 13C n.m.r. spectroscopy. The total 13C signal recovered after the various treatments was found to be correlated, in order of decreasing significance, with the mineral fraction present in the sample, the organic carbon/Fe ratio and the mass magnetic susceptibility. The expression (organic carbon/Fe) -0.147 (mineral fraction present in the sample) +0.043 (1/mass magnetic susceptibility), accounted for 85.3% of the variation in the relative visibility of the 13C signal. Prior to soil state CP/MAS 13C n.m.r. analysis, the recommended pretreatment for surface soils containing Fe involves a sequence of 2% HF extractions in the order 5 x 1 h, 2 x 16 h and 1 x 64 h. For soils high in Fe, this procedure allows CP/MAS 13C n.m.r. spectra to be acquired that would otherwise be difficult to obtain. It also results in a significant increase in sensitivity and in resolution of the 13C n.m.r. spectra of soils with moderate Fe contents.

The removal of magnetic materials from surface soils. A solid state 13C CP/MAS n.m.r. study

A method for the semiquantitative estimation of charcoal (char) in soils is reported. The technique, performed on the 90% of soil char in the <53 μm fraction. Because of the assumptions used, estimates of char by this approach must be considered to be semiquantitative, but are conservative and provide estimates of the minimum char content of a soil.

Estimation of charcoal (char) in soils

The Walkley‐Black Method is shown to recover charcoal carbon (C) from both charcoal samples made in the laboratory from a range of plant materials as well as from soils containing various amounts of relic charcoal. The rate of recovery of charcoal C depends on the nature of the material from which it is derived and its particle size but not on its surface area. From the data presented, it is clear that the Walkley‐Black Method recovers charcoal C with a high enough efficiency so that, at the concentrations of charcoal found in soil, given its fine particle size and the potentially diverse nature of its origin, it is not possible to differentiate between charcoal C and other organic forms found in soil by this method.

Does the walkley‐black method determine soil charcoal?

Abstract Vegetation indices (VI), especially the normalised difference vegetation index (NDVI), are used to determine management units (MU) and to explain quantity and quality of vineyard production. How do NDVI maps from different sensing technologies differ in a production context? What part of the variability of yield and quality can they explain? This study compares high-resolution multispectral, multi-temporal data from CropCircle, SpectroSense + GPS, Parrot Sequoia + multispectral camera equipped UAV, and Sentinel-2 imagery over two seasons (2019 and 2020). The objective was to assess whether the date of data collection (phenological growth stage) influences the correlations between NDVI and crop production. The comparison of vineyard NDVI data from proximal and remote sensing in both a statistical and a productive context showed strong similarities between NDVI values from similar sensors (0.69 &lt; r &lt; 0.96), but divergences between proximal and airborne/spaceborne observations. Exploratory correlation analysis between NDVI layers and grape yield and total soluble solids data (TSS) showed high correlations (maximum |r|= 0.91 and |r|= 0.74, respectively), with correlations increasing as the season progressed. No relationship with must titratable acidity or pH was found. Finally, proximal sensors explained better the variability in yield and quality for grapes in the early and late growth stages. The UAV's MUs described the yield of both years better than the other sensors. In 2019, the PCA-based MUs explained the TSS variability better than the UAV-related zones. Due to their coarse spatial resolution, the satellite data proved inconsistent in explaining the variability. 

/pdf/investigation-of-the-similarities-between-ndvi-maps-from-2xp795rb.pdf

JA Taylor

Papers

The chemistry and nature of protected carbon in soil

The removal of magnetic materials from surface soils. A solid state 13C CP/MAS n.m.r. study

Estimation of charcoal (char) in soils

Does the walkley‐black method determine soil charcoal?

Investigation of the similarities between NDVI maps from different proximal and remote sensing platforms in explaining vineyard variability