Many studies report that, under some circumstances, amending soil with biochar can improve field capacity and plant-available water. However, little is known about the mechanisms that control these ...improvements, making it challenging to predict when biochar will improve soil water properties. To develop a conceptual model explaining biochar's effects on soil hydrologic processes, we conducted a series of well constrained laboratory experiments using a sand matrix to test the effects of biochar particle size and porosity on soil water retention curves. We showed that biochar particle size affects soil water storage through changing pore space between particles (interpores) and by adding pores that are part of the biochar (intrapores). We used these experimental results to better understand how biochar intrapores and biochar particle shape control the observed changes in water retention when capillary pressure is the main component of soil water potential. We propose that biochar's intrapores increase water content of biochar-sand mixtures when soils are drier. When biochar-sand mixtures are wetter, biochar particles' elongated shape disrupts the packing of grains in the sandy matrix, increasing the volume between grains (interpores) available for water storage. These results imply that biochars with a high intraporosity and irregular shapes will most effectively increase water storage in coarse soils.
The addition of charcoal (or biochar) to soil has significant carbon sequestration and agronomic potential, making it important to determine how this potentially large anthropogenic carbon influx ...will alter ecosystem functions. We used column experiments to quantify how hydrologic and nutrient-retention characteristics of three soil materials differed with biochar amendment. We compared three homogeneous soil materials (sand, organic-rich topsoil, and clay-rich Hapludert) to provide a basic understanding of biochar-soil-water interactions. On average, biochar amendment decreased saturated hydraulic conductivity (K) by 92% in sand and 67% in organic soil, but increased K by 328% in clay-rich soil. The change in K for sand was not predicted by the accompanying physical changes to the soil mixture; the sand-biochar mixture was less dense and more porous than sand without biochar. We propose two hydrologic pathways that are potential drivers for this behavior: one through the interstitial biochar-sand space and a second through pores within the biochar grains themselves. This second pathway adds to the porosity of the soil mixture; however, it likely does not add to the effective soil K due to its tortuosity and smaller pore size. Therefore, the addition of biochar can increase or decrease soil drainage, and suggests that any potential improvement of water delivery to plants is dependent on soil type, biochar amendment rate, and biochar properties. Changes in dissolved carbon (C) and nitrogen (N) fluxes also differed; with biochar increasing the C flux from organic-poor sand, decreasing it from organic-rich soils, and retaining small amounts of soil-derived N. The aromaticity of C lost from sand and clay increased, suggesting lost C was biochar-derived; though the loss accounts for only 0.05% of added biochar-C. Thus, the direction and magnitude of hydraulic, C, and N changes associated with biochar amendments are soil type (composition and particle size) dependent.
The production of pyrogenic carbon (PyC; a continuum of organic carbon (C) ranging from partially charred biomass and charcoal to soot) is a widely acknowledged C sink, with the latest estimates ...indicating that ~50% of the PyC produced by vegetation fires potentially sequesters C over centuries. Nevertheless, the quantitative importance of PyC in the global C balance remains contentious, and therefore, PyC is rarely considered in global C cycle and climate studies. Here we examine the robustness of existing evidence and identify the main research gaps in the production, fluxes and fate of PyC from vegetation fires. Much of the previous work on PyC production has focused on selected components of total PyC generated in vegetation fires, likely leading to underestimates. We suggest that global PyC production could be in the range of 116–385 Tg C yr⁻¹, that is ~0.2–0.6% of the annual terrestrial net primary production. According to our estimations, atmospheric emissions of soot/black C might be a smaller fraction of total PyC (<2%) than previously reported. Research on the fate of PyC in the environment has mainly focused on its degradation pathways, and its accumulation and resilience either in situ (surface soils) or in ultimate sinks (marine sediments). Off‐site transport, transformation and PyC storage in intermediate pools are often overlooked, which could explain the fate of a substantial fraction of the PyC mobilized annually. We propose new research directions addressing gaps in the global PyC cycle to fully understand the importance of the products of burning in global C cycle dynamics.
To best use biochar as a sustainable soil management and carbon (C) sequestration technique, we must understand the effect of environmental exposure on its physical and chemical properties because ...they likely vary with time. These properties play an important role in biochar's environmental behavior and delivery of ecosystem services. We measured biochar before amendment and four years after amendment to a commercial nectarine orchard at rates of 5, 15 and 30tha−1. We combined two pycnometry techniques to measure skeletal (ρs) and envelope (ρe) density and to estimate the total pore volume of biochar particles. We also examined imbibition, which can provide information about soil hydraulic conductivity. Finally, we investigated the chemical properties, surface, inner layers atomic composition and C1s bonding state of biochar fragments through X-ray photoelectron spectroscopy (XPS). Ageing increased biochar skeletal density and reduced the water imbibition rate within fragments as a consequence of partial pore clogging. However, porosity and the volume of water stored in particles remained unchanged. Exposure reduced biochar pH, EC, and total C, but enhanced total N, nitrate-N, and ammonium-N. X-ray photoelectron spectroscopy analyses showed an increase of O, Si, N, Na, Al, Ca, Mn, and Fe surface (0–5nm) atomic composition (at%) and a reduction of C and K in aged particles, confirming the interactions of biochar with soil inorganic and organic phases. Oxidation of aged biochar fragments occurred mainly in the particle surface, and progressively decreased down to 75nm. Biochar surface chemistry changes included the development of carbonyl and carboxylate functional groups, again mainly on the particle surface. However, changes were noticeable down to 75nm, while no significant changes were measured in the deepest layer, up to 110nm. Results show unequivocal shifts in biochar physical and chemical properties/characteristics over short (~years) timescales.
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•Porosity and surface chemistry control many biochar-induced environmental services.•Ageing increased biochar skeletal and envelope density, but not porosity.•Changes in hydrologic behaviors were linked to surface changes.•Environmental exposure increased surface at% of O, S, N, Na, Al, Ca, Mn and Fe.•Oxidation included the development of O-containing functional groups down to 75nm.
It is clear that the density and porosity of biochar will impact its mobility in the environment, its interaction with the soil hydrologic cycle, and its suitability as an ecological niche for soil ...microorganisms. However, the wide range of biochar pore sizes complicates biochar porosity characterization, making it challenging to find methods appropriate to connect the fundamental physical properties of density and porosity to environmental outcomes. Here, we report the use of two fast, simple density measurement techniques to characterize biochar density and porosity. We measured biochar skeletal density by helium pycnometry and envelope density by displacement of a dry granular suspension. We found that biochar skeletal density ranged from 1.34 g cm−3 to 1.96 g cm−3, and increased with pyrolysis temperature. Biochar envelope density ranged from 0.25 g cm−3 to 0.60 g cm−3, and was higher for wood biochars than grass biochars—a difference we attribute to plant cell structures preserved during pyrolysis. We compared the pore volumes measured by pycnometry with those measured by nitrogen gas sorption and mercury porosimetry. We show that biochar pore volumes measured by pycnometry are comparable to the values obtained by mercury porosimetry, the current benchmark method. We also show that the majority of biochar pore volume is in macropores, and thus, is not measured by gas sorption analysis. These fast, simple techniques can now be used to study the relationship between biochar's physical properties and its environmental behaviors.
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•Biochar's density and porosity relate to its environmental interactions and transport.•Biochar porosity is difficult to characterize due the wide range of pore sizes.•We measured biochar porosity by combining skeletal and envelope density measurements.•Biochar skeletal density increases with pyrolysis temperature.•Biochar envelope density and porosity vary primarily with biomass feedstock.
Drylands soils worldwide are naturally colonized by microbial communities known as biocrusts. These soil microbiomes render important ecosystem services associated with soil fertility, water holding ...capacity, and stability to the areas they cover. Because of the importance of biocrusts in the global cycling of nutrients, there is a growing interest in describing the many microbial configurations these communities display worldwide. However, comprehensive 16S rRNA genes surveys of biocrust communities do not exist for much of the planet: for example, in the continents of South America and the northern part of Africa. The absence of a global understanding of biocrust biodiversity has lead us to assign a general importance to community members that may, in fact, be regional. Here we report for the first time the presence of biocrusts in Colombia (South America) through 16S rRNA genes surveys across an arid, a semi-arid and a dry subtropical region within the country. Our results constitute the first glance of the Bacterial/Archaeal communities associated with South American biocrust microbiomes. Communities where cyanobacteria other than
prevail, despite the latter being considered a key species elsewhere, illustrate differentiable results in these surveys. We also find that the coastal biocrust communities in Colombia include halo-tolerant and halophilic species, and that niche preference of some nitrogen fixing organisms deviate from previously described global trends. In addition, we identified a high proportion (ranging from 5 to 70%, in average) of cyanobacterial sequences that did not match any formally described cyanobacterial species. Our investigation of Colombian biocrusts points to highly diverse communities with climatic regions controlling taxonomic configurations. They also highlight an extensive local diversity to be discovered which is central to better design management and restoration strategies for drylands soils currently undergoing disturbances due to land use and global warming. Finally, this field study highlights the need for an improved mechanistic understanding of the response of key biocrust community members to changes in moisture and temperature.
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 biology literature that recognize the observed large variations of biochar 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 thereby inactivation of growth-inhibiting substances likely plays a role for 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 the need for symbionts. In the short term, the release of a variety of organic 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.
► Microbial biomass in most cases increases in the presence of biochar. ► Notable exceptions are mycorrhizae in situations of abundant nutrient supply. ► No direct negative effects of biochars on roots have been detected. ► Critical to further advances is the recognition and reporting of the diversity of biochars.
Fire‐derived black carbon (BC: charcoal and soot) has been thought to be a passive player in soils, contributing to the refractory soil organic carbon (SOC) pool, but playing no role in pedogenesis ...and regional short‐term carbon cycling. This model, however, is at odds with recent results on the role of charcoal in soil fertility and its detection in the dissolved organic carbon (DOC) pool. For example, if BC simply accumulated passively in soils, its pattern of accumulation should match a simple model correlating fire frequency to BC storage. Instead, soil type, climate, biota, and land use practices all appear to play roles in controlling whether BC accumulates or is lost from soils. We summarize current knowledge of BC‐soil interactions and construct a new paradigm describing the controls on BC storage in soils. We reconcile the refractory‐labile BC paradox by proposing a model where BC storage is controlled by (1) fire frequency, (2) ecosystem presence or absence of aromatic precursor carbon and appropriate combustion conditions, (3) biological or physical mixing to remove BC from the soil surface, where it is vulnerable to combustion in future fires, (4) the presence or absence of soil mineral fractions able to sorb BC into the long‐term stable carbon pool, and (5) the presence of microbial communities capable of degrading aromatic carbon. We also recognize that soil BC/SOC ratios are strongly influenced by land‐use practices and add (6) human activities as a final control.
•Fine biochar decreased hydraulic conductivity (K) due to decrease in soil grain size.•Coarse biochar reduced K due to bimodal grain size distribution.•Only a small fraction of biochar carbon moved ...as DOC (0.06–0.18wt%).
The amendment of soil with biochar can sequester carbon and alter hydrologic properties by changing physical and chemical characteristics of soil. To understand the effect of biochar amendment on soil hydrology, we measured the hydraulic conductivity (K) of biochar–sand mixtures as well as dissolved organic carbon (DOC) in leachate. Specifically, we assessed the effects of biochar concentration and particle size on K and amount of DOC in the soil leachate. To better understand how physical properties influenced K, we also measured the skeletal density of biochars and sand, and the bulk density, the water saturation, and the porosity of biochar–sand mixtures. Our model soil was sand (0.251–0.853mm) with biochar rates from 2 to 10wt% (gbiochar/gtotal soil×100%). As biochar (<0.853mm) concentration increased from 0 to 10wt%, K decreased by 72±3%.
When biochar particle size was equal to, greater than, and less than particle size of sand, we found that biochar in different particle sizes have different effects on K. For a 2wt% biochar rate, K decreased by 72±2% when biochar particles were finer than sand particles, and decreased by 15±2% when biochar particles were coarser than sand particles. When biochar and sand particle size were comparable, we observed no significant effect on K. We propose that the decrease of K through the addition of fine biochar was because finer biochar particles filled spaces between sand particles, which increased tortuosity and reduced pore throat size of the mixture. The decrease of K associated with coarser biochar was caused by the bimodal particle size distribution, resulting in more compact packing and increased tortuosity.
The loss of biochar C as DOC was related to both biochar rate and particle size. The cumulative DOC loss was 1350% higher from 10wt% biochar compared to pure sand. This large increase reflected the very small DOC yield from pure sand. In addition, DOC in the leachate decreased as biochar particle size increased. For all treatments, the fraction of carbon lost as DOC ranged from 0.06 to 0.18wt% of biochar. These experiments suggest that mixing sandy soils with biochar is likely to reduce infiltration rates, holding water near the surface longer with little loss of biochar-derived carbon to groundwater and streams.
Global warming accelerates decomposition of soil organic carbon (SOC) pools with varying rates and temperature sensitivities. Black carbon (BC) materials are among the slowest decomposing components ...of the SOC pool. Although BC is a large component of SOC in many systems, the influence of temperature on decomposition of BC bearing different chemical and physical structures remains poorly understood. Four BC materials, produced by carbonizing corn residue and oak wood at 350 and 600 °C (corn-350-BC, corn-600-BC, oak-350-BC, and oak-600-BC), were mixed with pure sand and incubated at 4, 10, 20, 30, 45, and 60 °C for 1 year. Corn-BC was more porous than oak-BC as determined by scanning electron microscopy (SEM). Increasing the charring temperature from 350 to 600 °C led to greater aromaticity with 5−15% more C in aromatic rings and a 39−57% increase in both nonprotonated aromatic C and aromatic bridgehead C quantified by nuclear magnetic resonance (NMR) spectroscopy and a greater degree of order and development of C layers as observed by transmission electron microscopy (TEM). With a temperature increase from 4 to 60 °C, C loss of corn-350-BC increased from 10 to 20%, corn-600-BC, from 4 to 20%, oak-350-BC, from 2.3 to 15%, and oak-600-BC from 1.5 to 14% of initial C content, respectively. Temperature sensitivity (Q 10) decreased with increasing incubation temperature and was highest in oak-600-BC, followed by oak-350-BC, corn-600-BC, and corn-350-BC, indicating that decomposition of more stable BC was more sensitive to increased temperature than less stable materials. Carbon loss and potential cation exchange capacity (CECp) significantly (p < 0.05) correlated with O/C ratios and change in O/C ratios, suggesting that oxidative processes were the most important mechanism controlling BC decomposition in this study.
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