Biochar is a charcoal that is used to improve land rather than as a fuel. Biochar is produced from biomass, usually through the process of pyrolysis. Due to the molecular structure and strength of the chemical bonds, the carbon in biochar is in a stable form and not readily mineralized to CO2 (as is the fate of most of the carbon in biomass). Because the carbon in biochar derives (via photosynthesis) from atmospheric CO2, biochar has the potential to be a net negative carbon technology/carbon dioxide removal option. Biochar is not a single homogeneous material. Its composition and properties (including longevity) differ according to feedstock (source biomass), pyrolysis (production) conditions, and its intended application. This variety and heterogeneity have so far eluded an agreed methodology for calculating biochar’s carbon abatement.
Meta-analyses increasingly summarize the effects of biochar in pot and field trials. These results illuminate that biochar may have important agronomic benefits in poorer acidic tropical and subtropical soils, with one study indicating an average 25% yield increase across all trials. In temperate soils the impact is modest to trivial and the same study found no significant impact on crop yield arising from biochar amendment. There is much complexity in matching biochar to suitable soil-crop applications and this challenge has defied development of simple heuristics to enable implementation. Biochar has great potential as a carbon management technology and as a soil amendment. The lack of technically rigorous methodologies for measuring recalcitrant carbon limits development of the technology according to this specific purpose.
Article
Biochar: An Emerging Carbon Abatement and Soil Management Strategy
Holly Morgan, Saran Sohi, and Simon Shackley
Article
Causes of Soil Salinization, Sodification, and Alkalinization
Elisabeth N. Bui
Driving forces for natural soil salinity and alkalinity are climate, rock weathering, ion exchange, and mineral equilibria reactions that ultimately control the chemical composition of soil and water. The major weathering reactions that produce soluble ions are tabled. Where evapotranspiration is greater than precipitation, downward water movement is insufficient to leach solutes out of the soil profile and salts can precipitate. Microbes involved in organic matter mineralization and thus the carbon, nitrogen, and sulfur biogeochemical cycles are also implicated. Seasonal contrast and evaporative concentration during dry periods accelerate short-term oxidation-reduction reactions and local and regional accumulation of carbonate and sulfur minerals. The presence of salts and alkaline conditions, together with the occurrence of drought and seasonal waterlogging, creates some of the most extreme soil environments where only specially adapted organisms are able to survive. Sodic soils are alkaline, rich in sodium carbonates, with an exchange complex dominated by sodium ions. Such sodic soils, when low in other salts, exhibit dispersive behavior, and they are difficult to manage for cropping. Maintaining the productivity of sodic soils requires control of the flocculation-dispersion behavior of the soil. Poor land management can also lead to anthropogenically induced secondary salinity. New developments in physical chemistry are providing insights into ion exchange and how it controls flocculation-dispersion in soil. New water and solute transport models are enabling better options of remediation of saline and/or sodic soils.
Article
The Family of HYDRUS Models
Jiří Šimůnek, Giuseppe Brunetti, Martinus Th. van Genuchten, and Miroslav Šejna
HYDRUS is a Windows-based modeling software package that can be used to analyze water flow and heat and solute transport in variably saturated porous media (e.g., soils or the vadose zone). The HYDRUS software includes an interactive graphics-based interface for data preprocessing, soil profile discretization, and graphic presentation of the results. Historically, HYDRUS consisted of two independent software packages. While HYDRUS-1D simulated flow and transport processes in one dimension and was a public domain software, HYDRUS (2D/3D; and earlier HYDRUS-2D) extended the simulation capabilities to the second and third dimensions and was distributed commercially. These two previously independent software packages were merged in 2023 into a single software package called HYDRUS.
The capabilities of the HYDRUS software packages have been significantly expanded by various standard and nonstandard specialized add-on modules. The standard add-on modules are fully incorporated and supported by the HYDRUS graphical user-friendly interfaces (GUIs) and documented in detail in the technical and user manuals. This is not the case for several additional nonstandard modules, which require additional work outside the GUI. A commonality of all HYDRUS add-on modules is that they simulate variably saturated water flow and heat and solute transport in porous media. The specialized add-on modules provide additional capabilities, such as considering general reactive transport (in the HPx models) or reactive transport with specific chemistry (notably the Wetland and UNSATCHEM modules). Other modules provide additional flow and/or transport modeling processes, such as to account for preferential flow (the DualPerm module), colloid-facilitated solute transport (the C-Ride module), or the transport of polyfluoroalkyl substances (PFAS; the PFAS module) or fumigant (the Fumigant module) compounds.
The HYDRUS models are among the most widely used numerical models for simulating processes in the subsurface. There are many thousands of HYDRUS users worldwide, with many applications (also several thousand) appearing in peer-reviewed international literature and many technical reports.
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Indigenous Polynesian Agriculture in Hawaiʻi
Noa Kekuewa Lincoln and Peter Vitousek
Agriculture in Hawaiʻi was developed in response to the high spatial heterogeneity of climate and landscape of the archipelago, resulting in a broad range of agricultural strategies. Over time, highly intensive irrigated and rainfed systems emerged, supplemented by extensive use of more marginal lands that supported considerable populations. Due to the late colonization of the islands, the pathways of development are fairly well reconstructed in Hawaiʻi. The earliest agricultural developments took advantage of highly fertile areas with abundant freshwater, utilizing relatively simple techniques such as gardening and shifting cultivation. Over time, investments into land-based infrastructure led to the emergence of irrigated pondfield agriculture found elsewhere in Polynesia. This agricultural form was confined by climatic and geomorphological parameters, and typically occurred in wetter, older landscapes that had developed deep river valleys and alluvial plains. Once initiated, these wetland systems saw regular, continuous development and redevelopment. As populations expanded into areas unable to support irrigated agriculture, highly diverse rainfed agricultural systems emerged that were adapted to local environmental and climatic variables. Development of simple infrastructure over vast areas created intensive rainfed agricultural systems that were unique in Polynesia. Intensification of rainfed agriculture was confined to areas of naturally occurring soil fertility that typically occurred in drier and younger landscapes in the southern end of the archipelago. Both irrigated and rainfed agricultural areas applied supplementary agricultural strategies in surrounding areas such as agroforestry, home gardens, and built soils. Differences in yield, labor, surplus, and resilience of agricultural forms helped shape differentiated political economies, hierarchies, and motivations that played a key role in the development of sociopolitical complexity in the islands.
Article
Infiltration of Water Into Soil
John Nimmo and Rose Shillito
The infiltration of water into soil has profound importance as a central component of the hydrologic cycle and as the means of replenishing soil water that sustains terrestrial life. Systematic quantitative study of infiltration began in the 19th century and has continued through to the present as a central topic of soils, soil physics, and hydrology. Two forces drive infiltration: gravity, and capillarity, which results from the interaction of air-water surface tension with the solid components of soil. There are also two primary ways water moves into and within the soil. One is diffuse flow, through the pores between individual soil grains, moving from one to the next and so on. The other is preferential flow, through elongated channels such as those left by worms and roots. Diffuse flow is slow and continues as long as there is a net driving force. Preferential flow is fast and occurs only when water is supplied at high intensity, as during irrigation, major rainstorms, or floods. Both types are important in infiltration. Especially considering that preferential flow does not yet have a fully accepted theory, this means that infiltration entails multiple processes, some of them poorly understood. The soil at a given location has a limit to how much water it can absorb—the infiltration capacity. The interplay between the mode and rate of water supply, infiltration capacity, and characteristics of the soil and surrounding terrain determines infiltration into the soil. Much effort has gone into developing means of measuring and predicting both infiltration capacity and the actual infiltration rate. Various methods are available, and research is needed to improve their accuracy and ease of use.
