The mechanisms of crop response are covered later and in the absence of long-term data (other than terra preta), development of predictive certainty for the longevity and durability of yield and other effects, particularly in relation to specific crop and soil types, is critical to guide selection of feedstock, production method, and application rate. Predictability and certainty are required to assign a financial value to the agronomic value of biochar and to open the possibility for large-scale deployment.
3.3. Impact on soil performance and resource implications
Both the mineral and the organic components of soil influence water-holding capacity. Although higher levels of soil organic matter increase water-holding capacity and can be deliberately managed, changes will be temporary unless a regime is maintained. Glaser et al. (2002) reported that water retention in terra preta was 18% higher than in adjacent soils where charcoal was low or absent, and likely a combined consequence of higher biochar content and higher levels of organic matter that appear to be associated with charcoal in these soils. As biochar is broadly stable in soil, it has potential to provide a direct and long-term modification to soil water-holding capacity through its often macroporous nature (predominantly μm-sized pores), reflecting cellular structures in the feedstock from which it is typically produced. The direct impact of particle size distribution in biochar added to the soil may have a direct impact on soil texture at the macroscale, but this effect must be short-lived as physically biochar appears to divide rapidly in soil to particles of silt size or less ( Brodowski et al., 2007), presumably by abrasion and the effects of shrink–swell or freeze-thaw, etc. This suggests that in the longer term the effect of biochar on available moisture will be positive in sandy soils ordinarily dominated by much larger pores than present in biochar, rather neutral in medium-textured soils, and potentially detrimental to moisture retention in clay soils—though since preferential flow is important in cracking clays, the impact of biochar on the nature of soil cracking might be important. The usual measure for pore size distribution in soil is the moisture release curve, which shows how quickly moisture is drawn from a soil under increasing tension. Although this method is well suited to discriminating soils of contrasting texture, it is not well able to discriminate the effects of subtle differences in soil management at a particular location: levels of replication have not been sufficient to attach statistical significance to differences in mean characteristics of amended and non-amended soils. In recent work, moisture release curves were determined for a loamy sand field soil to which up to 88 t ha− 1 biochar had been applied ( Gaskin et al., 2007). For soils where biochar has been added at rates up to 22 t ha− 1 there was no difference compared to non-amended soil, but at the highest rate there was a significant effect at water potentials in the range 0.01–0.20 MPa. At the highest potential the volumetric water content was double that of soil without biochar added. Soils of lower bulk density are generally associated with higher soil organic matter, and bulk density provides a crude indicator for how organic matter modifies soil structure and pore-size distribution. Many studies where the effect of biochar on crop yield has been assessed have cited moisture retention as a key factor in the results. Soil temperature, soil cover, evaporation, and evapotranspiration affect soil water availability, so comparison of volumetric water content between biochar-amended and control soils in field experiments may be confounded by indirect effects, that is, on plant growth and soil thermal properties. In addition to the chemical stabilization of nutrients, modification of the physical structure of the bulk soil may result in biochar not simply increasing the capacity of soil to retain water, but also nutrients in soil solution.
There are several reasons why biochar might be expected to decrease the potential for nutrient leaching in soils, and thus enhance nutrient cycling and also protect against leaching loss. In field studies where positive yield response to biochar application has been observed, enhanced nutrient dynamics has been frequently cited as an explanation. However, the underlying processes have not been demonstrated directly, and no empirical or mechanistic description has been established. In general, both mineral and organic fractions of soil contribute to cation exchange capacity (CEC) in soil, although not in a summative manner. The CEC largely controls the flush of positively charged ammonium ions after fertilizer or manure application, and rapid mineralization of soil organic matter under favorable environmental conditions. These relatively loose associations do not automatically preclude acquisition by the plant, but have an important effect on mitigating losses of nitrate by leaching, and consequently on agronomy and avoided eutrophication of aquatic and marine environments. Only certain inorganic components of the soil contribute significant CEC due to mineralogy, abundance, and particle size and surface area, with certain types of clay being most important. On a mass basis the exchange capacity of soil organic matter is up to 50 times greater than for any mineral, but is a small proportion of soil mass in most agricultural situations, particularly under tropical conditions. In heavy textured soils in climates favoring organic matter about one-third of total CEC may derive from organic matter (Stevenson, 1982). Since the mineralization of organic matter is also a major source of ammonium in soil, increasing organic matter inputs to increase soil organic matter can potentially increase rather than decrease leaching losses. Available evidence suggests that the specific CEC of biochar is consistently higher than that of whole soil, clay minerals, or soil organic matter and analogy can be drawn to the very high CEC associated with activated carbon that defines its function as a sorption medium for decolorizing and purifying solutions. Since secondary thermal treatment of charcoal is one method for activating charcoal substrate, it is expected that of the process parameters that appear to affect the CEC of biochar, temperature should be the most critical (Gaskin et al., 2007). This function of biochar arises from specific surface area, which increases with temperature through the formation of micropores (Bird et al., 2008), and the abundance of carboxyl groups on those surfaces. The apparent proliferation of carboxyl groups on char surfaces over time, within or outside the soil environment, suggests either partial oxidation of accessible surfaces by biotic and abiotic processes (Cheng et al., 2006) or, alternatively, chemisorption. To develop understanding of this process and the rate at which it proceeds, it may be necessary to perfect methods for recovery of larger samples of intact and increasingly aged biochar from field soils. Although information on the CEC of fresh pyrolysis products relates to limited feedstock and production conditions, and it appears that CEC can substantially develop prior to biochar application to soil. The inherent stability of biochar creates a distinction between the CEC that it provides, and CEC associated with soil organic matter. Importantly, there is no obvious constraint on the level of benefit that that could be attained with repeated addition, by incremental enhancement of CEC. Provided that biochar is biologically stable, the benefits of higher CEC could be achieved but without causing seasonal flushes of nitrate. It is possible that if biochar were proven to significantly impact retention of nutrients and benefit water dynamics at application rates feasible for strategic deployment in vulnerable catchments, it could assist in the mitigation of diffuse pollution from agriculture. It could also be possible to utilize its sorptive capacity to remove contamination in water treatment processes. Studies that demonstrate the capacity for biochar to remove nitrate (Mizuta et al., 2004) and phosphate (Beaton et al., 1960) have been cited in this context.