The intensification of agricultural production on a global scale is necessary in order to secure the food supply for an increasing world population. However, in most tropical environments, sustainable agriculture faces large constraints due to low nutrient content and accelerated mineralization of soil organic matter (SOM) [1
]. Therefore, the low cation exchange capacity (CEC) of the soils further decreases. Under such circumstances, the efficiency of applied mineral fertilizers is very low when the loss of mobile nutrients from the topsoil is enhanced by high rainfall [2
]. Additionally, coarse-structured soils with low clay content are characterized by a lack of both water retention and nutrient-holding capacities that are necessary for plant growth [3
]. Many farmers cannot afford the costs of regular applications of mineral fertilizers. Consequently, nutrient deficiency is prevalent in many crop production systems of the tropics [4
In contrast to these deficient soils, the famous Terra Preta
maintains its fertility, despite its 2000 years of age [5
].This is partly due to the tremendous nutrient levels and SOM stocks that act as a long-term, slow-release fertilizer [5
]. The physical and hydrological properties in this soil also contrast with adjacent soils. For instance, Glaser et al. [4
] verified that the water retention of Terra Preta
was 18% higher as compared to adjacent soils. The secret of the Terra Preta
is in the biochar; this type of earth contains on average 50 Mg ha−1
of biochar per hectare in the upper 50 cm soil depth. Adjacent reference soils contain only 4 Mg biochar, which is about 10 times less than that in Terra Preta
]. The existence of Terra Preta
in Amazonia today proves that it is possible to convert infertile soils’ insufficient physical and hydrological properties to sustainable, fertile soils with good physical and hydrological properties. It is evident that biochar is a key ingredient in making Terra Preta
so special [6
] and is the ingredient to improve the soil quality on intensive agriculture.
Biochar as key for Terra Preta
formation can improve physical and hydrological properties such as water retention, water available content, bulk density, and porosity [7
]. For example, the addition of 20 Mg ha−1
biochar to sandy soil in northeast Germany increased its water-holding capacity by 100% [10
]. At the same time, the incorporation of biochar into soil has been shown to enhance soil capacity to retain plant nutrients, decrease nutrient losses from leaching, and increase soil water holding capacity, pH and SOM [11
Many functions in one product are possible because the biochar is composed of condensed aromatic moieties that give biochar its black color and are responsible for its stability, which makes biochar an interesting compound for C sequestration [13
]. In addition, biological degradation and consequently partial oxidation results in the formation of functional groups on the edges of biochar, causing reactivity in soil such as nutrient retention or organomineral stabilization [15
]. The highly porous material leads to enhanced air and water storage in soil [6
Because of these functions, biochar can be used as a soil amendment to improve the quality of agricultural soils [4
]. The application of biochar to soil is considered as a win-win strategy to improve the soil physical conditions that influence soil hydraulic properties and water retention [2
] and increase soil fertility [6
The effects on the chemical and physical properties of soil are dependent on the biochar amount, pyrolysis temperature, biomass type, and biochar particle size [7
]. However, few studies have focused comprehensively on the effects of biochar particle size on hydraulic, physical and chemical properties [7
]. Understanding biochar particle size is important because it affects the interaction with the soil matrix. The greater and/or lesser interaction of biochar with the soil matrix may have a direct effect on its chemical, physical and hydrological properties [22
]. This interaction is dependent of biochar particle size and therefore can influence the physical and hydraulic properties of soil [7
Small biochar particles can more easily interact with soil particles to form aggregates than large biochar particles [23
]. In addition, the greater specific surface area per unit of mass increases the water retention [7
] and plant-available water [24
]. In another study, Głąb et al. [25
] found that bulk density decreased, total porosity increased, plant-available water content decreased, and water repellence decreased with an increase in the biochar size from 0.5 to 2 mm.
The role of biochar on temperate soils has been discussed in the literature [7
]. However, for tropical soil conditions, there is a lack of information on this promising soil conditioner. The present study, as far as we understand, is the first research that presents data on the fate of biochar application on the physical, chemical and hydrological properties of tropical soil.
We hypothesize that the biochar application has a positive effect on the physical, chemical and hydrological properties of soil under tropical conditions. However, this effect is dependent on particle size; the reduction of particle size causes an increase in water retention and total porosity and a decrease in available water content and bulk density. Therefore, the objective of this study was to determine the effect of biochar particle size on the physical, hydrological and chemical properties of soil. The knowledge of the relationship between soil’s physical, hydrological and chemical properties and biochar particle size is potentially useful in management applications, particularly those concerning irrigation and recovery of degraded areas.
The physical properties of soil, such as bulk density and total porosity, were dependent on the biochar size, especially in loamy soil. Small particles of biochar reduced the volume of soil pores (<0.15 mm diameter) but increased the volume of mesopores (0.15–0.50 mm diameter) and macropores (>0.50 mm diameter).
Biochar application improved the soil water characteristics by slightly increasing the plant-available water storage capacity, especially when the finest fraction was used in sandy soil. The biochar has a great potential to improve soil water retention in the finest fraction of loamy and sandy soils.
The benefits found in our research show that this material can be recommended for farmers as a soil amendment to improve the chemical, physical and hydrological quality of their soil.
For the farmers to obtain improvement in the chemical, physical and hydrological properties, the biochar can be used in the finest fraction <0.15 mm with rates of 25 Mg ha−1. Moreover, due to the difficulty of applying a small particle size in agricultural soils, the biochar can be co-composted before it is applied to soil.
Further investigations are recommended to better understand the influence of biochar particle size on hydraulic conductivity, rates and time of interaction, as well as cost-to-benefit ratios in sandy and loamy soils.