The application of biochar is promising for improving the physical, chemical and hydrological properties of soil. However, there are few studies regarding the influence of biochar particle size. This study was conducted to evaluate the effect of biochar size on the physical, chemical and hydrological properties in sandy and loamy tropical soils. For this purpose, an incubation experiment was conducted in the laboratory with eight treatments (control (only soil), two soils (loamy and sandy soil), and three biochar sizes (<0.15 mm; 0.15–2 mm and >2 mm)). Analyses of water content, bulk density, total porosity, pore size distribution, total carbon (TC) and total N (TN) were performed after 1 year of soil–biochar-interactions in the laboratory. The smaller particle size <0.15 mm increased water retention in both soils, particularly in the loamy soil. Bulk density slightly decreased, especially in the loamy soil when biochar > 2 mm and in the sandy soil with the addition of 0.15–2 mm biochar. Porosity increased in both soils with the addition of biochar in the range of 0.15–2 mm. Smaller biochar particles shifted pore size distribution to increased macro and mesoporosity in both soils. Total carbon content increased mainly in sandy soil compared to control treatment; the highest carbon amount was obtained in the biochar size 0.15–2 mm in loamy soil and <0.15 mm in sandy soil, while the TN content and C:N ratio increased slightly with a reduction of the biochar particle size in both soils. These results demonstrate that biochar particle size is crucial for water retention, water availability, pore size distribution, and C sequestration.

Biochar application has improved soil properties contributing to crop growth. This study evaluates the effect of biochar amount on soil physical, chemical and hydraulic properties in sandy (SD) and clay loam (CL) soils under tropical conditions. An incubation experiment was installed under laboratory conditions with eight treatments (control, two kinds of soils, SD and CL, and three biochar doses (6.25, 12.5, and 25 Mg ha−1). Analyses of soil water retention, bulk density (BD), total porosity (TP), pores size, total carbon (TC), and N were performed after one year. The BD slightly decreased by 0.035 and 0.062 Mg m−3 and TP increased by 1.87 and 2.31% in CL and SD soil respectively, upon 6.25 to 25 Mg ha−1 biochar application. TC increased in CL and SD by 6.5 and 4.2 kg kg−1, respectively, compared to control. The total nitrogen content increased upon biochar addition in CL soil than in SD soil. We found a positive effect of biochar on water availability, microporosity, and a small effect on water retention, especially for CL soil at high biochar application, but this influence did not occur for SD, possibly due to the short time of interaction.

Biochar is known for its effects on carbon sequestration and soil fertility. However, there is a lack of information about its effects on soil physical and hydraulic properties for tropical soils. We assessed the effects of biochar (BC) plus sugar cane filter cake (FC) rate, and time of interaction on soil physical and hydraulic properties under humid tropical conditions. For this purpose, a field experiment was installed at a loamy sandy soil with five treatments and four replicates: control (only soil), 25 Mg ha −1 sugar cane filter cake, and 25 Mg ha −1 sugar cane filter cake plus 6.25, 12.5, and 25 Mg ha −1 Miscanthus biochar, respectively, two soil depths (0-10 and 10-20 cm) and two times of interaction (9 and 18 months). Physical properties (aggregate stability, bulk density, total porosity, pores size distribution) and hydraulic properties (soil water holding capacity, hydraulic conductivity, plant-available water holding capacity) were measured after nine and eighteen months. The bulk density decreased slightly, and the porosity increased after nine months, for the biochar plus sugar cane filter cake (both 25 Mg ha −1). After 18 months, biochar plus filter cake interaction increase micropores, aggregate stability, and plant-available water content. Saturated hydraulic conductivity was not influenced by sugar cane filter cake. However, biochar significantly reduced saturated hydraulic conductivity when combined with sugar cane filter cake after 18 months. We concluded that sugar cane filter cake in combination with biochar modified the pore size distribution, slightly increased plant-available water holding capacity, and significantly decreased saturated hydraulic conductivity.

The impacts of biochar application in combination with compost on soil hydrological conditions such as infiltration, water-storage capacity, and methods of soil wettability, are not well understood. The aim of this study was to fill this knowledge gap and propose the best composition proportion of compost and/or biochar to improve sand soil water balance. For this purpose, we examined three different mixtures of sand and compost and six different mixtures of sand, compost, and biochar with respect to their wettability and water infiltration using tree methods, (I) Water Droplet Penetration Time Test (WDPT test), (II) Wilhelmy Plate Method (WPM) and (III) Capillary Rise Method (CRM) applied at different soil water potential (pF) levels. The different mixtures were characterized by texture, bulk density, field capacity, and soil water holding capacity through measurement of the water content of soil columns. Increasing biochar plus compost or compost alone had contributed to increasing the soil water holding capacity and water available content. The best composition with the highest water storage capacity, plant-available water, and wettability was: 0.6% of biochar, 5.4% soil organic matter (SOM), and 93.99% of sand. According to the WDPT test, the increased content of biochar contributed to improving the soil wettability, this test was preferred since 1) WDPT appeared to allow the best estimates of wetting behavior of the different mixtures, 2) the column samples were the least disturbed, and 3) the test was particularly feasible at different water content levels. However, WDPT was strongly dependent on the conductor of the experiment and the accurate measurement of short infiltration times (0s - 2s) proved challenging. The WDPT results indicate that biochar might enhance the infiltration capacity under natural conditions and the combined results indicate that biochar-compost-applications are not expected to cause ecologically critical hydrophobicity or a critical lowering of the sandy soil infiltration capacity. Graphical abstract

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