Biochar application has been reported to improve the physical, chemical, and hydrological properties of soil. However, the information about the size fraction composition of the applied biochar as a factor that may have an impact on the properties of soil-biochar mixtures is often underappreciated. Our research shows how sunflower husk biochar (pyrolyzed at 650 °C) can modify the water retention characteristics of arable sandy soil depending on the biochar dose (up to 9.52 wt.%) and particle size (<50 µm, 50–100 µm, 100–250 µm). For comparison, we used soil samples mixed with biochar passed through 2 mm sieve and an unamended reference. The addition of sieved biochar to the soil caused a 30% increase in the available water content (AWC) in comparing to the soil without biochar. However, the most notable improvement (doubling the reference AWC value from 0.078 m³ m⁻³ to 0.157 m³ m⁻³) was observed at the lowest doses of biochar (0.95 and 2.24 wt.%) and for the finest size fractions (below 100 µm). The water retention effects on sandy soil are explained as the interplay between the dose, the size of biochar particles, and the porous properties of biochar fractions.

The current Polish Agricultural Drought Monitoring System (ADMS) adopted Climatic Water Balance (CWB) as the main indicator of crop losses caused by drought conditions. All meteorological data needed for CWB assessment are provided by the ground meteorological stations network. In 2018, the network consisted of 665 stations, among which in only 58 stations full weather parameters were registered. Therefore, only these stations offered a possibility to estimate the exact values of potential evapotranspiration, which is a component of the CWB algorithm. This limitation affects the quality of CWB raster maps, interpolated on the basis of the meteorological stations network for the entire country. However, the interpolation process itself may introduce errors; therefore, the adaptation of satellite data (that are spatially continuous) should be taken into account, even if the lack of data due to cloudiness remains a serious problem. In this paper, we involved the remote sensing data from MODIS instrument and considered the ability to integrate those data with values determined by using ground measurements. The paper presents results of comparisons for the CWB index assessed using ground station data and those obtained from potential evapotranspiration as the product from Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing instrument. The comparisons of results were performed for specific points (locations of ground stations) and were expressed by differences in means values. Analysis of Pearson’s correlation coefficient (r), Mann–Kendal trend test (Z-index), mean absolute error (MAE) and root mean square error (RMSE) for ten years’ series were evaluated and are presented. In addition, the basic spatial interpretation of results has been proposed. The correlation test revealed the r coefficient in the range from 0.06 to 0.68. The results show good trend agreement in time between two types of CWB with constantly higher values of this index, which is estimated using ground measurement data. In results for 34 (from 43 analyzed) stations the Mann–Kendal test provide the consistent trend, and only nine trends were inconsistent. Analyses revealed that the disagreement between the two considered indices (determined in different ways) increased significantly in the warmer period with a significant break point between R7 and R8 that falls at the end of May for each examined year. The value of MAE varied from 80 mm to 135 mm.

Biochar is a carbon-rich material obtained from the process of biomass pyrolysis. Due to its desirable properties, it is discussed as a soil amendment to improve soil quality; for example, adding biochar can change soil water retention by modifying soil textural and structural properties. However, the optimal fabrication conditions and proportions of biochar particles sizes, that would improve soil properties are still not precisely known. In our research, we investigated the influence of grain size and a dose of biochar on water retention of sandy soil. For this purpose, water retention curves (pF) were measured, as it indicates such important properties as plant available water, field water capacity, wilting point. The studies were carried out on podzol soil samples taken from meadow located in Sekow, Poland, mixed with different percentage mass content of sunflower husk biochar produced in 650-750°C (0.95, 2.36, 4.76 and 9.52% of sample weight). Samples contain one of biochar granulometric fraction: 250-100, 100-50 or less than 50 µm. The control included soil samples with the addition of mixed fractions of biochar and soil without biochar. The research method we used allows obtaining information about plant available water content by comparing differences in water content between 0.06 and 5 bar pressure points which corresponding to a 1.85-3.7 pF. In this range, most plants can use water for their growth and development. Our results revealed that, surprisingly, soil with all fractions of biochar reduces the amount of available water for plants compared to the control (soil without biochar), regardless of the biochar dose applied. However, fractionated biochar can both increase or decrease the soil water content, depending on the particle size and dose. Small doses of sunflower husk biochar (0.95 and 2.36%) and the finest fraction (<50μm) have the most beneficial effects for water retention of investigated soil. Our research may strongly suggest the biochar producers that the production of biochar with the right fraction may be more favourable for increasing soil water retention. Research was partially conducted under the project “Water in soil - satellite monitoring and improving the retention using biochar” no. BIOSTRATEG3/345940/7/NCBR/2017 which was financed by Polish National Centre for Research and Development in the framework of “Environment, agriculture and forestry” – BIOSTRATEG strategic R&D programme.