Grain-Size Effects on Field Capacity of Soil-Biochar Mixtures

Abstract:

While it is known that under some circumstances amending soil with biochar improves soil hydrologic properties, the mechanisms driving these improvements are not well-understood. This is a serious gap: without mechanism understanding, it is much harder to predict when biochar will improve soil properties. Grain size likely plays a key role in controlling biochar effects on water movement and storage, because grain size (of both biochar and soils) plays a major role in soil porosity. Here we report water potential of soil-biochar mixtures in the context of grain size and porosity measurements to better understand the mechanisms controlling how pores in biochar and soil store water. Our soil-biochar mixtures are sand (0.251-0.853 mm) with 2 wt% biochar; we used two separate biochar grain size ranges. Comparison of pure sand with biochar+sand mixtures shows a field capacity (θ_fc, water content at soil water potential = 33 kPa) increase from 0.014 to 0.027 kg_water/kg_soil+water (increase of 93%) when biochar is 0.251-0.853 mm (same size as sand) and a field capacity increase from 0.014 to 0.032 kg_water/kg_soil+water (increase of 129%) when biochar is 0.853-2.00 mm (larger than sand). Thus when biochar is larger than sand, θ_fc of biochar+sand mixtures (0.032 kg_water/kg_soil+water) is higher than θ_fc of biochar+sand mixtures (0.027 kg_water/kg_soil+water) when biochar is the same size as sand. Biochar+sand mixtures have interporosity (porosity between grains) and intraporosity (pores inside biochar particles). The average intraporosity of our biochars is 0.61±0.01 (61±1% of the biochar volume is pore space). Based our results we interpret that this intraporosity can store water and drives the increase of θ_fc for biochar-amended sand. Field capacity also increases with pore size decreases because smaller pores tend to hold water tightly due to higher capillary forces. The increase of θ_fc with increased biochar grain size may be caused by the bimodal particle size distribution in the mixture of large biochar particles and sand, which results in reduced pore sizes. Extensions of this research will include measuring the permanent wilting point (water content at soil water potential = 1500 kPa) of biochar+sand mixtures to investigate how increased water storage in biochar+sand mixtures may increase plant available water