Abstract

Many studies report that, under some circumstances, amending soil with biochar can improve field capacity and plant-available water. However, little is known about the mechanisms that control these improvements, making it challenging to predict when biochar will improve soil water properties. To develop a conceptual model explaining biochar’s effects on soil hydrologic processes, we conducted a series of well constrained laboratory experiments using a sand matrix to test the effects of biochar particle size and porosity on soil water retention curves. We showed that biochar particle size affects soil water storage through changing pore space between particles (interpores) and by adding pores that are part of the biochar (intrapores). We used these experimental results to better understand how biochar intrapores and biochar particle shape control the observed changes in water retention when capillary pressure is the main component of soil water potential. We propose that biochar’s intrapores increase water content of biochar-sand mixtures when soils are drier. When biochar-sand mixtures are wetter, biochar particles’ elongated shape disrupts the packing of grains in the sandy matrix, increasing the volume between grains (interpores) available for water storage. These results imply that biochars with a high intraporosity and irregular shapes will most effectively increase water storage in coarse soils.

Highlights

  • Biochar is charcoal made for the purpose of soil amendment [1]

  • We develop a conceptual model of how these physical properties drive changes in water retention of biochar-sand mixtures

  • Biochar grain size played an important role in the water retention of sand-biochar mixtures

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Summary

Introduction

Amending soil with biochar is an approach to mitigate climate change [2] and to improve crop productivity [1, 3]. Biochar can affect plant growth by altering soil hydrologic properties [4,5,6,7] and nutrient availability [8]. Water movement and storage in soils are crucial for nutrient delivery and plant productivity. Biochar has the potential to alter soil hydrology and to drive shifts in the amount of water stored in soils. To understand how biochar amendment may influence water delivery to plants, we must understand how biochar affects soil hydrologic processes. While many studies report effects of specific biochars on specific soil water properties [9, 10], there is a dearth

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