The Bioeconomy & Circular Economy in Southern Arizona: Case Study 6.2

6.2 Biochar for Irrigated Desert Croplands

By Shelby Höglund, Consultant/Grant Writer, Desert Cedar Consulting, LLC
Joseph Blankinship, Associate Professor, Department of Environmental Science, University of Arizona

Increasing soil water retention and nutrient availability to improve grain yield in southern Arizona using pyrolyzed forestry waste.

Biochar is a stable, carbon-rich product produced from organic materials (Lehmann, 2007). Mixing biochar into soil has been shown to improve soil water-holding capacity and nutrient retention when combined with an additional nutrient source (Artiola et al., 2012; Omondi et al., 2016). Biochar itself can retain water and nutrients due to its many pores, large surface area, and surface reactivity (Liang et al., 2006; Omondi et al., 2016). Because the type of organic waste (e.g., softwood) and method of production can vary, different types of biochars have different characteristics. Biochar contains carbon that persists for hundreds to thousands of years (Keiluweit et al., 2010), so adding biochar to soil increases long-lasting soil carbon content. In addition, mixing biochar with compost or adding biochar during the composting process (referred to as co-composting) can improve the nutrient content of finished compost (Hagemann et al., 2017).

Increasing biochar application rate does not always lead to proportional increases in soil water and nutrient retention. A greenhouse study at the University of Arizona Controlled Environment Agriculture Center found that increasing biochar application rate did not proportionally increase the capacity of a desert cropland soil to retain water or nutrients for plants. This study was conducted using a commercially available biochar produced from softwood in a modified biomass reactor at 760 °C. The soil’s capacity to retain water for plants did not increase in treatments with biochar application rates lower than 73 Mg ha-1 (81 U.S. t ha-1). Lower application rates also lost more plant-available phosphate via leaching whereas higher application rates retained more plant-available phosphate compared to the control treatment without biochar.

Agricultural applications for biochar. In other regions of the world, biochar is used in soil to improve crop yield and certain soil qualities. Applying biochar to croplands can provide several environmental benefits, but not without limits. Site-specific research should guide decision-making. Because biochar improves soil water retention, its use in croplands in the Southwestern U.S. may help sustain food production as water resources become scarcer. This was investigated in a field study at the University of Arizona Campus Agricultural Center that compared three soil amendments: 1) biochar mixed with finished compost, 2) co-composted biochar, and 3) a control treatment with compost. Three irrigation treatments were implemented to investigate if biochar could maintain or increase crop yield when irrigation frequency was reduced.

When soils were frequently irrigated in this study, biochar and co-composted biochar produced 31% and 45% greater grain yield in the first year compared to the control treatment. However, biochar treatments did not improve grain yield compared to the control when irrigation frequency was reduced. In addition, biochar treatments only affected plant-available nutrients in the first year of this study; soils with co-composted biochar had greater available nitrate while soils with biochar and co-composted biochar reduced available phosphate compared to the control. Based on these results, biochar can increase grain yield, but may not alleviate irrigation water requirements. Because biochar increased crop yield per area, biochar can potentially reduce land area required for growing crops while maintaining total yield and therefore less land will need irrigation.

Furthermore, compost manufacturers can use biochar to improve the quality of finished compost. In the field study, co-composting biochar increased the concentration of nitrate and phosphate in finished compost. As mentioned, more nutrients in the finished compost did not result in greater nutrient availability when amendments were mixed with soil. Composting with biochar also accelerated the composting process. A shorter composting process will reduce equipment and labor requirements as well as move materials through compost facilities faster.

Biochar for Circularity. Producing biochar and co-composted biochar from local organic material for use as soil amendments is a way to reuse material and recycle nutrients (from compost) and carbon back into soil. Organic materials often used to produce biochar include wood (chipped wood, landscaping waste, forestry residue, etc.) and agricultural waste (straw, crop residue, etc.). Producing biochar and co-composted biochar diverts organic materials from landfills and has the potential to reduce the demand for chemical fertilizers.