Second-year effects of biochar, biosolids, and greenwaste on tall fescue under deficit irrigation: Part II

Authors: BARROS SILVA FILHO, JAIME - Us Forest Service (FS); MONTGOMERY, JONATHAN - Humboldt State University; Anderson, Raymond; MCGIFFEN, JR., MILTON - University Of California, Riverside

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2026
Publication Date: 6/25/2026
Citation: Barros Silva Filho, J., Montgomery, J., Anderson, R.G., McGiffen, Jr., M.E. 2026. Second-year effects of biochar, biosolids, and greenwaste on tall fescue under deficit irrigation: Part II. Agronomy. 16(13). Article 1230. https://doi.org/10.3390/agronomy16131230.
DOI: https://doi.org/10.3390/agronomy16131230

Interpretive Summary: Landscapes often rely on soil amendments such as biochar, composted biosolids, and greenwaste to improve water conservation and support healthy turfgrass. While these materials can help turf establish, it is less clear whether their benefits continue once the turf is mature. This study tested how different amendments affected soil water retention and tall fescue performance over a 108-day period under two irrigation levels. Results showed that amendment effects during the mature phase differed markedly from what was observed during establishment. A high rate of biochar, which previously improved early growth, created more fine soil pores in the mature stand. This increased the amount of water held too tightly for plants to use and ultimately reduced plant-available water. A moderate biochar rate maintained more stable soil pore structure and supported consistent water availability across irrigation treatments. Greenwaste and biosolids increased total soil water content, but much of this water was also held beyond plant-extractable levels. As a result, turf stands receiving these amendments declined late in the season despite appearing to have adequate moisture. Overall, the study demonstrates that effective drought mitigation depends not just on increasing soil water content, but on retaining water in a range plants can use. Soil amendments that perform well during establishment may not provide the same hydraulic advantages at maturity. For long-term turf resilience, selecting amendments that maintain stable water dynamics is more important than short-term establishment benefits. These results are of interests to agronomic and irrigation managers who are interested in using biochar to improve soil health and irrigation efficiency.

Technical Abstract: Soil amendments are widely applied to improve water conservation in urban turfgrass systems, yet whether their hydraulic benefits persist beyond the establishment phase remains an open question. This study evaluated the effects of biochar, composted biosolids, and greenwaste on soil water retention and tall fescue (Schedonorus arundinaceus) performance during a 108-day mature-stand trial under deficit (50% ET0) and moderate (85% ET0) irrigation. Canopy performance was assessed through visual quality, normalized difference vegetation index, percent green cover, and dark green color index, and linked to soil water retention curves fitted to the van Genuchten model. Results revealed a sharp departure from establishment-phase patterns reported in Part I. Whereas fescue grown with high-rate biochar (24.71 t ha'¹) excelled during establishment, its mature-stand behavior shifted toward fine-pore dominance, increasing the permanent wilting point (PWP) and reducing plant-available water (PAW). Moderate biochar (12.36 t ha'¹) maintained stable pore networks and consistent PAW across irrigation regimes. Greenwaste and biosolids increased total volumetric water content but elevated PWP plant wilting point sufficiently to reduce PAW, inducing late-season canopy decline despite apparent moisture abundance. Durable drought mitigation requires retaining water within the plant-extractable tension range. Because first-year success does not guarantee mature-stand resilience, amendment selection must prioritize hydraulic stability over initial establishment advantages.