Organic amendment additions to rangelands: A meta‐analysis of multiple ecosystem outcomes

Abstract

Interest in land application of organic amendments—such as biosolids, composts, and manures—is growing due to their potential to increase soil carbon and help mitigate climate change, as well as to support soil health and regenerative agriculture. While organic amendments are predominantly applied to croplands, their application is increasingly proposed on relatively arid rangelands that do not typically receive fertilizers or other inputs, creating unique concerns for outcomes such as native plant diversity and water quality. To maximize environmental benefits and minimize potential harms, we must understand how soil, water, and plant communities respond to particular amendments and site conditions. We conducted a global meta‐analysis of 92 studies in which organic amendments had been added to arid, semiarid, or Mediterranean rangelands. We found that organic amendments, on average, provide some environmental benefits (increased soil carbon, soil water holding capacity, aboveground net primary productivity, and plant tissue nitrogen; decreased runoff quantity), as well as some environmental harms (increased concentrations of soil lead, runoff nitrate, and runoff phosphorus; increased soil CO2 emissions). Published data were inadequate to fully assess impacts to native plant communities. In our models, adding higher amounts of amendment benefitted four outcomes and harmed two outcomes, whereas adding amendments with higher nitrogen concentrations benefitted two outcomes and harmed four outcomes. This suggests that trade‐offs among outcomes are inevitable; however, applying low‐N amendments was consistent with both maximizing benefits and minimizing harms. Short study time frames (median 1–2 years), limited geographic scope, and, for some outcomes, few published studies limit longer‐term inferences from these models. Nevertheless, they provide a starting point to develop site‐specific amendment application strategies aimed toward realizing the potential of this practice to contribute to climate change mitigation while minimizing negative impacts on other environmental goals.

The role of soil carbon in natural climate solutions

“…Organic biosolids from cities are a large pool of organic material that are often a pollution and waste disposal problem 47 , that could provide substrate to build soil health and sequester carbon in soils. Exogenous organic matter additions can stimulate rangeland productivity and sequester endogenous organic matter beyond the actual tonnage of compost/biosolids applied 48 , but may pose a risk to native plant biodiversity 49 and more research is needed (and is underway) to understand how universal these findings are. Early research from row-crop systems suggests endogenous vs. exogenous organic matter have similar effects 50 .…”

Supporting evidence varies for rangeland management practices that seek to improve soil properties and forage production in California

“…These amendments have been shown to improve on-ranch soil C sequestration and forage quantity and quality at two sites in Northern California (Ryals et al 2014;Ryals et al 2016). In addition, a recent global meta-analysis by Gravuer et al (2019) showed that rangeland forage production and soil C increase in the first few years following application of organic amendments. However, Gravuer et al (2019) caution that these benefits may come with trade-offs depending on the type of organic material used and the rates of application.…”

“…In addition, a recent global meta-analysis by Gravuer et al (2019) showed that rangeland forage production and soil C increase in the first few years following application of organic amendments. However, Gravuer et al (2019) caution that these benefits may come with trade-offs depending on the type of organic material used and the rates of application.…”

“…Given more time, changes in soil properties may start to emerge (Gravuer et al 2019). Indeed, modeling efforts have demonstrated the potential for compost to improve on-site and public ecosystem benefits across California over longer time frames (Flint et al 2018;Ryals et al 2015;Silver et al 2018).…”

“…Previous work indicates that these practices have the potential to influence plant dynamics and increase soil organic carbon (hereafter soil C) in some rangeland ecosystems (Byrnes et al 2018;Dybala, Matzek, et al 2019;Gravuer et al 2019) -and there is reason to believe they can also affect other soil properties, such as soil compaction and nitrogen (N) availability (Byrnes et al 2018;Dahlgren et al 1997;Zhou et al 2017). In California's arid and semiarid Mediterranean climate, however, the effects of rangeland management may be overwhelmed by strong interannual variability in precipitation (Jackson and Bartolome 2007), spatial variability in soil type and topography (Booker et al 2013;Graham and O'Geen 2016) and land-use history (Huntsinger et al 2007).…”