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.
Highlights
Organic amendments—materials of plant or animal origin that can be added to soil, such as manures, biosolids, green wastes, and composts—are applied to millions of acres of mesic or irrigated croplands each year, where they can boost soil carbon (C) and fertility (Diacono & Montemurro, 2010; Hargreaves, Adl, & Warman, 2008; Khaleel, Reddy, & Overcash, 1981; National Research Council, 2002)
Interest in organic amendment application to rangelands has been increasing in parallel to soil health efforts in croplands, bolstered by growing interest in using these materials to restore degraded rangelands, a need that may expand as global changes increasingly challenge these lands (Huang et al, 2017)
For every outcome we analyzed, the paucity of long‐term studies was a limitation to assessing the long‐term effects of organic amendment application
Summary
Organic amendments—materials of plant or animal origin that can be added to soil, such as manures, biosolids, green wastes, and composts—are applied to millions of acres of mesic or irrigated croplands each year, where they can boost soil carbon (C) and fertility (Diacono & Montemurro, 2010; Hargreaves, Adl, & Warman, 2008; Khaleel, Reddy, & Overcash, 1981; National Research Council, 2002). Most rangelands today are found in arid, semiarid, or Mediterranean climates, as mesic nonforested lands tend to be converted to croplands or pastures with higher economic returns (Sayre, 2017) In these dry climates, temperature and precipitation patterns—which are highly variable within and between years—exert stronger controls on vegetation productivity and composition than do human management actions (Booker, Huntsinger, Bartolome, Sayre, & Stewart, 2013; Westoby, Walker, & Noy‐Meir, 1989). Over time, decreases in plant diversity or increases in invasive species could potentially reduce primary productivity (Isbell et al, 2013) and/or the seasonal availability and quality of forage (Haferkamp, Grings, Heitschmidt, MacNeil, & Karl, 2001) These considerations suggest that analysis of the potential benefits and risks of organic amendment addition to rangelands—including which types of amendments and sites are likely to maximize benefits and minimize harms—could provide a valuable basis for decision‐making in light of increasing interest in this practice. After modeling how effect sizes for the eight most data‐rich outcomes varied according to climate zone, time since application, amount of amendment applied, and amendment N concentration, we used those models to compare benefits and harms for defined amounts of amendment applied and amendment N concentrations
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