Orchard recycling improves climate change adaptation and mitigation potential of almond production systems

minimizing the environmental footprint of agriculture while increasing adaptation to harsh climatic conditions [5]. Ambitious climate mitigation and adaptation policies are increasingly recognizing this potential and incentivizing the development and adoption of innovative man- agement practices that promote cycling and storage of SOC. However, uncertainties remain about the potential of management practices to help meet regional climate smart agriculture and soil health targets. Given the anticipated larger incentives to offset CO2 emissions by investing in increasing agricultural soil C storage, it is essential to estimate the potential for C gains and soil health co-benefits associated with specific management practices as well as the downstream impacts on adaptation to future climate conditions. Biomass recycling is gaining widespread interest as a means to use agriculture and its by- products as a resource for renewable energy production. Grinding and incorporating whole trees into soil during orchard turnover may also provide a unique opportunity to store C in soils and build synergies between mitigation and adaptation goals at large scales [6,7]. This may be of particular significance for drought-prone semi-arid agroecosystems with lower lev- els of SOC and water retention, and with high vulnerability to impending shifts in resource availability [8]. Perennial cropping systems, such as California almonds [Prunus dulcis (Mill.) D. A. Webb], usually have low SOC and are uniquely susceptible to the impacts of a changing

climate, including increased frequency of extreme weather events, constrained water resources, and reduced winter chill hours [9]. These production systems are prominent across the landscape and rapidly increasing with more than 530,000 hectares supplying 80% of global almond demand [10]. High commodity prices, costly and increasingly scarce water inputs, and more stringent air quality regulations have promoted turnover of less productive orchards to new almond plantings and renewed interest in using in situ biomass recycling as a climate adaptation and mitigation tool [11,12]. However, the potential for long-term C sequestration and practical tradeoffs associated with high C input practices such as whole orchard recycling (WOR) remain unclear in semi-arid orchard production systems where production relies heavily on irrigation. We tested the long-term viability of biomass recycling within test plots of a highly produc- tive almond orchard and evaluated its potential benefits for climate change adaptation. We hypothesized that addition of substantial amounts of high organic C woody biomass would lead to long term soil C sequestration and improvements in soil health characteristics with positive impacts on yields and orchard productivity. We also explored the implications for orchard adaptation to future climate conditions by testing how and to what extent potential improvements in soil quality, especially hydraulic properties, would impact tree and yield resistance to an acute water shortage (i.e. deficit irrigation). Although it is often claimed that

healthy soils have the potential to build resilience, this claim has seldom been empirically tested, especially in intensive perennial systems. This work provides novel insights on mitiga- tion potential of this practice and the soil health mediated mechanisms influencing orchard health and productivity. Materials and methods Trial establishment and experimental design This experiment was conducted in an irrigated almond orchard located at the University of California Kearney Agricultural Research and Extension Center in Parlier, CA (36˚35’59.4"N 119˚30’11.7"W) on a Hanford fine sandy loam site. The climate is Mediterranean, with precipi- tation levels below evapotranspiration (ET) requirements during most of the growing season. Long term (68 years) annual rainfall and temperature averages are 285 mm and 17˚C, respec- tively. The soil treatments were established in a complete randomized block design with seven PLOS ONE Whole orchard recycling PLOS ONE | https://doi.org/10.1371/journal.pone.0229588 March 27, 2020 2 / 22