Florida is the second leading producer of organic strawberries (Fragaria ×ananassa Duch.) in the United States, with a focus on winter season production (USDA-NASS, 2017). Ensuring nitrogen (N) supply meets crop demands throughout the crop cycle remains a major challenge in organic production systems (Gaskell and Smith, 2007), and this could be exacerbated in Florida, where soils at most production sites have a high sand content resulting in poor water holding capacity and cation exchange capacity. Nitrate (NO3-N) is the dominant N ion found in well-aerated soils, the main form of N absorbed by plant roots, and is highly prone to leaching in sandy soils. Moreover, optimizing N management in Florida is complicated by the low soil organic matter (SOM) content of most soils (less than 10 g·kg−1) and unpredictable N mineralization patterns from organic fertilizers and SOM (Cherr et al., 2007). Integrating cover crops into crop rotation may help build SOM while improving other soil characteristics for crop nutrient management (Haruna and Nkongolo, 2015; Thorup-Kristensen et al., 2003).
As a tropical legume, sunn hemp (Crotalaria juncea L.) is adapted to growth conditions found in the southeastern United States and Florida: sandy soils with low fertility characterized by a long warm season (Durairaj and Stute, 2018). Using sunn hemp as a rotational crop in organic strawberry production systems holds great potential for improving soil fertility and quality and environmental sustainability as this high-residue cover crop can help increase SOM, contribute to N supply through N-fixation, reduce weed pressure, and suppress sting nematodes (Belonolaimus longicaudatus), a major soilborne pest for strawberry production in Florida (Blanco-Canqui et al., 2012; Braz et al., 2016). As a green manure, sunn hemp can produce between 5000 and 11,500 kg·ha−1 dry weight of biomass with an estimated N input of 100 to 200 kg·ha−1 N (Ozores-Hampton, 2012; Schomberg et al., 2007). Although sunn hemp has been increasingly adopted as a summer cover crop by organic strawberry producers in Florida, there is limited information available on how much N sunn hemp residues provide to fall-planted strawberry crops. Considering the coarse texture of Florida’s sandy soils, there is also a need for reliable methods to help predict and monitor the dynamics of plant-available N (PAN) released from cover crop residues.
AEMs have been used to measure plant available soil NO3-N dynamically in agricultural fields (Hill et al., 2016; Nyiraneza et al., 2009). AEMs are semipermeable membranes containing positively charged head groups attached to polymer resins. The head groups in AEMs enable the adsorption of anions while repelling cations. Quaternary ammonium head groups are the commonly used anion-conducting groups of AEMs due to their simple synthesis process (Hagesteijn et al., 2018). When buried into the soil for a given period, AEM resins act as an anion sink and are capable of continuously adsorbing anion forms of nutrients present in soils through ion exchange reactions in the head groups. Thus, resins mimic ion interception by roots, while the adsorption and accumulation of nutrient anions are driven by anion movement in response to concentration gradients in the soil solution (Yang et al., 1991). By contrast, it is challenging to employ traditional soil sampling and chemical extraction methods to quantify NO3-N availability and reveal in situ N release pattern due to the dynamic nature of the N cycle and the disadvantage of using inherently static measurements in this context (Abrams and Jarrel, 1992). AEMs can be viewed as a diffusion-sensitive tool to monitor soil N availability because the process of nitrate accumulation on the resins integrates N transformations and N diffusion to sinks into the measurement of N availability (Qian and Schoenau, 2002). It is also considered an innovative and practical in situ method that is cost-effective and simple to deploy (Qian and Schoenau, 2002; Skogley and Dobermann, 1996). A previous pot study demonstrated that AEM measurements could be used as a sensitive indicator of NO3-N dynamics in soils with green manure amendments (León Castro and Whalen, 2016).
In addition to in situ measurements, laboratory incubation methods have been employed to effectively estimate the N mineralization potential of soils amended with cover crop residues (Johnson et al., 2012; Masunga et al., 2016). To our knowledge, the potential of AEMs as a tool for monitoring in-situ soil NO3-N availability has not been examined in Florida sandy soils amended with organic inputs. Moreover, few studies have attempted to use multiple methods to determine the N release pattern of soils amended with sunn hemp residues. In addition to understanding soil N availability dynamics as affected by sunn hemp as a preceding cover crop in organic strawberry production systems, the impact of sunn hemp residues on strawberry yield performance also needs to be examined as little information is currently available.
The objectives of this 2-year study established on Florida sandy soils were to 1) monitor soil NO3-N fluxes using AEMs in an organic strawberry production field where sunn hemp cover crop residues were incorporated before strawberry planting; 2) determine site-specific N availability patterns of soils amended with sunn hemp residues using a combined approach including AEMs, traditional soil testing, and laboratory incubations; and 3) examine the influence of sunn hemp residues on organic strawberry fruit yield components in the early season.
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