Organic apple production has experienced significant growth in the past 15 years, largely due to food and environmental safety concerns regarding synthetic pesticides and fertilizers used in conventional production. Increased organic fruit production can also be attributed to advancements in organic production practices, such as pheromone mating disruption for codling moth (Cydia pomonella), and yet significant challenges remain (Delate et al., 2008). Among the most significant challenges is nitrogen (N) supply because organic fertilizers are often bulky and expensive, and release of N can be slow and unpredictable. Improving the understanding of N cycling in organic systems and increasing organic fertilizer-use efficiency are critical to cost-effective organic fruit production.
Cultivation in the tree row is currently the most common understory management practice in organic apple production. Cultivation provides weed control but is costly and impairs soil quality and N availability (Cambardella and Elliot, 1992; Granatstein, 2004; Sanchez et al., 2007). Alternative groundcover strategies that reduce costs and improve N mineralization of organic fertilizers are needed. Possible groundcovers include organic mulches and leguminous cover crops.
The use of organic mulches such as wood chips, shredded paper, or alfalfa have increased soil N availability and mineralizable forms of N in multiple studies (Forge et al., 2003; Marsh et al., 1996; Sanchez et al., 2003; Yao et al., 2005). Elevated soil N availability may result from increased microbial activity and N turnover (Forge et al., 2003; Yao et al., 2005). Forge et al. (2003) reported that use of a high C:N ratio mulch in the tree row of an apple orchard did not result in net N immobilization or lower N supply and, to the contrary, increased N fertilizer-use efficiency. Neilsen et al. (2003) observed increased apple tree vigor and yield with organic mulches. In a comparison study of different orchard floor management systems, Shribbs and Skroch (1986) observed that apple trees under organic mulches had larger trunk diameter after four years compared with bare ground, cultivation, or a legume cover crop.
Sanchez et al. (2003) and Hoagland et al. (2008) reported that leguminous cover crops resulted in greater soil N availability and microbial activity than other orchard floor management strategies, including cultivation and wood chip mulch. Numerous studies, however, have reported reduced tree growth and yield due to increased competition between trees and cover crops (Hoagland et al., 2008; Larsson et al., 1997; Sanchez et al., 2003).
It is critical to ensure that N is supplied at the appropriate times to achieve adequate tree growth and yield. In-season N uptake and reserve N stored in perennial tissues (i.e., branches, trunk, and roots) from previous growth cycles are vital to apple tree nutrition. In late summer and autumn, trees move N from annual tissues to perennial tissues (Millard and Thompson, 1989; Neilsen et al., 2001b; Sanchez et al., 1992; Toselli et al., 2000). For instance, before leaf abscission, 35% to 70% of leaf N is relocated into perennial tissues as reserve N (Blasing et al., 1990; Munoz et al., 1993). Remobilization of these reserves from woody tissues is the main driver of early season fruit and leaf growth, and has been correlated with leaf canopy development (Cheng and Fuchigami, 2002; Khemira et al., 1998; Millard and Neilsen, 1989; Neilsen et al., 2001a; Titus and Kang, 1982).
Partitioning of in-season N uptake is affected by fertilizer timing. Early season N uptake is heavily partitioned into fruit and leaves (Cheng and Fuchigami, 2002; Khemira et al., 1998). Munoz et al. (1993) reported that N uptake in April was preferentially partitioned into fruit, whereas May N uptake was allocated into leaves of peach trees, with vegetative growth being greatest from May to August. Fertilizer N application during this period of vegetative growth can result in higher fruit N concentrations, having possible detrimental affects on fruit quality and storage (Sanchez et al., 1992; Toselli et al., 2000).
Previous mulching and cover cropping studies have focused on tree and soil responses but have not examined their effects on internal N cycling. Conversely, N partitioning and uptake studies have centered on inorganic fertilizer use and application timing. Interactions between N partitioning, organic fertilizers, and orchard floor management are not well understood. The objectives of this study were to determine the effects of cultivation, wood chip mulch, and a legume cover crop on tree growth, partitioning of compost N at different application timings, and fertilizer-use efficiency.
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