Organic Orchard Floor Management Impacts On Soil Nitrogen Cycling and Tree Nitrogen Uptake.

Productive capacity in organic peach orchards is dependent on effective management practices that promote the efficient cycling of nitrogen for tree uptake. Total soil nitrogen (N), monthly soil nitrate (NO₃^-) and ammonium (NH₄⁺), and the natural abundance of N isotopes in peach tree leaves was measured to assess N turnover and tree N uptake among different orchard management treatments. The certified organic peach trial was initiated in 2008 at the Utah Agricultural Experimental Station, Kaysville, Utah and includes six orchard floor treatments: a living-mulch (shallow rooted alyssum) with legume alleyway (LL), living-mulch (shallow rooted alyssum) with grass alleyway (LG), straw-mulch with legume alley (NL), straw-mulch with grass alleyway (NG), tillage (TG) and weed fabric (WG). Biomass inputs from mow and blow contributed an average 225 grams of N to LL and NL treatments in 2011. Total soil N increased significantly across all treatments from 2009 to 2011, and was significantly higher in NL compared to NG and TG treatments in 2011. Soil NO₃^- was significantly higher in WG in 2009, whereas the LL and NL treatments were highest from 2010-2011, suggesting a cumulative effect of biomass inputs over several seasons. Soil NH₄⁺ was non-significant between treatments. The NG and NL peach trees with straw-mulch had elevated ¹⁵N in comparison to other treatments with the same alleyway. Elevated ¹⁵N may indicate ¹⁵N depleted forms of N are immobilized by straw-mulch or microbial biomass. Significantly lower ¹⁵N enrichment in the LL treatment may result from uptake of soil N derived from N₂ fixation processes. Differences in soil NO₃^- and tree leaf ¹⁵N abundances may be attributed to immobilization by straw-mulch or microbial biomass, legume mow and blow inputs, or the uptake of legume N exudates by alleyway tree roots, suggesting N uptake and soil N turnover is altered by orchard floor management.