Blueberry Yield and Soil Mineral Nitrogen Response to Nitrogen Fertilizer and Nitrification Inhibitors under Drip-Fertigation Systems

In blueberry plantings, nitrification can result in losses of mineral nitrogen (N) through leaching because blueberries prefer ammonium (NH4+) over nitrate (NO3-). The objective of this study was to assess the effects of two rates of N fertilizer, mixed or not with nitrification inhibitors (NI) and applied through two fertigation systems, on berry yield and the concentrations of NH4+-N and NO3--N along the soil profile. Thus, nine combinations of treatments including two N fertilizer rates (60 and 120 kg N ha(-1)), two NI (with DCD + Nitra-pyrin or without) and two fertigation application methods (buried and suspended drip lines) and a control (0 kg N ha(-1)) were tested over three years (2016-2018) in a long-term blueberry planting. Berry yield was on average 47.32 Mg ha(-1) in 2016 and 26.86 Mg ha(-1) in 2018. In 2017, berry yield varied between 8.60 Mg ha(-1) under the control and 11.66 Mg ha(-1) with 120 kg N ha(-1) applied through suspended drip lines. Low berry yield in 2017 was due to a heavy pruning to rejuvenate the plants. In 2016, the concentration of NH4+-N in the sawdust mulch layer varied between 13.1 and 27.1 mg kg(-1) in the spring, 11.4 and 32.1 mg kg(-1) in the summer, and 7.9 and 72.9 mg kg(-1) in the fall; the concentration of high NH4+-N along the soil profile did not exceed 5 mg kg(-1). High concentrations of NH4+-N in the sawdust mulch layer were associated with NI, but did not translate to high berry yields. The concentration of NO3--N in the soil profile reached 42.6 mg kg(-1) in the summer and 39.0 mg kg(-1) in the fall and these high concentrations were associated with NI. In 2017 and 2018, there was no effect of NI on NH4+-N concentrations even in the layer of sawdust mulch which was not consistent with the results obtained in 2016. High concentrations of NO3--N were measured beneath the sawdust mulch layer with treatments including N fertilizer alone or mixed with NI. It is possible that NH4+ retained in the layer of sawdust mulch and not taken up by plant roots was subsequently oxidized to NO3- at the end of the residence time of NI in the soil. Our results showed that high concentrations of NO3--N along the soil profile occurred mainly during the summer which could be explained by irrigation water driving NO3- leaching.