Modelling the impact of planting density on water and nitrogen use by young almond tree

Abstract

Developing efficient water and nutrient use under an advanced production system requires a precise evaluation of water and nutrient use by the crop over multiple growing seasons. In this study, a numerical model (HYDRUS-2D) was used to assess almond production systems for water balance, salinity dynamics and nitrogen (N) transport in the soil under varied planting densities (308, 440, and 615 trees/ha) of Nonpareil and Shasta genotypes at Almond Centre of Excellence during the 2019-20 and 2020-21. The results showed that medium density Nonpareil planting (615 trees/ha) registered about 14% higher root water uptake (RWU) than 308 trees/ha whereas RWU by Shasta remained unaffected by density. Average RWU in Nonpareil increased by 16-22% of water applied during 2020-21 as compared to preceding season (40-42%). Evaporation losses were similar under both genotypes (18-21%), indicating an appreciable amount of unproductive water loss. Measured and predicted salinity dynamics remained below the crop threshold (ECsw = 3 dS/m) in the soil layer and shoot up rapidly at 60 and 90 cm depths. Simulated seasonal NO3-N concentrations in the soil varied from 0.0-500 mg/L, and an appreciable amount of NO3-N was detected at 90 cm depth. Model simulated NO3-N uptake in Nonpareil and Shasta averaged around 66%. Medium density plantings showed increased nitrogen use efficiency over low density plantings Nitrate leaching accounted for 14-20% of the applied nitrogen, and around 15-16% of applied N remained in the soil profile at the end of the simulation and was therefore susceptible to leaching in the ensuing winter. Estimated water productivity (WP) for irrigation (WPI), evapotranspiration (WPET) and transpiration (WPT) for 615 trees/ha plantings (high density) increased by 42, 49 and 62% respectively as compared to low density planting (308 trees/ha). The equivalent WP values for Shasta increased by 9-16% only. Estimated N use efficiency (NUE) in terms of kernel yield was 63% higher under high density as compared to low density planting. However, NUE in terms of N uptake against added N was similar under both genotypes and planting densities. The data can be further improved by strengthening the model calibration through robust and intensive field measurements of soil salinity and N dynamics in the soil at multiple locations.

Keywords: almond, planting density, water balance, water productivity, nitrogen use efficiency, leaching, soil salinity