Impact of Organic Matter Amendments on Integrated Nutrient Management in California Almond.

Use of chemical fertilizers for nitrogen (N), phosphorus (P), and potassium (K) nutrition results in beneficial outcomes for agronomic performance, but also comes with economic and environmental costs. Organic matter amendments (OMA) offer a viable option to supplement or partially substitute chemical fertilizers. We examined the effects of composted manure and green waste compost on nutrient availability under root exclusion where almond tree roots where excluded from the soil during the duration of the experiment. Soil cores (0 – 50 cm) were deployed with resin beads attached to the base of the core to measure nutrient movement through the active rooting zone from April to October 2015. Soil total organic carbon (C) and total N significantly increased in the soil cores (0 – 50 cm) from both OMA sources. There was no difference in the rate of N release between composted manure and green waste compost. For both materials, the N release rate was more rapid in April and progressively slowed during the course of the growing season, which followed an exponential decay. Inorganic N (NH₄⁺ + NO₃^-) adsorbed to resin stakes in the top soil (0 – 10 cm) were not different between the normally fertilized control and OMA treatments, however there were significantly greater P, K, Fe and Zn availability from the OMA treatments. There were no differences between net N mineralization in the soil cores (0 – 50 cm) and potential N leaching from inorganic N adsorbed to resin beads at the end of the growing season. These results suggest the N released from OMA is not being lost from the active rooting zone (0 – 50 cm) at a greater rate than the control. Our study demonstrates OMA play an important role in N immobilization that can retain N in the soil and reduce the risk of N leaching to groundwater. Furthermore, OMA supply readily available P, K and micronutrients that may be able to act as a substitute for other chemical fertilizers.