Intensive fertilizer use increases orchard N cycling and lowers net global warming potential
Sat Darshan S. Khalsa, David R. Smart, Saiful Muhammad, Christine M. Armstrong, Blake L. Sanden, Benjamin Z. Houlton, Patrick H. Brown
Abstract
Nitrogen (N) fertilizer use has simultaneously increased global food production and N losses, resulting in degradation of water quality and climate pollution. A better understanding of N application rates and crop and environmental response is needed to optimize management of agroecosystems. Here we show an orchard agroecosystem with high N use efficiency promoted substantial gains in carbon (C) storage, thereby lowering net global warming potential (GWP). We conducted a 5-year whole-system analysis comparing reduced (224 kg N ha−1 yr−1) and intensive (309 kg N ha−1 yr−1) fertilizer N rates in a California almond orchard. The intensive rate increased net primary productivity (Mg C ha−1) and significantly increased N productivity (kg N ha−1) and net N mineralization (mg N kg−1 soil d−1). Use of 15N tracers demonstrated short and long-term mechanisms of soil N retention. These low organic matter soils (0.3–0.5%) rapidly immobilized fertilizer nitrate within 36 h of N application and 15N in tree biomass recycled back into soil organic matter over five years. Both fertilizer rates resulted in high crop and total N recovery efficiencies of 90% and 98% for the reduced rate, and 72% and 80% for the intensive rate. However, there was no difference in the proportion of N losses to N inputs due to a significant gain in soil total N (TN) in the intensive rate. Higher soil TN significantly increased net N mineralization and a larger gain in soil organic carbon (SOC) from the intensive rate offset nitrous oxide (N2O) emissions, leading to significantly lower net GWP of −1.64 Mg CO2-eq ha−1 yr−1 compared to −1.22 Mg CO2-eq ha−1 yr−1 for the reduced rate. Our study demonstrates increased N cycling and climate mitigation from intensive fertilizer N use in this orchard agroecosystem, implying a fundamentally different result than seen in conventional annual cropping systems.