Satellite-observed decrease in the sensitivity of spring phenology to climate change under high nitrogen deposition

Spring phenology is a sensitive indicator of climate change and has substantial impacts on the carbon cycle. The global N cycle has been greatly disturbed by anthropogenic activities resulting in altered atmospheric N deposition worldwide. Research has been focused on the changes in the spring phenology and its covariations with climatic factors. However, the influences of N deposition on spring phenology have not been well documented to date. Herein, we investigated the effects of N deposition on the start of growing season (SOS) in continental United States (CONUS) during the years 1986-2015 using the normalized difference vegetation index (NDVI) datasets derived from both the third generation NDVI dataset and the Moderate Resolution Imaging Spectroradiometer (MODIS). We observed that N deposition could only explain approximately 5% of temporal variation in SOS in CONUS. However, the sensitivities of SOS in response to unit change in both temperature (ST) and precipitation (SP) showed clear decreasing spatial patterns with increasing N deposition. The ST generally decreased from −6 d/°C in low N deposition regions (4 kg ha−1. Furthermore, the positive SP also showed a continuously decreasing pattern with the increase in N deposition, but the negative SP was gradually weakened when N deposition was >1.0 kg ha−1. The results have important implications as it reveals the role of N deposition on spring plant phenology, and strongly suggest the consideration of N deposition effects when analyzing or predicting spring phenology in response to future climate change.