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 19862015 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 (<2 kg ha1) to 4 d/C in areas with N deposition >4 kg ha1. 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 ha1. 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.