The retention dynamics of early-spring N input in a temperate forest ecosystem: Implications for winter N deposition

Abstract

In N-limited temperate regions of China, rates of atmospheric N deposition remain high during winter due to industrial development and energy consumption. Winter-deposited N accumulates and is then released after snowmelt. However, little is known about the retention dynamics of early-spring N input in temperate forest ecosystems. We applied 15 N isotopic tracer after snowmelt, and then quantified 15 N dynamics in litter, soils, microbes and vascular plants over the following growing season in a warm temperate forest of northern China. In early spring (7 days after 15 N addition), approximately 80% of applied 15 N was retained in the ecosystem. The 15 N recovery was the highest in litter, followed by soils and microbes, with only trivial acquisition in vascular plants. After early spring, there was little change in total 15 N recovery over the following season, which indicated that the temperate forest ecosystem had high potential for the retention of early-spring N input. The 15 N levels gradually declined in litter and microbes, while they were gradually increased in the vascular plants. In late fall, substantial 15 N tracer retained in litter and was resorbed from senescing tissues to roots. Evergreen coniferous trees presented higher 15 N acquisition than deciduous broad-leaved trees. Our results suggest that substantial early-spring N input can be retained in warm temperate forest ecosystems. The findings highlight the importance of litter and plants in sustaining early-spring exogenous N resources, inferring the need to considering winter N deposition for a better understanding of N cycling in temperate ecosystems.