A research team led by Prof. LIU Lingli from the Institute of Botany, the Chinese Academy of Sciences (IBCAS), has identified a mean annual precipitation (MAP) threshold of approximately 700 mm, beyond which the dominant controls on ecosystem nitrogen retention shift.

The study, published in Nature Geoscience, utilized the natural abundance of soil nitrogen stable isotope (δ¹⁵N) as a time-integrated tracer to capture the long-term balance of nitrogen inputs and losses. By analyzing data from 31 sites across the National Ecological Observatory Network (NEON) in the United States, the researchers revealed a robust precipitation threshold that aligns with the iconic arid–humid divide of North America (the 100th Meridian, now shifting toward the 98th under climate change). Below this threshold, soil δ¹⁵N decreases as MAP rises, reflecting stronger nitrogen retention, while above it, soil δ¹⁵N increases with MAP, indicating a greater extent of nitrogen loss.

In dry ecosystems (MAP < 700 mm), higher MAP is associated with greater plant diversity, which intensifies competition with soil microbes for nitrogen, effectively "locking" nitrogen within the biomass and reducing its loss from the system. Plant community structure and microbial composition are the key regulators of nitrogen retention in these water-limited environments.

However, in humid regions (MAP>700 mm), further increases in MAP lead to nitrogen "leakiness" through enhanced hydrological leaching and microbial transformation (denitrification), with soil physicochemical properties such as carbon-to-nitrogen ratios, nitrate levels, and clay content playing a dominant role.

These findings advance mechanistic insight into how precipitation regulates ecosystem nitrogen retention by altering the balance among plant, microbial, soil, and hydrological processes. The study also highlights that shifts in hydroclimatic boundaries, including the eastward movement of the arid–humid divide in North America, could potentially reshape continental nitrogen cycling under climate extremes. By identifying a precipitation threshold associated with changes in dominant nitrogen-cycling controls, this work provides an important reference for improving ecosystem and Earth system model predictions of natural nitrogen dynamics under changing precipitation regimes.

Spatial distribution of sampling sites and precipitation-dependent patterns of soil δ¹⁵N. (Image by PENG Yong)

Contrasting controls of soil δ¹⁵N across precipitation regimes. (Image by PENG Yong)