Short-term nitrogen pulse affects nitrogen fixation and ammonia oxidation in biocrusts: New insights into dryland nitrogen dynamics

Abstract

In drylands, biocrusts function as essential components of the nitrogen cycle and display pronounced sensitivity to external nitrogen inputs. Episodic rainfall events can mobilize dry-deposited nitrogen into short-term pulse that influences nitrogen retention and transformation. However, the effects of short-term nitrogen pulse, commonly encountered in drylands, on biocrust nitrogen dynamics remain poorly understood. This study simulates rainfall-driven short-term nitrogen pulse to examine how varying pulse concentrations impact biocrusts nitrogen fixation, ammonia oxidation, and overall nitrogen balance under conditions of intensified nitrogen deposition after a 13-year nitrogen addition experiment in Gurbantunggut Desert. The nitrogen pulse sharply disrupted biocrusts' nitrogen cycling. Both nitrogen fixation and ammonia oxidation rates declined precipitously immediately after the pulse. However, within 14–21 days, these rates rebounded to or even surpassed pre-pulse levels. This pattern reflects the biocrusts' acute sensitivity to nitrogen perturbations, as well as their ecological resilience. Over 21 days, cumulative nitrogen fixation decreased by 47%–72% relative to the control, indicating suppression by nitrogen addition; in contrast, cumulative ammonia oxidation increased by 39%–91% but stabilized at moderate to high nitrogen inputs (⩾1.0 g N m^–2 yr^–1, N1.0), revealing a saturation threshold. Moreover, the risk of nitrogen loss increased with higher nitrogen concentrations and plateaued at approximately an 88% elevation under ⩾N1.0, indicating that high nitrogen inputs exacerbate biocrusts nitrogen-loss vulnerabi-lity. These findings elucidate the impacts of acute nitrogen pulse on nitrogen dynamics in biocrusts, highlighting that amid escalating global nitrogen deposition, precise evaluation of nitrogen balance in drylands must integrate pulse effects, biocrusts nitrogen-carrying capacity, input modes, concentrations, durations, and interactions with environmental conditions.

Graphical abstract

Keywords

biocrusts / nitrogen pulse / nitrogen fixation / ammonia oxidation

Highlight

	● Short-term nitrogen pulses significantly alter nitrogen cycling dynamics in biocrusts.
	● The mechanism of short-term sensitivity and recovery in biocrusts nitrogen dynamics to nitrogen disturbance was elucidated.
	● Nitrogen pulses exacerbate nitrogen loss risks, providing new insights for dryland nitrogen deposition research.
	● Nitrogen loss risks increase with nitrogen concentration, exposing the ecological vulnerability of drylands.

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