Wheat and maize canopies regulate ammonia cycling

Research Hypothesis:We hypothesized that in intensive cropping systems experiencing elevated atmospheric nitrogen loading, crop physiological regulation at the canopy scale can override climate-driven seasonality to dictate canopy–atmosphere ammonia (NH3) exchange. Specifically, we posited that crop alternation would lead to divergent canopy source-sink dynamics (alternating between wheat and maize) at the annual scale, and continuous year-round tracking would reveal critical source-sink transitions that single-season observations often fail to capture.Data Gathering Methodology:
The dataset consists of continuous, field-based micrometeorological measurements conducted in a highly intensive wheat-maize rotation system in the North China Plain across two full annual cycles (June 2015–June 2016 and June 2018–June 2019). Atmospheric NH3 concentrations were continuously monitored using a high-precision laser NH3 analyzer (LGR) equipped with a vertical profiling system sampling at five heights (0.5 m, 1.0 m, 2.0 m, 4.0 m, and 7.0 m above the ground). Concurrent 3D ultrasonic anemometer data were collected to determine atmospheric turbulence. Canopy-scale NH3 exchange fluxes were subsequently calculated using the Aerodynamic Gradient Method (AGM). Data were strictly quality-controlled, filtering out stable nighttime or low-turbulence periods to ensure flux validity.What the Data Shows & Notable Findings:This dataset specifically corresponds to Sections 3.1 to 3.3 (Figures 1 to 3) of our study:Annual & Vertical Concentration Dynamics (Fig. 1): The data demonstrates pronounced seasonal variability and clear vertical canopy gradients, driven by fertilization pulses and crop phenology.Diurnal Variations (Fig. 2): High-resolution diel profiles during key growth stages reveal that wheat tends to enhance daytime NH3 concentrations (acting as a source), whereas maize acts as an effective daytime sink, causing within-canopy concentration depletion.Canopy-Scale NH3 Fluxes (Fig. 3): The flux data uncovers contrasting source-sink roles. The wheat canopy acts as a persistent structural source (e.g., contributing 26.3 kg N ha⁻¹ in 2016) , while the maize canopy functions as a strong net sink. However, intense soil emissions mask the maize canopy's uptake, rendering the overall system a massive net NH3 source.Data Interpretation and Usage:
These data provide empirical evidence challenging the conventional, soil-centric views of agricultural NH3 emissions by quantifying the intricate source-sink transitions at the canopy-atmosphere interface