Response of ammonia volatilization from lawn canopy to shading and nitrogen fertilization and the source resolution

Objectives The aims of this study were to investigate the dynamics of ammonia volatilization from the canopy of lawns of different vegetation types and its response to shade and nitrogen fertilization, and to examine the sources of ammonia volatilization from the canopy of C3 and C4 plants and their contribution to ammonia volatilization from lawns.

Methods The ammonia volatilization from different lawns and soils was monitored in a pot experiment using a pump suction portable ammonia detector (PAD). The experiment was set up with three lawn plants (C3 plant Ophiopogon japonicus, Festuca elata Keng and C4 plant Cynodon dactylon), three shade conditions (100%, 45%, and 25% light penetration) and two nitrogen application rates (N 2.3 and 0 g/kg dry soil) in complete combinations, and there were total 18 treatments. Leaf nitrate reductase (NR), glycolate oxidase (GO), glutamine synthetase (GS), and phenylalanine deaminase (PAL) activity, plant physiological and biochemical indices, such as chlorophyll a, chlorophyll b, and carotenoids, as well as environmental indices, such as light intensity and rainfall, soil temperature, moisture and mineral N content, were also monitored. Different turfgrass canopy ammonia sources were explored by structural equation modeling.

Results Canopy ammonia volatilization varied significantly among lawns, the accumulated canopy ammonia volatilization of Festuca elata keng, Cynodon dactylon and Ophiopogon japonicus in two growing seasons were N 52.1, 34.0 and 4.0 kg/hm², respectively; the contribution rates of canopy ammonia volatilization to lawn ammonia volatilization were 71.1%, 62.8% and 66.8%, and nitrogen fertilizer emission factors were 4.3%, 4.4% and 0.4% respectively. Nitrogen application significantly increased lawn canopy ammonia volatilization by 35% (P<0.05); shading reduced ammonia emissions and ammonia emission coefficients of nitrogen fertilizer, but increased the contribution of canopy ammonia emissions to lawn ammonia volatilization. Glycolic acid oxidase (GO) and nitrate reductase (NR) activities of C3 plants were significantly higher than that of C4 plants. Chlorophyll content was positively correlated with canopy ammonia volatilization, and chlorophyll a and b and carotenoid contents in Ophiopogon japonicus were significantly lower than those of Cynodon dactylon and Festuca elata keng. Structural equation modeling showed that both lawn canopies ammonia emission was positively regulated by nitrogen application and negatively regulated by shading; the canopy ammonia volatilization from C3 and C4 plants was mainly regulated by the phenylpropane metabolic pathway and the photorespiratory metabolic pathway respectively.

Conclusions Nitrogen application significantly increases lawn canopy ammonia volatilization; shading significantly reduces the volatilization and fertilizer ammonia emission coefficients. The canopy ammonia volatilization of Festuca elata keng and Cynodon dactylon is significantly higher than that of Ophiopogon japonicus. The canopy ammonia valtilization sources of C3 plants are different from those of C4 plants and the former is mainly controlled by phenylpropane metabolism pathway, whereas the latter by photorespiration.