Soil nitric oxide emissions after nitrogen and phosphorus additions in two subtropical humid forests

Abstract

Soil nitric oxide (NO) emissions after the addition of nitrogen (N) and phosphorus (P) were studied at a broadleaf forest and a pine forest in Dinghushan Biosphere Reserve, south China. N was applied at 5 g N m−2 (or 50 kg N ha−1) and P was applied at 4 g P m−2 as NH4Cl (AN), NaNO3 (NN), NH4Cl + Na2HPO4 (AP), NaNO3 + Na2HPO4 (NP), and Na2HPO4 (P) in water solutions, respectively, in comparison with water and blank controls. Enhancement of NO emission by N addition alone was greater than that by combined addition of N and P in both forests due to the P‐limiting soil nutrient status. In both forests, temporal pattern of NO emission after adding NO3−‐N was different with that after adding NH4+‐N. NO3−‐N addition resulted in immediate NO fluxes for the broadleaf and to a lesser extent the pine forest site. In both forests, nitrification contributed more than denitrification to NO production although denitrification also played an important role in the broadleaf forest. N addition induced greater NO emission in the mature broadleaf forest than in the primary pine forest. Over the 50‐day experimental period, averaged NO fluxes in plots adding AN, NN, AP, and NP were 3.3, 2.3, 1.9, and 1.8 times that in the water control plots, respectively, for the broadleaf forest and 2.7, 1.6, 1.5, and 1.3 times that in the water control plots, respectively, for the pine forest. Applied N loss as NO‐N in AN, NN, AP, and NP plots were 2.8%, 1.6%, 1.1%, and 1.0%, respectively, in the broadleaf forest; and 2.6%, 1.0%, 0.8% and 0.4%, respectively, in the pine forest. On the average, N loss as NO from the forest floors was approximately 2% of the applied N as NH4+ or NO3− alone, quite similar to that measured by Hall and Matson (1999) from forest floors in the Hawaiian Islands.