Effects of amino acid addition on nitrogen mineralization and enzyme activity in dark brown soils of temperate forests.

To investigate the regulatory effects of different amino acid additions on nitrogen (N) mineralization in temperate dark brown soil, we conducted a 50-day indoor incubation experiment with dark brown soil under broadleaf forest, mixed coniferous-broadleaf forest, and coniferous forest near the Maoershan Forest Ecosystem National Field Scientific Observation and Research Station, Heilongjiang Province. Treatments included control, glycine, glutamic acid, and histidine additions (40 mg N·kg-1). We measured the soil inorganic N (NO3--N and NH4+-N) content, N mineralization rates (ammonification, nitrification, and net N mineralization rates), pH, and protease and urease activities throughout the incubation. The results showed that amino acid addition significantly increased soil NH4+-N content, ammonification rate, and net N mineralization rate, with increases of 15.5%-51.9%, 24.3%-93.6%, and 32.5%-59.3%, respectively, at the end of incubation. The responses of NO3--N content and the nitrification rate were delayed, becoming significantly higher than those of CK only from day 14 onward. Glycine induced significantly greater increases in NH4+-N content and ammonification rate than glutamic acid and histidine. Amino acid additions significantly increased soil pH, protease activity, and urease activity. Protease activity increased by 60.7%-85.9% on day 14 and urease activity increased by 11.2%-39.2% on day 7. Forest type significantly affected soil N transformation processes. Soil inorganic N content, nitrification rate, and urease activity were significantly higher in mixed conifer-broadleaf and broadleaf forests than in coniferous forests. During the late incubation period (days 28-50), ammonification rate and net N mineralization rate in mixed conifer-broadleaf and broadleaf forests were significantly higher than those in coniferous forests. Moreover, soil pH and protease activity followed the order of mixed conifer-broadleaf forest > broadleaf forest > coniferous forest. Both protease and urease activities were significantly and positively correlated with soil pH, NH4+-N content, and N mineralization rates. Overall, amino acid inputs significantly promoted N mineralization processes in temperate dark brown soil, and these effects were significantly moderated by forest type and soil conditions.