Early responses of soil ammonium and nitrate nitrogen to forest gap harvesting of a Pinus massoniana plantation in the upper reaches of Yangtze River

To understand the short-term effects of forest gap by human harvesting on soil available nutrient in Pinus massoniana plantations,the variations of soil ammonium nitrogen(NH_4~+-N) and nitrate nitrogen(NO_3~--N) concentrations in the gap center and gap edge during growing season were observed in seven gaps of different size(Gl:100 m~2;G2:225 m~2;G3:400 m~2;G4:625 m~2;G5:900 m~2;G6:1225 m~2;G7:1600 m~2) and pure understory of a 39-year-old masson pine plantation in a hilly area of the upper reaches of Yangtze River.The results showed that in the early stage of gap formation,the gap size had significant effect on NH_4~+-N,the season changes on NO_3~--N,and the interaction effect of gap size and seasonal variation on NH_4~+-N and NO_3~--N.The difference of NH_4~+-N and NO_3~--N between the gap center and gap edge was not significant.(1)The NH_4~+-N content was 4.30- 11.99 mg kg~(-1),and NO_3~--N content was 2.57- 10.81 mg kg~(-1).There was no obvious difference in NH_4~+-N and NO_3~--N among gaps of different size in early or late growing seasons,when both increased first and decreased afterwards in the middle of growing season.The gaps of 100-400 m~2 area had a higher content of available nitrogen.(2)The seasonal dynamic differed between NH_4~+-N and NO_3~--N,with the former lower in middle growing season whereas the latter higher in the middle growing season but lower in the end of growing season.The soil NH_4~+-N was higher than NO_3~--N in the early and late periods,but lower in the middle period.(3) The soil NH_4~+-N and NO_3~--N in parts of gaps were lower than understory in the early and late growing season.(4) Correlation analyses showed that NH_4~+-N had significant positive correlation with microbial biomass nitrogen(MBN),and NO_3~--N with soil temperature,MBN and organic matter.But the impact of soil water content on available nitrogen was not significant.These results suggested that soil temperature and microbial activity variation caused by gap harvesting are the main factors affecting soil available nitrogen content of Finns massoniana plantations.