EFFECT OF NITROGEN FERTILIZATION ON SOIL RESPIRATION IN LARIX GMELINII AND FRAXINUS MANDSHURICA PLANTATIONS IN CHINA

Aims Soil respiration is important in carbon flux and changes in carbon cycling in forest soil. Understanding belowground response to fertilization is critical in assessing soil carbon dynamics and atmospheric nitrogen deposition. The objectives of this study were to: 1) compare soil respiration rates between Larix gmelinii and Fraxinus mandshurica plantations in the same field site, 2) examine the effects of nitrogen fertilization on soil respiration rate in both plantations, and 3) analyze the relationships between soil respiration rate and soil temperature, fine root biomass and nitrogen contents. Methods In May 2002, we established six plots in each plantation type at the Maoershan Forest Research Station, and from 2003 to 2005 we fertilized three plots and left three unfertilized as the control. We placed ten soil chambers randomly in each plot in July 2004 and measured soil respiration rates using a soil chamber system from August 2004 to October 2005. At the same time, we estimated fine root standing biomass in each plot by sampling eight soil cores monthly, calculated dry mass and analyzed nitrogen content for each fine root sample. Important findings Fine root standing biomass of F. mandshurica (229 g·m-2) was greater than that of L. gmelinii (158 g·m-2), and the average difference in soil respiration rates during the growing season was 19.8%. Nitrogen fertilization significantly decreased fine root standing biomass 27.4% in F. mandshurica and 18.4% in L. gmelinii; soil respiration rate was decreased 25.8% and 34.9% respectively. However, nitrogen fertilization did not change fine root nitrogen contents. In both plantation types, soil respiration rate exhibited significant exponential relationships with soil temperature (R2=0.93-0.98). The range of Q10 values was 2.45-2.62 for F. mandshurica and 3.02-3.29 for L. gmelinii. Nonetheless, nitrogen fertilization did not impact Q10 values in either plantation type; the difference of Q10 between fertilized and unfertilized was less 1%. Nitrogen fertilization in L. gmelinii and F. mandshurica did not alter nitrogen contents in fine roots or Q10 values, but decreased fine root standing biomass, which suggests that reduction of soil respiration rate in fertilized plantations was caused by the decreases of fine root standing biomass.