Effects Of Simulated Nitrogen Deposition On Soil Respiration In The Typical Mixed Broadleaved-korean Pine Forest In Xiaoxing’an Mountains, China

The widespread N enrichment in the atmosphere and soil has considerably affected terrestrial carbon (C) cycle. However, the effects of simulated N deposition still remain controversial and have shown variable effects on soil respiration (Rs)-The aim of this study is to assess the impact of nitrogen addition on soil respiration in the typical mixed broadleaved-Korean pine (Pinus koraiensis) forest. Four levels of N treatments:control (NO,0 kg N·ha-1·a-1), low-N (N1,30 kg N·ha-1·a-1), medium-N (N2,60 kg N·ha-1·a-1), and high-N (N3,120 kg N·ha-1·a-1) were set in the present study. We measured soil CO2 efflux about every other week during the growing seasons from 2010 May to 2011 October using a LI-6400 portable CO2 infrared gas analyzer. The main results are as follows.(1) The seasonal variation of Rs from all treatment plots was strong and followed a bimodal curve, absolutely parallel to that of T5. Rs peaked in middle of June (3.84-4.55μmol CO2·m-2·s-1 1), and presented the lowest values in the early growing season (1.37-1.84 μmol CO2·m-2·s-1). Rs ranged from 1.36 to 4.41 μmol CO2·m-2·s-1 in NO,1.43 to 3.98μmol CO2·m-2·s-1 in N1, 1.31 to 4.46μmol CO2·m-2·s-1 in N2, and 1.38 to 4.55μmol CO2·m-2·s-1 in N3.(2) Rs was exponentially related to soil temperature at 5 cm (T5) or at 10 cm (T10) depth. T5 could explain 49.9% to 69.2% of seasonal variation in respiration and T10 could explain 8% to 39% of seasonal variation in respiration. In all treatment plots the models of Rs against T5 were better than those against T10. Furthermore, soil temperature (T5) and soil moisture (W5) and the interactions between them could explain 52.2% to 73.5% of seasonal variation in soil respiration, and incorporating soil moisture into the pure Rs-temperature model improved the prediction of Rs in NO and N2 treatment plots. The Q10 values of soil respiration from all treatment plots were 2.10,1.93,1.97, and 2.01, respectively.(3) The average soil respiration rates in the growing season for NO, N1, N2 and N3 treatment plots were 3.09,2.78,3.06, and 2.90μmol C02·m-2·s-1,respectively. The seasonal variation of litterfall from all treatment plots was strong and followed a single peaked curve, and the total litter fall produetion were estimated to be 377±54 g·m-2 for the control (NO),335±14 g·m-2 for N1,365±25 g·m-2 for N2 and 400±24 g·m-2 for N3, respectively. The average soil respiration rate, soil temperature, soil moisture and annual litterfall biomass in simulated N deposition plots were not significantly different compared with NO (P> 0.05).