No abstract

The intercropping system of tree with soybean in juvenile plantations, as a short-term practice, was applied at Lao Shan Experimental Station in Mao'er Shan Forest of Northeast Forestry University, Harbin, China. The larch (Larix gmelinii)/soybean (Glycine max.) and ash (Fraxinus mandshurica) intercropping systems were studied in the field to assess the effects of the intercropping on soil physicochemical properties. The results showed that soil physical properties were improved after soybean intercropping with larch and ash in one growing season. The soil bulk density in larch/soybean and ash/soybean systems was 1.112 g·cm -3 and 1.058 g·cm -3 , respectively, which was lower than that in the pure larch or ash plantation without intercropping. The total soil porosity also increased after intercropping. The organic matter amount in larch/soybean system was 1.77 times higher than that in the pure larch plantation, and it was 1.09 times higher in ash/soybean system than that in the pure ash plantation. Contents of total nitrogen and hydrolyzable nitrogen in larch/soybean system were 4.2% and 53.0% higher than those in the pure larch stand. Total nitrogen and hydrolyzable nitrogen contents in ash/soybean system were 75.5% and 3.3% higher than those in the pure ash plantation. Total phosphorus content decreased after intercropping, while change of available phosphorus showed an increasing trend. Total potassium and available potassium contents in the larch/soybean system were 0.6% and 17.5% higher than those in the pure larch stand. Total potassium and available potassium contents in the ash/soybean system were 56.4% and 21.8% higher than those in the pure ash plantation.

Knowledge of soil respiration and photosynthesis under elevated CO2 is crucial for exactly understanding and predicting the carbon balance in forest ecosystems in a rapid CO2-enriched world. Quercus mongolica Fischer ex Ledebour seedlings were planted in open-top chambers exposed to elevated CO2 (EC = 500 µmol mol−1) and ambient CO2 (AC = 370 µmol mol−1) from 2005 to 2008. Daily, seasonal and inter-annual variations in soil respiration and photosynthetic assimilation were measured during 2007 and 2008 growing seasons. EC significantly stimulated the daytime soil respiration by 24.5% (322.4 at EC vs. 259.0 mg CO2 m−2 hr−1 at AC) in 2007 and 21.0% (281.2 at EC vs. 232.6 mg CO2 m−2 hr−1 at AC) in 2008, and increased the daytime CO2 assimilation by 28.8% (624.1 at EC vs. 484.6 mg CO2 m−2 hr−1 at AC) across the two growing seasons. The temporal variation in soil respiration was positively correlated with the aboveground photosynthesis, soil temperature, and soil water content at both EC and AC. EC did not affect the temperature sensitivity of soil respiration. The increased daytime soil respiration at EC resulted mainly from the increased aboveground photosynthesis. The present study indicates that increases in CO2 fixation of plants in a CO2-rich world will rapidly return to the atmosphere by increased soil respiration.

For secondary forests, the major forest resources in China (accounting for more than 50% of the national total), soil respiration (R S ) and the relationship between R S and various biotic/abiotic factors are poorly understood. The objectives of the present study were to examine seasonal variations in soil respiration during the growing season, and to explore the factors affecting the variation in soil respiration rates for three forest types (Mongolian oak, Manchurian walnut and mixed forests) of temperate secondary forest in Northeast China. The results showed that (1) the maximum total R S rate occurred in July, following a bell-shaped curve with season, (2) for all forest types, the total R S was significantly influenced by soil temperature (P \ 0.01), and did not significantly correlate with soil moisture, (3) compared with fine root biomass, coarse root biomass was more closely related with the root respiration in mixed forest (R 2 = 0.711, P = 0.017) and in Manchurian walnut forest (R 2 = 0.768, P = 0.010), and (4) microbial biomass carbon (MBC) and nitrogen were significantly correlated with heterotrophic R S in Mongolian oak forest (R 2 = 0.664, P = 0.026; R 2 = 0.784, P = 0.008, respectively) and in mixed forest (R 2 = 0.918, P = 0.001; R 2 = 0.967, P = 0.001, respectively). We can conclude that in temperate secondary forests: (1) the R S rate and the relationships between R S and abiotic/biotic factors change greatly with forest types, and (2) R S is strongly influenced by soil temperature, MBC, microbial biomass nitrogen and coarse root biomass in temperate secondary forests.

There is, so far, no common conclusion about photosynthetic responses of trees to long-term exposure to elevated CO 2 . Photosynthesis and specific leaf area (SLA) of 1-year-old and current-year needles in Pinus koraiensis and P. sylvestriformis grown in open-top chambers were measured monthly for consecutive two growing seasons (2006, 2007) after 8-9 years of CO 2 enrichment in northeastern China, to better understand species-specific and needle agerelated responses to elevated CO 2 (500 lmol mol -1 CO 2 ). The light-saturated photosynthetic rates (P Nsat ) increased in both species at elevated CO 2 , but the stimulation magnitude varied with species and needle age. Photosynthetic acclimation to elevated CO 2 , in terms of reduced V cmax (maximum carboxylation rate) and J max (maximum electron transport rate), was found in P. koraiensis but not in P. sylvestriformis. The photosynthetic parameters (V cmax , J max , P Nsat ) measured in different-aged needles within each species responded to elevated CO 2 similarly, but elevated CO 2 resulted in much pronounced variations of those parameters in current-year needles than in 1-year-old needles within each species. This result indicated that needle age affects the magnitude but not the patterns of photosynthetic responses to long-term CO 2 enrichment. The present study indicated that different species associated with different physioecological properties responded to elevated CO 2 differently. As global change and CO 2 enrichment is more or less a gradual rather than an abrupt process, longterm global change experiments with different plant species are still needed to character and better predict the global change effects on terrestrial ecosystems.Keywords Global environmental change Á Photosynthetic acclimation Á Photosynthetic parameters Á Photosynthetic responses Á Specific leaf area Abbreviations AQE Apparent quantum yield P Nsat Light-saturated photosynthetic rate V cmax Maximum carboxylation rate J max Maximum electron transport rate SLA Specific leaf area