Soil Fauna Community Ecology And Effects Of Nitrogen Addition In Larix Gmelinii And Fraxinus Mandshurica Plantations

Soil fauna, which are main soil nutrition contributors, play an important role in soil carbon (C) and nutrient cycling in forest ecosystems. Soil fauna are sensitive to soil nutrients availability, but few studies focused on the interrelations between soil fauna density and nitrogen (N) availability in Northern forests. In this study, we used soil cores to study the effects of N addition on soil fauna densities, feeding habits and groups diversities in larch (Larix gmelinii) and ash (Fraxinus mandshurica) plantations, and used insecticide together with ingrowth cores to study the effects of soil fauna on root length density. The main results were below:(1) Soil fauna density in larch plantation was 26.1×103 ind m-2, which was belong to 8 classes,13 orders and 34 families. The corresponding values in ash plantation were 31.6×103 ind m-2,8 classes,15 orders and 43 families, respectively. In both plantations, seasonal peak of soil fauna density was October>May>July, but fauna species was October>July>May. There were higher density and number of groups in topsoil layer (0-10 cm) than that in subsoil layer (10-20 cm). Among the three feeding habits, the magnitude of soil saprophagous was accounted for over 80%, then the soil herbivores and predators were accounted for 10%and less than 5%, respectively.(2) The density of soil fauna had negative correlation with soil temperature (P<0.05), but positive correlation with soil pH and organic matte (P<0.01) in both plantations. However, N addition reduced these correlations. In contrast to other soil factors, soil moisture did not affect soil fauna densities, excepting in subsoil layer under the N treatment plots in ash plantation.(3) Diversity of soil fauna differed in plantations and treatments. Diversity index and evenness index in the N-addition plots were increased in larch plantation, but decreased in ash plantation comparing with the control plots. N addition also decreased the dominant index in larch plantation, but increased in ash plantation. These three indexes did change in seasonal patterns between the control and N-addition plots.(4) The effects of N addition on densities of soil fauna in both plantations differed among four sampling times in two soil depths, their densities was increased at the start time and decreased in latter time, so did fauna groups. In three feeding habits, the densities were decreased in soil saprophagous, increased in soil herbivores, and unchanged in soil predators after N fertilization.(5) Insecticide application significant reduced the densities of soil herbivores, with reduced by 97%(0-10 cm) and 100%(10-20 cm) in larch plantation, and reduced by 94%(0-10 cm) and 84%(10-20 cm) in ash plantation. The reduction of herbivore densities significant increased root length densities (P<0.05). These suggest that soil fauna, particularly soil herbivores, may be a critical in estimating soil C cycling in forest ecosystems.