Root Exuduation And Effect Of Root Exduate To Soil Nitrogen Transformation In Poplar And Alder Mixed Plantation

Soil nitrogen supply is the key factor limiting the productivity of poplar plantation, while the introducing of N-fixing tree species is an effective way to alleviate the situation and improve poplar growth. Trees take up nutrients directly from the rhizospheric zone. The root exudates could alter the physical, chemical, and biological environment, and therefore the nutrient transformation processes, in the rhizosphere, as a feedback, leading to the change of nutrient supply. In order to evaluate the nutrient dynamics in the rhizosphere, root distribution and low-molecular-weight organic carbon(LMWOC) secretion properties, as well as N transformation in the rhizosphere, were investigated in a mixed plantation of poplar and alder(established in 2012). Additional simulated incubation experiments were also designed to study the diversity effects of LMWOC on soil N transformation. The results showed that the root of poplar widely distributed and occupied about 85% of the total root biomass in the upper 20 cm soil layer in the mixing plantation. On the contrary, about 84.2% of the root of alder focused on an area of about 40 cm from its trunks. Root secretion rate, as gram of organic C per root surface area, was 1.27 times more for alder when compared to poplar. The main components of LMWOC in root exudates included glucose and turanose for poplar, and with other two organic acid as lactic acid and malic acid for alder. Soil p H and microbial biomass(MBC and MBN) in rhizosphere showed no difference from bulk soil for both poplar and alder. An obvious deficit of inorganic N was found in the rhizosphere of poplar, amount from 74% to 87% of the content in the bulk soil, while no significant difference existed for gross N mineralization between rhizosphere and bulk soil for both poplar and alder. Significant deficit ratio of 31% for available phosphorus was found in the rhizosphere of poplar when compared to bulk soil, while a slight enrichment was found for the rhizosphere of alder. The addition of LMWOC promoted soil microbial decomposition activity significantly, consequently reduced soil inorganic nitrogen and phosphorus contents significantly due to microbial uptake. The reducing effect of sugar was much more significant on N, while that of organic acid was significant on P. Increase in the diversity of LMWOC showed active result for relieving such reducing effects to a certain extent. Gross N mineralization was not obviously affected by the addition of LMWOC, while slight enhancement was found when the diversity of LMWOC, especially of organic acid, was increased.