Leaching Decreased Microbial Decomposition and Sorption of Easily Available Organic Substances in Soil.
Holger Fischer and Yakov Kuzyakov. Univ of Hohenheim, Emil-Wolff-Straße 27, Stuttgart, Germany
Low molecular weight organic substances exuded from roots or from lysis of microbial or plant residues into the soil exist only for a very short time free and unchanged in soil solution. They are subject to three competing processes: 1) leaching, 2) microbial uptake and decomposition, and 3) sorption by clay minerals and soil organic matter (uptake by plants can be neglected). Since the three processes are competitive, the intensity of one affects the amount of substance subjected to two others. To our knowledge, interactions of these processes were never compared in one study. In a column experiment under controlled conditions we tested the effect of increasing leaching intensity on the ratio of microbial decomposition, leaching and sorption of two typical substances: glucose as representative for sugars and glycine as representative for amino acids. To distinguish the added substances from similar ones already present in the soil we used uniformly 14C labeled glucose or glycine (5 kBq per container with 55 g soil) in rhizosphere relevant concentrations (10 µmol l-1). After addition of the substances to the soil, the containers were closed air tied and leached with water at rates ranging from 0.3 to 1.0 ml h-1 g-1soil. A complete 14C balance was calculated as all the three possible C pools (eluate, CO2 and residue in the soil) were measured for their radioactivity. The importance of the three processes alter depending on the substance investigated and the leaching rates. By leaching intensity up to 0.5 ml h-1 g-1 only 24% of the recovered radioactivity for glucose and 65% for glycine were eluted. The remainder was decomposed to CO2 (11% for glucose, and 9% for glycine) or remained in the soil (65% for glucose, and 26% for glycine). Under low leaching rates (0.3 – 0.5 ml h-1 g-1) far more glycine than glucose was eluted. By increasing the leaching rate up to 1 ml h-1 g-1 the amount of glucose recovered in exudates rose much stronger than that of glycine: The glycine recovery in eluate increased only from 65% at 0.5 ml h-1 g-1 to 89% at 1 ml h-1 g-1 compared to the increase of glucose in eluate from 24% to 75% for the same leaching rates . After 15 hours of glycine leaching no further distribution changes were observed, indicating a strong retention of the glycine remaining in the soil. Such strong retention was reached after about 20 hours for glucose and derived metabolites. This time was halved, when half of the soil was replaced by sand. As the retention of glycine was strongly reduced in sand:soil mixture compared to soil only (17% vs. 26%), we assume that the retention is been mainly controlled by sorption. Addition of sand did not affect glucose retention. Therefore, we assume that glucose was mainly taken into the microbial cells. We infer on these results that up to the leaching intensity of 0.5 ml h-1 g-1 not sorption but microbial incorporation is the most important process of glucose retention in the soil. If the precipitation intensity was higher than 0.5 ml h-1 g-1 then the most easily available organic substances were leached from the soil within 20 hours so that microbial uptake and decomposition were reduced. We conclude that despite very high leaching rates that may occur only on flooded areas or on sites with additional lateral inflow, considerable amount of sugars and amino acids is not leached but remains in soil sorbed to the soil matrix or are utilized by microorganisms.