Controls on the dynamics of dissolved organic matter in soils: A review

Dissolved organic matter (DOM) in soils plays an important role in the biog eochemistry of carbon, nitrogen, and phosphorus, in pedogenesis, and in the transport of pollutants in soils. The aim of this review is to summarize t he recent literature about controls on DOM concentrations and fluxes in soi ls. We focus on comparing results between laboratory and field investigatio ns and on the differences between the dynamics of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP). Both laboratory and field studies show that litter and humus are the most i mportant DOM sources in soils. However, it is impossible to quantify the in dividual contributions of each of these sources to DOM release. In addition , it is not clear how changes in the pool sizes of litter or humus may affe ct DOM release. High microbial activity, high fungal abundance, and any con ditions that enhance mineralization all promote high DOM concentrations. Ho wever, under field conditions, hydrologic variability in soil horizons with high carbon contents may be more important than biotic controls. In subsoi l horizons with low carbon contents, DOM may be adsorbed strongly to minera l surfaces, resulting in low DOM concentrations in the soil solution. There are strong indications that microbial degradation of DOM also controls the fate of DOM in the soil. Laboratory experiments on controls of DOM dynamics have often contradicted field observations, primarily because hydrology has not been taken into acc ount. For example, laboratory findings on the effects of plant species (con ifer vs. deciduous) on DOM release from forest floors and on the effects of substrate quality (e.g.: C/N ratio) or pH on DOC concentrations were often not confirmed in field studies. The high adsorption capacity of soil clay minerals and oxides for DOM shown in laboratory studies may not control the transport of DOM in soils in the field if macropore fluxes dominate under field conditions. Laboratory findings about the biodegradability of DOM als o await verification under field conditions. Studies that include DON and DOP dynamics in addition to DOC are few. The r ate of release and the fate of DOG, DON, and DOP in soils may differ to a f ar greater extent than previously assumed. Controls established for DOC mig ht thus be not valid for DON and DOP. Despite intensive research in the last decade, our knowledge of the formati on and fate of DOM in soils and its response to changing environmental cond itions is still fragmented and often inconsistent. Predictions at the field scale are still very uncertain, and most of the information available toda y is the result of studies on temperate soils and forest ecosystems. Thus, future research on controls of DOM dynamics should be extended to soils und er different land uses and in other climate zones. Emphasis should also be given to: (i) the effects of soil organic matter properties on the release of DOM (ii) environmental factors controlling DOM quantity and quality (iii ) the assessment of biological versus physico-chemical controls on the rele ase and retention of DOM in soils, and (iv) the differences between DOG, DO N, and DOP. Finally, if our goal is to predict DOM concentrations and fluxe s in soils, future research on the controls of DOM dynamics should have a s trong focus on field studies.