Article pubs.acs.org/est
Organic Matter Chlorination Rates in Different Boreal Soils: The Role of Soil Organic Matter Content Malin Gustavsson,†,* Susanne Karlsson,† Gunilla Ö berg,‡ Per Sandén,† Teresia Svensson,† Salar Valinia,§ Yves Thiry,∥ and David Bastviken† †
Department of Thematic Studies, Water and Environmental Studies, Linköping University, 58183 Linköping, Sweden. Institute for Resources, Environment and Sustainability, University of British Columbia, 428-2202 Main Mall Vancouver, British Columbia, Canada, V6T 1Z4. § Department of Aquatic Sciences and Assessment SLU, Swedish University of Agricultural Sciences, P.O. Box 7050, SE 75007 Uppsala, Sweden. ∥ Andra, Research and Development Division, 1/7 rue Jean-Monnet, 92298 Chatenay-Malabry Cedex, France. ‡
ABSTRACT: Transformation of chloride (Cl−) to organic chlorine (Clorg) occurs naturally in soil but it is poorly understood how and why transformation rates vary among environments. There are still few measurements of chlorination rates in soils, even though formation of Clorg has been known for two decades. In the present study, we compare organic matter (OM) chlorination rates, measured by 36Cl tracer experiments, in soils from eleven different locations (coniferous forest soils, pasture soils and agricultural soils) and discuss how various environmental factors effect chlorination. Chlorination rates were highest in the forest soils and strong correlations were seen with environmental variables such as soil OM content and Cl− concentration. Data presented support the hypothesis that OM levels give the framework for the soil chlorine cycling and that chlorination in more organic soils over time leads to a larger Clorg pool and in turn to a high internal supply of Cl− upon dechlorination. This provides unexpected indications that pore water Cl− levels may be controlled by supply from dechlorination processes and can explain why soil Cl− locally can be more closely related to soil OM content and the amount organically bound chlorine than to Cl− deposition.
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INTRODUCTION Chloride participates in a complex biogeochemical cycle, and details regarding this cycle have been discussed for several years. In the late 1980s and early 1990s1−3 it was revealed that large amounts of naturally formed chlorinated organic matter (Clorg) were present ubiquitously in the environment.2−6 Chlorinated organic compounds were earlier believed to originate from anthropogenic activities only.7 An increasing amount of evidence has confirmed that formation of Clorg occurs naturally and that Clorg is as abundant as the chloride ion (Cl−) in organic soils.3,6,8,9 Even though it is nowadays widely accepted that Cl− can be transformed to Clorg, the underlying processes are not well understood. However, microorganisms and enzymes appear be important for both the formation and degradation processes of Clorg.1,10,11 Imbalance between these processes seems to favor the build up of a soil pool of Clorg.12 It seems however as if the balance between the two processes can shift, leading to that Cl− is retained during some conditions and released from soil during other conditions.13,14 © 2011 American Chemical Society
Questions about how formation/mineralization of Clorg takes place and how fast these processes occur are of interest for a number of reasons. First, it is still often assumed that Cl− is inert and freely mobile in soil and Cl− has frequently been used in hydrological research and biogeochemical modeling. Typically, Cl− is used as an inexpensive and easily measured tracer for soil and groundwater movements when calculating budget and deposition estimates.15 Not accounting for the Cl− cycling including retention and release of Clorg can lead to bias when doing hydrological modeling. Second, the widespread perception that Cl− is inert in soil is also applied to radioactive chlorine (36Cl).16,17 36Cl is produced in the irradiated fuel assembly during reactor operations and has a half-life of 3.01 × 105 years. The mobility of Cl− in the environment and the long Received: Revised: Accepted: Published: 1504
September 12, 2011 December 7, 2011 December 22, 2011 December 22, 2011 dx.doi.org/10.1021/es203191r | Environ. Sci. Technol. 2012, 46, 1504−1510
Environmental Science & Technology
Article
Table 1. Soil and Vegetation Characteristics at the Sample Sites sample
land use
soil parent materiala
texture
vegetation field layer
canopy cover (%)
stand age (years)
F1 F2 F3 F4 P1 P2 P3 P4 A1 A2 A3
coniferous forest coniferous forest coniferous forest coniferous forest pasture pasture pasture pasture agricultural agricultural agricultural
till till till till till till till till till till till
coarse siltyb fine sandyc fine sandy coarse silty sandyd coarse silty sandy sandy clayeye clayey clayey
high - low herb types high − low herb types thin leaved grass types bilberry broad leaved grass types high − low herb types broad leaved grass types broad leaved grass types various ley barley
80 44 48 51 12 9 19 5 0 0 0
25 50 50 50 74 55 105 37
a Soil parent material comprises information on genesis and grain-size distribution of the parent material for soil formation. bSoil with a grain size between 0.06 and 0.02 mm. cSoil with a grain size between 0.2 and 0.06 mm. dSoil with a grain size between 0.6 and 0.2 mm. eSoil with a grain size
