Genesis and dynamics of dissolved organic matter in agricultural soils

The dissolved organic matter (DOM) is the term used for organic components of natural origin present in the soil solution. It is a complex mixture of molecules, generally dominated by humic substances. The DOM is a small fraction (< 1%) of the soil organic matter (SOM), however DOM is supposed to be the most mobile and probably the most reactive fraction. Consequently, it plays an important role in the biogeochemistry of the elements it contains (C, N, P, S, ) and the transport of heavy metals and organic pollutants with a high affinity for organic matter. Since biodegradation is mediated through the aqueous phase, DOM is supposed to be the most bioavailable organic matter pool. Understanding of organic matter cycling and of DOM-facilitated transport of solutes in mineral soils requires information about the origin and dynamics of DOM. The main goal of this research is to unravel the source and the dynamics of DOM in agricultural soils.No standard method exists to sample soil solution. Therefore dissolved organic carbon (DOC) concentration and quality in soil is operationally defined. Effluents of undisturbed soil columns of an agricultural Luvisol contained significantly less DOC than corresponding soil centrifugates (10 versus 20 mg C l-1). In contrast, inorganic solute concentrations did not vary between both sampling methods. The difference in DOC concentrations between both sampling methods did not depend on the water flow rate in the columns. Furthermore, the DOM in the centrifugates was less aromatic than the DOM in the leachates. All this suggests that the larger DOC concentration in centrifugates is unrelated to DOC concentration gradients in the pore matrix but that it is likely released from a fraction of the microbial biomass that is disrupted during centrifugation. Spectroscopic properties of DOM in the samples corroborate this suggestion. Hence, DOM in effluents of undisturbed soil columns is likely most representative of in situ soil DOM.The effect of varying soil properties and pre-treatments on the quantity and quality of DOC in the centrifuged soil solutions was investigated in an explorative study of 87 European agricultural soils. Soil solution DOC concentrations ranged between 12 and 104 mg C l-1 (10th - 90th percentile), median 33 mg C l-1. In freshly sampled agricultural soils, DOC was only weakly explained by soil properties whereas correlations were somewhat stronger in soils that had been equilibrated by incubation or cold storage before analysis. Overall, DOC concentrations weakly increased with increasing % soil organic carbon and decreased with increasing pH. The storage conditions affected DOC concentration and composition: rewetting dry soils released DOM with a low aromaticity that probably originated from decaying biomass rather than from humified organic matter.The source and dynamics of DOM in an agricultural soil after plant residue amendment were studied in a batch experiment. It revealed that residue applicationreleased a pulse of hydrophilic DOC that temporarily (< 3 days) dominated the soil DOM pool and the degradable organic matter. However, beyond that pulse the majority of DOM was derived from soil organic matter. The isotope signature of DOC differed from the isotope signature of microbial biomass C and respired C. This casts doubt that DOC measured in the soil solution represents the major bioavailable soil C pool.The source, dynamics and transport of DOM were subsequently studied in column experiments. First of all, the mechanisms of DOM release were investigated by leaching undisturbed soil columns and applying stop flows. Secondly, the sorption and mineralisation of DOM were examined by adding a 13C labelled DOM pulse to the columns. The release of DOM during leaching was controlled by two different processes. The baseline concentration of DOC in soil solutions was buffered and was independent of the water flow rate. An additional amount of DOC can be released by desorption, diffusion from micro- to macropores or lysis of microbial cells. These processes are kinetically controlled and the additional DOC concentration hence depends on the water fluxes. In agreement with the batch experiments, largest DOC concentrations were found at flow initiation after soil drying. This low aromaticity DOM mainly originated from decaying biomass. A plant derived DOM solution was very quickly and almost completely mineralised during transport through the soil.The results of the laboratory experiments were linked with those of a field trial, to interpret monitoring data of DOM leaching in an arable field subjected to 4 different land management treatments, i.e. conventional tillage, reduced tillage, waste water irrigation and manure application. Leachates were collected at 45 cm depth by passive capillary wick samplers. Over 95% of leachates collected during three successive years contained between 4 and 20 mg DOC l-1 and on average 32 kg DOC ha-1 y-1 was leached. The most consistent findings of the laboratory experiments were also observed in the field. The export of DOC (kg DOC ha-1 y-1) was mainly determined by the water flux, while no correlation was found between the water flow and DOC concentrations. At high water velocities, soil solutions contained small DOC concentrations, while DOC concentrations in soil solutions flowing at low water velocities varied largely. A baseline DOC concentration independent of water flow rates was observed. The largest DOC concentrations however, were inversely proportional to the mean water velocity between succeeding samplings. This indicates that the release of additional DOM is kinetically controlled. Higher concentrations were mostly found in the first leachates collected after a dry period and those solutions mainly contained DOM with a low aromaticity. The effect of environmental conditions and land management practices on DOC concentration or its spectroscopic properties was rather small.A model consisting of twotypes of DOM in aseven pool concept (labile and stable SOM, labile and stable DOM, microbial biomass, plant residues and CO2) summarised the most important findings and captured the general patterns in DOM dynamics rather well. Degradation reactions between the pools were described with first order kinetic reactions and the dynamic equilibrium between DOM and SOM pools was determined by a soil:solution distribution coefficient. The transport of DOM was described by the convection dispersion equation. It was found that soil solutions always contained a constant concentration of relatively stable, not easily degradable organic matter. The concentration of labile DOC was variable and depended on the water velocity, flow interruptions and the presence of plant residues.It can be concluded that soil solution DOM mainly originates from rather stable, more humified soil organic matter. This baseline DOM concentration is highly buffered and may depend on physicochemical soil properties. Flow interruptions or amending the soil with plant residues releases additional DOM. This extra DOM is very labile and quickly mineralised by the microbial biomass. Consequently, it will probably not reach deeper soil layers.Dankwoord ... i Summary ... iii Samenvatting ... vii List of abbreviations ... xi List of symbols ... xiii Table of contents ...xv Chapter 1 Introduction, hypotheses, research objectives and thesis outline ...1 Chapter 2 Comparison of column leaching and centrifugation for obtaining soil solution dissolved organic matter ... 11 Chapter 3 Factors controlling the dissolved organic matter concentration in pore waters of agricultural soils ... 25 Chapter 4 Tracing the source and fate of dissolved organic matter in soil after incorporation of a 13C labelled residue: a batch incubation study... 41 Chapter 5 Dynamics of dissolved organic matter during vertical transport: a column study ... 57 Chapter 6 The effect of environmental conditions and land management on dissolved organic matter dynamics in an agricultural soil: a field study ... 77 Chapter 7 Dynamics of dissolved organic matter in agricultural soils: a model ... 97 Chapter 8 General conclusions and future perspectives ... 129 References ...137 Appendix 1 Stable isotope analysis of DOC in soil solutions using a catalytic combustion TOC analyser – IRMS with cryofocusing interface ... 149 Appendix 2 Description of the model ... 171 List of publications ... 177nrpages: 206status: publishe