Dissolved organic matter (DOM) in soils plays an important role in the biogeochemistry of carbon, nitrogen, and phosphorus, in pedogenesis, and in the transport of pollutants in soils. The aim of this review is to summarize the recent literature about controls on DOM concentrations and fluxes in soi

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

http://andrewsforest.oregonstate.edu/pubs/pdf/pub1374.pdf

Stabilization and destabilization of soil organic matter: mechanisms and controls

Soil aggregate stability is a crucial soil property affecting soil sustainability and crop production A broad outline of the processes and agents of aggregate formation and aggregate stabilization are presented and discussed in this review Aggregate stability is difficult to quantify and interpret The aim of aggregate stability tests is to give a reliable description and ranking of the behavior of soils under the effect of water, wind and management Numerous methods have been used to determine aggregate stability with varying success The different methodologies complicate the comparison among aggregate stability data It is also difficult to obtain a consistent correlation between aggregate stability and other important soil properties such as soil erodibility or crusting potential This paper reviews the different methods of measurement of soil aggregate stability used in the literature, paying attention to the conditions of sample collection in the field and sample preparation and treatmen

Soil aggregate stability: a review

Long-term increases in surface water dissolved organic carbon: Observations, possible causes and environmental impacts

Statistical considerations show that the sediment, solute, or pollutant load of a river is likely to be underestimated by methods in which unmeasured concentrations are estimated from discharge using a least squares regression for the logarithm of concentration. The degree of underestimation increases with the degree of scatter about the rating curve and can reach 50%. A simple correction factor is proposed and tested successfully on simulated and real data sets.

River Loads Underestimated by Rating Curves

Several studies have highlighted an increase in DOC concentration in streams and lakes of UK upland catchments though the causal mechanisms controlling the increase have yet to be fully explained. This study, compiles a comprehensive data set of DOC concentration records for UK catchments to evaluate trends and test whether observed increases are ubiquitous over time and space. The study analysed monthly DOC time series from 198 sites, including 29 lakes, 8 water supply reservoirs and 161 rivers. The records vary in length from 8 to 42 years going back as far as 1961. Of the 198 sites, 153 (77%) show an upward trend in DOC concentration significant at the 95% level, the remaining 45 (23%) show no significant trend and no sites show a significant decrease in DOC concentration. The average annual increase in DOC concentration was 0.17 mg C/l/year. The dataset shows: (i) a spatial consistent upward trend in the DOC concentration independent of regional effects of rainfall, acid and nitrogen deposition, and local effects of land-use change; (ii) a temporally consistent increase in DOC concentration for period back as far as the 1960s; (iii) the increase in DOC concentration means an estimated DOC flux from the UK as 0.86 Mt C for the year 2002 and is increasing at 0.02 Mt C/year. Possible reasons for the increasing DOC concentration are discussed.

Trends in Dissolved Organic Carbon in UK Rivers and Lakes

SUMMARY

Stability changes during ageing (sterile) or incubation (non-sterile) of both natural field aggregates and remoulded aggregates from five soils were studied for periods up to 30 days. Growth of fungal hyphae, estimated by ergosterol measurement, corresponded to temporary stability increases in both types of aggregates during the first 15 days. Thereafter, fungal hyphae disappeared and were replaced by actinomycetes and bacteria. Increased stability due to entanglement by hyphae was comparable to that due to thixotropy in remoulded aggregates. Bacterial growth accompanied the fall in stability associated with fungal decline, but had little direct effect in stabilizing soil aggregates. Destruction of polysaccharides by periodate oxidation greatly diminished aggregate stability. The role of bacterial polysaccharides in soil aggregate stability is discussed.

Contributions by fungi and bacteria to aggregate stability of cultivated soils

Summary Soils were examined from each of six primary forest plots at altitudes of 100 m, 500 m, 1000 m, 1500 m, 2000 m and 2600 m on Volcan Barva, Costa Rica. All were on volcanic parent materials. There was evidence of decreasing intensity of weathering and organic matter decomposition with increasing altitude. Subsoil clay content decreased from 80% at 100 m altitude to less than 10% at 2000 m. Ratios of free to total iron decreased from 0.3 to less than 0.1. Organic matter in the surface horizons increased from 18% to 49% over the same altitude range. The uppermost site at 2600 m was formed on younger and less weathered material and had greater concentrations of exchangeable bases than all the other sites. There was no evidence that the nutrients measured were less well supplied at high altitudes, and forest changes with altitude were not caused by any soil changes examined in this study.

Soil variation with altitude on Volcan Barva, Costa Rica.

Human activities have had pronounced impacts on soil properties. Conifer afforestation in the uplands has caused significant decreases in soil pH and in the quality and turnover of organic matter. Acid deposition has increased soil acidity by a similar amount to conifer afforestation but has been shown to affect soils at greater depths. Acid deposition has also increased the mobility of trace metals in the soil and therefore increased metal concentrations in drainage waters. Applications of sewage sludge to the soil have been shown to increase metal concentrations, although most of the Scottish soils affected have high trace metal binding capacities. Intensification of arable cultivation in the lowlands has reduced organic matter concentrations, structural stability and soil workability, and has had effects on soil erodibility. Human trampling, while highly localised, affects sensitive mountain soils in popular areas, leading to loss of surface organic horizons, and therefore, carbon storage. The future impacts of human activities on the soil may be exacerbated by changing climate, and the need to monitor and predict these will not diminish.

Human impacts on soil properties and their implications for the sensitivity of soil systems in Scotland

SUMMARY. 1. Dissolved organic matter (DOM) was measured at 8h intervals over a 1 year period in a stream draining 51 ha of moorland with peaty soils.



2. DOM concentrations increased with increasing stream discharge from low flow values of 0–3 mg 1−1 to maximum values of 30 mg 1−1. There were also seasonal differences of up to 13mgl−1 between August (maximum) and February, and differences of about 2.5 mg l−1 between rising and falling stage samples.



3. Seasonal variation was closely related to mean temperature.



4. Total loss over the year was 168 kg ha−1 DOM (84 kg ha−1 C), larger than previous estimates for upland sites.

Ian C. Grieve

Papers

Trends in Dissolved Organic Carbon in UK Rivers and Lakes

Contributions by fungi and bacteria to aggregate stability of cultivated soils

Soil variation with altitude on Volcan Barva, Costa Rica.

Human impacts on soil properties and their implications for the sensitivity of soil systems in Scotland

Concentrations and annual loading of dissolved organic matter in a small moorland stream