Dissolved organic carbon (DOC) export from soils can play a significant role in soil C cycling and in nutrient and pollutant transport. However, information about DOC losses from agricultural soils as influenced by management practices is scarce. We compared the effects of mineral fertilizer (MF) and liquid hog manure (LHM) applications on the concentration and molecular size of DOC released in runoff and tile-drain water under corn (Zea mays L.) and forage cropping systems. Runoff and tile-drain water samples were collected during a 2-mo period (October to December 1998) and DOC concentration was measured. Characterization of DOC was performed by tangential ultrafiltration with nominal cut-offs at 3 and 100 kDa. Mean concentration of DOC in runoff water (12.7 mg DOC L(-1)) was higher than in tile-drain water (6.5 mg DOC L(-1)). Incorporation of corn residues increased the DOC concentration by 6- to 17-fold in surface runoff, but this effect was short-lived. In runoff water, the relative size of the DOC molecules increased when corn residues and LHM were applied probably due to partial microbial breakdown of these organic materials and to a faster decomposition or preferential adsorption of the small molecules. The DOC concentration in tile-drain water was slightly higher under forage (7.5 mg DOC L(-1)) than under corn (5.4 mg DOC L(-1)) even though the application rates of LHM were higher in corn plots. We suggest that preferential flow facilitated the migration of DOC to tile drains in forage plots. In conclusion, incorporation of corn residues and LHM increased the concentration of DOC and the relative size of the molecules in surface runoff water, whereas DOC in tile-drain water was mostly influenced by the cropping system with relatively more DOC and larger molecules under forage than corn.

No abstract

Managed grasslands are a predominant land use in northern temperate regions. They are often on poorly drained soils and contain large stocks of soil organic carbon (SOC). It is important to understand C dynamics in grasslands to better assess their role in regional C budgets. Two adjacent grassland plots, one amended with 100 m3 ha−1 of liquid swine manure (LSM) annually since 1978, and another unamended were killed by glyphosate in the autumn or (i) left with vegetation intact. Those killed were either (ii) left as an undisturbed chemical fallow, (iii) plowed by full‐inversion tillage (FIT) in the autumn, or (iv) in the spring. Following the autumn plowing, we monitored CO2 emissions from the fallow soil surface, CO2 concentrations in the soil profile, soil temperature, and soil water content for 1 yr. Changes in soil aggregation, the light fraction organic matter, and microbial biomass were also monitored. Plowing decreased the total microbial biomass on average by 27 g C m−2, the quantity of water‐stable aggregates by 7 to 12% and with it the concentration of light fraction organic matter. Respiration was also reduced by 142 g CO2‐C m−2 by autumn‐FIT and 175 g CO2‐C m−2 by spring‐FIT on unamended soils, and 90 g CO2‐C m−2 by autumn‐FIT and 98 g CO2‐C m−2 by spring‐FIT on amended soils. Regression analyses suggested that reductions in CO2 emissions were due to the placement of surface SOC at depth where soil temperature and oxygen availability were attenuated. In these soils, the cool humid conditions at depth in the soil profile may act to counter‐balance the physical effects of tillage thereby preserving C deep in the soil profile. Effective calculation of C budgets and changes in SOC stocks depends on the ability to reproduce the interaction between climate, soil type, land use, and management action. A more complete understanding of the effect of management actions that modify the vertical distribution of biogeochemical (particularly organic C) and environmental parameters on soil respiration in poorly drained soils is required.

No abstract

Lalande, R., Gagnon, B. and Royer, I. 2009. Impact of natural or industrial liming materials on soil properties and microbial activity. Can. J. Soil Sci. 89: 209Á222. Soil acidity is a major problem in agriculture because it limits plant growth and reduces crop productivity. The neutralizing potential of industrial by-products and their impact on soil properties were evaluated in two acidic soils characterized by contrasting textures, and submitted to intensive agriculture practices. Soil pH, microbial (dehydrogenase and alkaline phosphatase) activity, and Mehlich-3 extractable P, K, Ca and Mg were monitored in the year of soil incorporation of eight liming products and in the following 2 yr. In the sandy loam, liming products did not result in significant increases in soil pH in the 0-to 7.5-cm soil layer. Lime mud (LM) significantly increased soil pH by 0.4 units in the 7.5-to 20-cm layer compared with cement kiln dust (CKD). In the silty clay, calciumÁ phosphateÁmagnesium (CalPoMag) significantly raised pH by 0.65 units over both natural calcitic lime (NCa) and the magnesium dissolution product (MgD) in the first soil layer, and by 0.5 units over carbide lime (CL) treatment in the second soil layer. Activities of dehydrogenase and alkaline phosphatase were increased to various degrees by all liming materials, especially on the silty clay; LM and CalPoMag were the most beneficial materials. The exception was MgD, which did not result in any impact on microbial activity relative to the control. Both enzymatic activities were related to the increase in soil pH, particularly the alkaline phosphatase. Ion leaching was more pronounced in the sandy loam than in the silty clay soil, where large differences in the Ca and Mg ion levels were still detected in the 20-to 40-cm layer of the sandy loam. In this study, LM and CalPoMag are interesting liming products, particularly in the silty clay soil.