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Abstract: Better understanding of soluble carbon (C) dynamics in managed grasslands fertilised with animal manures is required to effectively estimate nutrient transport potential and C loss during land-use change. Two adjacent grassland plots, one amended with 100m3 ha−1 of liquid swine manure annually since 1978 and another unamended for the same period, were either killed by glyphosate in the autumn or left with vegetation intact. Those killed were either (i) left as an undisturbed chemical fallow, (ii) ploughed (20cm depth) by full inversion tillage (FIT) in the autumn, or (iii) ploughed (20cm depth) in the following spring. Microbial biomass C (MBC) and soluble C fractions: hot-water extractable organic C (HWEOC), cold-water extractable organic C (CWEOC), and dissolved organic C from tension lysimeters (DOC) were monitored during one full year following herbicide application and ploughing. On both unmanured and manured sites, HWEOC was positively correlated with MBC. However, on the manured grassland the slope of the HWEOC:MBC relationship was 55% higher than on the unmanured site indicating that the manured grassland accumulated more potentially soluble C per unit MBC than the unmanured grassland. Compared to the plots with intact vegetation and the chemical fallow, full inversion tillage reduced HWEOC and CWEOC by 10 to 30%, on both manured and unmanured soils, and reduced DOC in the soil solution of the manured soils. Peak DOC concentrations measured in the field occurred when soils were water-logged, particularly on the manured grassland. Our results suggest that manured grasslands on poorly-drained soils accumulate important quantities of readily soluble C which are prone to export as DOC when the soil becomes water-saturated. However, full inversion tillage of these grassland soils attenuated the amounts of readily soluble C and, in particular, the peaks in DOC. [Copyright &y& Elsevier]

Abstract: Managed grasslands occupy a large portion of the agricultural landbase, are rich in C and N and therefore represent a risk for emissions of N2O during landuse change. Two adjacent grassland plots, one amended with 100m3 ha−1 of liquid swine manure annually since 1978 and an unamended grassland were either (i) left with vegetation intact (Control) or killed by glyphosate in the autumn. Glyphosate-treated subplots were either (ii) left as an undisturbed chemical fallow, (iii) ploughed by full inversion tillage (FIT) in the autumn, or (iv) in the spring. Cumulative emissions of N2O over the measuring period (static chambers), soil solution NO3-N (tension lysimeters) and soil NO3-N (KCl extraction) were monitored following tillage for one fallow year. Emissions of N2O decreased in the order: amended chemical fallow (3.0gNm−2), amended spring-FIT (2.0gNm−2), amended autumn-FIT and unamended chemical fallow (1.7gNm−2, both), unamended spring-FIT (1.2gNm−2) and the unamended autumn-FIT subplots (0.9gNm−2) while grassland controls emitted 0.07 and 0.09gNm−2 for the unamended and amended plots, respectively. Emissions of N2O were positively correlated with soil solution NO3-N at 30-cm depth in the chemical fallow, but negatively with soil solution NO3-N at 45cm in FIT subplots. In chemical fallow soils, N2O was likely produced as the NO3 − mineralized at the soil surface moved down the soil profile, enriching denitrification sites at shallow depths during rainfall events. In contrast, FIT placed the C and N required for denitrification deeper in the soil profile, and complete reduction to N2 was likely favoured. On these poorly drained grassland soils, FIT reduced emissions of N2O relative to a chemical fallow by a factor of 2 to 3 for equivalent soil NO3-N. Emission factors accounting for the interaction between soil characteristics – in this case drainage conditions – and management practices may be important in providing accurate N2O emission estimates. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]