Soil soluble carbon dynamics of manured and unmanured grasslands following chemical kill and ploughing

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 100 m3 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 (20 cm depth) by full inversion tillage (FIT) in the autumn, or (iii) ploughed (20 cm 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.

Research Highlights
► Grassland plots either unfertilized or manured annually since 1978 were used.
► Grass stands were killed by glyphosate and left as chemical fallow or ploughed.
► Ploughing reduced water-extractable and dissolved organic C by 10 to 30% for 1 yr.
► Manured grasslands accumulated more C prone to export as DOC after chemical kill.
► Ploughing by full inversion tillage attenuated the risk of C export as DOC.