Short-term sequestration of slurry-derived carbon into particle size fractions of a temperate grassland soil

Abstract

Surface application of animal wastes in intensive grassland systems has caused growing environmental problems during the last decade and, therefore, increasing public and scientific concern. In the present study we examined if the natural abundance ¹³C stable isotope tracer techniques could be used to investigate a poorly defined aspect of waste application, i.e. incorporation of slurry-derived C and its distribution in soil organic matter (SOM) fractions with different turnover times of a pasture soil. C₃ and C₄ slurries (δ¹³C_V-PDB = −30.7‰ and −21.3‰, respectively) from cows fed either on a maize (C₄) or perennial ryegrass (C₃) diet were applied to a C₃ soil with a δ¹³C value of −30.0 ± 0.2‰. The cattle slurry was applied at 50 m³ ha⁻¹. Coarse sand, fine sand, silt, clay and fine clay were isolated from bulk soil samples (0–2 cm depth), freeze-dried and ground prior to total organic C (TOC) using elemental analysis and ¹³C natural abundance analysis by isotope-ratio mass spectrometry. The stable isotope tracer technique did allow to quantify the short-term sequestration of slurry-derived C in particle-size fractions of the grassland soil. Slurry-derived carbon was sequestered in various amounts in the five particle-size fractions, but most of it was sequestered in the coarse sand fraction during the two week experiment. The preferential input into the coarse sand fraction suggests that only the larger particulate slurry-derived materials were trapped into the soil during the experimental period. Less than 40% of the applied slurry-derived C was sequestered into the soil, suggesting a potential for large losses into the wider environment. The practice of surface spreading of slurry to temperate grassland soils is clearly not efficient, and improvements in slurry application methods, such as incorporation directly into the soil, should therefore be encouraged.

Keywords:

• Carbon 13
• Grassland
• Particle-size fractions
• Slurry
• Soil

IGER is grant aided by the Biotechnology and Biological Science Research Council (BBSRC). The German Research Foundation (Ze 154/46-1) for financially supporting the stable isotope measurements. The Bayreuth Rotary Club for a small grant which enabled Judith Moering to participate in the study. We also thank the two anonymous reviewers for their constructive comments to improve the paper.