Carbon and nitrogen cycling in Scottish upland grassland soils and the influence of excretal returns
Stack, Philip Eugene
Upland grasslands comprise a large proportion of the UK’s land area and are primarily used to graze sheep. These grasslands store large quantities of carbon (C). Changes in land use or climate could affect the ability of these soils to store C and the fluxes of other greenhouse gases associated with agricultural soils, nitrous oxide (N2O) and methane (CH4). Grazing substantially changes the cycling of C and nitrogen in grassland ecosystems, particularly through the deposition of rapidly degrading excreta, both dung and urine, on the soil. The major non-enteric greenhouse gas emissions associated with this type of extensive farming of ruminants are the emission of N2O and CH4 from soils affected by the animal’s excreta. This PhD project has investigated the cycling of sheep dung in two upland soils of different management regimes to investigate the effects imposed by the plant community. Dung incorporation was measured by capitalising on the natural difference in natural 13C abundance (δ13C ratios) between maize and native British vegetation, which permitted maize-derived sheep dung to be used as a 13C tracer of dung incorporation into soil. A physical and chemical soil fractionation methodology was used to isolate the distinct soil organic carbon (SOC) pools and ascertain the location of the dung C. There were differences between soils in dung C cycling, with more dung C being measured in semi-improved soils at experiment’s end. Throughout the one year timeframe of this experiment, most of the dung C was recovered in the particulate organic matter fraction. Changing the plant community did not have a measurable effect on dung C cycling within the experimental period. Urine patches in grazed pastures represent a major source of agriculture’s N2O emissions. The N2O, CH4 and CO2 fluxes from chambers treated with synthetic urine, synthetic urine and dung, or dung, and an untreated control in randomised block design at two sites were measured over one year. Relevant soil parameters were also measured at each sampling point. From this data N2O emission factors for sheep excreta at these sites were calculated. N2O emission factors were significantly different between sites, were different for dung and urine, and in all cases were less than the current default value used by countries utilising a Tier 1 methodology, according to the IPCC, to inventory N2O emissions derived from grazing livestock. Dietary manipulation has been proposed to increase certain components in urine that are thought to inhibit N2O emission with the aim of reducing livestock greenhouse gas emissions. One such urinary component is hippuric acid. Soil to which synthetic urine with incrementally increased quantities of hippuric acid were added were incubated, as were soils to which dung only and dung and synthetic urine had been added, as well as an untreated control. No significant effect of hippuric acid concentration was observed. N2O emissions from the dung only and dung and urine treatments were unusually high and surpassed those of the urine only treatments. This has been hypothesised to be due to fungal denitrification in the dung treatments or suppression of microbial activity due to ammonia toxicity in the urine-treated soils. The key conclusions from this PhD work are that the effect of dung deposition on SOC cycling may be quite small and appears to result in substitution of native SOC with dung C, rather than an increase in SOC; N2O emissions from sheep dung and urine deposition in semi-improved grasslands is likely to be very low and much lower than the current IPCC default value; and that in our incubation experiment there was no discernible impact of hippuric acid on N2O emissions, but it is possible that this is an experimental artefact.