Net carbon uptake of irrigated dairy pasture can offset its own nitrous oxide emissions
Measurement setup to determine greenhouse gas budgets of irrigated pasture. image - John Hunt
Many managed grazed pastures around the world slowly accumulate carbon (C) over time. In other words, on average their uptake of carbon dioxide (CO2) from the atmosphere plus other C imports (e.g. from animal excreta and fertilisers) exceeds the amount of C lost by respiration, grazing, and harvesting. The surplus C accumulates in the soil and belowground biomass. This is good for soil health and can also offset some of the greenhouse-gas (GHG) emissions associated with farm animals. However, this does not always hold for New Zealand's intensive pastoral systems.
Studies measuring soil C from the same pastures (flat or rolling) many years apart found either no change or decreases over time, while a NZ-wide study sampling neighbouring sites with and without irrigation revealed significantly less C under the irrigated pastures. The causes for observed losses of soil C are still unknown.
We have completed a 3-year experiment on a commercial dairy farm in mid-Canterbury, grazing 900 cows, in order to construct net C and net GHG budgets of an irrigated, intensively managed pasture on a well-drained stony soil (Lismore). The pasture was grazed 10 times per year and received fertiliser (mostly urea) after most grazing events, averaging 226 kg of nitrogen (N) per ha per year. A large pivot irrigator operated from late spring to autumn to ensure that soil volumetric water content did not fall below 30 %.
The GHG budgets include CO2, methane (CH4), and nitrous oxide (N2O). We determined the net exchange of these three gases continuously with micrometeorological methods. To do this, we measured horizontal and vertical wind speeds and their short-term variations, as well as the gas concentrations at two heights above the pasture surface (Fig. 1). Combining these data, we determined the half-hourly net exchanges of the gases representative of a surface area of a couple of hectares. As gas measurements were excluded for periods when cows grazed within this area, the annual budgets of measured CO2 and CH4 represent the pasture only, and exclude direct cow emissions.
To construct annual C budgets, we also needed the C inputs from excreta and fertiliser, and the biomass removed by grazing. We measured the pasture biomass before and after some grazing events to accurately determine the dry-matter weight, and with these data calibrated a rising-plate meter that was then used for the other grazing events. From the amounts of grazed biomass, plus amounts of supplements fed during milking, we estimated the amounts of C returned as dung and urine to the pasture. Also from feed intake, we calculated the amounts of N in the excreta, in order to interpret the observed N2O emissions. We further used the feed intake to calculate the direct CH4 emissions of the cows. Since these two variables are tightly linked, this approach is more accurate than micrometeorological measurements during grazing events would have been.
In each of the 3 years, we found a net uptake of C by the irrigated pasture (averages in Fig. 2). The pasture emitted CH4, throughout all seasons. These emissions were about 15 times greater than emissions expected just from cow dung, and similar to rates we measured from a nearby dryland pasture. This is a somewhat puzzling result as the soils are well drained and CH4-producing microbes usually require oxygen-free conditions; however, similar observations have occurred in other intensively managed grasslands, grazed or harvested. The emissions of N2O were roughly in line with expectations based on the N inputs from fertiliser and excreta. The global-warming potential of the N2O emissions (expressed as CO2-equivalent mass) was approximately equal to the C uptake. Hence, this irrigated dairy pasture was offsetting its own N2O emissions. However, the CH4 emissions directly from the cows were 2–3 times greater than the N2O emissions, and about 6 times greater than the pasture's CH4 emissions. These cannot be offset by C uptake, therefore the dairy system including pasture and cows is a net GHG producer.
Acknowledgements: We thank Anitra Fraser, Matti Barthel, Tony McSeveny, Graeme Rogers, Rebecca Phillips and Gabriel Moinet for their contributions to this project. Purata (formerly Synlait Farms Co.) provided detailed farming operation information. This research was undertaken with CRI Core Funding from MBIE.
JOHANNES LAUBACH AND JOHN HUNT – LANDCARE RESEARCH