The carbon balance of dryland and irrigated lucerne growing on stony soils
Figure 1. Measurement site in irrigated lucerne field, with weather station on left and CO2-exchange instrumentation on the right. The lucerne around the site has just been harvested, and the vegetation left standing is about to be cut by our team to determine the biomass-carbon accumulated since the previous harvest. (Photo: John Hunt)
Casual scanning of stories in the newspaper almost every day reveals concerns about the environmental consequences of the widespread expansion of intensive irrigated pastoral agriculture (particularly dairy farming). A major focus has been on Canterbury, which produces 64% of New Zealand’s primary sector exports, mainly from shallow, stony soils prone to leaching. On-farm solutions are required to reduce negative environmental consequences while maintaining productivity and profitability.
The basis of our new research programme is that increasing soil carbon inputs will favour microbiological communities that retain nitrogen for plant growth and reduce losses. We are testing this on small experimental plots, for a range of management options from grazed grassland to cut-and-carry crops. For scaling-up to paddock scale, we are using dryland and irrigated lucerne, at Lincoln University’s Ashley Dene Research & Development Station. In these two paddocks, we have installed large lysimeters to directly measure nutrient and carbon leaching losses.
To test our idea that nitrogen losses are linked to carbon inputs, we have estimated the annual carbon balance of the two lucerne paddocks, for two seasons starting with the conversion period. The two major terms contributing to the carbon balance are the net exchange of carbon dioxide (CO2) with the atmosphere and the removal of harvested biomass. The net CO2 exchange is the difference of CO2 uptake by photosynthesis and CO2 emission from respiration. This difference is obtained by measuring rapid variations in CO2 concentration and vertical wind speed at 2 m above ground (Fig. 1). The exported carbon in biomass is measured by weighing and analysing harvested samples. Minor contributions to the carbon budget are the amounts of carbon applied with effluent and fertilisers, and carbon leached.
Cumulative carbon balances are shown in Fig. 2, for the first and second year in the left- and right-hand panels, respectively. In the first year, both paddocks were converted from oats to lucerne by cultivation, and irrigation began in December. The carbon balance for the oat crop was neutral. Following that, respiration from the bare soil during the conversion period caused a carbon loss of 1.75 t C/ha. Once growing, the irrigated lucerne recovered 74% of this loss (while being harvested twice). By contrast, the dryland lucerne did not establish well and thus did not recover any of the conversion loss.
In the second year, both lucerne crops grew well throughout spring and summer. The irrigated and dryland lucerne were harvested five and four times, respectively. For the first three harvests, the carbon balance for both paddocks was positive but the fourth and fifth harvest periods turned this around into net losses. During the second year, carbon removed with harvested biomass for animal feed amounted to 3.14 and 5.78 t C/ha for the dryland and irrigated sites, respectively.
Our data show by how much irrigation increased net CO2 uptake and biomass production, compared with the dryland site. However, the irrigated lucerne also recorded greater net carbon losses than the dryland lucerne, which suggests the carbon from the additional net CO2 uptake was almost entirely allocated to above-ground biomass, which was then harvested as feed stock.
These results will be linked with other strands of our research programme to understand the below-ground carbon and nitrogen budgets. In order to analyse differences in microbiological activity, we extracted soil cores down to 1.7 m depth from both sites in early winter. We are yet to analyse the effects of irrigation on nitrogen leaching, because drainage at 1.5 m depth only started to occur in late autumn, following heavy rainfall. Our research combines measurements of carbon, water and nitrogen inputs and losses for stony soils where there are few data available. Our findings will inform policymakers and farmers of the environmental impacts of lucerne production on stony soils.
JOHANNES LAUBACH, JOHN HUNT, GWEN GRELET, ROWAN BUXTON, GRAEME ROGERS, DAVID WHITEHEAD
Manaaki Whenua – Landcare Research
E: LaubachJ@landcareresearch.co.nz
