Ecosystem service delivery from bioenergy shelterbelts on dairy farms

Abstract

The study concerns the production and agronomic recommendations for growing the bioenergy grass Miscanthus x giganteus (Mxg) Greef et Deu ex Hodkinson et Renvoize on intensive dairy farms on the Canterbury Plains in the South Island of New Zealand. Mxg was not planted as whole fields but was used to create shelterbelts specifically for the purpose of replacing those that had been removed during installation of centre-pivot irrigation. The Canterbury region is prone to drought conditions, suffering extreme soil moisture deficit for much of the growing season, and successful intensive agriculture is dependent upon irrigation. The region in recent times has seen a large-scale land use change from dryland sheep and beef farms and arable farms to dairying. Between 1980 and 2009 dairying increased from about 20,000 ha to nearly 190,000 ha and, until the recent fall in milk prices, it was estimated to double by the year 2030. Dairying largely relies on centre-pivot irrigation and to accommodate these extensive irrigation systems any shelterbelts present that interfere with the passage of the irrigator have been removed. The landscape effect of this is that the Canterbury Plains now comprise of large expanses of flat, treeless expanses of low-diversity pasture. This production system is low in ecosystem service (ES) provision, the most visual of which is low aesthetic value which creates a public perception of unsustainable dairy production. Planting Mxg as shelterbelts instead of growing it in whole fields delivers a number of ES: • Mxg is dependent upon irrigation to maximise yield potential if produced in Canterbury. Irrigation systems are expensive and in the absence of bioenergy subsidies in New Zealand installing irrigation specifically for Mxg production is uneconomic. Producing it on established irrigated farms, dairying in this instance, removes this problem. • Effective shelter is dependent upon the height of the shelter. The distance shelter benefits, such as improved pasture yield, extends into the field is 10 to 12 times the height of the shelter. Mxg is a sterile perennial and once established, in its third season, will grow to 4 m tall each year. It bends over and springs back when the centre-pivot passes through it and so does not interfere with its passage. • Replacing food-producing crops with bioenergy feedstocks is controversial in light of the ever-increasing world demand for food. The system developed in this study of using Mxg to create shelterbelts delivers a number of ES which offsets the removal of land from pasture production. For example, protecting pasture plants from drying northerly winds, a predominant feature of the study area, increased pasture production. • Producing bioenergy feedstock allows the potential for farming systems to become wholly or partly energy self-sufficient. An average dairy farm in New Zealand uses 75,000 kWh per year, excluding pasture irrigation, and use around 2.5% of all electricity generated. Electricity can be generated from Mxg biomass using bio-digesters and it can also be used to produce renewable diesel. Both these reduce the farm’s contribution to the production of greenhouse gases. Mxg can also be used to make biochar. Initial research suggests that when biochar is included in cattle rations, methane, a potent greenhouse gas, emissions are reduced. An important consideration when introducing a novel non-native plant into an established agricultural system is the potential of it to invade areas outside of the farm. Mxg is a sterile hybrid that has a vigorous upward growth habit. Its lateral spread, however, is slow and its potential movement into other areas is very low. The only conceivable means of spread is through dispersed rhizome pieces. However, these would need to be buried into soil soon after ‘escape’ before they desiccate and die. Also, initial successful establishment of Mxg is dependent upon adequate removal of weed competition. Of the ES and potential ecosystem disservices (EDS) studied, it was found that under irrigation Mxg should be able to produce upwards of 30 t DM ha yr-1. It was also found to be a very resilient plant recovering from unplanned grazing and drought conditions. Detectable improvement in pasture production of up to 18% was recorded in field areas protected from drying winds by Mxg shelterbelts. A key contributing factor to this was higher rates of stomatal conductance in sheltered plants. This also has implications for plant water use efficiency as sheltered zones of fields produced more dry matter (DM) under the same irrigation rate as less productive areas of the field. Pasture quality was not affected by the presence of shelterbelts. Mxg shelterbelts were found to be preferred nesting sites for bumblebees and shelter areas for skinks (New Zealand native lizards) compared to unsheltered field edges. Sheltered field areas had higher detectable mineralisation rates of organic matter and higher levels of earthworm activity compared to unsheltered areas. The amount of nitrogen (N) removed by Mxg at harvest time is an important consideration when considering the dynamics of N use on dairy farms. N removal is low if harvested for bioenergy feedstock as most of the N in the above-ground plant material has been translocated to the rhizome and the N content of the plant material is very low (around 0.28%) at harvest. This means for a crop yielding 30 t DM ha-1 N removal will be 90 kg ha-1. For traditional ryegrass clover systems, typical annual DM yield in New Zealand is 20 t DM ha-1 under irrigation and optimal N applied to achieve this is 200 kg ha-1. Creating shelterbelts from Mxg also generates possible EDS. Two EDS studied were the fire risk potential of Mxg, and whether Mxg shelterbelts harbour rodent populations. Mxg in its ‘green’ state, which is its normal state under irrigation until it senesces in the winter, is of low fire risk potential. The second EDS analysed did show higher populations of rodents in Mxg shelterbelts compared to unsheltered field areas but further analysis on their preference compared to alternative shelterbelts such as pampas grass, Cortaderia selloana, needs further study. In conclusion, this study indicates growing Mxg as shelterbelts on irrigated dairy farms has considerable potential for improving the sustainability of the farming system through the generation of a number of ES. A total of 16 possible ES were identified, of which eight have been assessed as part of this study. The results from this study provide enough evidence to consider planting all field boundaries on an irrigated milking platform (i.e. the area used for grazing during lactation) with Mxg shelterbelts and to measure the ES delivery from these. The predominant factor contributing to a number of the ES delivered in this study was through providing wind protection. The level of this protection should be greatly increased if all fields under the centre-pivot had Mxg shelterbelts along their boundary.