Tree-based intercropping systems, where agricultural crops are grown in rotation between widely-spaced rows of trees, have the potential to reduce N2O emissions and enhance carbon (C) sequestration in agricultural fields. To date, no research has addressed or documented this aspect of agroforestry in Canada.
In the recent past, many studies have identified tree-based land-use practices as a significant global opportunity to reduce the accumulation of CO2 in the atmosphere (e.g. Schroeder, 1993; Nair, 1993; Dixon, 1995; Young, 1997). The United Nations has also estimated that agroforestry based land-use practices on marginal or degraded lands could sequester 0.82-2.2 Pg C per year (Pg = 1015g) globally, over a 50-year time frame (Dixon et al., 1994).
In field trials at University of Guelph (Ontario, Canada), three crops (corn, soybeans, winter wheat) are intercropped at 2 tree row spacings (12.5m, 15m) with 10 species of trees. (Photo by N. Thevathasan)
(Condensed from an unpublished manuscript entitled "Enhancing greenhouse gas (GHG) sinks in agroecosystems through agroforestry based land-use practices in Canada.")
In 1987, the University of Guelph established a large field experiment on 30 ha of prime agricultural land in Wellington County, southern Ontario, to investigate various aspects of intercropping trees with agricultural crops. A variety of spacing, crop compatibility and tree growth and survival experiments were initiated at that time, utilizing 10 tree species within the genera Picea, Thuja, Pinus, Juglans, Quercus, Fraxinus, Acer, and Populus. Two between row-spacings (12.5 m or 15 m) and two within row-spacings (3 m, or 6 m) were utilized in conjunction with all possible combinations of three agricultural crops [soybean (Glycine max L.) corn (Zea mays L.), and either winter wheat (Triticum aestivum L.) or barley (Hordeum vulgare L.)].
Materials and Methods
A fast-growing tree, hybrid poplar clone DN-177, was selected for study. C sequestration rates will vary depending upon tree species; the values presented here might therefore represent the high end of sequestration potentials.
Both above and belowground C quantification was undertaken by destructive sampling. In order to obtain the above ground oven dry biomass, sub-samples were collected from each tree component and the moisture content was determined. Moisture content derived from these respective tree components was then used to convert the fresh weights of tree components into oven dry biomass. Oven dry biomass was then converted to carbon content.
In order to quantify tree below ground biomass, heavy equipment was used to dig a pit to a depth of 2m around a 3m x 3m block of earth containing the tree stump. This block of soil was then washed off with high-pressure water from a fire hose. For each tree, 45,000 to 55,000 liters of fresh water were used to expose the belowground root system. C content was then determined as described for the above ground tree components.
Soil C and litter fall quantification was determined using standard methods as described in Thevathasan and Gordon, (1997). It is acknowledged that fine root biomass, which can be extensive in certain systems, is not quantified in this manner. Fine root turnover was therefore estimated to be equal to the annual litter input (Gray, 2000).
1. Impact of tree-based intercropping systems on C sequestration in woody components and in soil
Thirteen-year-old trees were sampled in this study. The total mean C sequestered in permanent tree components of hybrid poplar clone DN-177 was 14 t ha-1. In addition, the C contribution to soil from leaf litter and fine root turn over for the last 13 years totals approximately 25 t ha-1. The total contribution in terms of C sequestration over the last 13 years at this experimental site is therefore approximately 39 t C ha-1.
Theoretically, this also implies that the system has immobilized 156 t of CO2 ha-1 in the last 13 years. However, about 70% of the C added, via leaf litter and fine roots, will be released back into the atmosphere through microbial decomposition processes.
Hence, the net sequestration potential from the trees alone is 1.65 t ha-1 y-1 or approximately 7 t of CO2 ha-1 y-1. It is also interesting to note that based on the known growth rate of this particular hybrid poplar clone, it is estimated that more than 43 t ha-1 C will be sequestered by age 40.
It can be seen that when trees are introduced into agricultural fields utilizing a tree-based intercropping system, C sequestration rates can be significantly enhanced. In a monocropped agricultural field (e.g. corn ), annual net C input to the soil is in the range of 400 to 600 kg ha-1C y-1 as compared to annual net C inputs as high as 2400 kg ha-1C y-1 in a tree-based intercropping system (4 times greater).
It is also important to note that 20% of the Canadian national target reduction in greenhouse gas (GHG) emissions could be achieved if at least 200 Kg C ha-1 is sequestered annually over the next 10 to 15 years on 45.5 million hectares of cropped land in Canada. In this context, it can be seen that the adoption of tree-based intercropping systems in geographically suitable regions of Canada could not only diversify farm income, bring about changes in biodiversity and enhance other environmental benefits but could also contribute towards embracing Kyoto (Thevathasan, 1998).
2. Impact of tree-based intercropping systems on nitrous oxide (N2O) emission reductions
Modeled data from the research site indicates that nitrate leaving the intercropping site can be potentially reduced by 50% over leaching losses associated with a conventional monocropped field (Thevathasan, 1998). A portion of the nitrate leached below the agricultural rooting zone will also be denitrified and lost as N2O. Oenema (1999), for example, estimates that 2.5% of the leached N is lost as N2O.
Based on the above assumption and from the modeled data it can be assumed that, as a result of reduced leaching, N2O emissions in tree-based intercropping systems could be potentially reduced by 0.50 kg N2O-N ha-1 relative to emissions from conventional agricultural fields. It is also believed that N2O-N emissions from Ontario agricultural fields need to be reduced by 2 kg N2O-N ha-1 over the next 10 years (2008-2012 first reporting period) in order to meet the terms of reference of the Kyoto Protocol.
In this context, our research suggests that tree-based intercropping systems could potentially reduce N2O emissions by 0.69 N2O-N Kg.ha-1y-1, 7 to 9 years after establishment of a fast-growing fibre tree species-based intercropping system in southern Ontario. As trees age and N-inputs in litterfall increase, N2O emissions could be further reduced.
Tree-based intercropping, a form of agroforestry, could potentially contribute towards remedial measures that are being discussed in relation to enhancement of C sequestration and reduction of GHG emissions from agroecosystems in Canada. Introduction of trees into agricultural landscapes may also qualify for "afforestation carbon credits", as proposed in the Green Cover Canada Program (Agriculture and Agri-Food Canada, 2003).
The United Nations has also estimated that agroforestry based land-use practices on marginal or degraded lands could significantly augment terrestrial carbon sinks in the next 50 years. Estimates of marginal or degraded land technically suitable for establishment of trees in North America are between 90 and 140 x 106 ha, and in Canada, 57 x 106 ha (agricultural land classes 3 through 6).
Agroforestry-based land use has a large potential in this regard and, therefore, more research is needed to quantify C sequestration and N2O emission reduction potentials in agroecosystems in different geographical regions of Canada. This information is vital for the design and development of such systems for many areas of Canada on a national level.
However, only a change in current tax policies that acknowledges non-tangible and societal-level benefits (e.g. improved water quality, reduced inorganic fertilizer use etc.) associated with agroforestry-based land-use practices will facilitate large-scale adoption rates in Canada.
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Thevathasan, N.V. 1998. Nitrogen dynamics and other interactions in a tree-cereal intercropping systems in southern Ontario. Ph.D Thesis. University of Guelph, Ontario, Canada. 230 p.
Thevathasan, N.V. and Gordon, A.M. 1997. Poplar leaf biomass distribution and nitrogen dynamics in a poplar-barley intercropped system in southern ontario, Canada. Agroforestry Systems 37:79-90.
Young, A. 1997. Agroforestry for soil management. CAB International, Wallingford, UK. 320 p.
By Naresh V. Thevathasan and Andrew M. Gordon
University of Guelph