Effects of agroforestry on yield, carbon sequestration and nitrogen leaching

Researchers at the University of Copenhagen have collected and analysed data from two organic fields representing agroforestry systems.

By Karen Munk Hansen

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The studies were conducted in and around 3-row windbreaks planted in the early 1990s. The aim was to investigate what effect tree planting on agricultural land can have on yields and the environment.

One field is used to grow cereals for human consumption (arable system) and the other to grow clover grass (grazing system). Both fields are located in Central Jutland and are fertilised and watered.

Conclusions

With a threshold value of 5% of the crop area values, there was lower grain yield up to 6 metres from the windbreak, higher carbon storage up to 15 metres from the fence, and less nitrogen leaching up to 7 metres from the fence in the arable system.

In the grazing system, there were lower yields up to 10 metres from the windbreak, reduced nitrogen leaching up to 5 metres from the fence and no difference in carbon storage.

The effects have not been studied on clay soil.

Effect on yields

Yields of both cereals (Figure 1) and clover grass (Figure 2) were lower close to the windbreak than further away. Thus, there is a competition zone between the windbreak and the crop. The width of this is different in the two systems. In the arable system, it is calculated to be 0.5 times the height of the fence. In the grazing system, it is 1.2 times the height of the fence.

The protein content of the grain was higher in the competition zone, but not enough to compensate for the lower grain yield in this zone. In clover grass, there was no clear systematic pattern in the nitrogen content, possibly because it was obscured by a large random variation.

Distance to trees is an important factor

Yields of both cereals and clover grass have been tested in relation to weather and soil conditions, among other things. The variation in yield can best be described as a function of distance to windbreaks and weather conditions. The results are in line with other research in agroforestry, where shade is the factor that plays the biggest role in yield loss.

Despite lower yields close to windbreaks, Danish studies from the 1900s and recent international research show that shade can increase yields at distances up to 20 times the height of a windbreak.

Figure 1. Cereal yield (spring barley and winter rye) over three growing seasons (bars) and protein content (lines) in an agroforestry system with crop production.

Figure 2. Clover grass yield over three growing seasons (bars) and average nitrogen content (lines) in an agroforestry system with grazing.

Effect on carbon storage

Soil carbon storage was higher under the windbreaks than in the surrounding field and highest under the windbreak in the arable system (Figure 3). The higher carbon content was especially visible in the 40-75 cm depth.

The windbreaks were similar in species composition and age, but the large difference can perhaps be explained by the fact that there have been windbreaks of different types in the crop production system since the 1950s. The addition of carbon from atmospheric CO2 has therefore occurred over a longer period of time, unlike in the grassland, where there have been no shelterbelts before the current one.

The difference may also be due to the fact that the clover grass is only rotated about every 5 years and provides a more stable input of carbon, so the system is closer to a stable carbon balance than the arable system.

The models developed show that up to 15 metres from the fence there was at least 5% higher soil carbon content in the arable system, while there was no significant variation in the grazing system (Figure 4).

Figure 3. Carbon storage in fields and windbreaks by soil depth 0-75 cm and tree/shrub biomass.

Figure 4 Carbon storage in soil and windbreaks in arable system (left) and grazing system (right)

Effect on nitrogen leaching

Soil nitrogen (N) content was very similar in the two systems. However, in the arable system, the amount of nitrogen was 34% lower in the windbreak and 16% lower 2 metres from the windbreak than further out in the field. In the grazing system, there was no difference between the windbreak and the field.

Nitrogen leaching in the windbreak compared to the field was 29% in the arable system and 27% in the grazing system.

Increasing leaching with increasing distance to trees

Leaching generally increased with increasing distance to the fence. Modelling shows that a 5% reduction in nitrogen leaching was achieved 7 m from the windbreak. This emphasises that windbreaks contribute as a safety net for nutrients in agroforestry. Based on the literature, a reduction in leaching can be expected especially when planting deciduous trees rather than conifers, species with high water uptake, and species with deep roots.

How the surveys were conducted

Measurements were taken in the windbreak and at four distances up to 25 metres from the fence (Figure 5). Soil water, soil moisture, soil properties, rainwater and irrigation, and crop yields were collected for three years. In addition, tree and shrub species were identified and biomass was measured.

Figure 5. Sketch of the surveys. All measurements were taken at four distances and in the centre of the windbreak. Soil water, rainwater and irrigation water were collected, soil moisture was measured and soil samples were taken down to 75 cm depth.