Abstract

In 2019, a field trial was carried out at Helgegården in Skepparslöv, near Kristianstad in northeastern Skåne, Sweden, to study how the date of establishment of multifunctional summer intermediate crops, integrated in organic crop rotations with vegetables, potatoes and cereals, affects carbon sequestration, weed control, plant nutrient balance and biomass production, e.g. for production of biogas and biogas digestate or as feed. The intermediate crops (buckwheat, phacelia, hemp, oil radish) were grown in pure stands or alternatively intercropped with the nitrogen-fixing legume crops; hairy vetch (Vicia villosa) or Persian clover (Trifolium resupinatum).

The intermediate crops were sown after harvest of spring barley at three times, on 11 July, 29 July and 31 August. In pure stand, the intermediate crops were unfertilized respectively fertilized with 40 kg of ammonium nitrogen per hectare, in the form of biogas digestate. The plots with the nitrogen-fixing intermediate crops were not fertilized. In each of the field trials, there were 12 experimental plots with intermediate crops, and these were repeated randomly in three experimental blocks. The trial was placed on a light sandy soil with spring barley as a precrop, either harvested as a whole crop or as grain kernel, depending on the spring barley's harvest date, before sowing the intermediate crops.

For the first two establishment dates, at the beginning and at the end of July, the best intermediate crops contributed an average of 310 kg of stable carbon per hectare, if the entire crop was mixed into the soil in the late autumn. When we harvested the intermediate crops, in September to November, which meant that only the roots and 10 cm stubble contributed to the soil carbon effect, then the contribution to stable carbon was halved to about 140 kg per hectare. Fertilized non-nitrogen-fixing intermediate crops gave 30% more soil carbon contribution compared to when they were unfertilized, but only 23% more soil carbon compared to when they were intercropped with a legume.

Intermediate crops that were sown on 11 July and 29 July had good weed-competing properties. The weed weight was 70 to 99% lower for unfertilized intermediate crops in pure stand, sown on 11 July and 29 July, compared to the control without any intermediate crop. This indicates that intermediate crops radically reduce the weight of annual weeds. However, the number of annual weeds was relatively high for buckwheat and hemp, when they were established on 31 August, especially in the fertilized plots, while phacelia and oil radish had good weedcompeting properties.

Our study shows that intermediate crops that are unfertilized and intercropped with legumes can generate large amounts of biomass, between 2 and 6 tonnes of dry matter per hectare, when sown before 1 August and harvested from mid-September to mid-November. In an earlier sowing, in mid-July, the intermediate crops yielded 3 to 6 tonnes of dry matter per hectare, when they were harvested from mid-September to mid-October.

Data of mineralized nitrogen (N-min) from soil sampling in the beginning of December 2019, after the three establishment dates for the intermediate crops, unfortunately indicates that none of the intermediate crops, except oil radish, gives any reduction of the easily leached NO3- nitrogen in the soil profile, 0-90 cm depth, compared to the control without any intermediate crop. The amount of NO3-nitrogen in the control plot was about 25 kg per hectare, about 10 kg per hectare for oil radish in pure stand, both fertilized and unfertilized, and in the range of 20 to 30 kg NO3-nitrogen per hectare for the other intermediate crops. Furthermore, there was no difference in the content of NH4-nitrogen in the soil profile, which was in the range of 40 to 50 kg per hectare, for all intermediate crops and for the control without any intermediate crop.

A carbon balance calculation shows that, from a climate gas point of view, it is 2.5-5 times better to harvest the fertilized intermediate crops in pure stand, to produce biogas in the form of vehicle gas and biogas digestate, compared to mix the entire intermediate crop into the soil, in the late autumn, to maximize soil carbon storage. For unfertilized intermediate crops, intercropped with the legumes, it is 2.5-3 times better to harvest the crop. When the intermediate crops are grown unfertilized in pure stand, it is 2-3 times better from a greenhouse gas point of view to use the intermediate crops in biogas production, compared to mulch them into the soil to maximize soil carbon storage.

Our carbon balance calculation, for harvested and mulched intermediate crops, shows that from a greenhouse gas point of view, it would be a very great advantage if the crops are harvested and used as a substrate for production of vehicle fuel and biogas digestate. In Sweden intermediate crops are approved as raw material for production of biofuels, which leads to a tax exemption for biofuels produced from this type of raw material.

Keywords

etableringstidpunkt; mellangröda; samodling; baljväxt; markkol; markolsuppbyggnad; kolinlagring; ogräs; ogräskontroll; mineraliserat kväve; skörd; biogassubstrat; fordonsgas; biogödsel; växthusgas; biodiversitet; ekosystemtjänster

Published in

Landskapsarkitektur, trädgård, växtproduktionsvetenskap: rapportserie
2022, number: 2022:1
ISBN: 978-91-576-9004-3, eISBN: 978-91-576-9003-6
Publisher: Fakulteten för landskapsarkitektur, trädgårds- och växtproduktionsvetenskap, Sveriges lantbruksuniversitet (LTV)

SLU Authors

• Prade, Thomas

  • Department of Biosystems and Technology, Swedish University of Agricultural Sciences
    

• Hansson, David

  • Department of Biosystems and Technology, Swedish University of Agricultural Sciences
    

• Svensson, Sven-Erik

  • Department of Biosystems and Technology, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Agricultural Science


Permanent link to this page (URI)

https://res.slu.se/id/publ/115849