Biogas production and improved nutrient supply in beetroot cultivation

The overall conclusion was that a cropping system with clover/grass ley can improve nitrogen utilization significantly if green biomass is harvested and digested and the digestate is re-used in the crop rotation. However, the soil buffering capacity is low on sandy soils, which could reduce nitrogen utilization of digestate handled in an inappropriate manner and disseminated to the crops at inappropriate times.

Is the digestate positive for nitrogen utilization?
In organic farming, green manure is used in crop rotations to get some new nitrogen into the crops through biological nitrogen fixation. A typical crop rotation may therefore contain a clover/grass ley and a few cash crops, as in Anita's field trials were beetroot and cereals. The overarching research questions were:

• Is the nitrogen utilization improved if biomass from the clover/grass ley is harvested and digested, rather than just cleaned and left?
• Is the nitrogen utilization and yield improved if digested ley biomass and beet tops are spread to the beetroot at a time when the beetroot needs nitrogen?

Anita tested these issues in a two-year field experiment on a sandy soil in the county of Halland. The aftereffect of clover/grass ley as green manure or from clover/grass ley harvested two or three times can be expressed in nitrogen uptake at harvest of beetroot and the amount of mineralized nitrogen down to 90 cm depth at the same time. The three crop strategies were compared with the aftereffect of a grain crop.

The aftereffect for clover/grass ley as green manure was 83 kg nitrogen more per hectare compared to grain crops. For clover/grass ley harvested two or three times the corresponding aftereffect was 42 and 74 kg more nitrogen per hectare.

Anita also compared the rate of utilization of the nitrogen applied as green manure with the utilization rate of the nitrogen applied as digested green mass plus beet tops. Fertilization strategies were tested in a field trial, also on a sandy soil in Halland. Results from the field experiment were used to simulate the equivalent net amount of mineral nitrogen, which was the sum of the preceding crop and effect of digestion, in three different crop rotations: A) green manure ley, beetroot, winter rye, or B) harvested forage, beetroot, winter rye; C) harvested ley, spring barley, beetroot. (A) represented a green manure system and (B) and (C) represented systems where the ley is harvested two or three times and the ley harvest and beet tops are digested in a biogas plant and the digestate is used in the crop rotation.

Higher yields and lower risk of leaching
Marketable yield of beetroot rose by 5.7 tonnes per hectare when digestate was supplied from the system where the ley was harvested twice. The beetroot yield increased with 9.1 tonnes per hectare when digestate was supplied from the system where the ley was harvested three times. Nitrate concentrations in beetroot were not higher than for beetroot fertilized with digestate instead of green manure. The amount of mineral nitrogen left in the soil down to 90 cm depth at harvest of beetroot, indicated that the risk of nitrate leaching was lower in the system where biomass was digested, compared with the green manure system. The amount of mineral nitrogen in the soil was 88 kg per hectare in unfertilized beetroot after green manure ley, 76 kg per hectare after low nitrogen input to the beetroot after ley harvested three times and 61 kg per hectare in the high nitrogen input to beetroot after barley.

Application of digestate, which added a moderate amount of nitrogen to beetroot after harvested ley, gave an unexpectedly low yield response in the field trials. With controlled nutrient experiments in climate chambers could Anita demonstrate that potassium access was more limiting for growth than nitrogen in the early growth stage of beetroot. To cultivate beetroot after ley, was more growth limiting than cultivating beetroot after spring barley, although digestate was applied in both systems.

In pot experiments with ryegrass Anita showed that net mineralization of organic nitrogen in the digestate corresponded to 12 percent.

Anita Gunnarsson at the Faculty of Landscape Planning, Horticulture and Agricultural Science, SLU, defended her thesis 'Plant -Based Biogas Production for Improved Nutrient Management of Beetroot In Stockless Organic Farming' November 9, 2012 and received a doctorate after well-executed presentation and discussion.