The effects of nutrient-loaded biochar on biological nitrogen fixation in faba beans

Overuse of fertiliser in global food production has led to severe environmental impacts, including nitrogen (N) pollution, water quality degradation, and eutrophication, particularly in lakes and coastal waters worldwide. These challenges highlight the need for sustainable agricultural practices that enhance N use efficiency and reduce environmental loading. Nutrient-loaded biochars (NLB) offer a promising solution by sequestering atmospheric carbon in the soil, providing slow, continuous nutrient release, and reducing nutrient loss. Research indicates that NLBs enhance N use efficiency and crop yields. Additionally, biochar and fertiliser combination increases soil N retention and microbial activity. Some biochars have also been shown to improve biological N fixation in legumes by improving soil structure, nutrient availability, and balance and optimising plant-microbe interactions. However, their specific effects on faba beans (Vicia faba L.) remain understudied.

This study aims to investigate the influence of NLBs on biological nitrogen fixation in faba beans and N use efficiency in boreal agricultural soils by using the 15N natural abundance method. The study was conducted on long-term biochar field experiments at the University of Helsinki's Viikki Research Farm with two distinct soil types: fertile Stagnosol and nutrient-poor Umbrisol. NLBs were prepared in three ways: spruce biochar was activated with nitric acid or tap water, then nutrient-loaded with organic N (from cattle slurry and meat bone meal) or inorganic N (from mineral fertiliser).

The study used split-plot designs, with three different biochar treatments in the Stagnosol and only slurry-activated biochar in the Umbrisol, applied at two rates: 0 biochar control and the highest rates (15 t ha^-1 in Stagnosol, 50 t ha^-1 in Umbrisol). Fertiliser treatments included two mineral levels (30% and 100% of recommended rates) for the Stagnosol and two types (no fertiliser and mineral fertiliser) for the Umbrisol. 72 plots were used: 48 in Stagnosol and 24 in Umbrisol, with additional 0 biochar control plots (6 + 7 respectively) established for growing oat (Avena sativa L.) as a non-N-fixing reference crop for soil N isotopic baseline to estimate N fixation in faba beans. Faba beans were cultivated in the experimental plots. We measured aboveground biomass, CN content, N yield, and isotopic ratios (¹⁵N/¹⁴N) by a Leco CN analyser and isotope ratio mass spectrometer at the flowering stage of faba bean and oats.

Preliminary results are expected to provide a good insight into the potential of NLBs to increase biological N fixation. The study findings could lead to innovative approaches for improving soil fertility and contributing to more sustainable practices in boreal agriculture and globally. Growing legumes in NLB-amended soils could be a good practice to increase plant available N and, thus, the subsequent crop yield, reducing farmers' need to purchase external mineral fertilisers. Moreover, the findings can also support climate change mitigation and ecological balance in boreal regions.