Improved Nitrate Retention and Reduced N2O Emissions with Aged Biochar.

We explored the potential of biochar to protect mineral N against leaching or N₂O emission loss, and to deliver it for plant growth in a number of different studies ranging from larger-scale agricultural field studies to lab studies using ¹⁵N labelled mineral N. The combined results suggest a surprisingly strong role of biochar in retaining mineral nitrogen, mostly in the form of nitrate, rather than ammonium (as could be expected). In field studies we observed significant nitrate retention in the top soil (0-15cm) where the biochar had been incorporated, while the subsoil nitrate concentrations (30–60 and 60–90cm) were significantly reduced. Biochar particles extracted from the top soil by forceps were considerably enriched with nitrate but only a fraction was extractable with conventional standard methods. Biochar improved plant growth only in zero- or low-N fertilized treatments, i.e. where mineral N was limiting but not when N fertilization was in excess of the plant demand. In a macrocosm leaching study with Vitis vinifera (cf. Riesling) pure biochar reduced nitrate leaching by 60% compared to pure sandy control soil, but the biochar-compost combination was most effective, reducing nitrate leaching to virtually zero.

Subsequent ¹⁵N labelling-tracing studies revealed that the untreated as well as aged (co-composted) biochars strongly sorbed mineral N, particularly nitrate. In a soil mixture of with 2%(wt/wt) biochar, up to 60% of the labelled nitrate-¹⁵N was retrieved with the (washed) biochar particles roughly 50 hours after ¹⁵N application. The co-composted biochar was more effective than the untreated biochar in sorbing nitrate; surprisingly it still reduced N₂O emissions significantly, although it carried dissolved organic carbon and nitrate as prerequisites for denitrification. Our results encourage further research into combining nutrient-rich agricultural waste streams with biochar post-treatment (i) to bring marginal soils into production, and (ii) to actively reduce groundwater pollution.