The rate dependent efficacy of biochar for crop yield and nutrition on Podzols newly converted from boreal forests.

Under climate change and local and global food security pressures conversion to agricultural lands on Podzols developed under boreal forests is accelerating across the global north. After conversion Podzols, acid and sandy, are infertile requiring significant management to allow economic production. Biochar, while less useful on fertile soils, may accelerate fertility gains on such marginal lands; it is already considered or land-applied by local farmers, under minimal guidance. A multi-year experiment assessed the utility of single or split application of biochar (BC), and of BC doses, for beet yields and nutrient uptake on newly converted Podzols. Time and rate-dependent progression of yields, macro- and micro-nutrients uptake, all parameters identified of interest by northern farmers, were evaluated. Kiln-derived hardwood biochar was added at 0, 10, 20, 40 or 80 Mg BC-C ha−1 (BC 0 to 80) either as single or as double application. Beet yields and elemental compositions were assessed over 4 years after biochar application. Work was done on a private farm in Labrador, Canada, under a scenario relevant to the boreal north. BC 0 could not sustain significant crop growth, suggesting that post-conversion mineralization of residual soil organic matter cannot support plant growth. In the first one to three years after application of biochar yields increased above the control, but statistically independent of application rates. A statistically significant positive biochar-rate to yield dependency was verifiable only in year 4. Nevertheless, biochar's positive effect on yields declined with time after application: e.g., BC 10 matched BC 0 yields in year 4. For the first three years, root nitrogen (N) concentrations were inversely correlated to yields and plant N% declined with biochar rate. In year 4 plant N% was directly correlated to plant biomass. In general, partitioning of nutrients between beet leaves and roots was not discernibly affected by the rate of biochar; an exception was boron whose preferential accumulation in leaves was directly related to the rate of biochar, indicating a role for the biochar for its mobilization. Biochar had an immediate positive effect on the beet yields and affected plant stoichiometry. Initially this was independent of the biochar rate. Biochar's utility declined with time at a rate inversely related to its application rate. Biochar is a recommendable fertility management tool for infertile, newly converted boreal Podzols; rates in the range of 10 Mg BC-C ha-1 may be repeated annually or every few years as practically feasible. Long term utilization requires further monitoring of micronutrients availability and uptake to inform any necessary management adjustments. Fertilizer management must consider the time since biochar's addition to soil. • Unmanaged Podzols on freshly converted boreal forest are infertile. • Biochar addition increases beet yield and yield quality, initially rate-independent. • A rate-dependent impact of biochar on yields and yield quality was noted in year 4. • Biochar's benefit fades over time at a rate inverse to its application rate. • Repeated, small biochar doses are recommendable over large single applications. [ABSTRACT FROM AUTHOR]