Young and century-old biochars strongly affect nutrient cycling in a temperate agroecosystem

Abstract

Biochar as a soil amendment is often described as a promising agricultural practice for climate change mitigation and adaptation. However, while recent research showed limited effects of biochar in temperate regions, its long-term impacts on nutrient management in-situ have been overlooked. Here we studied charcoal residues from pre-industrial kiln sites as a proxy to determine the effects of century-old biochar (~200 years old) on nutrient cycles of 3 land covers in a conventionally cropped field. We compared nutrient cycles of soil containing either century-old biochar (CoBC), recently pyrolyzed biochar (YBC) produced from similar feedstock (oak) and amended in similar amounts and a reference charcoal free soil (REF). For these three modalities, we characterized soil chemical properties, the pore water nutrient concentration evolution with time and depth using suction cups, and the crop nutrient uptakes. Our results revealed soil pore water nutrient concentrations strongly depended on biochar age. Indeed, YBC resulted in lower N-NO3- and K+ leaching but higher P-PO4– pore water concentrations in the topsoil Ap (0–30 cm) horizon. In CoBC higher K+ and Mg2+ concentrations occurred in the pore water than in REF for subsoil horizon E (30–60 cm) and Bt (60–100 cm). Beyond soil pore water, CoBC also strongly increased soil total N, available K+ and Ca2+ but decreased available P contents compared to REF and YBC. Finally, although no change in crop productivity occurred, lower N, K, Ca and higher Mg plant uptakes were observed for modalities with biochar. This resulted in no difference in terms of nutrient exports from the field in chicory but it significantly decreased N, K, Ca exports from biochar rich soil under winter wheat in straw. This study delivers the first field-based evidence that the effects of hardwood biochar on nutrient cycles change over its lifetime in a temperate Luvisol soil, whereby young biochar impacts mainly pore water nutrient concentrations and aged biochar mainly plant available contents. In such a strongly managed environment, no differences are noted in productivity despite strong changes in the nutrient cycle. Our study provides insights for addressing long-term effects of biochar in cultivated lands not only in terms of agronomic perspectives but through a biogeochemistry lens.