Standard Extraction Methods May Underestimate Nitrate Stocks Captured by Field-Aged Biochar

Biochar (BC) has been shown to increase the potential for N retention in agricultural soils. However, the form of N retained and its strength of retention are poorly understood. Here, we examined if the N retained could be readily extractable by standard methods and if the amount of N retained varied with BC field ageing. We investigated soil and field-aged BC (BC) particles of a field experiment (sandy soil amended with BC at 0, 15, and 30 t ha) under two watering regimes (irrigated and rain-fed). Throughout the study, greater nitrate than ammonium retention was observed with BC addition in topsoil (0-15 cm). Subsoil (15-30 cm) nitrate concentrations were reduced in BC treatments, indicating reduced nitrate leaching (standard 2 mol L KCl method). The mineral-N release of picked BC particles was examined with different methods: standard 2 mol L KCl extraction; repeated (10×) extraction in 2 mol L KCl at 22 ± 2°C and 80°C (M); electro-ultrafiltration (M); repeated water + KCl long-term shaking (M); and M plus one repeated shaking at 80°C (M). Nitrate amounts captured by BC particles were several-fold greater than those in the BC-amended soil. Compared with M, standard 2 mol L KCl or electro-ultrafiltration extractions retrieved only 13 and 30% of the total extractable nitrates, respectively. Our results suggest that "nitrate capture" by BC may reduce nitrate leaching in the field and that the inefficiency of standard extraction methods deserves closer research attention to decipher mechanisms for reactive N management.