Effects of fresh and aged biochars from pyrolysis and hydrothermal carbonization on nutrient sorption in agricultural soils

Abstract

Abstract. Leaching of nutrients from agricultural soils causes major environmental problems that may be reduced with biochar amendments to the soils. Biochars are characterised by a high adsorption capacity, i.e., they may retain nutrients such nitrate and ammonium. However, biochar properties strongly depend on feedstock and the production process. We investigated the nutrient retention capacity of biochars derived from pyrolysis (pyrochar) as well as from hydrothermal carbonization (hydrochar; produced at 200 and 250 °C) from three different feedstocks (digestates, Miscanthus, woodchips) mixed into different soil substrates (sandy loam and silty loam). Moreover, we investigated the influence of biochar degradation on its nutrient retention capacity using a seven-month in-situ field incubation of pyrochar and hydrochar. Pyrochars showed the highest ability to retain nitrate, ammonium and phosphate, with pyrochar from woodchips being particularly efficient in nitrate adsorption. Ammonium adsorption of pyrochars was controlled by the soil type of the soil-biochar mixture. We found some ammonium retention on sandy soils, but no pyrochar effect or even ammonium leaching from the loamy soil. The phosphate retention capacity of pyrochars strongly depended on the pyrochar feedstock with large phosphate leaching from digestate-derived pyrochar and some adsorption capacity from woodchip-derived pyrochar. Application of hydrochars to agricultural soils caused small, and often not significant, effects on nutrient retention. In contrast, some hydrochars did increase the leaching of nutrients compared to the non-amended control soil. We found a surprisingly rapid loss of the biochars' adsorption capacity after field application of the biochars. For all sites and for hydrochar and pyrochar, the adsorption capacity was reduced by 60–80% to less or no nitrate and ammonium adsorption. Thus, our results cast doubt on the efficiency of biochar applications to temperate zone soils to minimize nutrient losses via leaching.

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

“…Sugarcane filter‐cake BC, produced at >500°C, reduced dissolved organic carbon (DOC) but not NO 3 − leaching from a sugarcane field treated with vinasse (nutrient‐dense effluent containing 0.08 g N L −1 ) (Eykelbosh et al, 2015). Biochar from woodchips significantly increased mineral nitrogen (N min ) (NO 3 − , NH 4 + ) and phosphate retention in a laboratory study, whereas the same BC lost 60 to 80% of adsorption capacity after 7 mo of field incubation (Gronwald et al, 2015).…”

Valuing Waste – A Multi-method Analysis of the Use of Household Refuse from Cooking and Sanitation for Soil Fertility Management in Tanzanian Smallholdings

“…In particular, CO 2 and N 2 O add to the global warming potential, whilst NH 3 and P leaching contribute to eutrophication (ibid.). The fact that biochar captures NO 3and PO 4 3-, as shown, for example, by Agyarko-Mintah et al 2016, Gronwald et al (2015) and Kammann et al (2015), is promising in regard to reducing GHG emissions and nutrient leaching during composting and also after compost is added to the soil. There is also empirical evidence that adding biochar to composting can decrease N 2 O and CH 4 emissions (e.g.…”

Recycling Improves Soil Fertility Management in Smallholdings in Tanzania

“…In addition, biochar additions to composting can potentially decrease N 2 O and CH 4 emissions, e.g., [86][87][88][89], whilst NH 3 emissions potentially increase (e.g., [87]). Other studies [33,90] showed that biochar captures nitrate and phosphate during composting, which is promising, in order to reduce nutrient leaching. However, such potentially beneficial effects of co-composting biochar have not yet been included in our model (Section 3.5).…”