Hydrochar amendment promotes microbial immobilization of mineral nitrogen

Abstract

Hydrochars and biochars are products of the carbonization of biomass in different conversion processes. Both are considered suitable soil amendments, though they differ greatly in chemical and physical composition (e.g., aromaticity, inner surface area) due to the different production processes (pyrolysis, hydrothermal carbonization), thus affecting their degradability in soil. Depending on the type, char application may provide soil microorganisms with more (hydrochars) or less (biochars) accessible C sources, thus resulting in the incorporation of nitrogen (N) into microbial biomass. A soil‐incubation experiment was conducted for 8 weeks to determine the relationship between mineral‐N concentration in the soil solution and microbial‐biomass development as well as soil respiration. An arable topsoil was amended with two hydrochars from feedstocks with different total N contents. Biochars from the same feedstocks were used for comparison. Both char amendments significantly decreased mineral‐N concentration and promoted microbial biomass compared to the nonamended control, but the effects were much stronger for hydrochar. Hydrochar application increased soil respiration significantly during the first week of incubation, simultaneous with the strongest decrease in mineral‐N concentration in the soil and an increase in microbial biomass. The amount of N detected in the microbial biomass in the hydrochar treatments accounted for the mineral N “lost” from the soil during incubation. This shows that microbial immobilization is the main sink for decreasing mineral‐N concentrations after hydrochar application. However, this does not apply to biochar, since the amount of N recovered in microorganisms was much lower than the decrease in soil mineral‐N concentration. Our results demonstrate that while both chars are suitable soil amendments, their properties need to be considered to match the application purpose (C sequestration, organic fertilizer).

Initial and subsequent effects of hydrochar amendment on germination and nitrogen uptake of spring barley

“…In both cultivation periods, the N content depended on the hydrochar concentration in the soil. Incubation experiments with hydrochar addition to sandy soil confirmed the high N-immobilization potential of hydrochars and showed a promotion of soil microbial biomass after hydrochar addition (Bargmann et al, 2014a). Nitrogen immobilized during the first 8 weeks of cultivation was not released during the second cultivation period, and an additional fertilizer application was insufficient to cover the crop's N demand.…”

“…As demonstrated in earlier studies with hydrochars, soil amendment with hydrochars can negatively affect the germination of various plants (Bargmann et al, 2013) and decrease the mineral-N concentration in soil (Bargmann et al, 2014a), thus limiting the plant's N uptake (Bargmann et al, 2014b). Although these results are supported by the studies of , Busch et al (2012), and Nelson et al (2011), and have also been reported for biochars (Clough and Condron, 2010), there is currently no information available regarding the persistence of these effects.…”

Impact of quality and quantity of biochar and hydrochar on soil Collembola and growth of spring wheat

“…Compared to control soils increasing amounts of fermented HTC lead to a higher shoot biomass and shoot: root ratio, likely due to its low C:N ratio that fostered microorganisms and in turn wheat growth (Bargmann et al, 2014). Moreover, P. fimata abundance declined by about half, and, as roots can serve as diet (Endlweber and Scheu, 2006;Ngosong et al, 2014), reduced grazing pressure may have additionally enhanced wheat growth.…”

“…Compared to biochar, hydrochar decomposition in soil is generally faster due to the less aromatic structure and higher content of labile C fractions (Steinbeiss et al, 2009;Cao et al, 2010;Libra et al, 2011). Thereby hydrochar provides often more accessible C and N resources to microorganisms (Bargmann et al, 2014), which also may affect their animal grazers.…”

Microbial community shifts 2.6 years after top dressing of Miscanthus biochar, hydrochar and feedstock on a temperate grassland site

“…HTC was found to have positive effects on growth, root colonization and spore germination of mycorrhizal fungi (Rillig et al 2010;Salem et al 2013b) as well as on the activity and abundance of SMB (Bargmann et al 2014). Negative effects of HTC on mycorrhiza have also been reported, pointing to the occurrence of toxic compounds, mostly present in the water soluble carbon fraction (George et al 2012), whereas collembola ingested and digested unwashed HTC without being negatively affected and may even derive nutritional benefits from it (Salem et al 2013a).…”