Methodological choices in size and density fractionation of soil carbon reserves - A case study on wood fiber sludge amended soils.

Soil organic carbon (SOC) is in the focus of research due to its central role in regulating climate and maintaining fertility and resilience of soils. Methodologically, shifting from whole soil C measurements to specific SOC fractions increases possibility to detect small changes in the vast SOC storage, and enhances estimation of SOC stability. However, SOC fractionation schemes are numerous and variable. In this study, deionized water and sodium hexametaphosphate (SHMP) were compared in soil dispersion by separating soils into coarse (0.25-2 mm), medium (0.063-0.25 mm) and fine (<0.063 mm) size fractions. The first two fractions were further separated by density (1.8 g cm-3) to obtain free particulate organic C (POC) and mineral associated organic C (MOC). The approach was applied to a clay and a silt loam soil with and without wood fiber sludge amendment to follow the added C. Aggregate disruption was enhanced with SHMP in comparison to water, but the effect was small and the use of SHMP decreased recovery of SOC, wherefore water was preferred. In both soils, 5-10 % of SOC occurred as coarse POC, 1-3% as coarse MOC, 5 % as medium POC, 10 % as medium MOC, and 70-85 % as fine MOC. The added C resided in the POC fractions with an indication of minor accumulation to the fine MOC in the clay soil. Longer time frame with repeated C additions would be needed to increase the stable MOC storages though saturation of the MOC reserve may hinder accumulation in the silt loam low in fines.

Slow pyrolysis liquid in reducing NH3 emissions from cattle slurry - Impacts on plant growth and soil organisms.

A substantial percentage of manure nitrogen (N) can be lost as gaseous ammonia (NH3) during storage and field spreading. Lowering slurry pH is a simple and accepted method for preserving its N. Efficiency of slow pyrolysis liquid (PL) produced from birch (Betula sp.) as an acidifying agent, and its ability to reduce NH3 emissions following surface application of cattle slurry, was studied in a field experiment. Untreated slurry (US) and slurries acidified with PL and sulfuric acid (SA) were applied to the second harvest of a grass ley. Immediate NH3 emissions, grass biomass, N-yield and possible toxic impacts on soil nematodes and enchytraeids were examined. Furthermore, the effects on soil respiration, nitrogen dynamics and seed germination were studied in subsequent laboratory experiments. In the field, over one third of the water-extractable ammonium-N (NH4-N) applied was lost through NH3 volatilization from US. SA and PL acidified slurries reduced NH3-N emission rate equally from 3.4 to <0.04 kg ha-1 h-1. Acidification with SA resulted in the highest and that with PL in the lowest grass dry matter (DM) and N yield. Neither SA nor PL acidification had negative effects on soil enchytraeids or nematodes. Reduced yield production, seed germination and delayed microbial activity after PL slurry application were most probably caused by the PL containing organic compounds. However, later increase in carbon dioxide (CO2) production and improved seed germination suggest that these compounds were rapidly volatilized and/or degraded by soil microbes. Though PL efficiently cut NH3 emission from surface-spread slurry, further studies on appropriate application methods and possible phytotoxicity are needed.

Author Correction: Neglecting diurnal variations leads to uncertainties in terrestrial nitrous oxide emissions.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Neglecting diurnal variations leads to uncertainties in terrestrial nitrous oxide emissions.

Nitrous oxide (N2O) is an important greenhouse gas produced in soil and aquatic ecosystems. Its warming potential is 296 times higher than that of CO2. Most N2O emission measurements made so far are limited in temporal and spatial resolution causing uncertainties in the global N2O budget. Recent advances in laser spectroscopic techniques provide an excellent tool for area-integrated, direct and continuous field measurements of N2O fluxes using the eddy covariance method. By employing this technique on an agricultural site with four laser-based analysers, we show here that N2O exchange exhibits contrasting diurnal behaviour depending upon soil nitrogen availability. When soil N was high due to fertilizer application, N2O emissions were higher during daytime than during the night. However, when soil N became limited, emissions were higher during the night than during the day. These reverse diurnal patterns supported by isotopic analyses may indicate a dominant role of plants on microbial processes associated with N2O exchange. This study highlights the potential of new technologies in improving estimates of global N2O sources.