Solid fraction of separated digestate as soil improver: implications for soil fertility and carbon sequestration

Purpose This study investigated the C and N mineralisation potential of solid fractions (SFs) from co-digestated pig manure after P-stripping (P-POOR SF) in comparison with P-rich SFs, as a means to estimate their organic matter stability in soil. Compost (COMP) and biochar (BCHR) (made from P-POOR SF) were also included in the study as reference biosolids. Methods The SFs were incubated in a sandy-loam soil under moist conditions to determine production of CO2 and mineral N. At specified intervals, CO2 evolution in the mixtures was measured via the alkali trap method and titration over a period of 81 days, while mineral N was measured using a flow analyser after KCl extraction over a period of 112 days. Results The various SFs showed similar patterns of C mineralisation (15–26% of added total C in 81 days) that were clearly higher than for COMP and BCHR (6% and 7%, respectively). Temporary N immobilisation was observed in biosolids with a high C/N ratio. The effective organic matter (EOM) of the SFs was calculated based on the C mineralisation data and varied between 130 and 369 kg Mg−1. Conclusions The SF with a reduced P content had a high EOM/P ratio which is beneficial in areas where P status of the soil is already high. Moreover, the N mineralisation patterns confirm that a high C/N ratio may also reduce risks for N leaching due to temporary N immobilisation.

Soil copper contamination effect on carbon mineralisation: evidence of a soil CO2 emission decrease from literature review

<p>Among all pollutants, copper (Cu) is of major environmental and toxicological concern with contamination from various origins. Moreover as a cation, Cu is easily complexed by the negatively charged soil organic matter (OM) inducing high concentrations in upper layers of soils where OM dominates. Due to its biotic and abiotic interactions with soil constituents Cu is expected to affect several soil processes among them the soil respiration, but studies provided contrasting results as soil respiration have been shown to decrease or increase with soil contamination depending on the studies.</p><p>In this study, we aimed at assessing how soil respiration is affected by Cu contamination in order to quantifying as a first approach the GHG emissions for a contaminated soil. We performed a quantitative review of literature focusing on soil heterotrophic respiration (thus excluding autotrophic respiration from plants) which aimed at 1) assessing the impact of a copper contamination on soil carbon (C) mineralisation and thus CO<sub>2</sub> emissions, and 2) hierarchizing the determinants of such an impact on C mineralisation compare to the influence of pedo-climatic soil parameters such as pH, clay percentage or the type of climate.</p><p>On the basis of a selection of roughly 390 literature data, global main results showed a decrease in soil CO<sub>2</sub> emission with an increase in soil Cu contamination. Data from ex situ spiking experiments could be easily differentiated from the ones originated from in situ natural contamination due to their sharper decrease in soil organic carbon mineralisation. Interestingly, ex situ spikes data on the short term provided a threshold: an increase in soil CO<sub>2</sub> emissions was noticed for data below total soil Cu content of 180 mg kg<sup>-1</sup> while a decrease was observed above this concentration. On the contrary, long-term in situ contamination due to anthropogenic activities (urbanisation, agriculture …) did not significantly impact soil carbon mineralisation except when we focused on the high inputs of industrial contamination (smelter, composted plant…). Soil pH was found as a variable of interest as acidic soils were more sensitive to Cu contamination for C mineralisation than neutral or alkaline soils, while the % of clay and the type of climate did not add explanation to the variation in C mineralisation. These results are discussed and the collected data allowed us to propose a general equation quantifying how soil respiration can be affected by a Cu contamination.</p>

Assessing the vulnerability of organic matter to C mineralisation in pasture and cropping soils of New Zealand

In New Zealand, pastoral soils have substantial organic carbon (OC) stocks, which may be vulnerable to loss from disturbance and environmental perturbations. We assessed OC vulnerability using two approaches. For the first approach, we postulated that the OC deficit of continuously cropped soils relative to nearby pastoral soils would provide a measure of the quantity of potentially vulnerable OC in pastures. As a test, soils were sampled to a depth of 15 cm at 149 sites and the total organic carbon (TOC) and particulate organic carbon (POC) contents were measured. The second approach involved measurement of OC mineralisation in a laboratory assay (98 day aerobic incubation at 25°C). For the pastoral soils, the mean TOC and POC was about twice that of the cropped soils. On average, 89% more OC was mineralised from the pastoral soils compared with the cropped counterparts. However, the quantity of OC mineralised in pasture soils was small relative to the potential for OC loss inferred from the difference in TOC between pastoral and cropped soils. Carbon mineralisation was explained using a two-pool exponential model with rate constants of the ‘fast’ and ‘slow’ pools equating to 0.36 ± 0.155 and 0.007 ± 0.003 day–1 respectively. The larger, slow OC pool correlated strongly with hot water extractable OC whereas the fast pool was related to OC extracted using cold water. Our results suggest that water extraction (using cold and hot water) can provide a rapid estimate of the quantity of mineralisable OC across a wide range of New Zealand soils.

Can worms be used to produce amendments with reduced CO₂ emissions during co-composting with clay and biochar and after their addition to soil?

In this study we evaluated CO2 emissions during co-composting and co-vermicomposting of green wastes with clay and/or biochar. The stability of the final products as well as their effect on C mineralization in soil have been evaluated. The aim of the study was to test the following hypothesis: (1) interactions between clay and biochar and organic wastes would lead to reduced CO2 emissions during the composting process, (2) these interactions would be enhanced in the presence of worms, and (3) more carbon would be sequestered in soil after the use of the resulting compost/vermicompost as amendments. We added two different doses of clay, biochar and their mixture to pre-composted green wastes and monitored C mineralisation during 21 days in presence or absence of worms (Eisenia species). The organic materials were then added to a loamy Cambisol and the CO2 emissions were monitored during 30 days in a laboratory incubation. Our results indicated that the addition of clay or clay/biochar mixture reduced carbon mineralization during co-composting without worms by up to 44 %. However, in the presence of worms, CO2 emissions increased for all treatments except for the low clay dose. The production conditions had more influence on C mineralization in soil for composts than for vermicomposts except for the low clay treatment, which showed a more reduced CO2 emissions compared to a regular compost. In summary, the addition of worms during co-composting with clay and biochar may be a promising technology for reducing CO2 emissions and increasing soil carbon storage. We suggest that the production of a low CO2 emission amendment requires optimisation of OM source, co-composting agents and worm species. The effect of the resulting material on soil fertility has to be evaluated.

Aerobic microbial activity in four tropical earthworm-soil systems. A mesocosm experiment

Soil organic matter (SOM) quality and carbon (C) availability may be major features influencing the effect of earthworms on the aerobic processes in clayey tropical soils. In this study, we assessed the effect of an anecic (Polypheretima elongata), an endogeic (Pontoscolex corethrurus) and an epigeic (Eudrilus eugeniae) earthworm on the aerobic microbial activity of two tropical soils, a calcic Vertisol and an acid Ferralsol, with clay content >70% and very different organic C content and SOM stability. The soil–earthworm interaction was studied in a 6-month mesocosm experiment in a greenhouse using soils with and without (control soil) earthworm addition. Potential C mineralisation, actual net nitrogen (N) mineralisation and dehydrogenase activity (DHA), as indicators of the aerobic activity of the soils, and phosphorus (P) availability were determined during the trial. DHA was used as an indicator of the global aerobic activity. Earthworms had little effect on potential C mineralisation but significantly increased actual net N mineralisation. The increase in N mineralisation in the Vertisol was twice as great as, and longer (6 v. 3 months) than for the Ferralsol. Differences between soils for N mineralisation were associated with a less recalcitrant SOM in the Vertisol. Available P increased 10% in the earthworm treatments. Earthworm activity improved N and P availability. DHA was 15 times higher for the Vertisol than for the Ferralsol, but the positive effect of earthworms on DHA was greater for the Ferralsol. This effect was greater for E. eugeniae, probably because of surface burrows generated by this epigeic earthworm, which favoured oxygen entry into the soil. Differences between the two soils were greater for DHA than for C and N mineralisation, and this was observed for the control soils as well as for the earthworm treatments. This indicates that earthworm activity modified the rate of the aerobic processes but it did not affect the intrinsic biological properties of these tropical soils, which were controlled mainly by SOM quality and C availability.

Effects of amendment of different biochars on soil enzyme activities related to carbon mineralisation

This study aimed to investigate the effects of different biochars on soil enzyme activities associated with soil carbon (C) mineralisation. Biochars were produced from two types of feedstock (fresh dairy manure and pine tree woodchip) at temperatures of 300°C, 500°C, and 700°C. Each biochar was mixed at a ratio of 5% (w/w) with a forest loamy soil and the mixture was incubated at 25°C for 180 days. Soil mineralisation rates, soil dissolved organic C, soil microbial biomass C, and five soil enzyme activities were measured during different incubation periods. Results showed that biochar addition increased soil enzyme activities at the early stage (mainly within the first 80 days) because biochar brought available nutrients to the soil and increased soil dissolved organic C and microbial activity. Soil enzyme activities were enhanced more by the dairy manure biochars than by the woodchip biochars (P < 0.05). The enhancement effect on enzyme activities (except catalase activity) was greater in the treatments with biochars produced at lower pyrolysis temperature (300°C). Linear relationships between some soil enzymes and C-mineralisation rates might indicate that the increased enzyme activities stimulated soil C mineralisation at the early stage. However, the biochar additions could result in great C sequestration in the long term, especially for the woodchip biochars pyrolysed at higher temperatures.

Soil Heterotrophic Respiration Potential and Maximum Respiration Rate of Differently Managed Meadows

In this study were compared heterotrophic respiratory potential (VDS/VMAX) expressing an increase in C mineralisation rate after drying and re-wetting the soil to 60% soil water content (v/w)(VDS) in relation to maximum respiration rate (VMAX) after glucose addition, and VMAX in organomineral soil (Ah horizon) of mod&shy;erately mown and for 11 years abandoned mountain meadows in Moravian-Silesian Beskids Mts. VDS/VMAX and VMAX were assessed in soil samples taken in 30-day intervals throughout the period of May&ndash;September 2004. The results obtained showed higher VDS/VMAX on the abandoned meadow throughout the whole experiment except the last sampling occasion, and higher VMAX throughout the whole experiment. Significantly (P &lt; 0.05) higher VDS/VMAX on the abandoned meadow was found in May and July, VMAX was significantly higher on the same meadow (P &lt; 0.05) only in September. From the parameters studied, the time of sampling had no significant (P &gt; 0.05) effect on VMAX when the data from the moderately mown meadow were evaluated. On the abandoned meadow, VMAX found was significantly (P &lt; 0.05) different when the samples from May and September or July and September were compared. A significant (P &lt; 0.05) effect of the sampling time on VDS/VMAX on the moderately mown meadow was presented by differences between May and other sampling times, on the abandoned meadow differences between September and other times of sampling except May were significant (P &lt; 0.05).