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Purpose: Digestates from anaerobic digestion are increasingly used in agriculture. There is thus a need for better understanding of digestates' value as organic soil amendments, and how this varies depending on the process parameters and post-treatments applied. Methods: A range of fifteen digestates (raw, solid, phase separated, and composted digestates), originating from farm and centralised anaerobic digestion sites were characterised. Carbon mineralisation and soil aggregate stability (AS) dynamics following the addition of the digestates to soil were monitored over a 182-day incubation period at 20 °C. Results: The mineralisation of carbon varied significantly between digestates, with the lowest percentage of carbon lost at 11.45% for composted digestate, and the highest at 49.27% for a solid batch digestate. Using hierarchical clustering of soil AS analysis at several sample dates, digestates were classified into four groups: those inducing (1) rapid but transient improvement of AS, (2) slower, temporary improvement of AS, (3) immediate improvement followed by a longer-lasting increase in AS, (4) minor or non-significant improvement in AS. Conclusion: Phase separation and composting of digestates were found to have more impact than other process parameters in determining the rate of mineralisation and dynamics in AS. Impact: Characterising and classifying digestates by their potential impact on soil structure would help to improve decision making for digestate production and sustainable use in agriculture. [ABSTRACT FROM AUTHOR]

A variety of organic feedstocks can be used for anaerobic digestion, resulting in digestates with different compositions, affecting the fertiliser value. Therefore, two experiments were conducted to assess (1) differences in the nitrogen (N) fertiliser value of seven digestates from different feedstocks in a 2-year field experiment with spring wheat, and (2) the degradability of organic matter (OM) in the digestates within an aerobic incubation experiment. In the field, mineral fertiliser equivalents were in a range of 18–60% (1st year) and 39–83% (2nd year). Fertiliser properties could describe 58.9–74.2% of the N offtake variance among digestates. In the incubation experiment, digestates produced 720–1900 mg CO2-C kg−1. After 56 days, 61% of organic C added by food waste digestate has been mineralised, compared to 16–22% for the other digestates. Digestate composition (C/N, Corg/Norg, carbonate, cellulose, lignin, and crude fibre) could explain 90.4% of the CO2 evolution. In both experiments, digested food waste stood out among digestates with the highest N offtake and highest OM mineralisation. In conclusion, differences in fertiliser value and OM degradability could be related to compositional variations. However, apart from food waste, the composition had only minor influence on digestate performance after soil application. [ABSTRACT FROM AUTHOR]

Monsoon-induced coastal upwelling, land run-off, benthic and atmospheric inputs make the western Indian shelf waters biologically productive that is expected to lead to high rates of mineralisation of organic matter (OM) in the sediments. Dissimilatory sulphate reduction (SR) is a major pathway of OM mineralisation in near-shore marine sediments owing to depletion of other energetically more profitable electron acceptors (O, NO , Mn and Fe oxides) within few millimetres of the sediment-water interface. We carried out first ever study to quantify SR rates in the inner shelf sediments off Goa (central west coast of India) using the S radiotracer technique. The highest rates were recorded in the upper 10 cm of the sediment cores and decreased gradually thereafter below detection. Despite significant SR activity in the upper ∼12 to 21 cm at most of the sites, pore water sulphate concentrations generally did not show much variation with depth. The depth integrated SR rate (0.066-0.46 mol m year) decreased with increasing water depth. Free sulphide was present in low concentrations (0-3 μM) in pore waters at shallow stations (depth <30 m). However, high build-up of sulphide (100-600 μM) in pore waters was observed at two deeper stations (depths 39 and 48 m), 7-11 cm below the sediment-water interface. The total iron content of the sediment decreased from ∼7 to 5 % from the shallowest to the deepest station. The high pyrite content indicates that the shelf sediments act as a sink for sulphide accounting for the low free sulphide levels in pore water. In the moderately organic rich (2-3.5 %) sediments off Goa, the measured SR rates are much lower than those reported from other upwelling areas, especially off Namibia and Peru. The amount of organic carbon remineralised via sulphate reduction was ∼0.52 mol m year. With an estimated average organic carbon accumulation rate of ∼5.6 (±0.5) mol m year, it appears that the bulk of organic matter gets preserved in sediments in the study region. [ABSTRACT FROM AUTHOR]

The availability of soil nutrients, which are recycled through the decomposition of crop residues, is important for the management of cropped soils. However, knowledge regarding the influence of contact between crop residues and soil on the dynamics of carbon (C) and nitrogen (N) in soil is limited. In particular, the effect of particle size on decomposition is not well-known, and conceptual approaches for modelling the soil-residue contact in a decomposition model remain scarce. Therefore, we analysed and modelled the effect of maize stem particle length on decomposition. We incubated maize stem residues with particle sizes of 0.02, 0.5, 2 and 5 cm in length in a loamy soil at 25 °C over 301 days. We continuously measured the mineralisation of C and N and determined the chemical evolution of the remaining particles. We used the decomposition model CANTIS which takes into account the soil-residue contact, using a contact factor, K. The decomposition rates of smaller maize particles were higher than those of larger particles during the early phases of decomposition. However, these differences were not maintained after 301 days. These results suggest that a larger size of the maize particles only slowed the rate of mineralisation in the short term but did not modify decomposition in the medium term. We proposed a new formalism for expressing the changes in soil-residue contact with different particle sizes. The contact factor K was calculated using the standardised specific surface area and can be applied more widely to residues that differ in morphology and density. [ABSTRACT FROM AUTHOR]

An incubation study determined the effect of one dry–rewetting (DRW) event on the turnover of carbon (C), phosphorus (P) and nitrogen (N). Thirty-two soils were collected from different climatic regions of southern Australia, varying in soil type, land use and agronomic management history. We hypothesised that respiration and nutrient pulses are related to soil physio-chemical properties. Respiration (CO2 release) was measured intensively for 90 h after rewetting. C mineralisation ( Cmin) model fitting was used to describe the amount of mineralisable C (Co90 h) and the proportional mineralisation rate ( k). Compared to constantly moist soils, 13 soils showed increases in both Co90 h and k, indicating that DRW increased the amount of mineralisable C and the rate at which C was mineralised over the 90-h period. In 17 soils, k was increased but not Co90 h, showing an increase in C mineralisation rate but no change in the amount of mineralisable C. Two soils showed a reduction in k with no change in Co90 h, possibly due to low C contents and small microbial biomass. Only one soil exhibited no change in either Co90 h or k. Multiple linear regression analysis indicated that the magnitude of the increase in mineralisable C in response to the DRW event (∆Co90 h = Co90 h DRW − Co90 h moist) was primarily explained by clay content (39%); however, inclusion of nine soil physio-chemical properties explained more of the variation in ∆Co90 h than any of the properties alone. Five of the nine physio-chemical variables present in the multiple-regression model were related to C content or composition. Pulses in available N and P were not related to ∆Co90 h. [ABSTRACT FROM AUTHOR]

Here we test the hypotheses that 19 years of simulated pollutant N deposition increases both losses of carbon (C) and the ability of plants to access P from organic material in upland heathland. The grass, Dactylis glomerata, and the dwarf shrub, Calluna vulgaris, were grown in soil containing microbial-derived organic matter labelled with 14C and 33P. We found that both soil and root-surface phosphatase activity increased significantly in response to N deposition. We also found a significant positive relationship between root-surface phosphatase activity and 33P uptake for Calluna, but a negative relationship for Dactylis. Efflux of 14C from the microbial-derived organic matter was strongly dependent on an interaction among plant presence, plant species and N deposition. Our results show that mineralisation of C and P, and subsequent plant uptake of P from organic sources is decoupled. In our experimental conditions, stimulation of P turnover coupled with subsequent plant uptake through up-regulation of root phosphatases is little affected by N addition. However, our data indicate that root-surface phosphatases are likely to be more important for uptake of P derived from organic sources for Calluna than for Dactylis. [ABSTRACT FROM AUTHOR]

Wood ants ( Formica rufa group) often bring large quantities of conifer resin to their mounds. The aim of this study was to test the hypothesis that the resin acts as a fungicide and thereby reduces C and N mineralisation. Two laboratory incubation experiments were carried out using two different materials: F/H layer from a Scots pine ( Pinus sylvestris) stand and mixed litter from Scots pine and Norway spruce ( Picea abies) stands. We estimated the effects of resin addition on fungal biomass and on the rates of C and N mineralisation. Addition of resin to the F/H material caused an increase in fungal biomass and C mineralisation, whereas N mineralisation decreased. Addition of resin to litter material did not significantly affect fungal biomass or C and N mineralisation. The results indicate that rather than having a fungicidal effect, resin acts as a C source that increases C mineralisation (mainly from the resin itself) and decreases net N mineralisation. The latter factor might be important in preventing plants dependent on inorganic N from invading and covering the ant mounds. [ABSTRACT FROM AUTHOR]