Differential impacts of sewage sludge and biochar on phosphorus-related processes: An imaging study of the rhizosphere.

Recycling of phosphorus (P) from waste streams in agriculture is essential to reduce the negative environmental effects of surplus P and the unsustainable mining of geological P resources. Sewage sludge (SS) is an important P source; however, several issues are associated with the handling and application of SS in agriculture. Thus, post-treatments such as pyrolysis of SS into biochar (BC) could address some of these issues. Here we elucidate how patches of SS in soil interact with the living roots of wheat and affect important P-related rhizosphere processes compared to their BC counterparts. Wheat plants were grown in rhizoboxes with sandy loam soil, and 1 cm Ø patches with either SS or BC placed 10 cm below the seed. A negative control (CK) was included. Planar optode pH sensors were used to visualize spatiotemporal pH changes during 40 days of plant growth, diffusive gradients in thin films (DGT) were applied to map labile P, and zymography was used to visualize the spatial distribution of acid (ACP) and alkaline (ALP) phosphatase activity. In addition, bulk soil measurements of available P, pH, and ACP activity were conducted. Finally, the relative abundance of bacterial P-cycling genes (phoD, phoX, phnK) was determined in the patch area rhizosphere. Labile P was only observed in the area of the SS patches, and SS further triggered root proliferation and increased the activity of ACP and ALP in interaction with the roots. In contrast, BC seemed to be inert, had no visible effect on root growth, and even reduced ACP and ALP activity in the patch area. Furthermore, there was a lower relative abundance of phoD and phnK genes in the BC rhizosphere compared to the CK. Hence, optimization of BC properties is needed to increase the short-term efficiency of BC from SS as a P fertilizer.

Is wood ash amendment a suitable mitigation strategy for N2O emissions from soil?

Wood ash, the by-product of biomass combustion to energy, can return important nutrients back to the soil and counteract acidification. However, the application of wood ash may affect the emission of greenhouse gases. Here, the effect of wood ash application on nitrous oxide (N2O) emissions from different soil environments were investigated in a 40 days incubation experiment comprising ten different soil types amended with five different wood ash concentrations (0, 3, 9, 20, and 54 t ash ha-1). The emitted N2O was measured continuously, and initial soil properties without ash application (carbon (C), nitrogen (N), ammonium (NH4+), nitrate (NO3-), and pH) and resulting soil properties (pH, NH4+, and NO3-) were measured prior and after the incubation period, respectively. The Random Forests (RF) model was used to identify which factors (initial and resulting soil properties, vegetation, management, wood ash doze, and respiration rate) were the most important to predict the development of emitted N2O after ash application. Wood ash either increased, decreased, or had no effect on the amount of emitted N2O depending on soil type and ash dose. The RF model identified the final resulting pH as the most important factor for the prediction of emitted N2O. The results suggest that wood ash can mitigate N2O emissions from soil, however, this effect depends on soil type where a mitigating effect of wood ash application was observed mainly in low pH soils with high soil organic matter whereas an increase in N2O emissions was observed in mineral soils that had previously received N fertilization. This study emphasises the importance of pH manipulation in regards to N2O emissions from soil.

Contrasting effects of biochar on phosphorus dynamics and bioavailability in different soil types.

We investigated how two different biochars (wood biochar - WBC and straw biochar - SBC) affected P dynamics and bioavailability in five different soils differing in pH, C%, texture, Fe, Al, Ca, and Mg giving a range of soils with low (S1 and S2), intermediate (S4), and high (S3 and S5) P sorption capacities. Langmuir and Freundlich equations were fitted to the sorption data of soil and soil/biochar mixtures. P fertilizer applied to all treatments was fractioned into strongly sorbed P (qS), easily available sorbed P (qA) and solution P (c) by determining the anion exchange resin (AER)-extractable P in samples from the sorption experiment. A pot experiment was conducted to measure P uptake by maize grown in S1, S2 and S3 amended with WBC or SBC at two P fertilizer levels (0 or 70mgPkg-1). Only WBC could sorb P from solution partly due to a high content of calcite. SBC did not have any effect on P sorption isotherms, whereas WBC increased the P sorption in S1, S2, and S4, yet decreased P sorption in acidic soil S5. qS increased in S1, S2, and S4, and decreased in S5 in WBC treatments, whereas, qS decreased in SBC treatments in soils S2, S4, and S5. Accordingly, there was a significant interaction between soil type and biochar on maize growth and P uptake. Biochar had no effect in an alkaline soil (S3), whereas, WBC and SBC had positive effects on maize growth in slightly acidic soils S1 and S2, depending on the soil P status, however, the P uptake was lower in WBC compared to SBC treatments. Biochar and soil properties and the P status of the soil affect P bioavailability. The study provides useful information for optimizing the use of biochar in agricultural P management.