Critical Transitions in Lake Ecosystem State May Be Driven by Coupled Feedback Mechanisms: A Case Study from Lake Erhai, China

Critical transitions between ecosystem states can be triggered by relatively small external forces or internal perturbations and may show time-lagged or hysteretic recovery. Understanding the precise mechanisms of a transition is important for ecosystem management, but it is hampered by a lack of information about the preceding interactions and associated feedback between different components in an ecosystem. This paper employs a range of data, including paleolimnological, environmental monitoring and documentary sources from lake Erhai and its catchment, to investigate the ecosystem structure and dynamics across multiple trophic levels through the process of eutrophication. A long-term perspective shows the growth and decline of two distinct, but coupled, positive feedback loops: a macrophyte-loop and a phosphorus-recycling-loop. The macrophyte-loop became weaker, and the phosphorus-recycling-loop became stronger during the process of lake eutrophication, indicating that the critical transition was propelled by the interaction of two positive feedback loops with different strengths. For lake restoration, future weakening of the phosphorus-recycling loop or a reduction in external pressures is expected to trigger macrophyte growth and eventually produce clear water conditions, but the speed of recovery will probably depend on the rates of feedback loops and the strength of their coupling.

Phosphorus Governance within Planetary Boundaries: The Potential of Strategic Local Resource Planning in The Hague and Delfland, The Netherlands

In recent years, a growing body of research has explored the urban dimension of the critical resource phosphorus with a focus on urban metabolism analysis, recovery technologies and governance frameworks. However, there has been no tangible and holistic attempt at choosing between available measures and instruments for their implementation in the urban realm. With the growing and increasingly urban world population, cities have become more and more important as actors in phosphorus governance by fueling global phosphorus flows, e.g., via the consumption of food, agricultural products, and phosphorus recycling. Globally, a future-proof phosphorus metabolism may be contributed to by strategic local phosphorus planning. This article systematically explores the purposive potential of local phosphorus planning using a case study of The Hague, The Netherlands. Looking across multiple administrative and spatial dimensions, the study combines quantitative analysis of phosphorus flows with qualitative investigation of their drivers, the actors involved, current regulations and local scope for action. The research reveals the feasibility of meaningfully impacting phosphorus flows through urban action. The potential for strategic local resource planning is demonstrated in a grid of policy options and in the assessment of their socio-economic and environmental benefits. Additionally, the study draws up a list of key recommendations to transfer to other urban settings. It encourages further research aimed at closing data gaps for local phosphorus inventories, collaborative approaches in strategic resource planning, scope for action in other cities or jurisdictions, as well as improved quantification of the outreach of policy effects.

Crystalline phase analysis and phosphorus availability after thermochemical treatment of sewage sludge ash with sodium and potassium sulfates for fertilizer production

Phosphorus rich sewage sludge ash is a promising source to produce phosphorus recycling fertilizer. However, the low plant availability of phosphorus in these ashes makes a treatment necessary. A thermochemical treatment (800–1000 °C) with alkali additives transforms poorly plant available phosphorus phases to highly plant available calcium alkali phosphates (Ca,Mg)(Na,K)PO4. In this study, we investigate the use of K2SO4 as additive to produce a phosphorus potassium fertilizer in laboratory-scale experiments (crucible). Pure K2SO4 is not suitable as high reaction temperatures are required due to the high melting point of K2SO4. To overcome this barrier, we carried out series of experiments with mixtures of K2SO4 and Na2SO4 resulting in a lower economically feasible reaction temperature (900–1000 °C). In this way, the produced phosphorus potassium fertilizers (8.4 wt.% K, 7.6 wt.% P) was highly plant available for phosphorus indicated by complete extractable phosphorus in neutral ammonium citrate solution. The added potassium is, in contrast to sodium, preferably incorporated into silicates instead of phosphorus phases. Thus, the highly extractable phase (Ca,Mg)(Na,K)PO4 in the thermochemical products contain less potassium than expected. This preferred incorporation is confirmed by a pilot-scale trial (rotary kiln) and thermodynamic calculation.

Starve to Sustain—An Ancient Syrian Landrace of Sorghum as Tool for Phosphorous Bio-Economy?

Phosphorus (P) is an essential macronutrient, playing a role in developmental and metabolic processes in plants. To understand the local and systemic responses of sorghum to inorganic phosphorus (Pi) starvation and the potential of straw and ash for reutilisation in agriculture, we compared two grain (Razinieh) and sweet (Della) sorghum varieties with respect to their morpho-physiological and molecular responses. We found that Pi starvation increased the elongation of primary roots, the formation of lateral roots, and the accumulation of anthocyanin. In Razinieh, lateral roots were promoted to a higher extent, correlated with a higher expression of SbPht1 phosphate transporters. Infrared spectra of straw from mature plants raised to maturity showed two prominent bands at 1371 and 2337 cm−1, which could be assigned to P-H(H2) stretching vibration in phosphine acid and phosphinothious acid, and their derivates, whose abundance correlated with phosphate uptake of the source plant and genotype (with a higher intensity in Razinieh). The ash generated from these straws stimulated the shoot elongation and root development of the rice seedlings, especially for the material derived from Razinieh raised under Pi starvation. In conclusion, sorghum growing on marginal lands has potential as a bio-economy alternative for mineral phosphorus recycling.

Model-Based Evaluation of Hydrothermal Treatment for the Energy Efficient Dewatering and Drying of Sewage Sludge

This research paper evaluates hydrothermal carbonization (HTC) as a possible treatment for sewage sludge, including phosphorus recycling. German governmental requirements force a high number of wastewater treatment plants (WWTP) to recover phosphorus from sewage sludge above limits of 20 g kgTS−1 before further disposal (e.g., co-incineration). The results show that pH reduction has a positive effect on shifting phosphorus to the liquid phase during HTC. Although the experimental results of this research do not yet achieve the necessary phosphorus reduction, various calculations are made to achieve this goal in future experimental studies. In order to be able to assess the energy benefits of HTC, Aspen Plus modeling was used to show the positive impact of implementing this technology in a WWTP. It is shown that the mechanical dewaterability of sewage sludge (SS) increases after HTC enabling energy savings by means of subsequent thermal drying. A heat optimized HTC is able to cut energy expenses by half, further providing a phosphorus-depleted hydrochar for extensive energetic use.