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Soil organic carbon (SOC) turnover in subsoils is assumed to be limited to spatially restricted microsites where fresh substrate inputs occur. Vice versa, the growth and activity of microorganisms outside of such hotspots may be limited by easily available substrates. The apparent long-term stability of subsoil organic carbon could thus be a result of microbial inactivity in these vast "cold regions" outside of hotspots. The aim of this study was to obtain realistic data about the in situ distribution of microbial hotspots in deep soil using soil zymography for three extracellular enzymes on undisturbed soil slices sampled from 0 to 161 cm depth. The results showed that most enzyme-driven turnover processes were concentrated to small portions of <1 to 10% of the subsoil volume, while enzymes in the major part of subsoils were barely active. In a second step, soil slices were homogenously sprayed with $^{14}$C glucose, incubated for 2 weeks and again analyzed with soil zymography. After glucose application, enzyme activities greatly increased in non-hotspot areas, thus confirming that substrate availability limits microbial activity in most of the subsoil volume. This implies that substrate limitation is a controlling factor for SOC stability in subsoils, suggesting that SOC in non-hotspots is persisting over long time periods until substrate becomes available and increases microbial activity

Biochar, compost, iron oxide, manure, and inorganic fertilizer affect bioavailability of arsenic and improve soil quality of an abandoned arsenic-contaminated gold mine spoil

Arsenic (As) contaminated mining spoils pose health threats to environmental resources and humans, and thus, mitigating this potential risk is worth investigating. Here, we studied the impacts of biochar, compost, iron oxide, manure, and inorganic fertilizer on the non-specifically (readily bioavailable)- and specifically- sorbed As and soil quality improvement of an abandoned mine spoil highly contaminated with As (total As = 1807 mg/kg). Compost, iron oxide, manure, and biochar were each applied at 0.5%, 2%, and 5% (w/w) to the contaminated soil; and NPK fertilizer at 0.1, 0.2, and 5.0 g/kg. The non-specifically (readily bioavailable)- and specifically- sorbed As were extracted sequentially and available P, total C and N, dissolved organic carbon, soil soluble anions, and exchangeable cations were extracted after 1- and 28-day incubation. Compost, manure, and biochar at 5% improved the total C and N and exchangeable $K^{+}$, $Mg^{2+}$ and $Na^{+}$. However, manure, compost, and iron oxide at 5% reduced available P from 118.5 to 60.3, 12.6, and 7.1 mg/kg, respectively. As compared to the untreated soil, the addition of iron oxide doses reduced the readily bioavailable As by 93%; while compost, manure, inorganic fertilizers, and biochar increased it by 106–332%, 24–315%, 19–398%, and 28–47%, respectively, with a significantly higher impact for the 5% doses. Furthermore, compost reduced specifically-sorbed As content (14–37%), but the other amendments did not significantly affect it. The impacts of the amendments on the readily bioavailable As was stronger than on specifically-sorbed As; but these were not affected by the incubation period. Arsenic bioavailability in our soil increased with increasing the soil pH and the contents of Cl-, DOC, and exchangeable $K^{+}$ and $Na^{+}$. We conclude that iron-rich materials can be used to reduce As bioavailability and to mitigate the associated environmental and human health risk in such mining spoils. However, the carbon-, and P-rich and alkaline materials increased the bioavailability of As, which indicates that these amendments may increase the risk of As, but can be used to enhance phytoextraction efficiency of As in the gold mining spoil

Short-term effect of biochar on microbial biomass, respiration and enzymatic activities in wastewater irrigated soils in urban agroecosystems of the West African savannah

Irrigated urban agriculture (UA) supports the economy and health of urban inhabitants in low-income countries. This system is often characterized by high nutrient inputs and mostly utilizes wastewater for irrigation. Biochar has been proposed to increase crop yields and improve soil properties. In this study, we assessed the transient effect of rice husk biochar (20 t $ha^{−1}$) and/or fertilizer (NPK: 15-15-15) on microbial respiration, microbial biomass carbon and enzyme activities of irrigated (wastewater and tap water) soil from an UA field experiment in the Guinea savannah zones of Ghana. Our results showed an increase by up to 123% in soil organic carbon (SOC) after a year of biochar application, while hot water extractable carbon (HWEC) was increased by only 11 to 26% and microbial biomass carbon (MBC) by 34%. Basal respiration was significantly increased in mineral fertilized soil by up to 46% but decreased by 12–45% under wastewater irrigation. Overall, the metabolic quotient ($\it q$$CO_{2}$) indicated less stress for the microbial community and increased carbon use efficiency with biochar application and wastewater irrigation. Total enzymes activity was increased under wastewater irrigation and biochar treated soils exhibit a more diverse composition of C-cycling enzymes and a higher activity of aminopeptidases. Biochar and wastewater showed positive effects on biological soil properties and contributed to soil fertility. Our results suggest beneficial effects of biochar on non-biochar SOC stocks in the long term

Microspheres as surrogate helminth eggs

Re-use of water containing helminth eggs during irrigation for agricultural purposes poses health risks, and likewise during research, due to the potential of spreading on contact. Therefore, polystyrene latex microspheres could be used as surrogates for chemical or biological species during colloidal transport. The aim here is to compare the settling velocities of microspheres having varied surface coatings - that is, proteins A, G and A/G; with that of real helminth eggs obtained from literature. The settling velocities of the microspheres were experimentally determined in tap- and wastewater, as well as theoretically in tap water; which was found to be within the range of mean values for those experimentally determined. There were no differences amongst the microspheres types used for settling in wastewater (i.e., A = 0.072 $\pm$ 0.02; G = 0.060 $\pm$ 0.03; A/G = 0.053 $\pm$ 0.01 mm/s). The same applied for settling in tap water (i.e., A = 0.068 $\pm$ 0.02; G = 0.047 $\pm$ 0.004; A/G = 0.095 $\pm$ 0.02 mm/s), except for microsphere G being different from microsphere A/G. All three types of microspheres settled at velocities lower than that of the wastewater particles (=0.118 $\pm$ 0.03). T-test analyses of settling velocities of microspheres in both tap- and wastewater, versus that from literature (i.e., $\it Ascaris$, $\it Trichuris$ and $\it Oesophagostomum$), showed that microsphere A and A/G may surrogate for $\it Ascaris$ in tap water, the same as A/G for $\it Oesophagostomum$. In wastewater however, both microspheres A and G are a good fit for $\it Trichuris$

Effects of biochar, waste water irrigation and fertilization on soil properties in West African urban agriculture

In large areas of sub-Saharan Africa crop production must cope with low soil fertility. To increase soil fertility, the application of biochar (charred biomass) has been suggested. In urban areas, untreated waste water is widely used for irrigation because it is a nutrient-rich year-round water source. Uncertainty exists regarding the interactions between soil properties, biochar, waste water and fertilization over time. The aims of this study were to determine these interactions in two typical sandy, soil organic carbon (SOC) and nutrient depleted soils under urban vegetable production in Tamale (Ghana) and Ouagadougou (Burkina Faso) over two years. The addition of biochar at 2 kg m$^{−2}$ made from rice husks and corn cobs initially doubled SOC stocks but SOC losses of 35% occurred thereafter. Both biochar types had no effect on soil pH, phosphorous availability and effective cation exchange capacity (CEC) but rice husk biochar retained nitrogen (N). Irrigation with domestic waste water increased soil pH and exchangeable sodium over time. Inorganic fertilization alone acidified soils, increased available phosphorous and decreased base saturation. Organic fertilization increased SOC, N and CEC. The results from both locations demonstrate that the effects of biochar and waste water were less pronounced than reported elsewhere

Evaluation of soil sensitivity towards the irrigation with treated wastewater in the Jordan River region

An assessment of soil sensitivity was carried out regarding the soil suitability for wastewater reuse. This was done based on digital soil maps joined with spatial data on soil properties using Geographic Information Systems (GIS). Six major risks of primarily agricultural significance were defined in close collaboration with regional experts. The changes in particular soil and groundwater properties as a result of irrigation with low water quality were evaluated and discussed. Based on the local soil parameters, the specific sensitivity and suitability grades were assessed for the respective soil unit concerning irrigation with treated wastewater (TWW) using standard and specially developed methods. In conclusion, with regard to soil suitability criteria, sensitivity and suitability maps, including the aggregated total sensitivity, were presented for supporting sustainable irrigation practices

Impact of fecal sludge and municipal solid waste co-compost on crop growth of Raphanus Sativus\textit {Raphanus Sativus} L. and Capsicum Anuum\textit {Capsicum Anuum} L. under stress conditions

Co-composted dewatered faecal sludge (FS) with organic fractions of municipal solid waste (MSW) has a high potential to be used as an agricultural resource in Sri Lanka. In addition to options for cost recovery in waste management, closing the nutrient and carbon cycles between urban and rural areas, substitution of mineral fertilizers, reduced pollution. and the restoration of degraded arable land are possible with important benefits. Up to now little is known about the usage of FS-MSW as fertilizer and it needs to be studied in order to achieve a better understanding and generate application recommendations. The aim of these experiments has been to evaluate the possibility of substituting mineral fertilization. Two field experiments were conducted on sandy loam to assess the effects of MSW compost and FS-MSW co-compost, its pelletized forms, and mineral-enriched FS-MSW on crop growth. As a short-term crop $\textit {Raphanus sativus}$ "Beeralu rabu" (radish) was studied for 50 days in a randomized complete block design (RCDB). Results show that, under drought conditions, FS-MSW co-compost increased the yield significantly, while MSW and FS-MSW compost enabled the highest survival rate of the plants. Similarly, the second field trial with a long-term crop, $\textit {Capsicum anuum}$ "CA-8" (capsicum), was planted as RCBD, using the same treatments, for a cultivation period of 120 days. Results display that during a drought followed by water saturated soil conditions co-compost treatments achieved comparable yields and increased the survival rate significantly compared to the control, fertilized with urea, triple super phosphate, and muriate of potash. Cost-benefit analysis (CBA) revealed that pelletizing decreased the monetary benefits if only fertilizer value is considered. It can be concluded that, under drought and water stress, co-compost ensures comparable yields and enables more resistance, but might not be economical viable as a one-crop fertilizer. These findings need to be validated with further trials under different climate regimes and soil