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Four biochar types, produced by slow pyrolysis of poultry litter (PL) and pine chips (P) at 400 or 500 °C, were added to two adjacent soils with contrasting soil organic matter (SOM) content (8.9 vs. 16.1 g C kg−1). The N mineralization rate was determined during 14-week incubations and assessments were made of the microbial biomass C, dehydrogenase activity, and the microbial community structure (PLFA-extraction). The addition of PL biochars increased the net N mineralization (i.e., compared to the control treatment) in both soils, while for treatments with P biochars net N immobilization was observed in both soils. Increasing the pyrolysis temperature of both feedstock types led to a decrease in net N mineralization. The ratio of Bacterial to Fungal PLFA biomarkers also increased with addition of biochars, and particularly in the case of the 500 °C biochars. Next to feedstock type and pyrolysis temperature, SOM content clearly affected the assessed soil biological parameters, viz. net N mineralization or immobilization, MBC and dehydrogenase activity were all greater in the H soil. This might be explained by an increased chance of physical contact between the microbial community activated by SOM mineralization upon incubation and discrete biochar particles. However, when considering the H soil's double C and N content, these responses were disproportionally small, which may be partly due to the L soil's, somewhat more labile SOM. Nonetheless, increasing SOM content and microbial biomass and activity generally appears to result in greater mineralization of biochar. Additionally, higher N mineralization after PL addition to the H soil with lower pH than the L soil can be due to the liming effect of the PL biochars. [ABSTRACT FROM AUTHOR]

Paddy soil indigenous N supply is often poorly related to N status and our aim was to assess its linkage to reduction of Fe 3+ and Mn 4+ , primary terminal electron acceptors in submerged soils. Transplanted rice was grown in the greenhouse in four Bangladeshi paddy soils with distinct SOC to Fe ox ratio (1.6 to 4.9) for 72 days firstly under continuous and then intermittent flooding. Solution buildup rates of Fe 2+ and Mn 2+ in the first 2–3 weeks of flooding correlated negatively with soil organic carbon (SOC) to NH 4 -oxalate extractable Fe (Fe ox ) and Mn (Mn ox ) ratios (p < 0.01). Mössbauer analysis detected ferrihydrite and goethite in all soils and with Mn 3 O 4 these are the likely source minerals. An electron (e − ) balance calculated from soil C-emission rates suggested reductive Fe and Mn dissolution to relevant e − -accepting processes, probably responsible for no >50% of e − capture, though. Reduction of abundantly present octahedral Fe 3+ in chlorites and vermiculite and their interstratified forms in these floodplain silty Inceptisols is hypothesized to also support microbial activity. Notwithstanding, a close temporal synergy existed between solution Fe and soil mineral N and their build-up rates were correlated (r: 0.77 to 0.90; p < 0.01) and with that of dissolved OC (DOC) (r: 0.84 to 0.96; p < 0.01), C emission rate (r: 0.99; p < 0.01) and SOC:Fe ox (r: −0.71; p < 0.01). These correlations suggest Fe 3+ reduction to be a relevant intermediary step in soil N mineralization, possibly through release of associated DOC, N or both. After switching to intermittent flooding dissolution of Fe, Mn and DOC were decoupled from mineral N release but since Eh remained in the Fe 3+ -reduction range in three out of four soils, possibly clay-Fe 3+ alternated with O 2 as e − -acceptor. Most importantly, in all soils N release slowed or halted after only 2 weeks of flooding but recommenced with intermittent flooding. As a next step, field experiments could verify if indigenous soil N supply also benefits from non-continuous irrigation management. Lastly, experimental proof is pending for release of clay-bound N and interlayer NH 4 + following reduction of octahedral Fe 3+ with consequent increased negative charge or structural destabilization, possibly an important process in floodplain paddy soils in Bangladesh. [ABSTRACT FROM AUTHOR]

In rice production, water-saving irrigation management is expanding and likely alters depth profiles of soil moisture, redox potential (Eh) and microbial activity. It is, however, unclear how such conditions then impact net soil N-release and availability to the rice crop, because we do not know well enough how water-saving irrigation management shapes depth-distribution of Eh and reductive processes, and microbial activity. A field experiment with rice was laid out on a typical young floodplain paddy soil of Bangladesh with three irrigation schemes, viz. continuous flooding (CF), safe alternate wetting and drying (AWD) and direct seeded rice (DSR), with 120 kg N ha −1 (N 120 ) or without (N 0 ) urea application. We evaluated changes in soil mineral N and plant N uptake, CH 4 and CO 2 emissions and soil pH, and at multiple depths soil Eh and temperature, dissolved C, Fe and Mn throughout 2015 dry (Boro) season (Jan–Apr). Eh stayed at or above ∼+300 mV except for sudden drops to ∼−200 mV with irrigation events in DSR. Eh quickly dropped to methanogenic conditions, under both AWD and CF; rises to ∼+200 mV were observed during AWD-drainage events but were restricted to upper 5.5 or 12.5 cm depths. Throughout the growing season there was a pronounced increase in reductive dissolution of Fe and Mn (hydro-) oxides, buildup of dissolved C, and CH 4 effluxes under AWD and CF but not DSR, likely at least partially driven by the gradual soil warming from ∼20 °C till 28 °C. Predominant aerobic conditions under DSR lead to a nearly doubled C-emissions (CO 2  + CH 4 ) compared to AWD and CF, suggesting more soil organic matter (OM) degradation in the former case, while soil mineral N plus plant N build-up rate followed an opposite order. Urea application did not raise soil exchangeable N levels, even prior to significant plant uptake from 28 DAT (days after transplanting), and we forward temporal abiotic NH 4 + -fixation and N-removal processes as explanations. We conclude that regardless of some distinctions in temporal evolutions of puddle layer Eh, solution C, Fe and Mn, and CH 4 -emission, soil N-supply was quite comparable under AWD and CF, as was rice yield. In the context of N availability, AWD could be safely adopted for rice growth in the Bangladeshi Boro season. The eventual fertilizer N recovery efficiency was higher for CF (42%) than for AWD (32%), but AWD saved 12% irrigation water. While DSR saved 45% water there was a large yield penalty, likely due to drought stress but also by poor germination caused by cold night temperatures in mid-January, while seedling transplantation in CF and AWD plots was only later on 28 January. Further research should be conducted to investigate the fast and pronounced removal of exchangeable inorganic N after initial N buildup by soil OM mineralization, especially in CF and AWD. At this moment most likely candidate processes appear clay-NH 4 + fixation and anaerobic NH 4 + -oxidation. [ABSTRACT FROM AUTHOR]

Abstract: Biochar produced during pyrolysis of biomass has the potential to reduce greenhouse gas (GHG) emissions from soils. In order to evaluate the effect of four different biochar additions on the emission of the greenhouse gases CO2 and N2O, two incubation experiments were established in a temperate sandy loam soil. Digestate, a waste-product of the wet fermentation of swine manure, and willow wood was slowly pyrolyzed at 350 °C and 700 °C, yielding four biochar types (DS350, DS700, WS350 and WS700). In the first incubation experiment (117 days), C mineralization was monitored in soil amended with biochar at a quantity of 10 Mg ha−1 on an area-basis (biochar to soil ratio of 1:69 on a mass basis) at 50% water filled pore space (WFPS). CO2 emissions from the 350 °C biochar treatments were significantly higher than the control (no biochar) treatment, while we observed no significantly different net C mineralization in the treatments with the 700 °C biochars compared to the control. After fitting a combined zero- plus first-order model to the cumulative C mineralization data, the parameter for the easily mineralizable C pool (CAf) positively correlated with the volatile matter (VM) contents of the biochars. Microbial biomass carbon consistently increased due to all biochar additions, while the dehydrogenase activity increased in the 350 °C biochar treatments but decreased in the 700 °C biochar treatments. Principal component analysis (PCA) of the extracted phospholipid fatty acids (PLFAs) demonstrated that divergent microbial community structures established after the addition of all biochars. The markers for Gram-positive and Gram-negative bacteria were more abundant in the 350 °C biochar treatments compared to the control and to the other biochar treatments. Net N mineralization was higher in the digestate biochar treatments than in the willow biochar treatments and decreased with increasing pyrolysis temperatures and increasing C:N ratio. In a second incubation experiment (15 days) N2O emissions were measured at WFPS of 70% and the same biochars were added in the same quantity as for C mineralization, with the addition of 40 mg KNO3–N kg−1. The cumulative N2O emission after 15 days was positively correlated with the volatile matter content of the biochars and was significantly lower in the 700 °C biochar treatments compared to the control, while no significant differences were found for the 350 °C biochar treatments. This study suggests that volatile matter content could be an important property of biochars in explaining short-term CO2 and N2O emissions from biochar-amended soils. [Copyright &y& Elsevier]

We have studied the possibilities of manipulating N mineralization from high N vegetable crop residues by the addition of organic materials, with the aim of initially immobilizing the mineralized residue N with a view to stimulating remineralization at a later stage. Residues of leek (Allium porrum) were incubated with soil, alone and in combination with straw, two types of green waste compost (with contrasting C:N ratios) and tannic acid. Evolution of mineral N was monitored by destructive sampling. After 15 weeks, molasses was added to part of the samples in each treatment, and incubation continued for another 12 weeks. All materials added during the first incubation stage, except the low C:N compost, resulted in significant immobilization of the residue N. The immobilization with the high C:N compost (41.4 mg N kg−1 soil) was significantly larger than with tannic acid and straw (both immobilized about 26 mg N kg−1 soil). In the straw treatment, remineralization started in the first stage of incubation from day 50 onwards. The addition of molasses caused a strong and significant remineralization in the second stage (equivalent to 73% of the N initially immobilized) in the treatment with the high C:N ratio compost. In the case of tannic acid, there was no consistent effect on mineralization from addition of molasses. This was attributed to the fact that the immobilization observed was due to chemical rather than biological fixation of the residue N. A number of non-toxic organic wastes could be considered for use in mediating release of immobilized N from high N crop residue materials in an attempt to synchronize residue N availability with crop N demand. [Copyright &y& Elsevier]