Biochar suppresses N2O emissions while maintaining N availability in a sandy loam soil

Nitrous oxide (N₂O) from agricultural soil is a significant source of greenhouse gas emissions. Biochar amendment can contribute to climate change mitigation by suppressing emissions of N₂O from soil, although the mechanisms underlying this effect are poorly understood. We investigated the effect of biochar on soil N₂O emissions and N cycling processes by quantifying soil N immobilisation, denitrification, nitrification and mineralisation rates using ¹⁵N pool dilution techniques and the FLUAZ numerical calculation model. We then examined whether biochar amendment affected N₂O emissions and the availability and transformations of N in soils.Our results show that biochar suppressed cumulative soil N₂O production by 91% in near-saturated, fertilised soils. Cumulative denitrification was reduced by 37%, which accounted for 85–95 % of soil N₂O emissions. We also found that physical/chemical and biological ammonium (NH₄⁺) immobilisation increased with biochar amendment but that nitrate (NO₃⁻) immobilisation decreased. We concluded that this immobilisation was insignificant compared to total soil inorganic N content. In contrast, soil N mineralisation significantly increased by 269% and nitrification by 34% in biochar-amended soil.These findings demonstrate that biochar amendment did not limit inorganic N availability to nitrifiers and denitrifiers, therefore limitations in soil NH₄⁺ and NO₃⁻ supply cannot explain the suppression of N₂O emissions. These results support the concept that biochar application to soil could significantly mitigate agricultural N₂O emissions through altering N transformations, and underpin efforts to develop climate-friendly agricultural management techniques.     

Wheat straw and its biochar have contrasting effects on inorganic N retention and N2O production in a cultivated Black Chernozem

A laboratory incubation experiment was conducted to investigate the effects of direct incorporation of either wheat straw or its biochar into a cultivated Chernozem on gross N transformations calculated by the ¹⁵N pool dilution technique and nitrous oxide (N₂O) production rates. Incorporation of wheat straw stimulated gross NH ₄ ⁺ (ammonium) and NO ₃ ^? (nitrate) immobilization rates by 302 and 95.2?%, respectively, suppressed gross nitrification rates by 32.2?%, and increased N₂O production by 37.7?%. In contrast, the addition of a biochar produced from the wheat straw did not influence any of the above N cycling processes. Therefore, application of biochar could be a possible management strategy for long-term C sequestration (through soil storage of stable C contained in the biochar) in soils without increasing N₂O production rates, but could not effectively immobilize NO ₃ ^? in the soil.     

Wheat straw and its biochar had contrasting effects on soil C and N cycling two growing seasons after addition to a Black Chernozemic soil planted to barley

Application of crop residues and its biochar produced through slow pyrolysis can potentially increase carbon (C) sequestration in agricultural production systems. The impact of crop residue and its biochar addition on greenhouse gas emission rates and the associated changes of soil gross N transformation rates in agricultural soils are poorly understood. We evaluated the effect of wheat straw and its biochar applied to a Black Chernozemic soil planted to barley, two growing seasons or 15 months (at the full-bloom stage of barley in the second growing season) after their field application, on CO₂ and N₂O emission rates, soil inorganic N and soil gross N transformation rates in a laboratory incubation experiment. Gross N transformation rates were studied using the ¹⁵N isotope pool dilution method. The field experiment included four treatments: control, addition of wheat straw (30 t ha^?1), addition of biochar pyrolyzed from wheat straw (20 t ha^?1), and addition of wheat straw plus its biochar (30 t ha^?1 wheat straw + 20 t ha^?1 biochar). Fifteen months after their application, wheat straw and its biochar addition increased soil total organic C concentrations (p?=?0.039 and <0.001, respectively) but did not affect soil dissolved organic C, total N and NH₄ ⁺-N concentrations, and soil pH. Biochar addition increased soil NO₃ ^?-N concentrations (p?=?0.004). Soil CO₂ and N₂O emission rates were increased by 40 (p?p?=?0.03), respectively, after wheat straw addition, but were not affected by biochar application. Straw and its biochar addition did not affect gross and net N mineralization rates or net nitrification rates. However, biochar addition doubled gross nitrification rates relative to the control (p?2 and N₂O emissions and enhance soil C sequestration. However, the implications of the increased soil gross nitrification rate and NO₃ ^?-N in the biochar addition treatment for long-term NO₃ ^?-N dynamics and N₂O emissions need to be further studied.     

Effect of labile and recalcitrant carbon on heterotrophic nitrification in a subtropical forest soil

Carbon (C) is an important factor controlling heterotrophic nitrification in soil, but the effect of individual C components (e.g., labile and recalcitrant C) is largely unclear. We carried out a C amendment experiment in which either labile C (glucose) or a recalcitrant C (cellulose and biochar) was added to a subtropical forest soil. A ¹⁵N-, ¹³C-tracing and MiSeq sequencing study was performed to investigate soil gross heterotrophic nitrification rates, carbon utilization for soil respiration and microbial biomass production and microbial composition, respectively. After 2 days, results showed a significant increase of gross heterotrophic nitrification rate in glucose (GLU) (on average 3.34 mg N kg⁻¹ day⁻¹), cellulose (CEL) (on average 0.21 mg N kg⁻¹ day⁻¹) and biochar (BIO) (on average 0.13 mg N kg⁻¹ day⁻¹) amendment in comparison with the unamended soil (CK) (on average 0.01 mg N kg⁻¹ day⁻¹; p < 0.05). The contribution of heterotrophic nitrification to total soil nitrification was significantly larger in GLU (average 85.86%), CEL (average 98.52%) and BIO (average 81.25%) treatments compared with CK (average 33.33%; p < 0.01). After 2-month amendment, the gross rates remarkably decreased in GLU (average 0.02 mg N kg⁻¹ day⁻¹), and the contribution to total nitrification (average 8.73%) were significantly lower than that in CK (p < 0.05). A decrease in the proportion of heterotrophic nitrification to total nitrification in soil was also observed in CEL (average 38.40%) and BIO (6.74%) treatments. Nevertheless, BIO amendment (compared to CK, GLU and CEL) showed the highest gross heterotrophic nitrification rate, accompanied by a notably higher abundance of specific heterotrophic nitrifiers, i.e. Trichoderma, Aspergillus and Penicillium. These results point to a stimulatory effect of C addition on soil heterotrophic nitrification in the short term, while the stimulatory impact of C amendment diminishes with the decline in easily available C. In addition, a shift of the microbial composition in the long term can possibly be sustained for longer if additional recalcitrant C is available to heterotrophic nitrifiers. The dynamic response of heterotrophic nitrification to labile and recalcitrant C in this study offered an explanation for the positive effect of plantation and plant root exudation on the process.     

Importance of heterotrophic nitrification and dissimilatory nitrate reduction to ammonium in a cropland soil: Evidences from a 15N tracing study to literature synthesis

Future climate change is predicted to influence soil moisture regime, a key factor regulating soil nitrogen (N) cycling. To elucidate how soil moisture affects gross N transformation in a cultivated black soil, a ¹⁵N tracing study was conducted at 30%, 50% and 70% water-filled pore space (WFPS). While gross mineralization rate of recalcitrant organic N (N_rec) increased from 0.56 to 2.47 mg N kg⁻¹ d⁻¹, the rate of labile organic N mineralization declined from 4.23 to 2.41 mg N kg⁻¹ d⁻¹ with a WFPS increase from 30% to 70%. Similar to total mineralization, no distinct moisture effect was found on total immobilization of ammonium, which primarily entered the N_rec pool. Nitrate (NO₃⁻) was mainly produced via autotrophic nitrification, which was significantly stimulated by increasing WFPS. Unexpectedly, heterotrophic nitrification was observed, with the highest rate of 1.06 mg N kg⁻¹ d⁻¹ at 30% WFPS, contributing 31.8% to total NO₃⁻ production, and decreased with WFPS. Dissimilatory nitrate reduction to ammonium (DNRA) increased from near zero (30% WFPS) to 0.26 mg N kg⁻¹ d⁻¹ (70% WFPS), amounting to 16.7–92.9% of NO₃⁻ consumption. A literature synthetic analysis from global multiple ecosystems showed that the rates of heterotrophic nitrification and DNRA in test soil were comparative to the forest and grassland ecosystems, and that heterotrophic nitrification was positively correlated with precipitation, soil organic carbon (SOC) and C/N, but negatively with pH and bulk density, while DNRA showed positive relationships with precipitation, clay, SOC, C/NO₃⁻ and WFPS. We suggested that low pH and bulk density and high SOC and C/N in test soil might favor heterotrophic nitrification, and that C and NO₃⁻ availability together with anaerobic condition were crucial for DNRA. Overall, our study highlights the role of moisture in regulating gross N turnover and the importance of heterotrophic nitrification for NO₃⁻ production under low moisture and DNRA for NO₃⁻ retention under high moisture in cropland.     

Mechanisms of soil N dynamics following long-term application of organic fertilizers to subtropical rain-fed purple soil in China

In this study, a ¹⁵N tracing incubation experiment and an in situ monitoring study were combined to investigate the effects of different N fertilizer regimes on the mechanisms of soil N dynamics from a long-term repeated N application experiment. The field study was initiated in 2003 under a wheat-maize rotation system in the subtropical rain-fed purple soil region of China. The experiment included six fertilization treatments applied on an equivalent N basis (280 kg N ha⁻¹), except for the residue only treatment which received 112 kg N ha⁻¹: (1) UC, unfertilized control; (2) NPK, mineral fertilizer NPK; (3) OM, pig manure; (4) OM-NPK, pig manure (40% of applied N) with mineral NPK (60% of applied N); (5) RSD, crop straw; (6) RSD-NPK, crop straw (40% of applied N) with mineral NPK (60% of applied N). The results showed that long-term repeated applications of mineral or organic N fertilizer significantly stimulated soil gross N mineralization rates, which was associated with enhanced soil C and N contents following the application of N fertilizer. The crop N offtake and yield were positively correlated with gross mineralization. Gross autotrophic nitrification rates were enhanced by approximately 2.5-fold in the NPK, OM, OM-NPK, and RSD-NPK treatments, and to a lesser extent by RSD application, compared to the UC. A significant positive relationship between gross nitrification rates and cumulative N loss via interflow and runoff indicated that the mechanisms responsible for increasing N loss following long-term applications of N fertilizer were governed by the nitrification dynamics. Organic fertilizers stimulated gross ammonium (NH₄⁺) immobilization rates and caused a strong competition with nitrifiers for NH₄⁺, thus preventing a build-up of nitrate (NO₃⁻). Overall, in this study, we found that partial or complete substitution of NPK fertilizers with organic fertilizers can reduce N losses and maintain high crop production, except for the treatment involving application of RSD alone. Therefore, based on the N transformation dynamics observed in this study, organic fertilizers in combination with mineral fertilizer applications (i.e. OM, OM-NPK, and RSD-NPK treatments) are recommended for crop production in the subtropical rain-fed purple soils in China.     

Effects of long-term annual inputs of straw and organic manure on plant N uptake and soil N fluxes

This study investigated the effects of long‐term annual inputs of animal manure and straw on the rate of gross nitrogen (N) mineralization–immobilization turnover (MIT), net N mineralization and potential nitrification, and examined how these N transformation rates affect plant N availability. The experiment was conducted during May–June 2001 in long‐term field experiments in Askov, Denmark, where organic manure and barley straw had been applied annually for 11 and 20 years prior to the year 2000, respectively. Thus, any differences could be attributed to residual effects from the previous years of application. Inputs of straw and organic manure to soil increased soil organic matter (SOM)‐N content in soil in the order: without straw, without manure < without straw, with manure < with straw, without manure < with straw, with manure. The inputs did not change net N mineralization in the soil. There was a distinct but non‐significant trend towards higher gross N mineralization with increasing SOM‐N. Gross N immobilization was enhanced by straw inputs and to a lesser extent by organic manure inputs, while potential nitrification was enhanced by both amendments. The results show that long‐term annual inputs of straw and organic manure can increase MIT rate and potential nitrification rate without influencing net N mineralization rate. MIT and potential nitrification explained 23–31% of the variation in plant N uptake, while net N mineralization rate only explained 1%. Plant N uptake therefore seems to be more influenced by MIT rate and potential nitrification rate than by net mineralization rate, presumably because mineral N in the transition between gross N mineralization and gross N immobilization is available for assimilation by plants.     

Gross N transformation rates and net N mineralisation rates related to the C and N contents of soil organic matter fractions in grassland soils of different age

We investigated the relationship between soil organic matter (SOM) content and N dynamics in three grassland soils (0-10 and 10-20 cm depth) of different age (6, 14 and 50 y-old) with sandy loam textures. To study the distribution of the total C and N content the SOM was fractionated into light, intermediate and heavy density fractions of particulate macro-organic matter (150-2000 μm) and the 50-150 μm and <50 μm size fractions. The potential gross N transformation rates (mineralisation, nitrification, NH₄⁺ and NO₃⁻ immobilization) were determined by means of short-term, fully mirrored ¹⁵N isotope dilution experiments (7-d incubations). The long-term potential net N mineralisation and gross N immobilization rates were measured in 70-d incubations. The total C and N contents mainly tended to increase in the 0-10 cm layer with increasing age of the grassland soils. Significant differences in total SOM storage were detected for the long-term (50 y-old) conversion from arable land to permanent grassland. The largest relative increase in C and N contents had occurred in the heavy density fraction of the macro-organic matter, followed by the 50-150 and <50 μm fractions. Our results suggest that the heavy density fraction of the macro-organic matter could serve as a good indicator of early SOM accumulation, induced by converting arable land to permanent grassland. Gross N mineralisation, nitrification, and (long-term) gross N immobilization rates tended to increase with increasing age of the grasslands, and showed strong, positive correlations with the total C and N contents. The calculated gross N mineralisation rates (7-d incubations) and net N mineralisation rates (70-d incubations) corresponded with a gross N mineralisation of 643, 982 and 1876 kg N ha⁻¹ y⁻¹, and a net N mineralisation of 195, 208 and 274 kg N ha⁻¹ y⁻¹ in the upper 20 cm of the 6, 14 and 50 y-old grassland soils, respectively. Linear regression analysis showed that 93% of the variability of the gross N mineralisation rates could be explained by variation in the total N contents, whereas total N contents together with the C-to-N ratios of the <50 μm fraction explained 84% of the variability of the net N mineralisation rates. The relationship between long-term net N mineralisation rates and gross N mineralisation rates could be fitted by means of a logarithmic equation (net m=0.24Ln(gross m)+0.23, R²=0.69, P<0.05), which reflects that the ratio of gross N immobilization-to-gross N mineralisation tended to increase with increasing SOM contents. Microbial demand for N tended to increase with increasing SOM content in the grassland soils, indicating that potential N retention in soils through microbial N immobilization tends to be limited by C availability.     

Effect of straw biochar application on soil carbon,greenhouse gas emissions and nitrogen leaching: A vegetable crop rotation field experiment

Intensive vegetable crop systems are rapidly developing, with consequences for greenhouse gas (GHGs) emissions, nitrogen leaching and soil carbon. We undertook a field trial to explore the effect of biochar application (0, 10, 20 and 40 t ha⁻¹) on these factors in lettuce, water spinach and ice plant rotation. Our results show that the 20 and 40 t ha⁻¹ soil treatments resulted in the SOC content being 26.3% and 29.8% higher than the control (0 t ha⁻¹), respectively, with significant differences among all treatments (p < .05). Biochar application caused N₂O emissions to decrease during the lettuce and water spinach seasons, by 1.5%–33.6% and 12.4%–40.5%, respectively, compared the control, with the 20 t ha⁻¹ application rate resulting in the lowest N₂O emissions. Biochar also decreased the dissolved nitrogen (DN) concentration in leachate by 9.8%–36.2%, following a 7.3%–19.9% reduction in dissolved nitrogen in the soil. Similarly, biochar decreased the nitrate (NO₃⁻) concentrations in leachate by 3.9%–30.2%, following a 3.8%–16.7% reduction in the soil nitrate level. Overall, straw biochar applied at rate of 20 t ha⁻¹ produced the lowest N₂O emissions and N leaching, while, increasing soil carbon.     

Different rates of biochar application change 15N retention in soil and 15N utilization by maize

Biochar application to soil may impact soil nitrogen (N) dynamics, but the effects on N uptake and utilization by crop remain largely unknown, especially the effects of the rate of biochar application. To investigate the effects of biochar on soil ¹⁵N retention rate and ¹⁵N utilization efficiency (¹⁵NUE) by maize, a six-month ¹⁵N isotope tracer technique combined with in situ pot experiment was conducted in Mollisol. The experiment included four treatments: no biochar applied (CK) and biochar applied at the rates of 12 t ha⁻¹ (P12), 24 t ha⁻¹ (P24) and 48 t ha⁻¹ soil (P48). Compared with CK, biochar application reduced soil bulk density and ¹⁵N loss rate, and significantly improved total N and ¹⁵N retention amount in the 0–30 cm soil depth. The P24 treatment had the largest increase in ¹⁵N retention rate throughout the 0–40 cm depth. After biochar application, the ¹⁵N uptake and ¹⁵NUE were significantly increased in the grain and leaf, which promoted grain yields. Contrary to this, the P48 treatment appeared to lower ¹⁵N uptake and ¹⁵NUE compared with P12 and P24. In conclusion, biochar application improves the potential of the soil to retain N and the improvement in ¹⁵N uptake and utilization are more pronounced in maize leaves and grain. Moreover, biochar application promotes ¹⁵N utilization in maize plant and improves maize yield. However, when biochar application rate is high (i.e. P48 treatment), the ¹⁵N retention by the soil and ¹⁵N utilization by the maize are reduced markedly compared with P12 and P24.     

Gross nitrogen transformation rate in soil is affected by coastal wetland cultivation

Abstract

A ¹⁵N tracing experiment was carried out to investigate gross N dynamics in coastal wetland soils. The results showed that the cultivation of coastal wetland has resulted in a significant increase in the gross mineralization rate, gross nitrification rate and gross microbial immobilization of ammonium rate (p<0.01). However, the effect of cultivation on the gross microbial immobilization of nitrate rate was not significant. The gross nitrification rate was much higher than nitrate immobilization (p<0.01), indicating that cultivation of coastal wetland could increase the risk of losses of N from the soil through leaching.     

Effects of cotton (Gossypium hirsutum L.) straw and its biochar application on NH3 volatilization and N use efficiency in a drip-irrigated cotton field

Reducing ammonia (NH₃) volatilization is a practical way to increase nitrogen (N) fertilizer use efficiency (NUE). In this field study, soil was amended once with either cotton (Gossypium hirsutum L.) straw (6 t ha^?1) or its biochar (3.7 t ha^?1) unfertilized (0 kg N ha^?1) or fertilized (450 kg N ha^?1), and then soil inorganic N concentration and distribution, NH₃ volatilization, cotton yield and NUE were measured during the next two growing seasons. In unfertilized plots, NH₃ volatilization losses in the straw-amended and biochar-amended treatments were 38–40% and 42–46%, respectively, less than that in control (i.e., unamended soil) during the two growing seasons. In the fertilized plots, NH₃ volatilization losses in the straw-amended and biochar-amended treatments were 30–39% and 43–54%, respectively, less than that in the control. Straw amendment increased inorganic N concentrations, cotton yield, cotton N uptake and NUE during the first cropping season after application, but not during the second. In contrast, biochar increased cotton N uptake and NUE during both the first and the second cropping seasons after application. Furthermore, the effects of biochar on cotton N uptake and NUE were greater in the second year than in the first year. These results indicate that cotton straw and cotton straw biochar can both reduce NH₃ volatilization and also increase cotton yield, N uptake and NUE. In addition, the positive effects of one application of cotton straw biochar were more long-lasting than those of cotton straw.     

Immobilization and remineralization of N following addition of wheat straw into soil: determination of gross N transformation rates by N-ammonium isotope dilution technique

N dynamics in soil where wheat straw was incorporated were investigated by a soil incubation experiment using ¹⁵N-labelled nitrate or ¹⁵N-labelled wheat straw. The incubated soils were sampled after 7, 28, 54 days from the incorporation of wheat straw, respectively, and gross rates of N transformations including N remineralization and temporal changes in the amount of microbial biomass were determined.Following the addition of wheat straw into soils, rapid decrease of nitrate content in soil and increase of microbial biomass C and N occurred within the first week from onset of the experiment. Both the gross rates of mineralization and immobilization determined by ¹⁵N-ammonium isotope dilution technique were remarkably enhanced by the addition of wheat straw, and gradually decreased with time. Remineralization rate of N derived from ¹⁵N-labelled nitrate, and mineralization rate of N derived from ¹⁵N-labelled wheat straw was estimated by ¹⁵N isotope dilution technique using non-labelled ammonium. Remineralization rates of N derived from ¹⁵N-labelled nitrate were calculated to be 0.71 mg N kg⁻¹ d⁻¹ after 7 days, 0.55 mg N kg⁻¹ d⁻¹ after 28 days, and 0.29 mg N kg⁻¹ d⁻¹ after 54 days.Nearly 10% of the ¹⁵N-labelled N originally contained in the wheat straw was held in the microbial biomass irrespective of the sampling time. The amount of inorganic N in soil which was derived from ¹⁵N-labelled wheat straw ranged between 1.93 and 2.37 mg N kg⁻¹.Rates of N transformations in soil with ¹⁵N-labelled wheat straw were obtained by assuming that the k value was equal to the ¹⁵N abundance of biomass N, and the obtained values were considered to be valid.     

Impact of maize straw biochar and tied-ridge-furrow rainwater harvesting on soil erosion and soil quality in a semiarid region

In dryland areas, integrating biochar soil amendment with in situ rainwater harvesting systems may decrease soil erosion, improve soil quality, and increase crop productivity and yield. This study was conducted to investigate the effect of maize straw biochar amendment and ridge-furrow rainwater harvesting systems on run-off, sediment yield and the physico-chemical properties of a Calcic Cambisol soil in semiarid areas. The experiment was conducted on alfalfa (Medicago sativa) production land at the Anjiagou Catchment experimental station in Gansu province, China. The experimental layout was a split-plot design with three replications. Biochar was applied at a rate of 0 and 30 t ha⁻¹, respectively. The tillage treatments were flat planting, open-ridging, and tied-ridging (TR). Overall, the integration of maize straw biochar with TR decreased soil bulk density at 0–40 cm depth. Biochar application reduced run-off by 37.8% and soil loss by 55.5% during alfalfa-growing seasons compared to the control. In general, biochar addition increased soil total potassium, but the same effect was not observed for soil pH, total nitrogen, total phosphorus, and available phosphorus. These findings demonstrate the potential of integrating maize straw biochar and tillage systems to reduce soil erosion and improve soil quality for rainfed crop production in semiarid areas. Further studies on the effect of biochar-tillage system interaction are warranted to improve soil conditions for plant growth and increase crop yield in dryland areas.     

Temporal variations of crop residue effects on soil N transformation depend on soil properties as well as residue qualities

Purple soils (Eutric Regosols) are widely distributed in humid subtropical Southwest China. They are characterized by high nitrification activities, with risks of severe NO₃^? leaching. Incorporation of crop residues is considered an effective method to reduce NO₃^? loss. In the present study, we compared the effects of alfalfa, rice straw, and sugarcane bagasse on gross N transformation turnover in a purple soil (purple soil, pH 7.62) compared with those in an acid soil (acid soil, pH 5.26), at 12 h, 3 months, and 6 months after residue incorporation. The gross N transformation rates were determined by ¹⁵N tracing. All tested crop residues stimulated the gross N mineralization rates, but reduced the net mineralization rates in both soils at 12 h after residue incorporation; however, the extent of the effect varied with the crop residue qualities, with rice straw having the strongest effects. Crop residues reduced net nitrification rates by depressing gross autotrophic nitrification rates and stimulating NO₃^? immobilization rates in the purple soil, particularly after rice straw incorporation (net nitrification rate decreased from 16.72 mg N kg^?1 d^?1 in the control to ??29.42 mg N kg^?1 d^?1 at 12 h of residue incorporation); however, crop residues did not affect the gross autotrophic nitrification rates in the acid soil. Crop residue effects subsided almost completely within 6 months, with sugarcane bagasse showing the longest lasting effects. The results indicated that crop residues affected the N transformation rates in a temporal manner, dependent on soil properties and residue qualities.     

Influence of nitrogen fertilizer forms and crop straw biochars on soil exchange properties and maize growth on an acidic Ultisol

Effects of repeated application of urea (UN) and calcium nitrate (CN) singly and together with crop straw biochars on soil acidity and maize growth were investigated with greenhouse pot experiments for two consecutive seasons. Canola straw biochar (CB), peanut straw biochar (PB) and wheat straw biochar (WB) were applied at 1% of dried soil weight in the first season. N fertilizers were applied at 200 mg N kg^?1. In UN treatments, an initial rise in pH was subjected to proton consumption through urea hydrolysis, afterwards nitrification of NH₄⁺ caused drastic reductions in pH as single UN had soil pH of 3.70, even lower than control (4.27) after the 2nd crop season. Post-harvest soil analyses indicated that soil pH, soil exchangeable acidity, NH₄⁺, NO₃^? and total base cations showed highly significant variation under N and biochar types (P < 0.05). Articulated growth of plants under combined application with biochars was expressed by 22.7%, 22.5%, and 35.7% higher root and 25.6%, 23.8%, and 35.9% higher shoot biomass by CB, PB and WB combined with CN over UN, respectively. Therefore, CN combined with biochars is a better choice to correct soil acidity and improve maize growth than UN combined with biochars.     

Estimates of Gross Transformation Rates of Dairy Manure N Using 15N Pool Dilution

Abstract

Most measurements of dairy manure nitrogen (N) availability depend on net changes in soil inorganic N concentration over time, which overlooks the cycling of manure N in the soil. Gross transformations of manure N, including mineralization (m), immobilization (i), and nitrification (n), can be quantified using ¹⁵N pool dilution methods. This research measures gross m, n, and i resulting from application of four freeze‐dried dairy manures that had distinctly different patterns of N availability. A sandy loam soil (coarse‐loamy, mixed, frigid Typic Haplorthod) was amended with four different freeze‐dried dairy manures and incubated at 25°C with optimal soil water content. The dilution of ¹⁵ammonium (NH4⁺) during a 48‐h interval (7–9 d and 56–58 d after manure application) was used to estimate m, whereas the dilution of ¹⁵nitrate (NO₃ ^?) was used to estimate n. Gross immobilization was calculated as gross minus net mineralization. Gross mineralization in the unamended soil was similar at 7‐ to 9‐d and 56‐ to 58‐d intervals and was significantly increased by the application of manures. For both amended and unamended soil, m was much greater (i.e., three‐ to nine‐fold) than estimated net mineralization, illustrating the degree to which manure N can be cycled in soil. At the early interval, both m and i were directly related to the manure C input, demonstrating the linkage between substrate C availability and N utilization by soil microbes. This research clearly shows that the application of dairy manures stimulates gross N transformation rates in the soil, improving our understanding of the impact of manure application on soil N cycling.     

Evidence of carbon stimulated N transformations in grassland soil after slurry application

High nitrification rates which convert ammonium (NH₄⁺) to the mobile ions NO₂⁻ and NO₃⁻ are of high ecological significance because they increase the potential for N losses via leaching and denitrification. Nitrification can be performed by chemoautotrophic or heterotrophic organisms and heterotrophic nitrifiers can oxidise either mineral (NH₄⁺) or organic N. Selective nitrification inhibitors and ¹⁵N tracer studies have been used in an attempt to separate heterotrophic and autotrophic nitrification. In a laboratory study we determined the effect of cattle slurry on the oxidation of mineral NH₄⁺-N and organic-N by labelling the NH₄⁺ or NO₃⁻ pools separately or both together with ¹⁵N. The size and enrichment of the mineral N pools were determined at intervals. To calculate gross N transformation rates a ¹⁵N tracing model was developed. This model consists of the three N-pools NH₄⁺, NO₃⁻ and organic N. Sub-models for decomposition of degradable carbon in the soil and the slurry were added to the model and linked to the N transformation rates. The model was set up in the software ModelMaker which contains non-linear optimization routines to determine model parameters. The application of cattle slurry increased the rate of nitrifcation by a factor of 20 compared with the control. The size and enrichment of the mineral N pools provided evidence that nitrification was due to the conversion of NH₄⁺ to NO₃⁻ and not the conversion of organic N to NO₃⁻. There was evidence that slurry-enhanced oxidation of NH₄⁺ to NO₃⁻ was due to a combination of autotrophic and heterotrophic transformations. Slurry application increased the mineralisation rate by approximately a factor of two compared with the control and the rate of immobilisation of NH₄⁺ by approximately a factor of three.     

Effects of biochar and fertilizer sources on nitrogen uptake by chilli pepper plants under Mediterranean climate

This study determined N uptake by serrano chilli pepper for two years and evaluated the effects of biochar amendment or organic N (org-N) fertilizer on N use under a Mediterranean climate. A field experiment was conducted using microplots from 2016 to 2017 in California, USA. Treatments included biochar amendment rates [0 (control), 10, 30 and 50 tons (t) ha⁻¹] biochar, all with 100% inorganic N fertilizer (165 kg N ha⁻¹), and org-N fertilizer applications at 50%, 75% and 100% of the total available N supply. Pepper yield, vegetative biomass, N uptake, ammonia (NH₃) volatilization and changes in soil organic carbon (SOC), and nitrate were determined. Pepper yield was highest in the 50% org-N and lowest in the 50 t ha⁻¹ biochar treatment during the first year. There were no differences in fruit yield among the organic treatments during the second year, and all were higher than that from the control. The 100% org-N treatment had less NH₃ volatilization than all other treatments during the first year. The two-year results showed that chilli pepper plants sequestered 4.6‒6.1 kg N to produce one ton fresh pepper fruits. During the first year, the 50% org-N treatment resulted in the highest N productivity or yield with lowest projected N fertilizer application requirements as compared to other treatments although there were no differences among all treatments in the second year. Thus, a combination of inorganic and org-N fertilizers can be an effective strategy to improve soil N productivity in long-term management.