The responses of nitrification on intensively managed agricultural soils following long-term biochar (BC) amendment are poorly understood. The nitrification potential, abundance, and composition of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in acidic oxisols and alkaline cambosols following a 3-year BC treatment were investigated using 42-day aerobic incubation, quantitative polymerase chain reaction (qPCR), and clone library approach, respectively. Fresh soils were collected from a wheat/millet rotated pot trial in which 0 (control), 2.25, and 22.5 Mg ha−1 rice straw BCs were added for six consecutive crop seasons. The 22.5 Mg ha−1 BC (BC22.5) treatment enhanced nitrification in oxisols and even altered nitrification pattern from zero-order to first-order reaction model. AOA and AOB gene copies in the BC22.5 treatment were 9.55 and 22.0 times, respectively, compared with those in the BC0 treatment. The relative abundance of operational taxonomic units (OTUs) in AOA group 1.1a changed due to BC application, and that of OTU-20 was high in group 1.1b-related under the BC22.5 treatment. AOB community composition shifted toward Nitrosospira cluster 3 and 3-related group under the BC22.5 treatment. Basal nitrification was already high in cambosols, and BC had minimal effect on nitrification or AOA/AOB abundance. However, the BC22.5 treatment increased the relative abundance of OTU-9 in Nitrosospira cluster 3 group and that of OTU-13 and OTU-16 in Nitrosospira cluster 3-related groups both being AOB. The BC amendment had minimal effect on ammonia oxidizer composition in cambosols but influenced ammonia oxidizer composition and stimulated nitrification activity in oxisols.

The efficiency of classical mineral NPK fertilizers is usually low because a major part of these fertilizers does not reach plant roots and ends up polluting groundwaters with nitrates and phosphates. Recently, a novel polymer-coated urea made from recycled plastics was proposed to enhance N availability in cereal production. To evaluate the efficiency of this polymer for rice production, we set up field plots, microplots, and pot experiments with 15N tracing. We compared rice yield, N uptake, and N loss between conventional three split applications of urea and a single basal application of four derivatives from the polymer-coated urea. The four derivatives included a blend with 70 % of N from 6 % (w/w) coated urea and 30 % from urea and three coated urea fertilizers with 6, 8, and 12 % coating at an identical N application rate during two rice-growing seasons. Results show that 6 % coated urea improved 15N recovery, reduced 15N loss, and increased grain yield slightly due to an initial 15N burst occurring at high field temperatures after basal fertilization; 8 or 12 % coated urea better met plant N demand from transplanting to heading, greatly enhanced 15N recovery, and decreased 15N loss and NH3 volatilization. Nevertheless, unlike a significant increase of yield for 12 % coated urea, 8 % coated urea did not increase yield due to 15N release and excessive 15N uptake by plants at ripening. Overall, our findings show that a single basal polymer-coated urea application improves N use efficiency and reduces N loss in rice agroecosystem.

Few studies have examined the effects of biochar on nitrification of ammonium-based fertilizer in acidic arable soils, which contributes to NO3− leaching and soil acidification.We conducted a 42-day aerobic incubation and a 119-day weekly leaching experiment to investigate nitrification, N leaching, and soil acidification in two subtropical soils to which 300 mg N kg−1 ammonium sulfate or urea and 1 or 5 wt% rice straw biochar were applied.During aerobic incubation, NO3− accumulation was enhanced by applying biochar in increasing amounts from 1 to 5 wt%. As a result, pH decreased in the two soils from the original levels. Under leaching conditions, biochar did not increase NO3−, but 5 wt% biochar addition did reduce N leaching compared to that in soils treated with only N. Consistently, lower amounts of added N were recovered from the incubation (KCl-extractable N) and leaching (leaching plus KCl-extractable N) experiments following 5 wt% biochar application compared to soils treated with only N.Incorporating biochar into acidic arable soils accelerates nitrification and thus weakens the liming effects of biochar. The enhanced nitrification does not necessarily increase NO3− leaching. Rather, biochar reduces overall N leaching due to both improved N adsorption and increased unaccounted-for N (immobilization and possible gaseous losses). Further studies are necessary to assess the effects of biochar (when used as an addition to soil) on N.

A pot study spanning four consecutive crop seasons was conducted to compare the effects of successive rice straw biochar/rice straw amendments on C sequestration and soil fertility in rice/wheat rotated paddy soil.We adopted 4.5 t ha−1, 9.0 t ha−1 biochar and 3.75 t ha−1 straw for each crop season with an identical dose of NPK fertilizers.We found no major losses of biochar-C over the 2-year experimental period. Obvious reductions in CH4 emission were observed from rice seasons under the biochar application, despite the fact that the biochar brought more C into the soil than the straw. N2O emissions with biochar were similar to the controls without additives over the 2-year experimental period. Biochar application had positive effects on crop growth, along with positive effects on nutrient (N, P, K, Ca and Mg) uptake by crop plants and the availability of soil P, K, Ca and Mg. High levels of biochar application over the course of the crop rotation suppressed NH3 volatilization in the rice season, but stimulated it in the wheat season.Converting straw to biochar followed by successive application to soil is viable for soil C sequestration, CH4 mitigation, improvements of soil and crop productivity. Biochar soil amendment influences NH3 volatilization differently in the flooded rice and upland wheat seasons, respectively.

Biochar has been increasingly used as a method for C sequestration and soil improvement. To understand how feedstock and pyrolysis conditions affect biochar characteristics, we investigated two wood-based biochars (bamboo and elm) and five crop-residue-based biochars (wheat straw, rice straw, maize straw, rice husk, and coconut shell), which were pyrolyzed at 500 or 700 °C and remained at that temperature for 4, 8, and 16 h under oxygen-limited conditions. For a given feedstock, increasing pyrolysis temperature from 500 to 700 °C resulted in increases in ash content, BET surface area, pH, and total P and Ca contents (P < 0.05) and decreases in yield, cation exchange capacity (CEC), total acid, and total N (P < 0.01). Prolonging residence time (from 4 to 8 or 16 h), the BET surface area and ash content of biochars increased (P < 0.05), whereas the yield decreased (P < 0.01). Fourier-transform infrared spectroscopy (FTIR) analysis showed that more recalcitrant and aromatic structures were formed in the biochars with increased temperature. The three straw-based biochars consistently exhibited far greater ash percentage (14.5–40.3 wt %), CEC (14.1–34.8 cmol kg–1), and the contents of total N (0.24–2.81 wt %), P (0.60–8.41 wt %), Ca (0.63–1.48 wt %), and Mg (0.24–0.63 wt %) and generally had higher yield (19.0–37.6 wt %), pH (9.2–11.1), and contents of total acid (0.15–0.53 mmol g–1), C (41.7–55.1 wt %), Na (0.27–6.72 wt %), and K (6.56–28.1 wt %) than the two wood-based biochars. The BET surface area of straw-based biochars with 700 °C pyrolysis temperature could be mostly as high as 112–378 m2 g–1, a comparable level with that of wood-based biochars. Despite the high variability in biochar properties, these results demonstrate that biochars from crop straw may be more effective and desirable for improving soil fertility and C sequestration in Chinese vast soils.

The rice-wheat rotation in southern China is characterized by frequent flooding-draining water regime and heavy nitrogen (N) fertilization. There is a substantial lack of studies into the behavior of dissolved organic nitrogen (DON) in the intensively managed agroecosystem. A 3-year in situ field experiment was conducted to determine DON leaching and its seasonal and yearly variations as affected by fertilization, irrigation and precipitation over 6 consecutive rice/wheat seasons. Under the conventional N practice (300 kg N ha−1 for rice and 200 kg N ha−1 for wheat), the seasonal average DON concentrations in leachate (100 cm soil depth) for the three rice and wheat seasons were 0.6–1.1 and 0.1–2.3 mg N L−1, respectively. The cumulative DON leaching was estimated to be 1.1–2.3 kg N ha−1 for the rice seasons and 0.01–1.3 kg N ha−1 for the wheat seasons, with an annual total of 1.1–3.6 kg N ha−1. In the rice seasons, N fertilizer had little effect (P > 0.05) on DON leaching; precipitation and irrigation imported 3.6–9.1 kg N ha−1 of DON, which may thus conceal the fertilization effect on DON. In the wheat seasons, N fertilization had a positive effect (P < 0.01) on DON. Nevertheless, this promotive effect was strongly influenced by variable precipitation, which also carried 1.8–2.9 kg N ha−1 of DON into fields. Despite a very small proportion to chemical N applied and large variations driven by water regime, DON leaching is necessarily involved in the integrated field N budget in the rice-wheat rotation due to its relatively greater amount compared to other natural ecosystems.

•Meta-analysis linking drainage N losses with maize yield.•Yield-scaled drainage N losses increased exponentially with estimated N surplus.•Matching N rate with crop demand maintained high yield and low yield-scaled N loss.•Above-average precipitation increased N loss on area- and yield-scaled bases.Subsurface nitrogen (N) losses represent a major environmental concern in agriculture, particularly from fields containing artificial drainage to prevent saturated soil conditions and increase crop production. To develop sustainable intensification strategies and achieve high yields with minimal environmental impacts, N losses are increasingly evaluated with respect to crop productivity on a “yield-scaled” basis, yet little information is available to address the current challenge of balancing crop yields and drainage N losses from intensive maize production systems in the U.S. Midwest by using this metric. In the present study, a meta-analysis was conducted using 31 studies with 381 observations from a publicly available nutrient loss drainage database (Measured Annual Nutrient loads from Agricultural Environments, MANAGE) to address this issue. Results showed that increasing N rates enhanced yields but had weak effects on area- and yield-scaled drainage N losses. In contrast, yield-scaled drainage N losses responded exponentially to N surplus (estimated as N application rate minus above-ground crop N uptake). Relative precipitation during the drainage monitoring period strongly influenced area- and yield-scaled drainage N losses. Maize-soybean rotations and silt loam soils had lower yield-scaled drainage N losses compared to continuous maize and clay loam soils, respectively, whereas tillage practices had little impact on yield-scaled drainage N losses. To meet the growing challenge of achieving high yields with minimal impacts on water quality, these results suggest that evaluating drainage N losses on a yield-scaled basis may complement the more conventional approach of evaluating N losses on an areas basis for maize systems in this region.

•We examined the short-term effects of biochar and NH4+-N addition on acid soils.•Application of biochar enhanced nitrification and restored nitrifier activity.•Ammonia-oxidizing bacteria may play a key role in biochar-enhanced nitrification.•Biochar greatly reduced N2O emissions under our short-term incubation conditions.Nitrification rates in Oxisols vary with soil pH and substrate availability. Biochar can be used to improve acid soils. The aim of this study was therefore to investigate the interactive impacts of 1% and 5% (w/w) rice-straw biochar application on nitrification, ammonia oxidizer populations and nitrous oxide (N2O) emissions over short periods of microcosm incubation in two agricultural Oxisols derived from granite (RGU) and tertiary red sandstone (RTU), respectively. We measured soil nitrate (NO3−) and ammonium (NH4+) concentrations during the incubation and used nitrification kinetic model to assess the response of nitrification to biochar addition. We also performed real-time quantitative polymerase chain reaction (qPCR) to quantify the copies of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) genes, and collected N2O gas at various intervals during the 56-day incubation. The addition of ammonium sulfate ((NH4)2SO4-N) stimulated nitrification in both soils. In RGU, biochar treatments altered soil nitrification patterns to a first-order reaction model; this stimulation was more pronounced with the increase of biochar application rates. In RTU, 1% biochar treatment increased nitrification rate constants, and 5% biochar treatment altered nitrification patterns from a zero-order to a first-order reaction model. Treating the two soils with 5% biochar rates significantly increased AOB gene copy numbers up to 7.88- and 14-fold compared with the no biochar controls in RGU and RTU, respectively, while the treatments had little or reduced effect on AOA gene copy numbers. Biochar addition significantly reduced cumulative N2O emissions up to 37.6% in RGU and 46.4% in RTU, respectively. These results underscore the potential of biochar in the restoration of nitrification and the reduction of greenhouse gas N2O emission in Oxisols.

The effects on nitrification and acidification in three subtropical soils to which (NH4)2SO4 or urea had been added at rate of 250 mg N kg−1 was studied using laboratory-based incubations. The results indicated that NH4+ input did not stimulate nitrification in a red forest soil, nor was there any soil acidification. Unlike red forest soil, (NH4)2SO4 enhanced nitrification of an upland soil, whilst urea was more effective in stimulating nitrification, and here the soil was slightly acidified. For another upland soil, NH4+ input greatly enhanced nitrification and as a result, this soil was significantly acidified. We conclude that the effects of NH4+ addition on nitrification and acidification in cultivated soils would be quite different from in forest soils. During the incubation, N isotope fractionation was closely related to the nitrifying capacity of the soils.

A laboratory-based aerobic incubation was conducted to investigate nitrogen (N) isotopic fractionation related to nitrification in five agricultural soils after application of ammonium sulfate ((NH4)2SO4). The soil samples were collected from a subtropical barren land soil derived from granite (RGB), three subtropical upland soils derived from granite (RQU), Quaternary red earth (RGU), Quaternary Xiashu loess (YQU) and a temperate upland soil generated from alluvial deposit (FAU). The five soils varied in nitrification potential, being in the order of FAU > YQU > RGU > RQU > RGB. Significant N isotopic fractionation accompanied nitrification of NH+4. δ15N values of NH+4 increased with enhanced nitrification over time in the four upland soils with NH+4 addition, while those of NO+4 decreased consistently to the minimum and thereafter increased. δ15N values of NH+4 showed a significantly negative linear relationship with NH+4-N concentration, but a positive linear relationship with NO+4-N concentration. The apparent isotopic fractionation factor calculated based on the loss of NH+4 was 1.036 for RQU, 1.022 for RGU, 1.016 for YQU, and 1.020 for FAU, respectively. Zero- and first-order reaction kinetics seemed to have their limitations in describing the nitrification process affected by NH+4 input in the studied soils. In contrast, N kinetic isotope fractionation was closely related to the nitrifying activity, and might serve as an alternative tool for estimating the nitrification capacity of agricultural soils.