Effects of soil moisture on gross N transformations and N2O emission in acid subtropical forest soils

Soil moisture changes, arising from seasonal variation or from global climate changes, could influence soil nitrogen (N) transformation rates and N availability in unfertilized subtropical forests. A ¹⁵?N dilution study was carried out to investigate the effects of soil moisture change (30–90 % water-holding capacity (WHC)) on potential gross N transformation rates and N₂O and NO emissions in two contrasting (broad-leaved vs. coniferous) subtropical forest soils. Gross N mineralization rates were more sensitive to soil moisture change than gross NH₄ ⁺ immobilization rates for both forest soils. Gross nitrification rates gradually increased with increasing soil moisture in both forest soils. Thus, enhanced N availability at higher soil moisture values was attributed to increasing gross N mineralization and nitrification rates over the immobilization rate. The natural N enrichment in humid subtropical forest soils may partially be due to fast N mineralization and nitrification under relatively higher soil moisture. In broad-leaved forest soil, the high N₂O and NO emissions occurred at 30 % WHC, while the reverse was true in coniferous forest soil. Therefore, we propose that there are different mechanisms regulating N₂O and NO emissions between broad-leaved and coniferous forest soils. In coniferous forest soil, nitrification may be the primary process responsible for N₂O and NO emissions, while in broad-leaved forest soil, N₂O and NO emissions may originate from the denitrification process.     

Tea plantation destroys soil retention of NO3− and increases N2O emissions in subtropical China

The intensive conversion from woodland to tea plantation in subtropical China might significantly change the potential supply processes and cycling of inorganic Nitrogen (N). However, few studies have been conducted to investigate the internal N transformations involved in the production and consumption of inorganic N and N₂O emissions in subtropical soils under tea plantations. In a ¹⁵N tracing experiment, nine tea fields with different plantation ages (1-y, 5-y and 30-y) and three adjacent woodlands were sampled to investigate changes in soil gross N transformation rates in humid subtropical China. Conversion of woodland to tea plantation significantly altered soil gross N transformation rates. The mineralization rate (M_Norg) was much lower in soils under tea plantation (0.53–0.75 mg N kg⁻¹ d⁻¹) than in soil sampled from woodland (1.71 mg N kg⁻¹ d⁻¹), while the biological inorganic N supply (INS), defined as the sum of organic N mineralized into NH₄⁺ (M_Norg) and heterotrophic nitrification (O_Nrec), was not significantly different between soils under woodland and tea plantation, apart from soil under 30-y tea plantation which had the largest INS. Interestingly, the contribution of O_Nrec to INS increased from 19.6% in soil under woodland to 65.0–82.4% in tea-planted soils, suggesting O_Nrec is the dominant process producing inorganic N in tea-planted soils. Meanwhile, the conversion from woodland to tea plantation destroyed soil NO₃⁻ retention by increasing O_Nrec, autotrophic nitrification (O_NH4) and abiotic release of stored NO₃⁻ while decreasing microbial NO₃⁻ immobilization (I_NO3), resulting in greater NO₃⁻ production in soil. In addition, long-term tea plantation significantly enhanced the potential release of N₂O. Soil C/N was positively correlated with M_Norg and I_NO3, suggesting that an increase in soil C/N from added organic materials (e.g. rice hull) is likely to reduce the increased production of NO₃⁻ in the soils under tea plantation.     

中国亚热带丘陵区马尾松林氧化亚氮排放

The forest ecosystem plays a pivotal role in contributing greenhouse gases to the atmosphere.In order to characterize the temporal pattern of nitrous oxide(N_2O) emissions and identify the key factors affecting N_2O emissions from a Masson pine forest in a hilly red-soil region in subtropical central China,we measured the N_2O emissions in Jinjing of Hunan Province using the static chambergas chromatographic method for 3 years(2010-2012) and analyzed the relationships between the N_2O fluxes and the environmental variables.Our results revealed that the N_2O fluxes over the 3 years varied from-36.0 to 296.7 μg N m~(-2) h~(-1),averaging 18.4±5.6 μg N m~(-2) h~(-1)(n=3).The average annual N_2O emissions were estimated to be 1.6±0.3 kg N ha~(-1) year~(-1).The N_2O fluxes exhibited clear intra-annual(seasonal) variations as they were higher in summers and lower in winters.Compared with other forest observations in the subtropics,N_2O emissions at our site were relatively high,possibly due to the high local dry/wet N deposition,and were mostly sensitive to variations in precipitation and soil ammonium N content.In this work,a multiple linear regression model was developed to determine the influence of environmental factors on N_2O emissions,in which a category predictor of "Season" was intentionally used to account for the seasonal variation of the N_2O fluxes.Such a model explained almost 40%of the total variation in daily N_2O emissions from the Masson pine forest soil studied(P0.001).     

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.     

Earthworm-induced N2O emissions in a sandy soil with surface-applied crop residues

Earlier research with endogeic and epigeic earthworm species in loamy arable soil has shown that both earthworm groups can increase nitrous oxide (N₂O) emissions, provided that crop residue placement matches the feeding strategy of the earthworm ecological group(s). However, it is not yet clear whether these effects also occur in sandy soils which typically contain less soil organic matter and have low soil aggregation levels. Here, we aimed to quantify N₂O emissions as affected by endogeic and/or epigeic earthworm species, and to relate changes in N₂O emissions to earthworm-induced changes in soil properties in a sandy soil. A 90 day mesocosm study was conducted with sandy soil and ¹⁵N-labeled radish (Raphanus sativus cv. Adagio L.) residue applied on top. Treatments included: (i) no earthworm addition, (ii) addition of the endogeic species Aporrectodea caliginosa (Savigny), (iii) addition of the epigeic species Lumbricus rubellus (Hoffmeister), and (iv) both species combined. An additional treatment was included without earthworms and with residue manually incorporated into the soil. L. rubellus significantly increased cumulative N₂O emissions from 228 to 859 μg N₂O–N kg^?1 (F_1,12 = 83.12, P < 0.001), whereas A. caliginosa did not affect N₂O emissions. In contrast to earlier studies in loamy soil, no positive interaction between both species with regard to N₂O emissions was found. This was probably related to high competition for organic resources in the relatively poor soil and a low potential for stable soil aggregate formation (and associated anaerobic microsites) by endogeic worms in sandy soil. ¹⁵N isotope analysis revealed that the activity of L. rubellus significantly increased (F_1,12 = 6.20, P = 0.028) the recovery of ¹⁵N in the 250–8000 μm size fraction, indicating incorporation of crop residues into the mineral soil. When residues were manually incorporated, N₂O emissions were significantly (P < 0.008) lower (509 μg N₂O–N kg^?1) than when incorporated by L. rubellus. The high N₂O emissions in the presence of L. rubellus, when compared to manual mixing, suggest a stimulation of microbial activity and/or changes in the microbial community composition. Insights on the earthworm effects on N₂O emission from such soils are discussed.     

Responses of CO2 and N2O emissions from soil-plant systems to simulated warming and acid rain in cropland

Journal of Soils and Sediments - Climate warming is anticipated to change the terrestrial carbon/nitrogen cycle through its impact on the fluxes of greenhouse gases such as CO2 and N2O. This study...     

Gross nitrogen transformations and related N2O emissions in uncultivated and cultivated black soil

A better understanding of the nitrogen (N) cycle in agricultural soils is crucial for developing sustainable and environmentally friendly N fertilizer management and to propose effective nitrous oxide (N₂O) mitigation strategies. This laboratory study quantified gross nitrogen transformation rates in uncultivated and cultivated black soils in Northeast China. It also elucidated the contribution made by nitrification and denitrification to the emissions of N₂O. In the laboratory, soil samples adjusted to 60 % water holding capacity (WHC) were spiked with ¹⁵NH₄NO₃ and NH₄ ¹⁵NO₃ and incubated at 25 °C for 7 days. The size and ¹⁵N enrichment of the mineral N pools and the N₂O emission rates were determined between 0 and 7 days. The results showed that the average N₂O emission rate was 21.6 ng N₂O-N kg^?1 h^?1 in cultivated soil, significantly higher than in the uncultivated soil (11.6 ng N₂O-N kg^?1 h^?1). Denitrification was found to be responsible for 32.1 % of the N₂O emission in uncultivated soil, and the ratio increased significantly to 43.2 % in cultivated soil, due to the decrease in soil pH. Most of the increase in net N₂O-N emissions observed in the cultivated soil was resulting from the increased production of N₂O through denitrification. Gross nitrification rate was significantly higher in the cultivated soil than in the uncultivated soil, and the ratio of gross nitrification rate/ammonium immobilization rate was 6.87 in cultivated soil, much larger than the uncultivated soil, indicating that nitrification was the dominant NH₄ ⁺ consuming process in cultivated soil, and this will lead to the increased production of nitrate, whereas the increased contribution of denitrification to N₂O emission promoted the larger emission of N₂O. This double impact explains why the risk of N loss to the environment is increased by long-term cultivation and fertilization of native prairie sites, and controlling nitrification maybe effective to abate the negative environmental effects.     

Different responses of soil CO2 and N2O emissions to simulated N deposition in forests with divergent N transformation characteristics

CO₂ and N₂O are important greenhouse gases that are related to soil mineralization–immobilization turnover and nitrification. To explore the responses of CO₂ and N₂O emissions to N deposition in forests with different N transformation characteristics, CO₂ and N₂O fluxes were measured in two NH₄NO₃ fertilized plots. One plot was in a temperate pine plantation in Heilongjiang Liangshui National Nature Reserve (LS) with slow and minimally coupled mineralization–immobilization turnover and a high nitrification rate. The other plot was in a subtropical bamboo forest in the Fujian Daiyun Mountain National Nature Reserve (DY) in China with rapid and coupled mineralization–immobilization turnover but a low nitrification rate. The results showed that CO₂ emissions in the DY with a high mineralization rate were greater than those in the LS. Cumulative CO₂ emissions were significantly enhanced by N addition in DY, but in LS, they were not affected. The mean N₂O fluxes in the control were 0.010 and 0.008 mg N m^?2 hr^?1 for LS and DY, respectively. High N addition stimulated N₂O emissions in both LS and DY, but the response ratio for N₂O flux in LS (8.6) was larger than that in DY (2.9). These results suggested that soils with rapid and coupled mineralization–immobilization turnover are beneficial to CO₂ emissions and their positive response to N deposition. A high nitrification rate contributed to high N₂O emissions and the sensitive response of N₂O emissions to N deposition.     

Quantifying soil N pools and N2O emissions after application of chemical fertilizer and straw to a typical chernozem soil

Biology and Fertility of Soils - An incubation experiment with equivalent N rates was conducted for 56&nbsp;days using a typical black soil amended with chemical fertilizer with or without...     

Multistate assessment of wetland restoration on CO2 and N2O emissions and soil bacterial communities

Over the last 200 years, wetlands have been converted to other land uses leading to the loss of approximately 53% of wetlands in the continental United States. In the late 1980's, policies were instated to mitigate further wetland loss through wetland creation and restoration. Restored wetlands provide important ecosystem services, such as filtration of nutrients and wildlife habitat. However, these benefits could be offset by increased greenhouse gas production. We assessed the impact of wetland conversion to agriculture and restoration on CO₂ and N₂O emissions and microbial communities in three land use types: wetlands with native vegetation (natural); wetlands converted to agricultural management (converted); and restored wetlands (restored). Soil properties varied among land use types. Most notably, soils from restored and converted sites had the lowest C and N, and higher pH. Multivariate analysis of soil properties showed the pocosin wetlands in North Carolina separating from all other locations, regardless of land use. Soil bacterial communities showed a similar trend with communities from North Carolina soils separating from the others with no significant effect of land use or season. Furthermore, land use did not have a significant effect on CO₂ or N₂O emissions, although there was significant temporal variation in CO₂ emissions. These findings indicate that while wetland conversion and restoration may alter some soil properties, these alterations do not appear to be great enough to override the underlying geographic and edaphic influences on soil bacterial communities. Furthermore, wetland restoration did not lead to increased N₂O emission at the dates sampled.     

Effect of calcareous and siliceous amendments on N2O emissions of a grassland soil

Liming of acidic agricultural soils has been proposed as a strategy to mitigate nitrous oxide (N₂O) emissions, as increased soil pH reduces the N₂O/N₂ product ratio of denitrification. The capacity of different calcareous (calcite and dolomite) and siliceous minerals to increase soil pH and reduce N₂O emissions was assessed in a 2-year grassland field experiment. An associated pot experiment was conducted using homogenized field soils for controlling spatial soil variability. Nitrous oxide emissions were highly episodic with emission peaks in response to freezing–thawing and application of NPK fertilizer. Liming with dolomite caused a pH increase from 5.1 to 6.2 and reduced N₂O emissions by 30% and 60% after application of NPK fertilizer and freezing–thawing events, respectively. Over the course of the 2-year field trial, N₂O emissions were significantly lower in dolomite-limed than non-limed soil (p < .05), although this effect was variable over time. Unexpectedly, no significant reduction of N₂O emission was found in the calcite treatment, despite the largest pH increase in all tested minerals. We tentatively attribute this to increased N₂O production by overall increase in nitrogen turnover rates (both nitrification and denitrification) following rapid pH increase in the first year after liming. Siliceous materials showed little pH effect and had no significant effect on N₂O emissions probably because of their lower buffering capacity and lower cation content. In the pot experiment using soils taken from the field plots 3 years after liming and exposing them to natural freezing–thawing, both calcite (p < .01) and dolomite (p < .05) significantly reduced cumulative N₂O emission by 50% and 30%, respectively, relative to the non-limed control. These results demonstrate that the overall effect of liming is to reduce N₂O emission, although high lime doses may lead to a transiently enhanced emission.     

不同种植模式对黄壤烟田 N2O排放的影响

为探明烤烟-玉米轮作和烤烟连作在黄壤烟田N_2O排放变化特征,并探明不同环境因子对其排放通量的影响。2016年于贵州省烟草科学研究院龙岗长期养分定位试验站,采用静态箱—气相色谱法测定了N_2O含量,试验共设3个处理,不施任何肥料+轮作(CK);常规施肥+轮作(CR);常规施肥+连作(CC)。试验结果表明,烤烟生长季,各施肥处理的黄壤烟田N_2O排放均有2次高峰,峰值主要出现在烤烟施基肥后3～5 d和追肥后1～7 d,其中施基肥后N_2O排放累积量占整个生长季N_2O排放累积量的比例较高,范围为16.26%～18.30%。各个处理N_2O平均排放通量的顺序是CCCRCK,其平均排放通量分别为0.044、0.022和0.013 mg·m~(-2)·h~(-1);相同施肥量条件下,烤烟连作较轮作N_2O排放总量提高了2倍,烤烟生物量降低了30.01%。相关分析表明,施肥+轮作的烟田N_2O排放通量与土壤硝态氮含量有关(P0.05)。黄壤烟田轮作后显著降低了土壤N_2O排放量,合理轮作是降低黄壤烟田温室气体N_2O排放的重要途径。     

Effect of crop residue returns on N2O emissions from red soil in China

For a long time, farmers in the red soil region of southern China have returned crop residues to the soil, but how various crop residues influence nitrous oxide (N₂O) emissions is not well understood. We compared the influence of returning different crop residues [rapeseed cake (RC), maize straw, rice straw and wheat straw (WS)] in combination with different levels of nitrogen (N) fertilizer (nil, low and high) on red soil N₂O emissions. Results confirmed the inverse relationship between cumulative N₂O emissions and residue C:N ratio in red soil under different levels of N fertilizer. However, N‐fertilizer application did not significantly influence N₂O emissions in the WS (which had the highest C:N ratio) and corresponding control treatments, while it enhanced N₂O emissions in the RC (which had the lowest C:N ratio) treatment and displayed significantly higher cumulative N₂O emissions with low N fertilizer application. This phenomenon may be attributed to the poor nutrient content in red soil, which leads to ‘Liebig's Law of the Minimum’ on available C. N fertilizer application provided sufficient available N, while the readily available C, which was mainly dependent on the degradability of the residue, became the crucial factor influencing N₂O emissions. Additional experiments, which showed that the addition of glucose and sucrose could increase N₂O emissions when N () was sufficient, confirmed this hypothesis. Thus, to reduce N₂O emissions when returning residues to red soil, we suggest that both the residue C:N ratio and the quality should be considered when deciding whether to apply N fertilizer.     

Nitrogen addition change soil N pools with litter removal or not in subtropical forest

ABSTRACT

There are many nitrogen (N) pools in soil, so their availability and different status can give information about bulk soil response to N deposition. However, the different size of N pools in forest soils and the relationship between them have not been well studied under N deposition when considering the role of litter. Here soil in an N-deposition experiment carried out for 5 years in a broad-leaved forest was used as an object to study the response of N pools to N deposition by stepwise extraction using water or solutions containing 0.5 M K₂SO₄, 2.5 M H₂SO₄ (LPI), or 13 M H₂SO₄ (LPII), and calculation of recalcitrant (RC) N pool. Under N control (CT), soil with the presence of litter had a higher N of 23.8–106.8% in the first four pools, but lower of 80.6% in recalcitrant N pool compared with soil with the absence of litter. In the absence of litter, N addition increased soil N in labile pool but decreased N in the RC pool compared to CT and these impacts were greater at high added N (HN) than low-added N (LN) rates. However, in the presence of litter, LN increased the amount of N in the K₂SO₄- extracted pool and HN reduced that in the water extracted pool. Additionally, LN and HN increased TN in the RC pool and HN increased the total soluble N (TSN) in the LPI and LPII pool. N changes in the water extraction pool were attributed to inorganic N, whereas they were NH₄ ⁺ and soluble organic N (SON) in the K₂SO₄-extracted, LPI, and LPII pools. In the presence of litter, HN increased the SON concentration in the K₂SO₄, LPI, and LPII extractions; thus, SON may be a potentially important N form for N availability. These results suggested that N additions improve the accumulation of N in RC pool with the presence of litter. The different effects of N additions on soil N pool or N form in each pool depend on litter present or not.