Response of organic N monomers in a sub-alpine soil to a dry–wet cycle

Cycles of soil drying followed by rewetting occur in most terrestrial ecosystems, but there is conflicting evidence as to the role of osmolytes in dry–wet cycles. The broad aim of this experiment was to determine how N-containing osmolytes and other organic N monomers are affected by rewetting of a moderately dry soil. In a sub-alpine grassland, experimental plots were irrigated with 50 mm of water near the conclusion of a typical late-summer drying cycle. Twelve putative osmolytes (proline, 8 quaternary ammonium compounds, trimethylamine N-oxide, ectoine, hydroxyectoine) and 60 other organic N monomers were identified and quantified by capillary electrophoresis-mass spectrometry of the free/exchangeable pool of soil water (0.5 M K₂SO₄ extracts) and microbial biomass (via chloroform fumigation extraction). The total concentration of organic N monomers was 25-times greater in fumigated than unfumigated extracts. Differences in relative abundance of compound classes and compounds between fumigated and unfumigated extracts suggested some compounds were localized to the free/exchangeable pool; others were predominantly microbial, whereas many were shared between pools. A striking feature of the free/exchangeable pool was that on an N-basis alkylamines were the most abundant compound class and accounted for 34% of the pool of organic N monomers. There was no evidence that osmolytes were the primary means soil microbes coped with dry–wet cycles. Instead, the pool of osmolytes was an invariant 4% of the pool of CE-MS detected monomers in K₂SO₄ extracts and 7% of the pool of CE-MS detected monomers in the chloroform-labile (microbial) fraction. The absence of substantial amounts of osmolytes may be because water stress was too mild or brief, or because osmolyte synthesis was limited by availability of energy, N or C and some alternative strategy was used to cope with water deficits.     

Rapid estimation of microbial biomass in grassland soils by ultra-violet absorbance

A reliable and simple technique for estimating soil microbial biomass (SMB) is essential if the role of microbes in many soil processes is to be quantified. Conventional techniques are notoriously time-consuming and unreproducible. A technique was investigated that uses the UV absorbance at 280 nm of 0.5 M K₂SO₄ extracts of fumigated and unfumigated soils to estimate the concentrations of carbon, nitrogen and phosphorus in the SMB. The procedure is based on the fact that compounds released after chloroform fumigation from lysed microbial cells absorb in the near UV region. Using 29 UK permanent grassland soils, with a wide range of organic matter (2.9–8.0%) and clay contents (22–68%), it was demonstrated that the increase in UV absorbance at 280 nm after soil fumigation was strongly correlated with the SMB C (r=0.92), SMB N (r=0.90) and SMB P (r=0.89), as determined by conventional methods. The soils contained a wide range of SMB C (412–3412 μg g⁻¹ dry soil), N (57–346 μg g⁻¹ dry soil) and P (31–239 μg g⁻¹ dry soil) concentrations. It was thus confirmed that the UV absorbance technique described was a rapid, simple, precise and relatively inexpensive method of estimating soil microbial biomass.     

Fe,Al and Si species and organic matter leached off a ferrallitic and podzolic soil system from Central Amazonia

The geochemistry of soil formation in central Amazonia, Brasil, was investigated by studying the waters draining off small podzolic, ferrallitic or mixed catchment areas. Dissolved, colloidal and particulate fractions were obtained by cascade filtration and tangential-flow filtration. The organic carbon, Fe, Si, Al concentrations and the complexing capacity with regard to Cu²⁺ were determined for each fraction. In the waters draining podzolic areas, bulk concentrations were in the range 25.0–38.1 mg L^− 1 for organic carbon, 240–280 μg L^− 1 for Fe, 130–630 μg L^− 1 for Al and around 0.9 mg L^− 1 for Si. Fe mainly migrates as organo-metallic complexes, while Al migrates roughly half as inorganics in the particulate fraction and half as small species likely inorganic in the dissolved fraction. The result is the leaching of all elements and the relative accumulation of residual quartz. In the waters draining ferrallitic areas, bulk concentrations were in the range 1.2–1.9 mg L^− 1 for organic carbon, 45–55 μg L^− 1 for Fe, 106–220 μg L^− 1 for Al and around 1.9 mg L^− 1 for Si, this later concentration remaining below saturation with quartz. Most elements were transported in the dissolved fraction, except 10% of Si which was in the particulate fraction, likely as quartz, and 40–45% of Al which was in the colloidal fractions, likely as Al-hydroxides. The result is a relative enrichment of the soil in Si with regard to Al. The soils strongly control the physico-chemical characteristics of the forest stream waters, and their transport capacity with regard to complexable metals. Moreover, our results showed that the behaviour of Al with regard to organic matter was different from the behaviour of Fe.     

Comparison of methods for extraction of organic N monomers from soil microbial biomass

One way of investigating the function of soil is via the pool of low molecular weight organic compounds in the soil microbial biomass. This is because low molecular weight organic compounds have key roles in metabolism of soil microbes, can function in osmotic adjustment and other stress responses, and are intermediates in the breakdown of polymers to inorganic nutrients. Methods for measuring low molecular weight microbial metabolites in soil rely upon extracting total metabolites and then subtracting the contribution from metabolites in the soil extracellular matrix (i.e. microbial = total − extracellular). Recent studies have tested methods for extracting organic N monomers from the extracellular matrix of soil, but there has not been similar testing of methods for extracting total organic N monomers. The aims of this study were to examine methods for extracting total organic N monomers by a) contrasting chloroform gas fumigation with chloroform direct extraction, and b) examining whether it is possible to extract soil with two methods that combine quenching of metabolic activity with extraction, namely cold methanol/chloroform/water and hot aqueous ethanol. To evaluate methods, organic N compounds were extracted from soil and then capillary electrophoresis–mass spectrometry identified and quantified 42 organic N monomers including amino acids, quaternary ammonium compounds, nucleobases and nucleosides, amines and polyamines. Absolute concentrations of 32 out of the 42 quantified organic N monomers were significantly different between soil extracted by chloroform gas fumigation and chloroform direct extraction. These differences were probably a function of gains and losses of compounds due to oxidation, hydrolysis and deamidation during the two-day chloroform gas fumigation. Cold methanol/chloroform/water yielded large amounts of the extremely labile compound ergothioneine, probably because the extraction method rapidly quenched metabolic activity. The primary limitation of extraction with methanol/chloroform/water is that it was ineffective at extracting strongly cationic compounds (e.g. polyamines). Extraction with hot aqueous ethanol was unsuccessful with soil presumably because soil microbes are difficult to lyse. It is recommended that future studies examining organic N monomers in soil microbial biomass use chloroform direct extraction or cold methanol/chloroform/water rather than chloroform gas fumigation.     

Wood biochar increases nitrogen retention in field settings mainly through abiotic processes

Nitrogen (N) is an essential element associated with crop yield and its availability is largely controlled by microbially-mediated processes. The abundance of microbial functional genes (MFG) involved in N transformations can be influenced by agricultural practices and soil amendments. Biochar may alter microbial functional gene abundances through changing soil properties, thereby affecting N cycling and its availability to crops. The objective of this study was to assess the effects of wood biochar application on N retention and MFG under field settings. This was achieved by characterising soil labile N and their stable isotope compositions and by quantifying the gene abundance of nifH (nitrogen fixation), narG (nitrate reduction), nirS, nirK (nitrite reduction), nosZ (nitrous oxide reduction), and bacterial and archeal amoA (ammonia oxidation). A wood-based biochar was applied to a macadamia orchard soil at rates of 10 t ha⁻¹ (B10) and 30 t ha⁻¹ (B30). The soil was sampled after 6 and 12 months. The abundance of narG in both B10 and B30 was lower than that of control at both sampling months. Canonical Correspondence Analysis showed that soil variables (including dissolved organic C, NO₃⁻–N and NH₄⁺–N) and sampling time influenced MFG, but biochar did not directly impact on MFG. Twelve months after biochar application, NH₄⁺–N concentrations had significantly decreased in both B10 (4.74 μg g⁻¹) and B30 (5.49 μg g⁻¹) compared to C10 (13.9 μg g⁻¹) and C30 (17.9 μg g⁻¹), whereas NO₃⁻–N concentrations increased significantly in B30 (24.7 μg g⁻¹) compared to B10 (12.7 μg g⁻¹) and control plots (6.18 μg g⁻¹ and 7.97 μg g⁻¹ in C10 and C30 respectively). At month 12, significant δ¹⁵N of NO₃⁻–N depletion observed in B30 may have been caused by a marked increase in NO₃⁻–N availability and retention in those plots. Hence, it is probable that the N retention in high rate biochar plots was mediated primarily by abiotic factors.     

Erosion rates and nutrient losses affected by composted cattle manure application in vineyard soils of NE Spain

Land preparation for mechanisation in vineyards of the Anoia–Alt Penedès region, NE Spain, has required major soil movements, which has enormous environmental implications not only due to changes in the landscape morphology but also due to soil degradation. The resulting cultivated soils are very poor in organic matter and highly susceptible to erosion, which reduces the possibilities of water intake as most of the rain is lost as runoff. In order to improve soil conditions, the application of organic wastes has been generalised in the area, not only before plantation but also every 3–4 years at rates of 30–50 Mg ha^− 1 mixed in the upper 30 cm.These organic materials are important sources of nutrients (N and P) and other elements, which could reduce further fertilisation cost. However, due to the high susceptibility to sealing of these soils, erosion rates are relatively high, so a higher nutrient concentration on the soil surface increases non-point pollution sources due to runoff.The aim of this study is to analyse the influence of applied composted cattle manure on infiltration, runoff and soil losses and on nutrients transported by runoff in vineyards of the Alt Penedès–Anoia region, NE Spain. In the two plots selected for the analysis, composted cattle manure had been applied in alternate rows 1 year previous to the study. In each plot soil surface samples (0–25 cm) were taken and compared to those of plots without manure application. The study was carried out at laboratory scale using simulated rainfall. Infiltration rates were calculated from the difference between rainfall intensity and runoff rates, and the sediment and total nitrogen and phosphorus were measured for each simulation. In addition, the influence of compost was investigated in the field under natural rainfall conditions by analysing the nutrient concentration in runoff samples collected in the field (in the same plots) after seven rainfall events, which amount different total precipitation and had different erosive character.Compost application increases infiltration rates by up to 26% and also increases the time when runoff starts. Sediment concentration in runoff was lower in treated (13.4 on average mg L^− 1) than in untreated soils (ranging from 16.8 to 23.4 mg L^− 1). However, the higher nutrient concentration in soils produces a higher mobilisation of N (7–17 mg L^− 1 in untreated soils and 20–26 mg L^− 1 in treated soils) and P (6–7 mg L^− 1 in untreated soils and 13–19 mg L^− 1 in treated soils). A major part of the P mobilised was attached to soil particles (about 90% on average) and only 10% was dissolved. Under natural conditions, higher nutrient concentrations were always recorded in treated vs. untreated soils in both plots, and the total amount of N and P mobilised by runoff was higher in treated soils, although without significant differences. Nutrient concentrations in runoff depend on rainfall erosivity but the average value in treated soils was twice that in untreated soils for both plots.     

Coarse particulate organic matter is the primary source of mineral N in the topsoil of three beech forests

Forest soils contain a variable amount of organic N roughly repartitioned among particles of different size, microbial biomass and associated with mineral compounds. All pools are alimented by annual litter fall as main input of organic N to the forest floor. Litter N is further subject to mineralization/stabilization recognized as the crucial process for the turnover of litter N. Although it is well documented that different soil types have different soil N stocks, it is presently unknown how different soil types affect the turnover of recent litter N. Here, we compared the potential mineralization of the total soil organic N with that of recent litter-released N in three beech forests varying in their soil properties. Highly ¹⁵N-labelled beech litter was applied to stands located at Aubure, Ebrach, Collelongo, which differ in humus type, soil type and soil chemistry. After 4-5 years of litter decomposition, the upper 3 cm of the organo-mineral A horizon was sampled and the net N mineralization was measured over 112 days under controlled conditions. The origin of mineralized N (litter N versus soil organic N) was calculated using ¹⁵N labeling. In addition, soils were fractionated according to their particle size (>2000 μm, 200-2000 μm, 50-200 μm, <50 μm) and particulate organic matter (POM) was separated from the mineral fraction in size classes, except the <50 μm fraction. Between 41 and 69% of soil organic N was recovered as POM. Litter-released ¹⁵N was mainly to be found in the coarse POM fractions >200 μm. On a soil mass basis, N mineralization was two-fold higher at Aubure and Collelongo than at Ebrach, but, on a soil N basis, N mineralization was the lowest at Collelongo and the highest at Ebrach. On a soil N (or ¹⁵N) basis, mineralization of litter ¹⁵N was two to four-fold higher than mineralization of the average soil N. Furthermore, the δ¹⁵N of the mineral N produced was closer to that of POM than to that of the mineral-bound fraction (<50 μm). Highest rates of ¹⁵N mineralization happened in the soil with the lowest N content, and we found a negative relationship between accumulations of N in the upper A horizon and the mineralization of ¹⁵N from the litter. Our results show that mineral N is preferentially mineralized from POM in the upper organo-mineral soil irrespective of the soil chemistry and that the turnover rate of litter N is faster in soils with a low N content.     

Forms and buffering potential of aluminum in tropical and subtropical acid soils cultivated with Pinus taeda L

Purpose

Several interactions between Al and the solid phase of soil influence Al buffering in soil solution. This work evaluated soils cultivated with Pinus taeda L. to determine Al forms in organic and mineral horizons using various extraction methods and to relate acidity with clay mineralogy.

Materials and methods

Organic and mineral horizons of 10 soil profiles (up to 2.1 m deep) in southern Brazil were sampled. Organic horizons were separated into fresh, aged, and fermented/humified litter. The following Al extraction methods were utilized: 0.5 mol L^?1 pH 2.8 CuCl₂–Al complexed in organic matter; 1.0 mol L^?1 KCl–exchangeable Al; water–Al soluble in soil solution; HF concentrated?+?HNO₃ concentrated?+?H₂O₂ 30% (v/v)–total Al. Six sequential extractions were carried out to isolate different forms of amorphous minerals that can buffer Al on soil solution: 0.05 and 0.1 mol L^?1 sodium pyrophosphate; 0.1 and 0.2 mol L^?1 ammonium oxalate; 0.25 and 0.5 mol L^?1 NaOH. Samples of clay were also analyzed by XRD.

Results and discussion

There was a clear effect of litter age on increasing total Al concentration. In the aged litter and fermented and/or humified litter, levels of total Al were 1.4 to 3.8 and 1.5 to 7.8 times greater than in fresh litter, respectively. The CuCl₂ method had higher Al extraction capacity than the KCl method for litter. The lowest Al–pyrophosphate values were observed in the Oxisol, which also had a predominance of gibbsite and the lowest levels of Al–KCl and Al–CuCl₂. There was an inverse relationship between degree of soil weathering and soluble and exchangeable Al in soils. Available Al increased with higher Si proportion in minerals of the clay fraction (2:1?>?1:1?>?0:1).

Conclusions

The worst scenario was soils with the combination of high soluble and exchangeable Al levels and high concentrations of amorphous forms of Al minerals. The best predictors of Al accumulation in the youngest litter horizon were extractions of amorphous minerals with pyrophosphate and NaOH. These extractors are normally used to predict the level of Al buffering in soils. Organic matter had less influence on Al dynamics in soils.

     

Experimental nitrogen deposition alters the quantity and quality of soil dissolved organic carbon in an alpine meadow on the Qinghai-Tibetan Plateau

Dissolved organic matter (DOM) plays a central role in driving biogeochemical processes in soils, but little information is available on the relation of soil DOM dynamics to microbial activity. The effects of NO₃⁻ and NH₄⁺ deposition in grasslands on the amount and composition of soil DOM also remain largely unclear. In this study, a multi-form, low-dose N addition experiment was conducted in an alpine meadow on the Qinghai–Tibetan Plateau in 2007. Three N fertilizers, NH₄Cl, (NH₄)₂SO₄ and KNO₃, were applied at four rates: 0, 10, 20 and 40 kg N ha⁻¹ yr⁻¹. Soil samples from surface (0–10 cm) and subsurface layers (10–20 cm) were collected in 2011. Excitation/emission matrix fluorescence spectroscopy (EEM) was used to assess the composition and stability of soil DOM. Community-level physiological profile (CLPP, basing on the BIOLOG Ecoplate technique) was measured to evaluate the relationship between soil DOC dynamics and microbial utilization of C resources. Nitrogen (N) dose rather than N form significantly increased soil DOC contents in surface layer by 23.5%–35.1%, whereas it significantly decreased soil DOC contents in subsurface layer by 10.4%–23.8%. Continuous five-year N addition significantly increased the labile components and decreased recalcitrant components of soil DOM in surface layer, while an opposite pattern was observed in subsurface layer; however, the humification indices (HIX) of soil DOM was unaltered by various N treatments. Furthermore, N addition changed the amount and biodegradability of soil DOM through stimulating microbial metabolic activity and preferentially utilizing organic acids. These results suggest that microbial metabolic processes dominate the dynamics of soil DOC, and increasing atmospheric N deposition could be adverse to the accumulation of soil organic carbon pool in the alpine meadow on the Qinghai-Tibetan Plateau.     

Organic nitrogen stimulates the heterotrophic nitrification rate in an acidic forest soil

Many previous studies have demonstrated that heterotrophic nitrification processes play an important role in the production of NO₃⁻ in acidic soils. However, it is not clear whether a low concentration of nitrogenous organic compounds support heterotrophic nitrification processes in natural soils. In this study, we performed an ¹⁵N tracer experiment with a glycine concentration gradient (20, 40, 80, and 160 mg N kg⁻¹) to investigate the effect of the organic nitrogen concentration on the heterotrophic nitrification rate and its relative contribution to the total nitrification of the studied acidic forest soil. This experiment demonstrated that ¹⁵N–NO₃⁻ accumulated over time with all nitrogen treatments in the presence of acetylene, confirming that heterotrophic nitrification occurred even at a low organic nitrogen concentration (20 mg kg⁻¹) in the studied acidic forest soil. In the presence of acetylene, the ¹⁵N–NO₃⁻ concentration in the 20 and 40 mg kg⁻¹ glycine-N treatments was significantly lower than in the 80 and 160 mg kg⁻¹ glycine-N treatments (p < 0.05), indicating that a high organic nitrogen concentration stimulated the heterotrophic nitrification rate. There was no significant difference in the average contribution of heterotrophic nitrification to total nitrification among the different nitrogen treatments, suggesting that the organic nitrogen concentration did not affect the relative contribution of heterotrophic nitrification to total nitrification in the studied acidic soil. Our results confirmed that a low concentration of organic N (20 mg kg⁻¹) supported heterotrophic nitrification in the studied soil. The organic nitrogen concentration stimulates the heterotrophic nitrification rate, but does not affect the relative contribution of heterotrophic nitrification to total nitrification in the studied acidic soil.     

Soil Na+ concentration controls salt-affected soil organic matter components in Hetao region China

Purpose

There is little knowledge on the organic matter fractions of salt-affected soil aggregates. This study aimed at investigating characteristics of salt-affected soil organic carbon components and the relationships between soil salt concentration and soil organic carbon component content.

Materials and methods

Five typical salt-affected soils in Hetao region China were collected and analyzed for light (LF) and heavy fraction (HF) in different water-stable aggregates. And the soil organic carbon components were measured by Fourier transform infrared (FTIR) and pyrolysis-gas chromatography/mass spectrometer (Py–GC/MS).

Results and discussion

The results showed that the salt-affected soils were dominant in 53–10-μm water-stable aggregates, 61–80% in the bulk soil, and very low in >?250-μm macro-aggregates, less than 7.06% in the bulk soil. The proportions of >?250-μm macro-aggregates and the mean weight diameter (MWD) were negatively correlated to Na⁺ concentration (p?<?0.05). Furthermore, the macro-aggregates were generally higher in total organic carbon (TOC) and accordingly higher C/N ratio than those in micro-aggregates. Heavy fractions (HF) from both >?53 μm and <?53-μm soil aggregates accounted for 99.30–99.83% of the bulk soil and contained 89.6–98.5% lower TOC and accordingly 49.2–84.8% lower C/N ratio than those in light fractions (LF). The LFs were high in lignin (7.27–34.02% in total pyrolysis products, 19.89% on average) and alkane/alkene-derived compounds (9.51–37.21%, 23.18% on average), but low in N-containing compounds (0–3.64%, 1.71% on average), while HFs were high in both alkane/alkene (4.38–27.46%, 15.06% on average) and N-containing compounds (7.45–26.45%, 13.98% on average), but low in lignin-derived compounds (1.13–8.75%, 3.86% on average).

Conclusions

The tested salt-affected soils were predominant in 53–10-μm micro-aggregates, which was caused by the Na⁺ dispersion effect on soil aggregates. Most SOM was stored in HF that contained high N-containing compounds and low C/N ratios. Our results suggested that the components of SOM were mainly controlled by the soil Na⁺ concentration.

     

Amino acids and amino sugars extracted by EUF from a sandy soil incubated with green manure,bacterial biomass or cellulose

The extraction of soils by the electro-ultrafiltration (EUF) method yields organic N which has been used as an index for mineralisable N in soils. This EUF extractable organic fraction contains a mixture of various N compounds not yet completely identified. It has been proposed that the amino N compounds are more indicative for the potentially mineralisable N in soils than the total organic N extracted (Mengel et al., 1999). An amendment of soils with easily mineralisable organic matter may, therefore, alter the amino N concentrations of the organic N extracted. Our determination of the amino N compounds aimed to prove this hypothesis. The principle of our experiment was to mix soil with green manure, bacterial biomass and cellulose, respectively, and to incubate the treated soil aerobically for 80 days at 20°C in the laboratory. Control treatments without organic amendment were also incubated. Soil samples were taken several times during the incubation period and analysed for the inorganic N (NO₃⁻-N and NH₄⁺-N) and for the EUF extractable organic N. Amino acids and amino sugars were determined in the hydrolysed EUF extracts. The concentrations of amino acids and amino sugars in the organic N extracted varied with time and differed between the treatments. Glutamic acid has been found to be the most relevant amino acid in the EUF extracts and was particularly indicative for the existence of mineralisable green manure in the soil. Glucosamine was the most relevant amino sugar in the EUF extracts and this amino sugar appears to be indicative for the easily mineralisable relics of microbial cells in the soil.     

Soil organic matter and water-stable aggregates under different tillage and residue conditions in a tropical dryland agroecosystem

Changes in the proportions of water-stable soil aggregates, organic C, total N and soil microbial biomass C and N, due to tillage reduction (conventional, minimum and zero tillage) and crop residue manipulation (retained or removed) conditions were studied in a tropical rice–barley dryland agroecosystem. The values of soil organic C and total N were the highest (11.1 and 1.33 g kg⁻¹ soil, respectively) in the minimum tillage and residue retained (MT+R) treatment and the lowest (7.8 and 0.87 g kg⁻¹, respectively) in conventional tillage and residue removed (CT−R) treatment. Tillage reduction from conventional to minimum and zero conditions along with residue retention (MT+R,ZT+R) increased the proportion of macroaggregates in soil (21–42% over control). The greatest increase was recorded in MT+R treatment and the smallest increase in conventional tillage and residue retained (CT+R) treatment. The lowest values of organic C and total N (7.0–8.9 and 0.82–0.88 g kg⁻¹ soil, respectively) in macro- and microaggregates were recorded in CT−R treatment. However, the highest values of organic C and total N (8.6–12.6 and 1.22–1.36 g kg⁻¹, respectively) were recorded in MT+R treatment. The per cent increase in the amount of organic C in macroaggregates was greater than in microaggregates. In all treatments, macroaggregates showed wider C/N ratio than in microaggregates. Soil microbial biomass C and N ranged from 235 to 427 and 23.9 to 49.7 mg kg⁻¹ in CT−R and MT+R treatments, respectively. Soil organic C, total N, and microbial biomass C and N were strongly correlated with soil macroaggregates. Residue retention in combination with tillage reduction (MT+R) resulted in the greatest increase in microbial biomass C and N (82–104% over control). These variables showed better correlations with macroaggregates than other soil parameters. Thus, it is suggested that the organic matter addition due to residue retention along with tillage reduction accelerates the formation of macroaggregates through an increase in the microbial biomass content in soil.     

Chronic N deposition does not apparently alter the biochemical composition of forest floor and soil organic matter

Future rates of atmospheric N deposition have the potential to slow litter decay and increase the accumulation of soil organic matter by repressing the activity of lignolytic soil microorganisms. We investigated the relationship between soil biochemical characteristics and enzymatic responses in a series of sugar maple (Acer saccharum)-dominated forests that have been subjected to 16 yrs of chronic N deposition (ambient + 3 g NO₃⁻–N m⁻² yr⁻¹), in which litter decay has slowed and soil organic matter has accumulated in sandy spodosols. Cupric-oxide-extractable lignin-derived phenols were quantified to determine the presence, source, and relative oxidation state of lignin-like compounds under ambient and experimental N deposition. Pools of respired C and mineralized N, along with rate constants for these processes, were used to quantify biochemically labile substrate pools during a 16-week laboratory incubation. Extracellular enzymes mediating cellulose and lignin metabolism also were measured under ambient and experimental N deposition, and these values were compared with proxies for the relative oxidation of lignin in forest floor and surface mineral soil. Chronic N deposition had no influence on the pools or rate constants for respired C and mineralized N. Moreover, neither the total amount of extractable lignin (forest floor, P = 0.260; mineral soil, P = 0.479), nor the relative degree of lignin oxidation in the forest floor or mineral soil (forest floor P = 0.680; mineral soil P = 0.934) was influenced by experimental N deposition. Given their biochemical attributes, lignin-derived molecules in forest floor and mineral soil appear to originate from fine roots, rather than leaf litter. Under none of the studied circumstances was the presence or relative oxidation of lignin correlated with the activity of cellulolytic and lignolytic extracellular enzymes. Although chronic atmospheric N deposition has slowed litter decay and increased organic matter in our experiment, it had little effect on biochemical composition of lignin-derived molecules in forest floor and surface mineral soil suggesting organic matter has accumulated by other means. Moreover, the specific dynamics of lignin phenol decay is decoupled from short-term organic matter accumulation under chronic N deposition in this ecosystem.     

Dissolved organic carbon and nitrogen in precipitation,throughfall, soil solution,and stream water of the tropical highlands in northern Thailand

Dissolved organic matter (DOM) is important for the cycling and transport of carbon (C) and nitrogen (N) in soil. In temperate forest soils, dissolved organic N (DON) partly escapes mineralization and is mobile, promoting loss of N via leaching. Little information is available comparing DOC and DON dynamics under tropical conditions. Here, mineralization is more rapid, and the demand of the vegetation for nutrients is larger, thus, leaching of DON could be small. We studied concentrations of DOC and DON during the rainy seasons 1998–2001 in precipitation, canopy throughfall, pore water in the mineral soil at 5, 15, 30, and 80 cm depth, and stream water under different land‐use systems representative of the highlands of northern Thailand. In addition, we determined the distribution of organic C (OC) and N (ON) between two operationally defined fractions of DOM. Samples were collected in small water catchments including a cultivated cabbage field, a pine plantation, a secondary forest, and a primary forest. The mean concentrations of DOC and DON in bulk precipitation were 1.7 ± 0.2 and 0.2 ± 0.1 mg L^–1, respectively, dominated by the hydrophilic fraction. The throughfall of the three forest sites became enriched up to three times in DOC in the hydrophobic fraction, but not in DON. Maximum concentrations of DOC and DON (7.9–13.9 mg C L^–1 and 0.9–1.2 mg N L^–1, respectively) were found in samples from lysimeters at 5 cm soil depth. Hydrophobic OC and hydrophilic ON compounds were released from the O layer and the upper mineral soil. Concentrations of OC and ON in mineral‐soil solutions under the cabbage cultivation were elevated when compared with those under the forests. Similar to most temperate soils, the concentrations in the soil solution decreased with soil depth. The reduction of OC with depth was mainly due to the decrease of hydrophobic compounds. The changes in OC indicated the release of hydrophobic compounds poor in N in the forest canopy and the organic layers. These substances were removed from solution during passage through the mineral soil. In contrast, organic N related more to labile microbial‐derived hydrophilic compounds. At least at the cabbage‐cultivation site, mineralization seemed to contribute largely to the decrease of DOC and DON with depth, possibly because of increased microbial activity stimulated by the inorganic‐N fertilization. Similar concentrations and compositions of OC and ON in subsoils and streams draining the forested catchments suggest soil control on stream DOM. The contribution of DON to total dissolved N in those streams ranged between 50% and 73%, underscoring the importance of DOM for the leaching of nutrients from forested areas. In summary, OC and ON showed differences in their dynamics in forest as well as in agricultural ecosystems. This was mainly due to the differing distribution of OC and ON between the more immobile hydrophobic and the more easily degradable hydrophilic fraction.     

Responses of enzymatic activities within soil aggregates to 9-year nitrogen and water addition in a semi-arid grassland

Soil microorganisms secrete enzymes used to metabolize carbon (C), nitrogen (N), and phosphorus (P) from the organic materials typically found in soil. Because of the connection with the active microbial biomass, soil enzyme activities can be used to investigate microbial nutrient cycling including the microbial response to environmental changes, transformation rates and to address the location of the most active biomass. In a 9-year field study on global change scenarios related to increasing N inputs (ambient to 15 g N m⁻² yr⁻¹) and precipitation (ambient to 180 mm yr⁻¹), we tested the activities of soil β-glucosidase (BG), N-acetyl-glucosaminidase (NAG) and acid phosphomonoesterase (PME) for three soil aggregate classes: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm) and microaggregates (<250 μm). Results showed higher BG and PME activities in micro-vs. small macroaggregates whereas the highest NAG activity was found in the large macroaggregates. This distribution of enzyme activity suggests a higher contribution of fast-growing microorganisms in the micro-compared with the macroaggregates size fractions. The responses of BG and PME were different from NAG activity under N addition, as BG and PME decreased as much as 47.1% and 36.3%, respectively, while the NAG increased by as much as 80.8%, which could imply better adaption of fungi than bacteria to lower soil pH conditions developed under increased N. Significant increases in BG and PME activities by as much as 103.4 and 75.4%, respectively, were found under water addition. Lower ratio of BG:NAG and higher NAG:PME underlined enhanced microbial N limitation relative to both C and P, suggesting the repression of microbial activity and the accompanied decline in their ability to compete for N with plants and/or the accelerated proliferation of soil fungi under elevated N inputs. We conclude that changes in microbial activities under increased N input and greater water availability in arid- and semi-arid grassland ecosystems where NPP is co-limited by N and water may result in substantial redistribution of microbial activity in different-sized soil particles. This shift will influence the stability of SOM in the soil aggregates and the nutrient limitation of soil biota.     

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.     

Glomalin-related soil protein responses to elevated CO2 and nitrogen addition in a subtropical forest: Potential consequences for soil carbon accumulation

According to the economy theory, plants should preferentially allocate photosynthate to acquire below-ground resources under elevated atmospheric carbon dioxide (eCO₂) but decrease below-ground C allocation when nitrogen (N) is sufficient for plant growth. Arbuscular mycorrhizae (AM) represent a critical mechanism of below-ground nutrient acquisition for plants. The dynamics of arbuscular mycorrhizal fungi (AMF) could therefore reflect the response of plant C allocation under eCO₂ and N addition. We examined the responses of glomalin-related soil protein (GRSP) to eCO₂ (approximately 700 μmol mol⁻¹ CO₂) and/or N addition (100 kg N ha⁻¹ yr⁻¹ as NH₄NO₃) in a modeled subtropical forest to better understand its potential influence on soil C storage. We hypothesized that GRSP would increase under eCO₂ and decrease under N addition. Furthermore, the positive effects of eCO₂ on GRSP would be offset by extra N addition, and GRSP would remain unchanged under combined eCO₂ and N addition. Our results showed that the mean concentrations of easily extractable GRSP (EE-GRSP) and total GRSP (T-GRSP) were 0.35 ± 0.05 and 0.72 ± 0.13 mg C cm⁻³, respectively, which accounted for 2.76 ± 0.53% and 5.67 ± 0.92% of soil organic carbon (SOC) in the 0–10 cm soil layer. Elevated CO₂ significantly increased T-GRSP by 35.02% but decreased EE-GRSP by 5.09% in the top 10 cm soil layer. The opposite responses of T-GRSP and EE-GRSP to eCO₂ might result from an unchanged photosynthate investment to AMF with possible changes in their decomposition rates. The effect of N on GRSP was contrary to our hypothesis, i.e., there was a 1.72%–48.49% increase in T-GRSP and a slightly increase in EE-GRSP. Both EE-GRSP and T-GRSP concentrations increased under the combination of eCO₂ and N addition, which was inconsistent with our hypothesis. The significant increase of EE-GRSP under the combination of eCO₂ and N addition was partly caused by more rapid plant growth and reduced microbial diversity, and the marginal increase of T-GRSP indicated that the interaction between eCO₂ and N addition offset their independent effects. In addition, the relatively higher accumulation ratios of GRSP (22.6 ± 13.6%) compared with SOC (15.9 ± 9.4%) indicated that more rapid GRSP deposition in the soil might accelerate SOC accumulation under eCO₂ and N addition. Our results will improve the understanding of the functioning of GRSP in soil C sequestration under global environmental change scenarios.     

Spatial and seasonal variations of atmospheric N2O and CO2 fluxes from a subtropical mangrove swamp and their relationships with soil characteristics

Marine ecosystems are a known net source of greenhouse gases emissions but the atmospheric gas fluxes, particularly from the mangrove swamps occupying inter-tidal zones, are characterized poorly. Spatial and seasonal fluxes of nitrous oxide (N₂O) and carbon dioxide (CO₂) from soil in Mai Po mangrove swamp in Hong Kong, South China and their relationships with soil characteristics were investigated. The N₂O fluxes averaged from 32.1 to 533.7 μg m⁻² h⁻¹ and the CO₂ fluxes were between 10.6 and 1374.1 mg m⁻² h⁻¹. Both N₂O and CO₂ fluxes in this swamp showed large spatial and seasonal variations. The fluxes were higher at the landward site than the foreshore bare mudflat, and higher fluxes were recorded in warm, rather than cold, seasons. The landward site had the highest content of soil organic carbon (OC), total Kjeldahl nitrogen (TKN), nitrate (NO₃⁻–N) and total phosphorus (TP), while the bare mudflat had the highest ammonium nitrogen (NH₄⁺–N) concentration and soil denitrification potential activity. The N₂O flux was related, positively, to CO₂ flux. Soil NO₃⁻–N and TP increased N₂O flux, while soil OC and TP concentrations contributed to the CO₂ flux. The results indicated that the Mai Po mangrove swamp emitted significant amounts of greenhouse gases, and the N₂O emission was probably due to soil denitrifcation.     

Threshold concentration of glucose for bacterial growth in soil

The activity of heterotrophic soil microorganisms is usually limited by the availability and quality of carbon (C). Adding organic substances will thus trigger a microbial response. We studied the response in bacterial growth and respiration after the addition of low amounts of glucose. First we determined if additions of glucose, at concentrations which did not result in an exponential increase in respiration after the lag phase, still stimulated bacterial growth. The second aim was to determine the threshold concentration of glucose needed to induce bacterial growth. Adding glucose-C at 1000 μg g⁻¹ soil resulted in an increased respiration rate, which was stable during 12 h, and then decreased without showing any exponential increase in respiration. Bacterial growth, determined as leucine incorporation, did not change compared to an unamended control during the first 12 h, but then increased to levels 5 times higher than in the control. Thus, after the lag phase, a period with increasing bacterial growth, but at the same time decreasing respiration rates, was found. Similar results, but with a more modest increase in bacterial growth, were found using 500 μg glucose-C g⁻¹ soil. Adding 50–700 μg glucose-C g⁻¹ resulted in increased respiration during 24 h correlating with the addition rate. In contrast, bacterial growth after 24 h was only stimulated by glucose additions >200 μg C g⁻¹ soil. Thus, there was a threshold concentration of added substrate for inducing bacterial growth. Below the threshold concentration growth and respiration appear to be uncoupled.