Quantitative determination of hydroxycinnamic acids in wheat, rice, rye, and barley straws, maize stems, oil palm frond fiber, and fast-growing poplar wood.

A new method has been developed for the quantitative determination of hydroxycinnamic acids participating in ester or ether linkages to the cell wall polymers. The method is based on mild alkaline hydrolysis followed by acid hydrolysis or mild alkaline hydrolysis, which partially removed esterified phenolic acids, and high-temperature concentrated alkaline treatment, which cleaved both the ester and ether linkages. It was found that traditional mild alkaline hydrolysis and acid hydrolysis released only part of the ester- and ether-linked phenolic acids, respectively. Approximately half (44.0-47.9%) of the total ester-linked p-coumaric acid and 18.2-32.6% of the total esterified ferulic acid remained ester-linked to the mild alkali-soluble lignin polymers, and 55.0-72.0% of the total ether-linked p-coumaric acid and 37.5-53.8% of the total ether-linked ferulic acid remained ether-linked to the solubilized lignin molecules after the acid hydrolysis. To correct this, a second mild alkaline hydrolysis of the alkali-soluble lignin preparations and acid hydrolysis of the solubilized lignin fractions, obtained from the first acid hydrolysis of the cell wall materials, was investigated. On the basis of this new method, a majority of the cell wall p-coumaric acid (55.8-81.5%) was found to be ester-linked to cell wall components, mainly to lignin, and about half of the cell wall ferulic acid is etherified through its phenolic oxygen to the cell wall lignin component, whereas the remainder is esterified to the cell wall hemicelluloses and/or lignin in different plant materials.     

Impact of lignin structure and cell wall reticulation on maize cell wall degradability

In this study, eight maize recombinant inbred lines were selected to assess both the impact of lignin structure and the impact of cell wall reticulation by p-hydroxycinnamic acids on cell wall degradability independently of the main "lignin content" factor. These recombinant lines and their parents were analyzed for in vitro degradability, cell wall residue content, esterified and etherified p-hydroxycinnamic acid content, and lignin content and structure. Lignin structure and esterified p-coumaric acid content showed significantly high correlation with in vitro degradability (r=-0.82 and r=-0.72, respectively). A multiple regression analysis showed that more than 80% of cell wall degradability variations within these 10 lines (eight recombinant inbred lines and their two parents) were explained by a regression model including two main explanatory factors: lignin content and estimated proportion of syringyl lignin units esterified by p-coumaric acid. This study revealed new biochemical parameters of interest to improve cell wall degradability and promote lignocellulose valorization.     

Chemical composition of cover crops and soil organic matter pools in no-tillage systems in the Cerrado

In no-tillage systems (NTS), cover crops are recommended to increase the productivity of agricultural systems. Furthermore, a greater diversity of cover crops in NTS favours an increase in soil carbon (C) stocks. However, there are scarce published data on the relationship between the chemical composition of cover crops and the accumulation of labile and stable fractions of SOM. We evaluated the relationship between the chemical composition of cover crops and SOM fractions, C stocks and maize yield. Hemicellulose, cellulose and lignin contents were determined for Urochloa ruziziensis, Canavalia brasiliensis, Cajanus cajan and Sorghum bicolor, cultivated in the off-season of maize. Canavalia brasiliensis had high N (20.96 g kg⁻¹) and hemicellulose (185.67 g kg⁻¹) contents, lower lignin content (39.50 g kg⁻¹) and high dry matter yield (3,251 kg ha⁻¹). All these characteristics resulted in a better SOM quality. Urochloa ruziziensis, with higher hemicellulose and lower lignin contents, and low lignin/N ratio, was associated with accumulation of TOC (19.95 and 18.33 g kg⁻¹ in 0- to 10-cm and 10- to 20-cm layers, respectively) and mineral-associated organic C (on average, 16.68 g kg⁻¹) in the soil. Cover plants with N:lignin ratio lower than 2.0 are fundamental for soil C sequestration. In conclusion, it is recommended the adoption of Urochloa ruziziensis and Canavalia brasiliensis as cover plants improve maize production, soil organic matter quality and C sequestration in the Cerrado region.     

Plant lignin and nitrogen contents control carbon dioxide production and nitrogen mineralization in soils incubated with Bt and non-Bt corn residues

Bt (Bacillus thuringiensis) corn is reported to produce lignin-rich residues, compared to non-Bt (NBt) corn, suggesting it is more resistant to decomposition. As the Bt gene is expressed selectively in stem and leaf tissue, it could affect lignin distribution in corn, which naturally has greater lignin content in roots than in stems and leaves. Our objective was to evaluate the effects of corn plant components, the Bt gene and elevated-lignin inputs on decomposition. Roots, stems and leaves from Bt corn and NBt corn isolines enriched with ¹³C and ¹⁵N were finely ground and mixed separately with soil, then incubated at 20 °C for 36 weeks. The effect of elevated lignin on decomposition was tested by adding a commercial lignin source (indulin lignin) to half of the samples. In addition to weekly CO₂ analysis and regular measurement of N mineralization, the degree of lignin degradation was evaluated at 1 and 36 weeks from the acid to aldehyde ratio (Ad/Al) of vanillyl and syringyl lignin-derived phenols. The CO₂ production and N mineralization was lower in root-amended soils than stem- and leaf-amended soils. The Bt genetic modification increased CO₂ production from stem-amended soils (P < 0.05) and decreased N mineralization in root-amended soils. The ¹³C and ¹⁵N results also showed more residue-C and -N retained in soils mixed with NBt stem residues. After 36 weeks leaf- and stem-amended soils with indulin lignin had a lower Ad/Al ratio and were less degraded than soils without exogenous lignin. In conclusion, plant lignin and nitrogen contents were good predictors of CO₂ production and N mineralization potential. Corn roots decomposed more slowly than aboveground components emphasizing the importance of recalcitrant root residues in sustaining the organic matter content of soil.     

Soil biodegradation of maize root residues: Interaction between chemical characteristics and the presence of colonizing micro-organisms

Due to their direct contact with the soil, roots are exposed to colonizing micro-organisms that persist after the plant has died. These micro-organisms may affect intrinsic root-chemical quality and the kinetics of root residue decomposition in soil, or interact with soil micro-organisms during the decomposition process. The aims in this work were i) to determine the interactions between the presence of root-colonizing micro-organisms and root-chemical quality and ii) to quantify the effect of these micro-organisms on root decomposition. Roots were selected from six maize genotypes cultivated in the field and harvested at physiological maturity. The roots of two genotypes (F2 and F2bm1) had a higher N content, lower neutral sugars content and higher Klason lignin content than the other genotypes (F292, F292bm3, Mexxal, Colombus). Location of the root residue micro-organisms by scanning electron microscopy and transmission electron microscopy revealed that F2 and F2bm1 roots were more colonized than roots of the other genotypes. Electron Dispersive X-Ray microanalyses of in situ N confirmed a higher N content in the colonizing micro-organisms than in the root cell walls. Residues of F2 and F2bm1 roots decomposed more slowly and to a lesser extent than those of the other genotypes during incubation in a silty loam soil under controlled conditions (15 °C, −80 kPa). After 49 days, 40.6% of the total C from F292 was mineralized but only 20.7% of from F2bm1. These results suggest that residue-colonizing micro-organisms decompose the cell-wall sugars to varying extents before soil decomposition thereby modifying the chemical quality of the residues and their mineralization pattern in soil. Due to their high N content, colonizing micro-organisms also impact on the total N content of root residues, reducing their C to N ratio. Gamma sterilized root residues were incubated under the same conditions as non-sterilized residues to see if micro-organisms colonizing root residues could modify the action of soil micro-organisms during decomposition. Similar C mineralization rates were observed for both non-sterilized and sterilized residues, indicating that the residue micro-organisms did not quantitatively affect the activity of soil micro-organisms.     

Understanding the degree of condensation of phenolic and etherified C-9 units of in situ lignins

A novel approach for the quantification of the degree of condensation at the C(5) position of etherified and phenolic phenylpropane (C-9) units of in situ lignin is described. This is achieved by degrading unmethylated and methylated wood by thioacidolysis and analyzing the resultant product mixtures by quantitative (31)P NMR spectroscopy. Applying this new method to compression wood and normal wood from Pinus radiata showed that, whereas 41-47% of etherified guaiacyl C-9 units are condensed at the C(5) position, almost all phenolic guaiacyl C-9 units exist as uncondensed moieties. Analysis of milled wood lignin (MWL) isolated from the same wood by (31)P NMR spectroscopy before and after thioacidolysis showed that the phenolic guaiacyl C-9 units were more condensed than those in the in situ lignin. This is likely due to partial cleavage of the more condensed etherified linkages during the lignin isolation, leading to a relative increase in condensed phenolic guaiacyl C-9 units.     

Rhizodeposition of maize: Short‐term carbon budget and composition

The aim of this study was to assess differences in rhizodeposition quantity and composition from maize cropped on soil or on 1:1 (w/w) soil–sand mixture and distribution of recently assimilated C between roots, shoots, soil, soil solution, and CO₂ from root respiration. Maize was labeled in ¹⁴CO₂ atmosphere followed by subsequent simultaneous leaching and air flushing from soil. ¹⁴C was traced after 7.5 h in roots and shoots, soil, soil solution, and soil‐borne CO₂. Rhizodeposits in the leachate of the first 2 h after labeling were identified by high‐pressure liquid chromatography (HPLC) and pyrolysis–field ionization mass spectrometry (Py‐FIMS). Leachate from soil–sand contained more ¹⁴C than from soil (0.6% vs. 0.4%) and more HPLC‐detectable carboxylates (4.36 vs. 2.69 μM), especially acetate and lactate. This is either because of root response to lower nutrient concentrations in the soil–sand mixture or decreasing structural integrity of the root cells during the leaching process, or because carboxylates were more strongly sorbed to the soil compared to carbohydrates and amino acids. In contrast, Py‐FIMS total ion intensity was more than 2 times higher in leachate from soil than from soil–sand, mainly due to signals from lignin monomers. HPLC‐measured concentrations of total amino acids (1.33 μM [soil] vs. 1.03 μM [soil–sand]) and total carbohydrates (0.73 vs. 0.34 μM) and ¹⁴CO₂ from soil agreed with this pattern. Higher leachate concentrations from soil than from soil–sand for HPLC‐measured carbohydrates and amino acids and for the sum of substances detected by Py‐FIMS overcompensated the higher sorption in soil than in sand‐soil. A parallel treatment with blow‐out of the soil air but without leaching indicated that nearly all of the rhizodeposits in the treatment with leaching face decomposition to CO₂. Simultaneous application of three methods—¹⁴C‐labeling and tracing, HPLC, and Py‐FIMS—enabled us to present the budget of rhizodeposition (¹⁴C) and to analyze individual carbohydrates, carboxylates, and amino acids (HPLC) and to scan all dissolved organic substances in soil solution (Py‐FIMS) as dependent on nutrient status.     

Influence of non-cellulose structural carbohydrate composition on plant material decomposition in soil

The C mineralisation pattern during the early stage of decomposition of plant materials is largely determined by their content of different carbohydrates. This study investigated whether detailed plant analysis could provide a better prediction of C mineralisation during decomposition than proximate analysis [neutral detergent solution (NDF)/acid detergent solution (ADF)]. The detailed analysis included sugars, fructans, starch, pectin, cellulose, lignin and organic N. To determine whether differences in decomposition rate were related to differences in hemicellulose composition, the analysis particularly emphasised the concentrations of arabinose and xylose in hemicelluloses. Carbon dioxide evolution was monitored hourly in soil amended with ten different plant materials. Principal component and regression analysis showed that C mineralisation during day 1 was closely related to free sugars, fructans and soluble organic N components (R ² = 0.83). The sum of non-cellulose structural carbohydrates (intermediate NDF/ADF fraction) was not related to C mineralisation between days 1 and 9. In contrast, a model including starch and protein in addition to the non-cellulose structural carbohydrates, with the hemicelluloses replaced by arabinose and xylose, showed a strong relationship with evolved CO₂ (R ² = 0.87). Carbon mineralisation between days 9 and 34 was better explained by xylan, cellulose and lignin (R ² = 0.72) than by lignocellulose in the ADF fraction. Our results indicated that proximate analyses were not sufficient to explain differences in decomposition. To predict C mineralisation from the range of plant materials studied, we propose a minimum set of analyses comprising total N, free sugars, starch, arabinose, xylan, cellulose and lignin.     

镉安全水稻材料根系细胞壁果胶对镉的响应特征

【目的】研究镉(Cd)处理下水稻根系细胞壁果胶对Cd胁迫的响应,进一步深化Cd安全水稻材料根系细胞壁Cd的固持机制。【方法】以Cd安全水稻材料D62B为研究对象,普通材料Luhui17为对照进行水培试验。设4个Cd质量浓度处理：0 mg/L (CK)、0.5 mg/L (Cd0.5)、1.0 mg/L (Cd1)、2.0 mg/L (Cd2)。在水稻分蘖期采集根系样品,分析细胞壁多糖中果胶、半纤维1、半纤维2以及残渣部分的Cd含量,测定果胶糖醛酸含量、果胶酯化度、果胶甲酯酶(PME)活性、根系过氧化氢(H₂O₂)含量以及细胞壁过氧化物酶(POD)活性,进而分析根系细胞壁果胶对Cd的响应特征。【结果】1) Cd胁迫下,D62B和Luhui17根系细胞壁果胶合成增加,根系细胞壁低酯化和高酯化果胶糖醛酸含量均表现为D62B高于Luhui17。Cd处理下D62B根系细胞壁低酯化和高酯化果胶糖醛酸含量较对照分别增加了13.21%～71.82%和22.10%～64.27%,Luhui17分别增加了24.14%～137.86%和13.12%～41.26%。...     

Sodium amalgam reduction of humic acid—II. application of the method

The nature and distribution of the phenolic compounds found in humic acid on sodium amalgam reduction resemble those reported to be found in microbial cultures grown on lignifiedplanttissue.lt has also been shown that the more humified and decayed fractions of soil organic matter yield humic acids that bear less resemblance to lignin than do the humic acids from less decayed plant remains. The aromatic portion of humic acid would seem to consist of biologically modified and transformed lignin, together with phenolic units synthesized by soil microbes. Sodium amalgam reduction can be employed to estimate the degree of transformation, but cannot, however, be used to “finger print” humic acids.     

Influence of biological aggregating agents associated with microbial population on soil aggregate stability

In this study, we investigated the effects of plant residue decomposition and biological aggregating agents (microbial extracellular polysaccharides and fungal hyphae) on soil aggregate stability and determined the microbial population at different stages of soil aggregate stabilization. Experiments were conducted in a 40 days incubation period with the following six treatments: the control (soil only), soil + fungicide, soil + bactericide, soil + maize residues, soil + maize residues + fungicide, and soil + maize residues + bactericide. The maize residues treatments greatly enhanced the formation of macroaggregates. In the residue treatments, the addition of fungicide led to a significant suppression of fungal biomass and activity as well as a reduction of soil aggregate stability, which demonstrated the profound influence of fungal activity on aggregate formation. The addition of bactericide also significantly reduced soil aggregate stability, indicating that bacterial activity also played an important role in the macroaggregate formation. However, the effect of microbial extracellular polysaccharides on soil aggregate stability was not significant, which might be attributable to the fast wet sieving method used for aggregate separation. For the treatments of soil + residues and soil + residues + bactericide, the temporal variations of soil aggregate formation with two peak values suggested that other factors, such as hydrophobic compounds and phenolic acids, might be involved in the soil aggregate stabilization process.     

基于长期定位试验的松嫩平原还田玉米秸秆腐解特征研究

为了研究松嫩平原还田玉米秸秆的腐解特征和养分释放规律,该试验采用尼龙网袋法,设置埋土和覆盖地表2种玉米秸秆还田方式,进行连续4 a的定位观测。结果表明:1)还田玉米秸秆的腐解速率和养分释放率都表现为埋土处理大于覆盖地表。秸秆腐解主要集中在还田的前3年,3 a累计腐解率达到91.70%和81.96%,其中第1年腐解率分别为60.63%和45.53%。2)还田玉米秸秆中养分释放的快慢顺序为KPCN。埋土和覆盖处理秸秆中钾的释放主要在还田第1年,释放率达到了96.26%和84.04%;而磷、碳和氮的释放则主要集中在还田前3年,其中磷释放率为92.03%和83.29%;碳释放率为90.96%和82.06%;氮释放率为91.70%和81.96%。3)还田玉米秸秆中半纤维素的腐解速度快于纤维素,木质素最慢。其中埋土和覆盖处理秸秆半纤维素2 a腐解率为88.78%和86.30%;纤维素2 a腐解率为80.42%和70.86%;而木质素3 a累计腐解率为78.63%和66.48%。     

Effect of phenolic structures on the degradability of cell walls isolated from newly extended apical internode of tall fescue (Festuca arundinacea Schreb.)

Apical internodes of tall fescue (Festuca arundinacea Schreb. var. Clarine) harvested at flowering were sectioned into 5 or 10 equal parts to study in situ degradability and cell wall composition, respectively. The basal (youngest) section had the greatest primary wall content. Cell walls in the upper (older) sections had the highest xylose/arabinose ratio and lignin content and a lignin rich in syringyl units, all typical of extensive secondary wall development. Almost all of the p-coumaric (p-CA) and about half of the ferulic acid (FA) were released by 1 M NaOH and presumed to be ester-linked. The total FA content was approximately double that of p-CA in all sections other than the youngest with a distribution similar to that of total p-CA. However, the ratio of esterified to ether and ether plus ester linked (Et & Et+Es) FA differed with age. Whereas the esterified form remained essentially constant ( approximately 4.5 g/kg of cell wall), Et & Et+Es ferulate increased with increasing age of the tissue and was significantly related to lignin deposition (r = 0.79, P < 0.01). The extent of cell wall degradation after 48 h of incubation in the rumen was inversely related to maturity, falling from 835 g/kg of dry matter in the youngest section to 396 g/kg in the oldest. Both the rate and extent of cell wall degradation were significantly negatively related to the ratio of xylose to arabinose, lignin content, proportion of syringyl units present in lignin, and concentration of Et & Et+Es FA present. A positive relationship between Et & Et+Es FA was also found, with the rate (P < 0.01) being better correlated than the extent (P < 0.05) of cell wall degradation. Application of the newly extended internode model to fescue produced results consistent with the view that both the lignin content and the extent to which lignin was covalently bound to the other wall polymers crucially influenced the rate and extent of degradation.     

Green mulch decomposition and nitrogen release from leaves of two Inga spp. in an organic alley-cropping practice in the humid tropics

Inga edulis Mart and Inga samanensis Uribe are promising yet little studied legume trees for use in agroforestry on acidic soils. The objective of this study was to analyze the decomposition and N release processes of green mulch from these species. Litterbags filled with leaves from each species were placed on the ground in an organic maize (Zea mays L.) alley-cropping experiment at the time of maize sowing and collected every 2 weeks over a 20 week period, and measured for dry matter, N, hemicellulose, cellulose, and lignin content. Three types of models were applied to the data, according to the characteristics of each component, to analyze the decomposition dynamics of whole leaves and leaf components: a negative exponential decay function, an inverted Michaelis-Menten function, and a linear regression. Initial decay of I. samanensis mulch was faster than I. edulis mulch. However, the recalcitrant fraction was about half of the initial litter mass in both Inga spp. Hemicellulose disappeared almost completely from the litter during the 20-week incubation period, while no significant lignin decay occurred. After a slow start, cellulose partially decayed following linear kinetics. The half-life of labile N, estimated as a Michaelis-Menten parameter, was 10 weeks in I. samanensis and ca. 24 weeks in I. edulis litter. Polyphenol content was significantly higher in I. edulis. Litter of I. edulis and I. samanensis may be classified as ‘low-quality’ and ‘medium-quality’ mulch, respectively. Due to the relatively large recalcitrant mulch fraction, both Inga spp. may promote C sequestration and long-term N accumulation in soil.     

Characterization and distribution of phenolics in carrot cell walls

Carrot cell walls have been shown to contain significant quantities of esterified p-hydroxybenzoic acid, which is presumed to be esterified to cell wall polymers. The purpose of this study was to investigate the distribution of p-hydroxybenzoic acid and related phenolics among carrot cell wall polysaccharides. Cell wall material was prepared from fresh carrot root tissues and extracted sequentially with water, imidazole, cyclohexane- trans-1,2-diamine- N, N, N', N'-tetraacetate, Na 2CO 3, and KOH (0.5, 1, and 4 M) to leave a cellulose-rich residue. The fractions were analyzed for their carbohydrate and phenolic acid components. Selected soluble fractions were subfractionated further by graded precipitation in ethanol. The majority of the polymer fractions comprised pectic polysaccharides, with varying quantities of neutral sugars (arabinose and galactose). Hemicellulosic polymers were generally found only in the strong alkali extracts (4 M KOH). p-OH-benzoic acid was the predominant phenolic ester and was associated with most fractions analyzed; p-OH-benzaldehyde was also detected in the fractions at much lower levels. Principal components analysis of the chemical data indicated that the p-OH-benzoic acid was associated predominantly with the branched pectic polysaccharides, in contrast to the p-OH-benzaldehyde. The possible roles and functional properties of these phenolics are discussed.     

Incorporation of natural monoacids from plant residues into an hydromorphic forest podzol

The monoacid part of soil lipids was studied in a hydromorphic sandy podzol under pine trees (Pinus maritima sp.). The undecomposed forest-litter layer (L), the fragmented mycelium-invaded litter layer (F) and the A1 soil horizon were sampled, and analysed for total lipid and total monoacid contents. Total monoacids were separated into straight-chain components and terpenic components. Straight-chain monoacids were determined as free acids and esters of fatty alcohols and of glycerol. Among these components, branched alkanoic acids occurred in the A1 soil horizon. Alkenoic acids were mainly determined as glycerides. Free n-alkanoic acids were mainly produced in soil from terminal oxidation of plant n-alkanes and plant n-alkanols. Free alkanoic acids with the longest carbon chains and alkanoic acids esterified as glycerides were concentrated in the A1 soil horizon. Terpenic monoacids were mainly diterpenic components from pine resin. Their concentration decreased markedly during the decomposition of plant debris.     

Mycorrhizal colonization associated with roots of field-grown maize does not decline with increasing plant-available phosphorus

Maize roots are colonized by arbuscular mycorrhizal fungi, but less mycorrhizal symbiosis is expected as the plant-available phosphorus (P) concentration of soil increases, based on greenhouse and growth bench experiments. The objective of this study was to evaluate maize root colonization by arbuscular mycorrhizal fungi in a sandy loam soil with a gradient of plant-available P concentrations resulting from P fertilizer inputs. The field experiment received inorganic and organic P fertilizers for 3 years, and this created a 20-fold difference in the plant-available P concentration, from 12 to 204 mg Mehlich-3 extractable P kg⁻¹. The proportion of maize roots colonized with arbuscular mycorrhizal fungi increased from 26 ± 2% during vegetative growth (V8 and VT growth stages) to 46 ± 2% in the reproductive R2 and R6 stages. The P fertilizer input did not affect maize root colonization by arbuscular mycorrhizal fungi. More arbuscular mycorrhizal fungi colonization of maize roots occurred in soil with increasing plant-available P concentrations (r = .12, p = .05, n = 237), and this was associated with greater P uptake in the maize shoots (r = .53, p < .001, n = 240). We conclude that the root-mycorrhizal symbiosis was more strongly related to maize growth than the plant-available P concentration under field conditions.     

长期氮肥施用对玉米根茬矿化分解的影响

通过室内培养和田间分解试验,研究了施氮量为N 0、120和240 kg/hm2处理的玉米根茬(R0、R120、R240)在15和45 cm两个肥力条件不同的土层中有机碳矿化分解特性及其对土壤活性有机碳组分的影响。结果表明,在室内矿化培养条件下,根茬CO2累积释放量和潜在碳矿化量均为R120R240R0;R120和R240根茬碳矿化率在表层土壤(15 cm)和底层土壤(45 cm)中分别较R0提高21.1%、12.7%和45.3%、33.7%。在田间埋藏分解条件下,分解386 d后R0、R120和R240根茬碳残留率在表层土壤中分别为36.3%、25.2%和28.7%,在底层土壤中分别为38.4%、30.6%和31.1%;根茬碳残留率与其C/N、木质素含量以及木质素/N正相关,而与根茬全氮含量呈负相关关系,表明根茬分解率随着其本身全氮含量的增加而提高;添加玉米根茬显著增加土壤微生物量碳含量143%~297%,增加土壤可溶性有机碳含量19.9%~118.2%。综上可见,长期施用氮肥影响作物根系的养分组成,显著提高其全氮含量,在评价土壤碳、氮养分循环时,应注重长期氮肥施用对作物残茬养分累积及其在土壤中分解、转化的影响。     

In search of a maize ideotype for cell wall enzymatic degradability using histological and biochemical lignin characterization

Grass cell wall degradability is conventionally related to the lignin content and to the ferulic-mediated cross-linking of lignins to polysaccharides. To better understand the variations in degradability, 22 maize inbred lines were subjected to image analyses of Fasga- and M?ule-stained stem sections and to chemical analyses of lignins and p-hydroxycinnamic acids. For the first time, the nearness of biochemical and histological estimates of lignin levels was established. Combination of histological and biochemical traits could explain 89% of the variations for cell wall degradability and define a maize ideotype for cell wall degradability. In addition to a reduced lignin level, such an ideotype would contain lignins richer in syringyl than in guaiacyl units and preferentially localized in the cortical region rather than in the pith. Such enrichment in syringyl units would favor wall degradability in grasses, contrary to dicots, and could be related to the fact that grass syringyl units are noticeably p-coumaroylated. This might affect the interaction capabilities of lignins and polysaccharides.     

Long-term nitrogen additions increased surface soil carbon concentration in a forest plantation despite elevated decomposition

Forests cover one-third of the Earth’s land surface and account for 30-40% of soil carbon (C). Despite numerous studies, questions still remain about the factors controlling forest soil C turnover. Present understanding of global C cycle is limited by considerable uncertainty over the potential response of soil C dynamics to rapid nitrogen (N) enrichment of ecosystems, mainly from fuel combustion and fertilizer application. Here, we present a 15-year-long field study and show an average increase of 14.6% in soil C concentration in the 0-5 cm mineral soil layer in N fertilized (defined as N+ hereafter) sub-plots of a second-rotation Pinus radiata plantation in New Zealand compared to control sub-plots. The results of ¹⁴C and lignin analyses of soil C indicate that N additions significantly accelerate decomposition of labile and recalcitrant soil C. Using an annual-time step model, we estimated the soil C turnover time. In the N+ sub-plots, soil C in the light (a density < 1.70 g cm⁻³) and heavy fractions had the mean residence times of 23 and 67 yr, respectively, which are lower than those in the control sub-plots (36 and 133 yr in the light and heavy fractions, respectively). The commonly used lignin oxidation indices (vanillic acid to vanillin and syringic acid to syringaldehyde ratios) were significantly greater in the N+ sub-plots than in the control sub-plots, suggesting increased lignin decomposition due to fertilization. The estimation of C inputs to forest floor and δ¹³C analysis of soil C fractions indicate that the observed buildup of surface soil C concentrations in the N+ sub-plots can be attributed to increased inputs of C mass from forest debris. We conclude that long-term N additions in productive forests may increase C storage in both living tree biomass and soils despite elevated decomposition of soil organic matter.