Modelling low molecular weight organic acid dynamics in forest soils

Low molecular weight organic acids such as citrate and oxalate have been hypothesized to play a key role in rhizosphere ecology and pedogenesis. A mathematical site-specific model, DYNLOW, was constructed to describe the temporal and spatial dynamics of these organic acids in coniferous forest soils using the modelling software STELLA^®. Experimentally derived values for biodegradation, adsorption, and daily values of soil temperature, moisture and hydrological flow were used to parameterize the model. The model describes the dynamics and downward movement of oxalate and citrate through the horizons (O, AE, E, Bhs, Bs) of three podzolic soil profiles in Sweden. After calibration, the model predicted average soil solution organic acid concentrations ranging from <1 to 90 μM, which was in agreement with experimental measurements (<1 to 116 μM). The model results indicated that microbial degradation of organic acids was in quantitative terms the biggest process regulating soil solution concentrations. Primary production rates of organic acid in the soil were predicted to be high (<1 to 1250 nmol g⁻¹ soil d⁻¹) in comparison to the amount present at steady state in the soil solution pool (<0.1 to 240 nmol g⁻¹ soil). The downward transfer of organic acids between soil horizons due to mass flow was predicted to be a small flux (<0.1 to 3% of the total loss) compared to that lost by microbial biodegradation. The model predicted that the amount of basal soil respiration that could be attributable to the microbial turnover of organic acids was on average 19±22% of the basal CO₂ production across all sites and horizons for citrate and 7±7% for oxalate. The model results are discussed in the context of pedogenesis, forest soil respiration and organic matter production.     

反应时间对对硝基氯苯在土壤上的吸附与解吸行为的影响

对硝基氯苯(p-.Nitrochlorobenzene,p-NCB)在东北黑土上吸附不同时间(1天和36天)的样品吸附等温线重合,表明吸附在1天内达到平衡。然而,不同吸附时间的两份样品解吸等温线存在显著差异,推测可能是吸附达到表观平衡后,随接触时间的增加,吸附质(p-NCB)分子和土壤有机质之间发生酶催化反应中的"诱导-契合"作用,从而使二者的结合更加紧密。高浓度p-NCB可能会造成黑土有机质发生结构或形态的改变,造成解吸延滞作用显著。     

Factors controlling decomposition of soil organic matter in fallow systems of the high tropical Andes: A field simulation approach using C- and N-labelled plant material

N-rich (C:N=27) and N-poor (C:N=130) wheat straw, labelled with ¹⁴C and ¹⁵N, was incubated for 2 yr in two major ecosystems of the upper elevation belt of cultivation in the high Andes: the moist Paramo (precipitation=1329 mm, altitude=3400 m asl, Andes of Merida, Venezuela) and the dry Puna (precipitation=370 mm, altitude=3800 m asl, Central Altiplano, Bolivia). The experiment was installed in young (2 yr) and old (7 yr) fallow plots. The following soil analyses were performed at nine sampling occasions: soil moisture, total-¹⁴C and -¹⁵N, and Microbial Biomass (MB)-¹⁴C and -¹⁵N. The measured data were fitted by the MOMOS-6 model (a process based model, with five compartments: labile and stable plant material, MB, and labile (HL) and stable humus (HS)) coupled with the SAHEL model (soil moisture prediction) using daily measured and/or predicted meteorological data. The aim was to understand how (1) the climatic conditions, (2) the quality of plant material, (3) the fallow age and (4) the soil properties affect the cycling of C and N within the soil organic matter system.The fallow age (2 and 7 yr) did not affect the measured data or the model predictions, indicating that in these systems the decomposition potential is not affected by fallow length. During the short initial active decomposition phase, the labile plant material was quickly exhausted, enabling a build up of MB and of HL. During the low activity phase, that covered 4/5 of the time of exposure, the MB size decreased slowly and the HL pool was progressively exhausted as it was reused by the MB as substrate. The HL compartment was directly or indirectly the major source for the inorganic ¹⁵N production. If the C:N ratio of the added plant material increased, the model predicted (1) a reduction of the decomposition rates of the plant material (essentially the stable plant material) and (2) an increased mortality of the MB which increased the production of HL (microbial cadavers and metabolites). Thus the essential effect of the slower decomposition due to the N-poor plant material was a higher accumulation of C and N in the HL and its slower recycling by the MB during the low activity phase. The labelling experiment allows to understand the higher soil native organic matter content in Paramo soils compared to Puna. The large sequestration of organic matter generally observed in the Paramo soils can be explained by two abiotic factors: the unfavourable soil microstructure and the accumulation of free aluminium linked to the climatic and acid soil conditions, inhibiting the microbial activity physically and chemically.     

The dynamics of lucerne (Medicago sativa L.) yield components in response to defoliation frequency

The productive life of lucerne (Medicago sativa L.) stands depends on the rate of mortality of individual plants. However, self-thinning of plant populations may be compensated for by increases in other yield components, namely shoots/plant and individual shoot mass. Frequent defoliation reduces lucerne yield but it is unclear whether this is caused by an acceleration of plant mortality or changes in these other yield components. To investigate this, crops with contrasting shoot yields were created using constant 28 or 42-day regrowth cycles applied to a ‘Kaituna’ lucerne crop in Canterbury, New Zealand during the 2002/2003 and 2003/2004 growth seasons. Two further treatments switched from 28 to 42 or 42 to 28 days grazing frequency in mid-summer (4th February) of each year. The annual yield of shoot dry matter (DM) ranged from 12 to 23 t/ha for the treatments defoliated consistently each 28 or 42 days, respectively. Plant population was unaffected by treatments and declined exponentially from 130 plants/m² in June 2002 to 60 plants/m² in September 2004. The dynamics of plant and shoot population were associated with the light environment at the base of the canopy. The slope of the size/density compensation (SDC) of plants was −1.67 for the treatment defoliated each 42 days, near the expected self-thinning slope of −1.5 for stands at constant leaf area index (LAI). Self-thinning of shoots resumed after each defoliation when the LAI reached 2.1 and the transmission of photosynthetically active radiation (PAR_t) was 0.20. At this point the proportion of aerial DM in the tallest (dominant) shoots increased non-linearly from 30 to >80%, due to the mortality of intermediary and suppressed shoots. The average maximum shoot population in each rotation was 780 shoots/m² and unaffected by the decline in plant population due to a compensatory increase from 6 to 13 shoots/plant as the stand thinned. A lower asymptote of 43 plants/m² was estimated as the minimum plant population at which yield component compensation would maintain the productive potential of these ‘Kaituna’ stands. Differences in shoot yield were explained (R² = 0.97) by changes in the individual shoot mass (ISM) that were consistently lowered by frequent defoliation treatments. Frequent defoliations reduced crop productivity by limiting the assimilation of biomass into each individual shoot with negligible impact on shoot appearance rate, the number of shoots per plant at an LAI of 2.1 or the rate of plant population decay. Inter-specific competition for light was proposed as the main factor controlling self-thinning of plants and shoots regardless of their individual C:N status.     

CPUE standardisation and the construction of indices of stock abundance in a spatially varying fishery using general linear models

Construction of annual indices of stock abundance based on catch and effort data remains central to many fisheries’ assessments. While the use of more advanced statistical methods has helped catch rates to be standardised against many explanatory variables, the changing spatial characteristics of most fisheries data sets provide additional challenges for constructing reliable indices of stock abundance. After reviewing the use of general linear models to construct indices of annual stock abundance, potential biases which can arise due to the unequal and changing nature of the spatial distribution of fishing effort are examined and illustrated through the analysis of simulated data. Finally, some options are suggested for modelling catch rates in unfished strata and for accounting for the uncertainties in the stock and fishery dynamics which arise in the interpretation of spatially varying catch rate data.     

Estimating leaf area index of mature temperate forests using regressions on site and vegetation data

Canopy gap fraction and leaf area index (LAI) were measured using hemispherical photography in 91 mature forests across Switzerland, including coniferous, broadleaved and mixed stands. The gap fraction and LAI derived from five photographs per site could be reproduced with a high coefficient of determination (R² > 0.7) by regression against simple stand parameters obtained from vegetation surveys: coverages of the tree, shrub and herb layers, and tree height. The method appeared to be robust across the different types of forests. Applied to 981 sites across Switzerland, the regression model produced LAI values ranging from 1.4 to 6.7. These predictions were compared with site variables not included in the regression. LAI appeared limited by the altitude, with maximal values decreasing by one third from 400 to 2000 m above see level. Water availability was also clearly a limitation at sites with a negative water balance, i.e. where the yearly potential evapotranspiration exceeded the precipitation. High or low values of a humidity index based on the ground vegetation also corresponded to a limitation of the LAI, with shorter trees at dry sites and more open canopies at wet sites. Compared to optical measurements (including hemispherical photography), our regression method is fast and inexpensive. Such an approach appears very promising for obtaining reliable estimates of LAI for many sites with low costs. These estimates can then be fed into process models at the stand level.     

活化过硫酸钠氧化土壤对挥发性有机污染物吸附特性的影响

活化过硫酸钠(Sodium persulfate,SPS)氧化技术是一种新型的土壤修复技术。为了更科学地评价化学氧化处理后土壤的环境风险,本文通过亚铁离子活化过硫酸钠法对有机质(Organic matter,OM)含量存在显著差别的两种土壤进行氧化处理,比较了活化过硫酸钠氧化前后两种土壤样品对3种挥发性有机污染物的吸附特性。结果表明,亚铁活化的SPS能够氧化土壤中腐殖酸和胡敏素类的有机质。对OM含量较高的1号土,SPS氧化对有机质的去除率为71.9%。而对OM含量较低的2号土,SPS氧化对有机质的去除率为49.9%。1号土样对3种挥发性有机物的吸附以分配作用为主,氧化后的1号土样对3种物质的吸附机制不变,但吸附量有所增加;2号土样对3种挥发性有机污染物的吸附有一定的非线性,而氧化后的2号土样对3种物质的吸附线性特征增强。吸附数据用对数形式的Freundlich方程拟合得到分配系数lg Kf值,比较有机碳标化后的分配系数lg Kfoc,氧化后的土壤有机质对3种挥发性有机污染物的吸附特性有所提高。分析表明,SPS氧化了有机质中较多的极性组分(如羧基及羟基等),从而使处理后的土壤中有机质的非极性增强,强化了对非极性化合物的吸附。     

Sorption–desorption of phenolic acids as affected by soil properties

Phenolic acids have been implicated in the process of allelopathy and are, therefore, of interest in plant management as a basis for new herbicide structures. The potential bioavailability of phenolic acids is controlled by sorption–desorption processes in soil. Sorption–desorption of p-coumaric acid (4-hydroxycinnamic acid), ferulic acid (3-methoxy-4-hydroxycinnamic acid), veratric acid (3,4-dimethoxybenzoic acid), vanillic acid (3-methoxy-4-hydroxybenzoic acid), and p-hydroxybenzoic acid (4-hydroxybenzoic acid) was characterized on soils with varying physicochemical properties. The phenolic acids sorbed quickly (<8 h) and in high proportions to the amount applied (average 84% of applied was sorbed). Sorption was irreversible with the batch desorption method used (0.01 N CaCl₂ extraction). Pretreatment of soils to remove organic matter and free metal oxides from the soils decreased sorption, particularly in soils with free oxides removed. Statistical analysis suggested that sorption of p-coumaric and ferulic acids was correlated with soil clay content and veratric acid sorption was correlated with several soil factors. In contrast, no consistent relationship between soil characteristics and vanillic and p-hydroxybenzoic acid sorption was found. Based on the results of these experiments, i.e. the high reactivity of the phenolic acids, it is unlikely that these chemicals would be transported far from their point of origin, limiting their range of influence. It appears that, for phenolic acids to have allelopathic effects on plants, they would have to persist in the soil for long periods, resulting in a buildup of the chemical to high enough levels so that sufficient chemical would be in solution to cause the allelopathic effects, even though they may be strongly sorbed at lower concentrations.     

土壤锌吸附动力学

采用流动法研究了红壤、黄棕壤锌吸附动力学特性，重点研究了溶液ｐＨ，锌溶液的浓度、温度对两种土壤锌吸附动力学特性的影响。实验结果表明：溶液ｐＨ、锌的浓度对土壤锌吸附动力学影响明显；两种土壤对锌的吸附表现上可分为化学吸附和物理吸附。０．０５ｍｏｌ·Ｌ＾－１ＣａＣｌ２解吸试验表明，Ｚｎ－Ｃａ交换几乎完全可逆。     

影响农药在土壤与沉积物上吸附作用的研究

农药是环境中重要的有机污染物之一，吸附作用是农药在土壤/沉积物环境中的重要迁移转化行为之一。从农药的性质结构、土壤/沉积物的组成、温度、pH值和共存有机化合物等几方面，阐述了上述因素对农药在土壤/沉积物上吸附作用及其机理的影响，并提出了该领域存在的问题及今后发展的方向。