Labile Carbon Content Research Articles

Temperature sensitivity of organic matter decomposition in two boreal forest soil profiles

Controversial conclusions from different studies suggest that the decomposition of old soil organic matter (SOM) is either more, less, or equally temperature sensitive compared to the younger SOM. Based on chemical kinetic theory, the decomposition of more recalcitrant materials should be more temperature sensitive, unless environmental factors limit decomposition. Here, we show results for boreal upland forest soils supporting this hypothesis. We detected differences in the temperature sensitivity 1) between soil layers varying in their decomposition stage and SOM quality, and 2) inside the layers during a 495 day laboratory incubation. Temperature sensitivity increased with increasing soil depth and decreasing SOM quality. In the organic layers, temperature sensitivity of decomposition increased during the early part of a 495 day laboratory incubation, after respiration rate and SOM quality had notably decreased. This indicates that decomposition of recalcitrant compounds was more temperature sensitive than that of the labile ones. Our results imply that Q 10 values for total heterotrophic soil respiration determined from short-term laboratory incubations can either underestimate or overestimate the temperature sensitivity of SOM decomposition, depending on soil layer, initial labile carbon content and temperature range used for the measurements. Using Q 10 values that ignore these factors in global climate models provides erroneous estimates on the effects of climate change on soil carbon storage.

Carbon mineralization rates at different soil depths across a network of European forest sites (FORCAST)

SummaryMost of the carbon (C) in terrestrial ecosystems is stored in the mineral soil layers. Thus, the response of the mineral soil to potential increases in temperature is crucial for the prediction of the impact of climate change on terrestrial ecosystems. Samples from three mineral soil layers were collected from eight mature forest sites in the European network CARBOEUROFLUX and were incubated at four temperatures (4, 10, 20 and 30°C) forc. 270 days. Carbon mineralization rates were related to soil and site characteristics. Soil water holding capacity, C content, nitrogen (N) content and organic matter all decreased with soil depth at all sites, with significantly larger amounts of organic matter, C and N in the top 0–5 cm of mineral soil than in the deeper layers. The conifer forest soils had significantly lower pH, higher C/N ratios and carbon contents in the top 5 cm than the broadleaf forest soils. Carbon mineralization rates decreased with soil depth and time at all sites but increased with temperature, with the highest rates measured at 30°C for all sites. Between 50 and 70% of the total C respired after 270 days of incubation came from the top 5 cm. The percentage C loss was small in all cases, ranging from 1 to 10%. A two‐compartment model was fitted to all data to derive the labile/active and slow/recalcitrant fractions, as well as their decomposition constants. Although the labile fraction was small in all cases, we found significantly larger amounts of labile C in the broadleaf forest soils than in the conifer forest soils. No statistically significant differences were found in the temperature sensitivity parameterQ10among sites, soil layers or between conifer and broadleaf soils. The averageQ10for all soils was 2.98 (± 0.10). We found that despite large differences among sites, C mineralization can be successfully predicted as a combined function of site leaf area index, mean annual temperature and content of labile carbon in the soil (R2 = 0.93).

Soil tillage and fertilization of Orthic Luvisol and their influence on chemical properties, soil structure stability and carbon distribution in water-stable macro-aggregates

We studied the influence of different soil tillage and fertilization on chemical parameters, soil structure stability and carbon distribution in water-stable macro-aggregates (WSA ma) of loamy Orthic Luvisol. In 1994, the Department of Plant Production of the Slovak Agricultural University in Nitra established a long-term field experiment in locality Dolná Malanta. In 1994–2007, the soil samples were collected from the depth 0–0.3 m. The field experiment included two types of soil tillage (conventional tillage—CT and reduced tillage—RT) and three variants of fertilization (1. C o—without fertilization, 2. PR + NPK—crop residues together with added NPK fertilizers, 3. NPK—with added NPK fertilizers). Different tillage and fertilization had statistically significant influence on changes of the soil pH and soil sorptive complex. The values of pH were more favourable in RT than in CT. In NPK (by 26%) and in PR + NPK (by 21%) decreased values of hydrolytic acidity. On the other hand it increased the sum of basic cations. This led to the increase of cation exchangeable capacity. In comparison to CT, a higher total carbon concentration ( C t) was determined in RT. According to vulnerability coefficient ( K v), the soil structure stability was better in RT (4.64 ± 1.54) than in CT (5.15 ± 1.75). Average value of WSA ma was higher by 9% in RT and it led to increasing of the sum of mean weight diameters of water-stable aggregates (MWD-WSA) by 11% and increasing of index stability ( S w) by 12%. We determined linear dependences between C t and critic level of soil organic matter concentration ( S t) in CT and RT as well as in PR + NPK and NPK. The negative correlation between Ca 2+ and S t (−0.507 **) and positive correlation between Ca 2+ and crusting index (0.525 **) were detected in CT. The values of Ca 2+ were in positive correlation with crusting index (0.363 *) in RT. We observed higher concentrations of C t and labile carbon content ( C L) in water-stable micro-aggregates (WSA mi) and WSA ma in the size fractions from 25 × 10 −4 to 3 × 10 −3 m in RT. There were also higher concentrations of C t and C L in WSA ma in the size fractions >3 × 10 −3 m in CT. The application of crop residues together with NPK fertilizers increased the concentration of C t in all fractions of WSA ma. On the other hand, C t concentration decreased by 7% in WSA mi. In PR + NPK, the highest concentration of C L was observed in WSA ma in the size fraction 2 × 10 −3 to 3 × 10 −3 m.

Assessing the effects of short-term Spartina alterniflora invasion on labile and recalcitrant C and N pools by means of soil fractionation and stable C and N isotopes

An exotic grass Spartina alterniflora was intentionally introduced to Jiuduansha wetlands in Yangtze River estuary in 1997, and since then it had rapidly replaced native plant Scirpus mariqueter that used to dominate the estuarine salt marshes. We investigated consequences of S. alterniflora invasion to soil labile and recalcitrant C and N compared to the native S. mariqueter using soil fractionation and stable C and N isotopes. Results showed that S. alterniflora increased soil labile carbon (LC), recalcitrant carbon (RC), and soil recalcitrant nitrogen (RN) contents significantly ( P < 0.05) in the upper soil layers (0–60 cm) compared to the S. mariqueter soil. Soil labile nitrogen (LN) in the S. alterniflora soil, however, remained lower than that in the S. mariqueter soil ( P < 0.01), except for the surface soil layer (0–20 cm). The LC accounted for, on average, 36–38% of soil organic matter (SOM) in both communities, while labile N accounted for 32% of SOM in S. alterniflora soil and 48% in S. mariqueter soil. The δ 13C values in S. alterniflora soil showed that S. alterniflora contributed on average 8.6% and 3.3% to the LC and RC pools, respectively, within the 0–100-cm soil layer. The greatest labile C contribution derived from S. alterniflora was found at the 40-cm soil whereas the proportion of recalcitrant C originating from S. alterniflora showed a decreasing trend with soil depth. These changes appeared to be associated with vertical distributions of roots and rhizodeposition. We also found that the δ 15N values of SOM were more enriched in S. alterniflora soil compared to S. mariqueter soil, suggesting that greater SOM input by S. alterniflora residues would stimulate microbial activity rates that could lead to increased N turnover rates in S. alterniflora soil.

Deoxygenation potential of the Richmond River Estuary floodplain, northern NSW, Australia

Periodic deoxygenation events (DO < 1 mg/L) occur in the Richmond River Estuary on the east coast of Australia following flooding and these events may be accompanied by total fish mortality. This study describes the deoxygenation potential of different types of floodplain vegetation in the lower Richmond River catchment and provides a catchment scale estimate of the relative contribution of floodplain vegetation decomposition to deoxygenation of floodwaters. Of the major vegetation types on the floodplain slashed pasture was initially (first 5 to 7 h) the most oxygen demanding vegetation type after inundation (268 ± mg O2 m−2 h−1), followed by dropped tea tree cuttings (195 ± 18 mg O2 m−2 h−1) and harvested cane trash (110 ± 8 mg O2 m−2 h−1). However, 10 h after inundation the oxygen consumption rates of slashed pasture (105 ± 5 mg O2 m−2 h−1) and tea tree cuttings (59 ± 7 mg O2 m−2 h−1) had decreased to a rate less than the harvested cane trash (110 ± 8 mg O2 m−2 h−1). The oxygen demands of the different floodplain vegetation types when inundated were highly correlated with their nitrogen content (r2 = 0.77) and molar C:N ratio (r2 = 0.82) reflecting the dependence of oxygen demand of vegetation types on their labile carbon content. The floodplain of the lower Richmond River (as flooded in February 2001) has the potential to deoxygenate about 12.5 × 103 mL of saturated freshwater at 25°C per day which is sufficient to completely deoxygenate floodwater stored on the floodplain with 3 to 4 days. In addition, oxidation of Fe2+ mobilized during the decomposition of floodplain vegetation via iron reduction and discharged from groundwater and surface runoff in acid sulfate soil environments could account for about 10% of the deoxygenation of floodwater stored on the floodplain. Management options to reduce floodplain deoxygenation include removing cuttings from slashed pasture and transporting off-site, reducing slashed pasture windrow loads by using comb-type mowers, returning areas of the floodplain to wetlands to allow the establishment of inundation tolerant vegetation and retaining deoxygenated floodwaters in low lying areas of the floodplain to allow oxygen consumption process to be completed before releasing this water back to the estuary. Copyright © 2006 John Wiley & Sons, Ltd.

Effects of vegetation cover on the tendency of soil to crust in South Africa

Abstract. Tendency to crust is a potentially useful index for assessing soil degradation and for assisting land use planning in South Africa. In this study, the influence of land use, geology and vegetation cover on the tendency of soil to form a surface crust was investigated in six vegetation types. Crusting at all sites was greater in exposed soils than soils under vegetation, as determined by infiltration rate, water dispersible clay and modulus of rupture. In Renosterveld, crusting was markedly greater in exposed soil than vegetation covered soil (mean infiltration 16 vs 44 mm h−1; dispersible clay 2.6 vs 2.2%; modulus of rupture 121 vs 64 kPa). Greater crusting in exposed soil was attributed to lower soluble salt and labile carbon (C) contents and an associated increase in the dispersion of clay. In Karoo, crusting of exposed, shale‐derived soils was greater than that of exposed, dolerite‐derived soils (infiltration 40 vs 83 mm h−1; dispersible clay 2 vs 1.2%), and a similar pattern was evident in Tall Grassveld (infiltration 18 vs 36 mm h−1; dispersible clay 1.2 vs 0.9%; modulus of rupture 31 vs 21 kPa). In Upland Grassland, cultivation of maize and rye enhanced crusting. In Thicket, crusting was greater in soils from open, degraded vegetation than intact, densely wooded sites (infiltration 19 vs 51 mm h−1; modulus of rupture 16 vs 34 kPa), probably due to lower content of soil C. In Bushveld, crusting was greater in annually burnt plots than unburnt plots (infiltration 109 vs 163 mm h−1; dispersible clay 0.9 vs 0.6% on granite‐derived soils; and infiltration 56 vs 72 mm h−1; dispersible clay 1.5 vs 1.3% on basalt‐derived soils). Greater crusting of soil from burnt plots was ascribed to a reduction in soil C and soluble salts as well as a greater exchangeable sodium percentage.

Disponibilidade de nitrogênio pela oxidação do carbono lábil com permanganato de potássio

O presente trabalho teve como objetivo avaliar a liberação do nitrogênio associado ao carbono lábil oxidado com KMnO4 333 mmol L-1 e verificar a capacidade supridora de nitrogênio pela sua correlação com o nitrogênio absorvido pelas plantas. O solo utilizado foi proveniente de um experimento com milho em um Argissolo Vermelho-Amarelo, cultivado com milho desde 1984, cuja produtividade vem sendo avaliada em função das doses de 0 e 40 m³ ha-1 ano-1 de composto orgânico (palha de soja e feijão com esterco bovino) combinadas com 0, 250 e 500 kg ha-1 ano-1 da fórmula 4-14-8 aplicados no plantio, e 0, 100 e 200 kg ha-1 ano-1 de sulfato de amônio em cobertura. Os resultados mostraram que o uso da matéria orgânica contribuiu para uma quantidade maior de nitrogênio solúvel em KMnO4 333 mmol L-1; o teor de nitrogênio solúvel em KMnO4 333 mmol L-1 foi superior ao obtido pelo método anaeróbico e se correlacionou significativamente com o carbono total, carbono lábil, nitrogênio total, teor de amônio e nitrato e com o nitrogênio biodisponível.

Organic matter and benthic metabolism in euphotic sediments along shallow sub-tropical estuaries, northern New South Wales, Australia

Sediment organic matter (OM), chloropigments and benthic metabolism (oxygen, carbon dioxide and alkalinity fluxes) were investigated along 3 sub-tropical Australian estuaries during different seasons. Significant rates of benthic productivity occurred throughout the year in the study estuaries, especially in the lower reaches where sediments were commonly net autotrophic. Benthic microalgae (BMA) contributed up to 20% of the total carbon content in the lower estuary sands, while organic matter was dominated by phytodetritus and allochthonous material in the middle to upper estuaries. Sediment oxygen demand (SOD) incubations were carried out on surface sediments to investigate labile carbon contents. SOD normalised to sediment carbon content (SOD mol C -1 ) confirmed a much higher proportion of labile OM in the lower estuary sediments due to a predominance of living BMA, while the OM pool in the upper and middle estuaries was relatively more degraded. SOD mol C -1 correlated well with both the C:N and the chlorophyll:pheophytin ratios of surface sediments, providing 3 independent measures of sediment OM quality. Comparisons between SOD and benthic community respiration rates (O 2 fluxes) measured in sediment core incubations suggested that oxygen demand due to the oxidation of reduced sulphur compounds formed a significant fraction of the O 2 flux, and that fluxes may be enhanced by bioturbation. O 2 and TCO 2 respiration rates were overall poorly correlated and suggested that benthic metabolism is characterised by periods of anaerobic mineralisation (TCO 2 :O 2 > 2) during periods of high OM supply, followed by aerobic respiration and reoxidation (TCO 2 :O 2 1 to 2) as OM supply waned. At many sites throughout the study TCO 2 :O 2 flux ratios were less than 1 and coupled with alkalinity uptakes, indicating that either sulphide oxidation or nitrification may be significant influences on the benthic fluxes. These processes cause a net drawdown on O 2 effluxes that can lead to significant underestimations of benthic productivity. As such, the use of CO 2 and O 2 fluxes yield distinctly different measures of benthic metabolism.