Protection Of Soil Organic Matter Research Articles

The Potential of Controlled Traffic Farming to Mitigate Greenhouse Gas Emissions and Enhance Carbon Sequestration in Arable Land: A Critical Review

The drive toward adoption of conservation agriculture to reduce costs and increase production sustainably causes concern due to the potentially negative effects of increased soil compaction. Soil compaction reduces aeration, water infiltration, and saturated hydraulic conductivity and increases the risk of waterlogging. Controlled traffic farming (CTF) is a system in which: (1) all machinery has the same or modular working and track width so that field traffic can be confined to the least possible area of permanent traffic lanes, (2) all machinery is capable of precise guidance along those permanent traffic lanes, and (3) the layout of the permanent traffic lanes is designed to optimize surface drainage and logistics. Without CTF, varying equipment operating and track widths translate into random traffic patterns, which can cover up to 85% of the cultivated field area each time a crop is produced. Nitrous oxide (N2O) is the greatest contributor to agriculture's greenhouse gas (GHG) emissions from cropping, and research suggests that its production increases significantly under conditions of high (>60%) water-filled porosity when nitrate (mainly from fertilizer N) and carbon (usually from crop residues) are available. Self-amelioration of soils affected by compaction occurs slowly from the surface downward; however, the rate of amelioration decreases with increase in depth. Consequently, all soils in non-CTF systems in mechanized agriculture are prone to some degree of compaction, which compromises water infiltration, increases the frequency and duration of waterlogged conditions, reduces gaseous exchange between soil and the atmosphere, inhibits root penetration and exploitation of nutrients and water in the subsoil, and enhances N2O emissions. Adoption of CTF increases soil porosity in the range of 5% to 70%, water infiltration by a factor of 4, and saturated hydraulic conductivity by a factor of 2. The greater cropping opportunity and enhanced crop growth for given fertilizer and rainfall inputs offered by CTF, coupled with no-tillage, provide potential for enhanced soil carbon sequestration. Reduced need and intensity of tillage, where compaction is avoided, also helps protect soil organic matter in stable aggregates, which may otherwise be exposed and oxidized. There is both circumstantial and direct evidence to suggest that improved soil structural conditions and aeration offered by CTF can reduce N2O emissions by 20% to 50% compared with non-CTF. It is not compaction per se that increases the risk of N2O emissions but rather the increased risk of waterlogging and increase in water-filled pore space. There may be an elevated risk of GHG emissions from the relatively small area of permanent traffic lanes (typically <20% of total cultivated area) if these are not managed appropriately. Quantification of the benefits of compaction avoidance in terms of GHG emissions may be possible through the use of well-developed models.

EFICIÊNCIA DE SOLUÇÕES DENSAS NO FRACIONAMENTO FÍSICO DA MATÉRIA ORGÂNICA DO SOLO

Os mecanismos de estabilização da matéria orgânica (MO) têm sido estudados em solos tropicais e subtropicais brasileiros; no entanto, poucos trabalhos avaliaram a influência da parte metodológica do uso das soluções nos resultados obtidos. Objetivou-se avaliar a eficiência de duas soluções salinas (iodeto de sódio - NaI 1,8 kg dm-3 e politungstato de sódio - PTS 2,0 kg dm-3) na separação de frações densimétricas da MO em dois solos (Argissolo Vermelho do sul do Brasil - 220 g kg-1 argila e Latossolo Vermelho do Cerrado - 630 g kg-1 argila) e o reflexo dessa eficiência na magnitude e importância do mecanismo de proteção física por oclusão em agregados no acúmulo de MO em solos brasileiros. Amostras de solo coletadas nas camadas de 0,00-0,05; 0,05-0,10; e 0,10-0,20 m foram submetidas ao fracionamento físico densimétrico e separadas as frações leve livre (FLL), leve-oclusa (FLO) e pesada (FP) da MO do solo. O uso de PTS 2,0 kg dm-3 aumentou o rendimento de carbono orgânico (CO) das FLL e FLO em ambos os solos em relação à solução de NaI 1,8 kg dm-3, sendo o efeito mais pronunciado na FLO. A utilização do sistema plantio direto (PD) aumentou os estoques de CO total na camada de 0,00-0,05 m do Argissolo em relação ao solo em preparo convencional (PC). O mecanismo de proteção física da MO por oclusão em agregados foi efetivo no aumento dos estoques de C do solo, sendo responsável por aproximadamente ⅓ do acúmulo na camada superficial do Argissolo (0,00-0,05 m). Para o Latossolo, não houve diferenças entre o estoque de CO total do PD e do PC, porém o solo sob PD apresentou acúmulo de aproximadamente ⅔ do estoque de C como FLO na camada superficial do solo. A proteção física por oclusão em agregados é um mecanismo expressivo na estabilização e sequestro de C em solos tropicais e subtropicais, cuja importância pode ser mascarada pela baixa eficiência da solução de NaI em estudos de fracionamento densimétrico da MO do solo.

Evaluation of seasonal variability of soil biogeochemical properties in aggregate-size fractioned soil under different tillages

An augment of soil organic matter (SOM) in agricultural lands is mandatory to improve soil quality and fertility and to limit greenhouse gases emissions. A better protection of SOM from degradation is seconded to its inclusion in aggregates and to the formation of organo–mineral interactions with the clay fraction within the soil matrix. Under Mediterranean conditions, conservation agriculture (CA) has been widely related with macro-aggregates formation, SOM protection, and to an improvement of soil fertility and crop yields.The objective of this work was to evaluate the biogeochemical properties of five aggregate-size fractions obtained by dry sieving of a Calcic Fluvisol of an experimental farm managed under three different tillages. Soil aggregates distribution, total organic carbon (TOC), labile carbon pools, and enzymatic activities were measured in 2 different periods of the same agricultural campaign. CPMAS 13C NMR analyses were also performed to elucidate the structure of preserved SOM.The results evidenced seasonal variability in aggregate distribution, labile carbon pools and dehydrogenase activity (DHA), whereas TOC, permanganate oxidizable carbon (POxC), and β-glucosidase activity demonstrated to be reliable soil quality indices for soil fractions. The NMR analyses showed a better SOM preservation under conservation tillages, due to higher plant litter inputs and/or higher amount of necromass derived compounds if compared with traditional tillage. Particularly interesting are the results of the Ø 0.5–1mm fraction, in which different trends were found for β-Glu and several organic compound classes if compared with the other fractions. Possibly, in this fraction are concentrated most of the products from cellulose depolymerization stabilized by organo–mineral interactions.

A warmer climate reduces the bioreactivity of isolated boreal forest soil horizons without increasing the temperature sensitivity of respiratory CO2 loss

Changes in the carbon (C) balance of boreal forest ecosystems may impact the global C cycle and climate. The degree to which antecedent temperature regime and mineral protection of soil organic matter (OM) influence the temperature response of boreal soil C pools remains unknown, however. To investigate these phenomena on time scales relevant to anthropogenic climate change, we quantified the temperature response of four soil C pools (L, F and H organic horizons and B mineral horizon) within soil profiles collected from replicated sites representing two regions along a climate transect (“regional warming”) during a 480-day incubation at 5 and 15 °C (“experimental warming”). We hypothesized that 1) warmer region soils would exhibit reduced bioreactivity, a measure of C lability assessed via cumulative soil C mineralization, relative to colder region soils, paralleling a decrease in bioreactivity with depth in both regions, and 2) temperature sensitivity of C mineralization (denoted as Q10) would increase with decreasing bioreactivity congruent with the “C quality-temperature” (CQT) hypothesis, with a smaller effect in mineral soil where physico-chemical protection likely occurs. Cumulative C mineralization decreased from surface L to deeper horizons and from the cold to warm region for organic F and H horizons only. This decrease in soil bioreactivity with depth was paralleled by an increase in Q10 with depth as expected, except in mineral soil where Q10 was similar to or lower relative to the overlying organic layer. The lower bioreactivity in F and H horizons of the warm relative to the cold region was not, however, associated with a greater Q10. A warmer regional climate in these otherwise similar forests thus resulted in reduced bioreactivity of isolated soil C pools without increasing the temperature sensitivity of soil C mineralization. This suggests that assumptions about temperature sensitivity of C mineralization based on the propensity for isolated organic C pools to undergo mineralization may not be valid in some organic-rich, boreal forest soils.

Stoichiometric controls upon low molecular weight carbon decomposition

Soil carbon (C) and nitrogen (N) cycles are inextricably linked, yet the impacts of N availability upon soil C sequestration and turnover are poorly understood. According to stoichiometric theory, in the absence of nutrient limitation substrate decomposition will reach maximum rates, with C assimilated into microbial biomass at the expense of CO2 production. In this study, we added a 14C labelled low molecular weight substrate (glucose) to a sandy soil along with eleven increasing levels of N, phosphorus (P), and sulphur (S) in relative proportions as required for microbial biomass production. Adding a simple soluble substrate allowed us to explicitly examine changes in microbial transformations of added C, rather than changes resulting from extracellular enzyme activity or the extent of substrate decomposition. We hypothesized that as nutrient addition increased, an increasing proportion of the glucose-C provided would be incorporated into microbial biomass at the expense of CO2 production and stabilized as soil organic carbon (SOC). Instead, CO2 production from glucose-C increased significantly with nutrient addition without measurable changes in glucose-derived microbial biomass or SOC. This suggests that if there was greater glucose-derived microbial biomass produced under higher nutrient addition it was offset by a higher rate of microbial biomass turnover. We also found greater soil-derived microbial biomass at lower nutrient addition levels, potentially supporting the concept of microbial mining of soil organic matter (SOM) for nutrients under low nutrient availability. In conclusion, our data suggest that in a sandy soil with low capacity for physical protection of SOM, nutrient addition does not immediately promote C sequestration in the soil microbial community, and that the interaction between C stabilization and nutrient addition requires further work, especially for predicting ecosystem responses.

Iron oxides as proxies for characterizing anisotropy in soil CO2 emission in sugarcane areas under green harvest

Soil CO2 emission (FCO2) is a main contributor of atmospheric carbon transfer and is the subject of research aimed at developing effective methods for characterizing and mitigating CO2 emissions. The FCO2 is related to various soil properties including porosity, density and moisture, which are in turn related to gas transfer, O2 uptake and CO2 release, as well as mineralogical components (particularly iron oxides, which are closely associated with aggregation and protection of soil organic matter). As estimated by diffuse reflectance spectroscopy (DRS), soil iron oxides such as hematite (Hm) and goethite (Gt) can be useful in determining FCO2. The main objective of this experiment was to assess the usefulness of the mineralogical properties Hm, Gt, and iron oxides extracted by dithionite–citrate–bicarbonate (Fed) to estimate the FCO2 in a sugarcane area under green harvest in southeastern Brazil. The experiment was conducted using an irregular 50m×50m grid containing 89 sampling points 0.50–10m apart to assess the soil properties. The FCO2 at each sampling point was measured at the beginning of crop growth and 54 days after planting with the use of two portable LI-COR LI-8100 Soil CO2 Flux Systems. The soil properties studied were found to be spatially dependent and exhibited well-defined anisotropy (particularly the mineralogical properties Hm, Gt and Fed). The first two components of a principal component analysis (PC1 and PC2) jointly accounted for 73.4% of the overall result variability with PC1 essentially related to the physical and mineralogical properties of the soil. Based on a multiple linear regression analysis, free water porosity (FWP) and Hm accounted for 71% of the FCO2 variability. Our results indicate that soil preparation and management practices in mechanically harvested sugarcane affect some factors inherent in the soil forming processes, including physical and mineralogical properties, which in turn affect FCO2. These results affirm the potential of DRS as an auxiliary tool for determination of properties that are typically associated with FCO2. In addition, the ensuing method allows for large-area FCO2 mapping to developing greenhouse gas emission inventories for agricultural soils.

Effects of flooding and warming on soil organic matter mineralization in Avicennia germinans mangrove forests and Juncus roemerianus salt marshes

Under a changing climate, coastal wetlands experience sea level rise, warming, and vegetation change, all of which may influence organic matter mineralization. In coastal wetlands of subtropical west-central Florida (USA), we investigated how soil carbon (C) and nitrogen (N) mineralization respond to soil water, temperature, and ecosystem type (Avicennia germinans mangrove forest vs. Juncus roemerianus salt marsh). We evaluated how soil respiration and mineral N concentration varied along a soil moisture gradient, and whether these relationships differed between ecosystem types. Then, we manipulated soils in a 28-d laboratory incubation to evaluate how potentially mineralizable C and N respond to temperature (23 vs. 27 °C), soil hydroperiod (inundated 4 vs. 20 h/d), and soil source. Soil saturation and inundation suppressed short-term (minutes to weeks) C mineralization from near-surface soils. Soil CO2 efflux declined by 65% as soil moisture increased from 75% to 85%, and potentially mineralizable C was 18% lower with a 20-h hydroperiod than with a 4-h hydroperiod. Organic C quality appears to be greater in A. germinans than in J. roemerianus soils, as A. germinans soils had higher field CO2 efflux rates and greater mineralizable C:N (despite lower total C:N). Increasing incubation temperature from 23 to 27 °C elevated potentially mineralizable C by 40%, indicating that two symptoms of climate change (increased inundation from sea level rise, and warming) may have opposing effects on soil C mineralization. Temperature sensitivity of C mineralization was high for long-hydroperiod soils, however, suggesting that protection of soil organic matter (SOM) due to prolonged inundation will be undermined by warming. Potentially mineralizable N was greater in J. roemerianus soils, although in situ mineral N was not different between ecosystems, instead correlating positively with SOM. These results indicate that models forecasting soil elevation responses to climate change might include inundation effects on mineralization rates.

The effects of earthworm species on soil behaviour depend on land use

This work is concerned with the effect of five earthworm species on short-term soil dynamics through carbon (C) mineralisation and large soil aggregate characteristics (production, stability, organic matter content and composition) in laboratory experiments. We hypothesised that the influence of earthworms on soil behaviour depends on individual species and land use history. Substrates were a sequence of three similar luvisols that had been subjected to different land use histories and represented increasing C contents from 10 mg kg−1 (cropped soil), to 20 mg kg−1 (5-year-old pasture soil) and 33 mg kg−1 (permanent pasture soil). We found that relative C mineralisation was slower in pasture than in cropped soils, which can be explained by differences in the physical protection of soil organic matter (SOM). Land use history influenced (i) the amount of large macro-aggregate production through earthworm behaviour (casting activity), and (ii) the size distribution of water stable soil fractions within the large macro-aggregate produced. No effect of earthworm species on SOM composition was observed with near infrared spectrometry indicating that the chemistry of large macro-aggregates was mainly driven by land use type. Earthworm species had more pronounced effects on physical (large aggregate production and stability) than on biological (C mineralisation promotion) or biochemical (SOM composition) features. This paper shows that, although earthworm effects on soil processes are now obvious, a number of factors may interact in such complex systems so that we should hesitate to make generalisations.

Response of soil nitrogen retention to the interactive effects of soil texture, hydrology, and organic matter

AbstractAdvances in nitrogen (N) saturation and retention theories have focused on soil organic matter (SOM) biogeochemistry in the absence of dynamic soil hydrology. Here we exploit two soil types with contrasting textures that span a hillslope gradient to test hypotheses that suggest N saturation symptoms are regulated by the interactive effects of soil texture, OM, and hydrology on N retention capacity (maximum pool size) and N retention kinetics (N retention rate). Down the hillslope gradient, soil solution nitrate (NO3) concentrations sampled with lysimeters increased, while 15NO3‐N retention decreased. Landscape location (upland, hillslope, and toeslope) and soil type interacted to affect soil solution NO3 concentrations so that the downslope increase in NO3 was greater in sandy versus silty soils. These patterns manifest despite a downslope increase in soil organic carbon (SOC) and C/N ratios. A positive correlation between saturated hydraulic conductivity and soil solution NO3 sampled in zero‐tension lysimeters during precipitation events suggested that high hydraulic conductivity promotes periodic rapid NO3 transport at rates that exceed retention kinetics. The downslope increase in soil solution NO3 in spite of a concomitant increase in SOC and C/N ratios provides an important contrast with previous N saturation research that highlights negative correlations between SOM C/N ratios and NO3 concentrations and suggests NO3 transport along connected hillslope flow paths may overwhelm stoichiometric sinks for inorganic N retention in SOM. Our results reveal important gaps in N retention theory based on SOM biogeochemistry alone and demonstrate how coupled biogeochemical and hydrological models can improve predictions of N saturation, particularly when considering periodic advective NO3 transport in the vadose zone. We show that in coarse‐textured soils, low capacity for protection of SOM N by association with fine mineral particles interacts with rapid hydrological flushing of NO3 to enhance the expression of ecosystem N saturation symptoms.

The effect of organic matter on aggregation under different soil management practices in a vineyard in an extremely humid year

Soil organic matter is an important agent responsible for binding soil mineral particles together and creating soil structure stability. Soil management can modify the content of soil organic matter and soil structure stability. Therefore, the aim of this study was to evaluate the effect of soil organic matter on soil structure stability under different soil management practices in Rendzic Leptosol, in an extremely humid year. In 2006, an experiment of the different management practices in a productive vineyard was established in the locality of Nitra-Dražovce (Slovakia). The following treatments were established: 1. control (grass without fertilization), 2. T (tillage), 3. T + FM (tillage + farmyard manure), 4. G + NPK3 (grass + NPK 120–55–195 kg·ha− 1), and 5. G + NPK1 (grass + NPK 80–35–135 kg·ha− 1). Soil samples were collected every month, during the year 2010. The results showed that the aggregation process and soil structure stability were mainly dependent on the stable form of soil organic matter or the physically protected soil organic matter inside of aggregates in all soil management practices in a vineyard. In the treatment with plowed farmyard manure, the soil structure stability decreased, during 2010. In this case, humic substances stability had a statistically significant influence on soil structure stability. The lower the values of color quotient of humic substances in soil were, the lesser was the decrease in soil structure stability.

Persistence of soil organic matter in eroding versus depositional landform positions

Soil organic matter (SOM) processes in dynamic landscapes are strongly influenced by soil erosion and sedimentation. We determined the contribution of physical isolation of organic matter (OM) inside aggregates, chemical interaction of OM with soil minerals, and molecular structure of SOM in controlling storage and persistence of SOM in different types of eroding and depositional landform positions. By combining density fractionation with elemental and spectroscopic analyses, we showed that SOM in depositional settings is less transformed and better preserved than SOM in eroding landform positions. However, which environmental factors exert primary control on storage and persistence of SOM depended on the nature of the landform position considered. In an annual grassland watershed, protection of SOM by physical isolation inside aggregates and chemical association of organic matter (complexation) with soil minerals, as assessed by correlation with radiocarbon concentration, were more effective in the poorly drained, lowest‐lying depositional landform positions, compared to well‐drained landform positions in the upper parts of the watershed. Results of this study demonstrated that processes of soil erosion and deposition are important mechanisms of long‐term OM stabilization.

Effect of compaction on microbial activity and carbon and nitrogen transformations in two oxisols with different mineralogy

The use of machinery in agricultural and forest management activities frequently increases soil compaction, resulting in greater soil density and microporosity, which in turn reduces hydraulic conductivity and O2 and CO2 diffusion rates, among other negative effects. Thus, soil compaction has the potential to affect soil microbial activity and the processes involved in organic matter decomposition and nutrient cycling. This study was carried out under controlled conditions to evaluate the effect of soil compaction on microbial activity and carbon (C) and nitrogen (N) mineralization. Two Oxisols with different mineralogy were utilized: a clayey oxidic-gibbsitic Typic Acrustox and a clayey kaolinitic Xantic Haplustox (Latossolo Vermelho-Amarelo ácrico - LVA, and Latossolo Amarelo distrófico - LA, respectively, in the Brazil Soil Classification System). Eight treatments (compaction levels) were assessed for each soil type in a complete block design, with six repetitions. The experimental unit consisted of PVC rings (height 6 cm, internal diameter 4.55 cm, volume 97.6 cm³). The PVC rings were filled with enough soil mass to reach a final density of 1.05 and 1.10 kg dm-3, respectively, in the LVA and LA. Then the soil samples were wetted (0.20 kg kg-1 = 80 % of field capacity) and compacted by a hydraulic press at pressures of 0, 60, 120, 240, 360, 540, 720 and 900 kPa. After soil compression the new bulk density was calculated according to the new volume occupied by the soil. Subsequently each PVC ring was placed within a 1 L plastic pot which was then tightly closed. The soils were incubated under aerobic conditions for 35 days and the basal respiration rate (CO2-C production) was estimated in the last two weeks. After the incubation period, the following soil chemical and microbiological properties were detremined: soil microbial biomass C (C MIC), total soil organic C (TOC), total N, and mineral N (NH4+-N and NO3--N). After that, mineral N, organic N and the rate of net N mineralization was calculated. Soil compaction increased NH4+-N and net N mineralization in both, LVA and LA, and NO3--N in the LVA; diminished the rate of TOC loss in both soils and the concentration of NO3--N in the LA and CO2-C in the LVA. It also decreased the C MIC at higher compaction levels in the LA. Thus, soil compaction decreases the TOC turnover probably due to increased physical protection of soil organic matter and lower aerobic microbial activity. Therefore, it is possible to conclude that under controlled conditions, the oxidic-gibbsitic Oxisol (LVA) was more susceptible to the effects of high compaction than the kaolinitic (LA) as far as organic matter cycling is concerned; and compaction pressures above 540 kPa reduced the total and organic nitrogen in the kaolinitic soil (LA), which was attributed to gaseous N losses.

Can composition and physical protection of soil organic matter explain soil respiration temperature sensitivity?

The importance of soil organic matter (SOM) in the global carbon (C) cycle has been highlighted by many studies, but the way in which SOM stabilization processes and chemical composition affect decomposition rates under natural climatic conditions is not yet well understood. To relate the temperature sensitivity of heterotrophic soil respiration to the decomposition potential of SOM, we compared temperature sensitivities of respiration rates from a 2-year long soil translocation experiment from four elevations along a ~3000 m tropical forest gradient. We determined SOM stabilization mechanisms and the molecular structure of soil C from different horizons collected before and after the translocation. Soil samples were analysed by physical fractionation procedures, 13C nuclear magnetic resonance (NMR) spectroscopy, and differential scanning calorimetry (DSC). The temperature sensitivity (Q 10) of heterotrophic soil respiration at the four sites along the elevation transect did not correlate with either the available amount of SOM or its chemical structure. Only the relative distribution of C into physical soil fractions correlated with Q 10 values. We therefore conclude that physical fractionation of soil samples is the most appropriate way to assess the temperature sensitivity of SOM.

Laboratory investigation of organic matter mineralization and nutrient leaching from earthworm casts produced by Amynthas khami

Earthworms are considered soil engineers due to their effects on soil properties including the creation of cast-aggregates which are usually enriched in organic matter compared to the surrounding soil. How earthworms influence the dynamics of soil organic matter (SOM) and mineral nutrients is still poorly understood, partly because the relationships between cast dynamics and SOM and mineral nutrient cycling are unknown. In this study, two laboratory experiments were carried out on free aggregates (casts or soil clods) sampled from the soil surface in a tropical fallow field before the rainy season (March 2008). In the first experiment, a 12h rainfall simulation (intensity: 44mmh−1) experiment was used to monitor fragmentation and nutrient leaching from Amynthas khami earthworm casts and soil clods. Control soil aggregates>5000μm were rapidly (∼2h) broken into large water-stable aggregates (>500μm) and then into smaller water-stable aggregates (<500μm). Conversely, the fragmentation of earthworm casts was progressive and more than 12h were needed to totally fragment the casts into small size aggregates (<500μm). Soil nutrient loss varied depending on aggregate size but it was always higher from cast than from control soil aggregates. NH4+ and NO3− releases showed linear dynamics vs. time, whereas K and P were tightly associated with the soil aggregates and their losses were significant only after 4h of rainfall. The total amount of N-NH4+, N-NO3−, K and P losses were, respectively, 8-, 7-, 12- and 2-fold greater from casts than from the control soil aggregates. In the second experiment, different aggregate sizes (50–250, 250–500 and 500–2000μm) from casts and control soil aggregates were sampled during the rainfall simulation and incubated at 28°C for 21 days. The total amount of K and P was not affected by soil aggregate size. The aggregate hierarchy concept was not confirmed because more C was observed in the smaller aggregate size fractions. Organic C and N mineralization, however, increased with increasing aggregate size for casts, but the opposite trend was observed for the control soil aggregates. This study therefore highlights the importance of earthworm activity for the transfer of mineral nutrients and the protection of SOM within microaggregates.

Physical protection of soil organic matter following mechanized forest operations in Pinus radiata D.Don plantations

Mechanized forest operations have a large impact on soil systems, and may cause disruption of aggregates and exposure of previously protected soil organic matter to microbial attack. In this study, we investigated how C storage, soil structure and unprotected, physically protected and resistant C pools recover 0, 7 and 20 years after mechanical harvesting and site preparation in second rotation Pinus radiata D.Don plantations. We detected differences in dry mean weight diameter (DMWD) of aggregates, with higher DMWD in 0- and 7-year-old pine stands (8.7 and 7.5 mm, respectively) than in 20-year-old stands (4.1 mm). This was attributed to the compaction induced by heavy machinery, as reflected by the positive relationship between DMWD and bulk density ( r = 0.85, P < 0.05). Organic C contents in the top 5 cm were similar 0 and 7 years after disturbance, but were twice as high after 20 years, with mean values of 25, 28 and 52 Mg C ha −1 , respectively. In addition, the resistant C pool was also reduced by up to 7% after clearance. In contrast, unprotected C and physically-protected C were greatest in the youngest stands, indicating that stand establishment and harvesting with heavy machinery may have broken soil aggregates and exposed the previously protected SOM to microbial decomposition and that forest operations may create new mega-aggregates able to protect organic residues. However, the lowest physically-protected C values in 20-year-old pine stands may also be attributed to differences in SOM quality. The alkyl C/O-alkyl C ratios were highest in the oldest stands, indicating that SOM in these stands was more resistant to further decomposition. Mechanized forest operations in radiata pine stands released at least 30 t C ha −1 from the first 5 cm of the soil profile immediately after clearance, caused significant alterations in the soil structure, which lasted for a minimum of seven years, and also reduced the resistant C pool. As the Kyoto Protocol encourages forest management practices that potentially increase carbon sequestration, mechanical harvesting and site preparation in these steep slopes should be reconsidered.

Decomposition and stabilization of root litter in top- and subsoil horizons: what is the difference?

Mechanisms leading to high mean residence times of organic matter in subsoil horizons are poorly understood. In lower parts of the soil profile root material contributes greatly to soil organic matter (SOM). The objective of this study was to elucidate the decomposition dynamics of root-derived C and N in different soil depths during a 3 year field experiment and to examine the importance of different protection mechanisms as well as abiotic factors for the decomposition dynamics. Additionally, we assessed the effect of root litter addition on native SOM. Our conceptual approach included the exposure of litterbags with 13C and 15N labeled wheat root material mixed to loamy agricultural soil at three different soil depths (30, 60 and 90 cm). During the incubation period, we monitored soil temperature, humidity and the incorporation of root derived C and N into the soil microbial biomass and physical SOM fractions. Our results showed that abiotic decay conditions were better in subsurface horizons compared to the topsoil. Root litter addition significantly increased the size of microbial biomass in all three soil horizons. In the topsoil, root-derived C decomposition was significantly higher in the first 6 months of incubation compared to subsoil horizons. In 60 and 90 cm depths, a lag phase with development of soil microbial biomass seemed to be prevailing before decomposition was activated. For root-derived N, similar decomposition kinetics could be observed in top- and subsoil horizons. Despite of higher SOM contents, better soil structure and higher microbial activity in the topsoil horizon compared to subsoil horizons, the amounts of root-derived C and N remaining after 3 years were similar for all three depths. Most of the root-derived C and N was present as organo-mineral complexes or occluded in soil aggregates (oPOM), illustrating similar importance of these two protection mechanisms in all three soil depths. Addition of fresh root litter caused small losses of native soil C whereas native N was retained. We conclude that despite of similar SOM protection mechanisms, there are distinct differences in decomposition dynamics of root litter between top- and subsoil horizons. In the long run, the better abiotic decay conditions prevailing in subsoil horizons may compensate for their poorer physico-chemical characteristics.

Experimental evidence for the attenuating effect of SOM protection on temperature sensitivity of SOM decomposition

AbstractThe ability to predict C cycle responses to temperature changes depends on the accurate representation of temperature sensitivity (Q10) of soil organic matter (SOM) decomposition in C models for different C pools and soil depths. Theoretically, Q10 of SOM decomposition is determined by SOM quality and availability (referred to here as SOM protection). Here, we focus on the role of SOM protection in attenuating the intrinsic, SOM quality dependent Q10. To assess the separate effects of SOM quality and protection, we incubated topsoil and subsoil samples characterized by differences in SOM protection under optimum moisture conditions at 25 °C and 35 °C. Although lower SOM quality in the subsoil should lead to a higher Q10 according to kinetic theory, we observed a much lower overall temperature response in subsoil compared with the topsoil. Q10 values determined for respired SOM fractions of decreasing lability within the topsoil increased from 1.9 for the most labile to 3.8 for the least labile respired SOM, whereas corresponding Q10 values for the subsoil did not show this trend (Q10 between 1.4 and 0.9). These results indicate the existence of a limiting factor that attenuates the intrinsic effect of SOM quality on Q10 in the subsoil. A parallel incubation experiment of 13C‐labeled plant material added to top‐ and subsoil showed that decomposition of an unprotected C substrate of equal quality responds similarly to temperature changes in top‐ and subsoil. This further confirms that the attenuating effect on Q10 in the subsoil originates from SOM protection rather than from microbial properties or other nutrient limitations. In conclusion, we found experimental evidence that SOM protection can attenuate the intrinsic Q10 of SOM decomposition.

Deep soil organic matter—a key but poorly understood component of terrestrial C cycle

Despite their low carbon (C) content, most subsoil horizons contribute to more than half of the total soil C stocks, and therefore need to be considered in the global C cycle. Until recently, the properties and dynamics of C in deep soils was largely ignored. The aim of this review is to synthesize literature concerning the sources, composition, mechanisms of stabilisation and destabilization of soil organic matter (SOM) stored in subsoil horizons. Organic C input into subsoils occurs in dissolved form (DOC) following preferential flow pathways, as aboveground or root litter and exudates along root channels and/or through bioturbation. The relative importance of these inputs for subsoil C distribution and dynamics still needs to be evaluated. Generally, C in deep soil horizons is characterized by high mean residence times of up to several thousand years. With few exceptions, the carbon-to-nitrogen (C/N) ratio is decreasing with soil depth, while the stable C and N isotope ratios of SOM are increasing, indicating that organic matter (OM) in deep soil horizons is highly processed. Several studies suggest that SOM in subsoils is enriched in microbial-derived C compounds and depleted in energy-rich plant material compared to topsoil SOM. However, the chemical composition of SOM in subsoils is soil-type specific and greatly influenced by pedological processes. Interaction with the mineral phase, in particular amorphous iron (Fe) and aluminum (Al) oxides was reported to be the main stabilization mechanism in acid and near neutral soils. In addition, occlusion within soil aggregates has been identified to account for a great proportion of SOM preserved in subsoils. Laboratory studies have shown that the decomposition of subsoil C with high residence times could be stimulated by addition of labile C. Other mechanisms leading to destabilisation of SOM in subsoils include disruption of the physical structure and nutrient supply to soil microorganisms. One of the most important factors leading to protection of SOM in subsoils may be the spatial separation of SOM, microorganisms and extracellular enzyme activity possibly related to the heterogeneity of C input. As a result of the different processes, stabilized SOM in subsoils is horizontally stratified. In order to better understand deep SOM dynamics and to include them into soil C models, quantitative information about C fluxes resulting from C input, stabilization and destabilization processes at the field scale are necessary.

Towards understanding of carbon stocks and stabilization in volcanic ash soils in natural Andean ecosystems of northern Ecuador

Volcanic ash soils contain very large stocks of soil organic matter (SOM) per unit area. Consequently, they constitute potential sources or sinks for the greenhouse gas carbon dioxide. Whether soils become a net carbon source or sink with climate and/or land‐use change depends on the stability of SOM against decomposition, which is influenced by stabilization mechanisms in the soil. To quantify organic carbon stocks and to clarify the importance of chemical and physical soil characteristics for carbon stabilization in volcanic ash soils, we applied selective extraction techniques, performed X‐ray diffraction analysis of the clay fraction and estimated pore‐size distribution of soils under natural upper montane forest and grassland (páramo) in the Ecuadorian Andes. Our results show that organic carbon stocks under both vegetation types are roughly twice as large as previously reported global averages for volcanic ash soils. SOM stabilization is suggested to be dominantly influenced by the following chemical and physical soil characteristics: (i) direct stabilization of SOM in organo‐metallic (Al‐humus) complexes, explaining at most 40% of carbon accumulation, (ii) indirect protection of SOM (notably aliphatic compounds) through low soil pH and toxic levels of Al, and probably also (iii) physical protection of SOM caused by a very large micro‐porosity. Moreover, in the case of the forest soils, inherent recalcitrance of OM itself was responsible for substantial accumulation in ectorganic horizons. Both vegetation types contributed to soil acidification, thus increasing SOM accumulation.

Does Physical Protection of Soil Organic Matter Attenuate Temperature Sensitivity?

Global climate change may induce accelerated soil organic matter (SOM) decomposition through increased soil temperature, and thus impact the C balance in soils. We hypothesized that compartmentalization of substrates and decomposers in the soil matrix would decrease SOM sensitivity to temperature. We tested our hypothesis with three short‐term laboratory incubations with differing physical protection treatments conducted at different temperatures. Overall, CO2 efflux increased with temperature, but responses among physical protection treatments were not consistently different. Similar respiration quotient (Q10) values across physical protection treatments did not support our original hypothesis that the largest Q10 values would be observed in the treatment with the least physical protection. Compartmentalization of substrates and decomposers is known to reduce the decomposability of otherwise labile material, but the hypothesized attenuation of temperature sensitivity was not detected, and thus the sensitivity is probably driven by the thermodynamics of biochemical reactions as expressed by Arrhenius‐type equations.