Modelling changes in soil organic matter after planting fast-growing Pinus radiata on Mediterranean agricultural soils

The Kyoto Protocol explicitly allows the storage of carbon (C) in ecosystems resulting from afforestation to be offset against a nation's carbon emissions and paves the way for carbon storage in soils to be eligible as carbon offsets in the future. More information is required about how afforestation affects carbon storage, especially in the soil. We report a study in which soil carbon storage in first‐rotation Mediterranean Pinus radiata plantations, established on former cereal fields and vineyards, was measured and modelled. Measurements were made on plantations of several ages, as well as repeat measurements at the same site after 5 years. We tested the ability of two widely used soil organic matter models (RothC and Century) to predict carbon sequestration in Mediterranean forest soils. Increases in the top 5 cm of soil of about 10 g C m^?2 year^?1 were observed after afforestation of former vineyards, but nitrogen (N) either remained the same or decreased slightly. During afforestation, most organic matter accumulated in the ectorganic layers at a rate of 19 g C m^?2 year^?1 in former vineyards and 41 g C m^?2 year^?1 in former cereal fields. The RothC and Century models were sensitive to previous land use and estimated a carbon sequestration potential over 20 years of 950 and 700 g C m^?2, respectively. The accurate simulation of the dynamics of soil organic matter by RothC, together with measured above‐ground inputs, allowed us to calculate below‐ground inputs during afforestation. The Century model simulated total C and N, including the ectorganic horizons, well. Simulations showed a depletion of N in the below‐ground fractions during afforestation, with N limitation in the former vineyard but not on former cereal land. The approach demonstrates the potential of models to enhance our understanding of the processes leading to carbon sequestration in soils.     

Effect of fertilization on respiration from different sources in a sandy soil of an agricultural long-term experiment

Annual changes in stocks of soil organic carbon may be detected by measurement of heterotrophic respiration, but field studies of heterotrophic respiration in long-term fertilization experiments on sandy soils are scarce. Our objectives were to: (1)investigate the influence of fertilizer type on mineralization of soil organic carbon and crop residue, and (2) show how fertilization treatments affect the annual C balance (net ecosystem carbon balance, NECB; negative values indicate a CO₂-source) in the sandy soil of the Darmstadt experiment. Treatments were long-term mineral fertilization with cereal straw incorporation (MSI) and application of rotted farmyard manure (FYM), both treatments receiving 14 g N m^?2 year^?1. This study used δ¹³C natural abundance after introduction of a C₄ crop to distinguish between different sources of respiration. Mineralization derived from C₃ sources was similar for MSI and FYM treatments (～270 g C m^?2 year^?1). The rate of residue mineralization in MSI treatments was higher, resulting in a mineralization of 49 and 37% of initial residue C in the soil of MSI and FYM treatments, respectively. The NECB (g C m^?2 year^?1) indicated the MSI treatment (approximately ?190) as a stronger source compared with the FYM treatment (～?30).     

Energy balances and SOC and N stocks as affected by organic amendments and inorganic N fertilization in a semi-arid environment (IOSDV-Madrid)

A long-term field experiment (1984–2011), was conducted on a Calcic Haploxeralf from semi-arid central Spain to evaluate the combined effect of three treatments: farmyard manure (FYM), straw and control without organic amendments (WOA) and five increasing rates of mineral N on: (1) some energetic parameters of crop production, and (2) the effect of the different treatments on soil organic carbon (SOC) and total N stocks. Crop rotation included spring barley, wheat and sorghum. The energy balance variables considered were net energy produced (energy output minus energy input), the energy output/input ratio and energy productivity (crop yield per unit energy input). Results showed small differences between treatments. Total energy inputs varied from 9.86 GJ ha^?1 year^?1 (WOA) to 11.14 GJ ha^?1 year^?1 in the FYM system. For the three crops, total energy inputs increased with increasing rates of mineral N. Energy output was slightly lower in the WOA (33.40 GJ ha^?1 year^?1) than in the two organic systems (37.34 and 34.96 GJ ha^?1 year^?1 for FYM and straw respectively). Net energy followed a similar trend. At the end of the 27-year period, the stocks of SOC and total N had increased noticeably in the soil profile (0–30 cm) as a result of application of the two organic amendments. Most important SOC changes occurred in the FYM plots, with mean increases in the 0–10 cm depth, amounting an average of 9.9 Mg C ha^?1 (667 kg C ha^?1 year^?1). Increases in N stocks in the top layer were similar under FYM and straw and ranged from 0.94 to 1.55 Mg N ha^?1. By contrast, simultaneous addition of increasing rates of mineral N showed no significant effect on SOC and total N storage.     

Soil organic carbon dynamics at the regional scale as influenced by land use history: a case study in forest soils from southern Belgium

Land use change (LUC) is known to have a large impact on soil organic carbon (SOC) stocks. However, at a regional scale, our ability to explain SOC dynamics is limited due to the variability generated by inconsistent initial conditions between sample points, poor spatial information on previous land use/land management history and scarce SOC inventories. This study combines the resampling in 2003–2006 of an extensive soil survey in 1950–1960 with exhaustive historical data on LUC (1868–2006) to explain observed changes in the SOC stocks of temperate forest soils in the Belgian Ardennes. Results from resampling showed a significant loss of SOC between the two surveys, associated with a decrease in variability. The mean carbon content decreased from 40.4 to 34.5 g C kg^?1 (10.6 to 9.6 kg C m^?2), with a mean rate of C change (ΔSOC) of ?0.15 g C kg^?1 year^?1 (?0.023 kg C m^?2 year^?1). Soils with high SOC content tended to loose carbon while conversely soils with low SOC tended to gain carbon. Land use change history explained a significant part of past and current SOC stocks as well as ΔSOC during the last 50 years. We show that the use of spatially explicit historical data can help to quantitatively explain changes in SOC content at the regional scale.     

Influence of land use (savanna,pasture, Eucalyptus plantations) on soil carbon and nitrogen stocks in Brazil

In Brazil, most Eucalyptus stands have been planted on Cerrado (shrubby savanna) or on Cerrado converted into pasture. Case studies are needed to assess the effect of such land use changes on soil fertility and C sequestration. In this study, the influence of Cerrado land development (pasture and Eucalyptus plantations) on soil organic carbon (SOC) and nitrogen (SON) stocks were quantified in southern Brazil. Two contrasted silvicultural practices were also compared: 60 years of short‐rotation silviculture (EUC_SR) versus 60 years of continuous growth (EUC_HF). C and N soil concentrations and bulk densities were measured and modelled for each vegetation type, and SOC and SON stocks were calculated down to a depth of 1 m by a continuous function. Changes in SOC and SON stocks mainly occurred in the forest floor (no litter in pasture and up to 0.87 kg C m^?2 and 0.01 kg N m^?2 in EUC_SR) and upper soil horizons. C and N stocks and their confidence intervals were greatly influenced by the methodology used to compute these layers. C/N ratio and ¹³C analysis showed that down to a depth of 30 cm, the Cerrado organic matter was replaced by organic matter from newly introduced vegetation by as much as 75–100% for pasture and about 50% for EUC_HF, poorer in N for Eucalyptus stands (C/N larger than 18 for Eucalyptus stands). Under pasture, 0–30 cm SON stocks (0.25 kg N m^?2) were between 10 and 20% greater than those of the Cerrado (0.21 kg N m^?2), partly due to soil compaction (limit bulk density at soil surface from 1.23 for the Cerrado to 1.34 for pasture). Land development on the Cerrado increased SOC stocks in the 0–30 cm layer by between 15 and 25% (from 2.99 (Cerrado) to 3.86 (EUC_SR) kg C m^?2). When including litter layers, total 0–30 cm carbon stocks increased by 35% for EUC_HF (4.50 kg C m^?2) and 53% for EUC_SR (5.08 kg C m^?2), compared with the Cerrado (3.28 kg C m^?2), independently of soil compaction.     

Changes in soil organic carbon and nitrogen in an area of Andisol following afforestation with Japanese cedar and Hinoki cypress

Abstract

To determine the rates of increase in C and N stocks in the soil and organic layers following afforestation in Andisols, we measured C and N densities in the organic and soil layers at depths of 0–5, 5–15 and 15–30?cm, together with a chronosequence analysis of 4-year-old, 14-year-old and 23-year-old Japanese cedar (Cryptomeria japonica) and 4-year-old, 12-year-old and 25-year-old Hinoki cypress (Chamaecyparis obtusa) plantations. The short-term changes in C and N were confirmed by repeated sampling 5?years after the first sampling. Tree growth, biomass accumulation and organic layers were much greater in Japanese cedar than in Hinoki cypress plantations. Soil C density (kg?m^?3) increased and bulk density decreased with stand age in the surface layer (0–5?cm). The average soil C accumulation rate was 22.9?g?C?m^?2?year^?1 for Japanese cedar and 21.1?g?C?m^?2?year^?1 for Hinoki cypress. Repeated sampling showed that the rate of increase in C in the surface soil was relatively slow in young stands and that soil C density (kg?m^?3) in the subsurface soil did not change over a 5-year period. Although N accumulated in the tree biomass and organic layers, the soil N density (kg?m^?3) did not change after afforestation. Although the andic properties of the soil and differences in the planted species did not influence the rate of increase in soil C, soil C density was expected to increase to a concentration greater than 80?g?kg^?1, possibly because of the large C accumulation capacity of Andisols.     

Effect of fire on solute release from organic horizons under larch forest in Central Siberian permafrost terrain

To evaluate the effects of forest fire and post-fire stand recovery on the organic layer chemistry and solute release within mound and trough microrelief elements (termed earth hummock microtopography) that mainly distribute permafrost affected area, we chose five fire plots (larch forests burned in 1951, 1981, 1990, 1994 and 2005) paired with adjacent control plots in mature larch forests in Central Siberian permafrost terrain. We determined total carbon, nitrogen and ash content in solid organic soils, and analyzed total carbon, nitrogen, bases and major anions in water extracts. There was a significant correlation between water-extracted organic carbon (WEOC) and total carbon (kg m⁻²) in area basis, implying that the quantity of total carbon was a major factor in WEOC production. WEOC correlated negatively with pH, indicating strong control by organic horizons (organic solute leaching) on soil acidity and base cation dynamics. The sum of water extractable base cations was also correlated significantly to total carbon, indicating that cations can be released through organic matter decomposition. Organic horizons in troughs in burned plots released greater amounts of Ca, Mg and K than those in mounds, probably due to greater content of organic matter as a cation source. Anions including nitrate and phosphate and WEOC also accumulated in trough depressions, due probably to organic matter degradation. The contrasting distribution of solutes between mounds and troughs in burned plots seems to be controlled by organic horizon development via changes in microtopography after forest fires.     

Impacts of land use and cropland management on soil organic matter and greenhouse gas emissions in the Brazilian Cerrado

The Brazilian Cerrado is a large and expanding agricultural frontier, representing a hotspot of land-use change (LUC) from natural vegetation to farmland. It is known that this type of LUC impacts soil organic matter (SOM) dynamics, particularly labile carbon (C) pools (living and non-living), decreasing soil health and agricultural sustainability, as well as increasing soil greenhouse gas (GHG) emissions, and accelerating global climate change. In this study, we quantified the changes in the quantity and quality of SOM and GHG fluxes due to changes in land use and cropland management in the Brazilian Cerrado. The land uses studied were native vegetation (NV), pasture (PA) and four croplands, including the following management types: conventional tillage with a single soybean crop (CT), and three no-tillage systems with two crops cultivated in the same year (i.e., soybean/sorghum (NT_SSo), soybean/millet (NT_SMi) and maize/sorghum (NT_MSo)). Soil and gases were sampled in the rainy season (November, December and January) and dry season (May, July and September). The highest soil C and nitrogen (N) stocks (6.7 kg C m⁻² and 0.5 kg N m⁻², 0–0.3-m layer) were found under NV. LUC reduced C stocks by 25% in the CT and by 10% in the PA and NT. Soil N stocks were 30% lower in the PA and NT_MSo and 15% lower in the croplands with soybean compared to NV. δ¹³C values clearly distinguished between the C-origin from NV (−25‰) and that from other land uses (−16‰). Soil (0–0.1 m) under NV also presented higher labile-C (625 g C m⁻²), microbial-C (70 g C m⁻²) and microbial-N (5.5 g N m⁻²), whereas other land uses presented values three times lower. GHG emissions (expressed as C-equivalent) were highest in the NV (1.2 kg m⁻² year⁻¹), PA (1.3 kg m⁻² year⁻¹) and NT_MSo (0.9 kg m⁻² year⁻¹) and were positively related to the higher SOM turnover in these systems. Our results suggest that in order to maintain SOM, it is necessary to adopt “best” management practices, that provide large plant residue inputs (above- and belowground). This can be seen as a pathway to achieving high food production with low GHG emissions.     

Controls on carbon accumulation and storage in the mineral subsoil beneath peat in Lakkasuo mire, central Finland

What processes control the accumulation and storage of carbon (C) in the mineral subsoil beneath peat? To find out we investigated four podzolic mineral subsoil profiles from forest and beneath peat in Lakkasuo mire in central boreal Finland. The amount of C in the mineral subsoil ranged from 3.9 to 8.1 kg m^?2 over a thickness of 70 cm and that in the organic horizons ranged from 1.8 to 144 kg m^?2. Rates of increase of subsoil C were initially large (14 g m^?2 year^?1) as the upland forest soil was paludified, but decreased to < 2 g m^?2 year^?1 from 150 to 3000 years. The subsoils retained extractable aluminium (Al) but lost iron (Fe) as the surrounding forest podzols were paludified beneath the peat. A stepwise, ordinary least‐squares regression indicated a strong relation (R² = 0.91) between organic C concentration of 26 podzolic subsoil samples and dithionite–citrate–bicarbonate‐extractable Fe (negative), ammonium oxalate‐extractable Al (positive) and null‐point concentration of dissolved organic C (DOC_np) (positive). We examined the ability of the subsoil samples to sorb dissolved organic C from a solution derived from peat. Null‐point concentration of dissolved C (DOC_np) ranged from 35 to 83 mg l^?1, and generally decreased from the upper to the lower parts of the profiles (average E, B and C horizon DOC_np concentrations of 64, 47 and 42 mg l^?1). The DOC_np was positively correlated with percentage of soil C and silt and clay content. The concentration of dissolved organic C in pore water in the peat ranged from 12 to 60 mg l^?1 (average 33 mg l^?1), suggesting that the sorptive capacity of the subsoil horizons for C had been exhausted. We suggest that the increase of C contents in the subsoil beneath mires is related to adsorption of dissolved organic C and slow mineralization under anaerobic conditions.     

Nitrogen deposition impacts on the amount and stability of soil organic matter in an alpine meadow ecosystem depend on the form and rate of applied nitrogen

The effects of atmospheric nitrogen (N) deposition on carbon (C) sequestration in terrestrial ecosystems are controversial. Therefore, it is important to evaluate accurately the effects of applied N levels and forms on the amount and stability of soil organic carbon (SOC) in terrestrial ecosystems. In this study, a multi‐form, small‐input N addition experiment was conducted at the Haibei Alpine Meadow Ecosystem Research Station from 2007 to 2011. Three N fertilizers, NH₄Cl, (NH₄)₂SO₄ and KNO₃, were applied at four rates: 0, 10, 20 and 40 kg N ha^?1 year^?1. One hundred and eight soil samples were collected at 10‐cm intervals to a depth of 30 cm in 2011. Contents and δ¹³C values of bulk SOC were measured, as well as three particle‐size fractions: macroparticulate organic C (MacroPOC, > 250 µm), microparticulate organic C (MicroPOC, 53–250 µm) and mineral‐associated organic C (MAOC, < 53 µm). The results show that 5 years of N addition changed SOC contents, δ¹³C values of the bulk soils and various particle‐size fractions in the surface 10‐cm layer, and that they were dependent on the amounts and forms of N application. Ammonium‐N addition had more significant effects on SOC content than nitrate‐N addition. For the entire soil profile, small additions of N increased SOC stock by 4.5% (0.43 kg C m^?2), while medium and large inputs of N decreased SOC stock by 5.4% (0.52 kg C m^?2) and 8.8% (0.85 kg C m^?2), respectively. The critical load of N deposition appears to be about 20 kg N ha^?1 year^?1. The newly formed C in the small‐input N treatment remained mostly in the > 250 µm soil MacroPOC, and the C lost in the medium or large N treatments was from the > 53 µm POC fraction. Five years of ammonium‐N addition increased significantly the surface soil POC:MAOC ratio and increased the instability of soil organic matter (SOM). These results suggest that exogenous N input within the critical load level will benefit C sequestration in the alpine meadow soils on the Qinghai–Tibetan Plateau over the short term.     

Impacts of nitrogen addition on the carbon balance in a temperate semiarid grassland ecosystem

Understanding carbon (C) cycling and sequestration in vegetation and soils, and their responses to nitrogen (N) deposition, is important for quantifying ecosystem responses to global climate change. Here, we describe a 2-year study of the C balance in a temperate grassland in northern China. We measured net ecosystem CO₂ exchange (NEE), net ecosystem production (NEP), and C sequestration rates in treatments with N addition ranging from 0 to 25 g N m^?2 year^?1. High N addition significantly increased ecosystem C sequestration, whose rates ranged from 122.06 g C m^?2 year^?1 (control) to 259.67 g C m^?2 year^?1 (25 g N). Cumulative NEE during the growing season decreased significantly at high and medium N addition, with values ranging from ?95.86 g C m^?2 (25 g N) to 0.15 g C m^?2 (5 g N). Only the highest N rate increased significantly cumulative soil microbial respiration compared with the control in the dry 2014 growing season. High N addition significantly increased net primary production (NPP) and NEP in both years, and NEP ranged from ?5.83 to 128.32 g C m^?2. The C input from litter decomposition was significant and must be quantified to accurately estimate NPP. Measuring C sequestration and NEP together may allow tracking of the effects of N addition on grassland C budgets. Overall, adding 25 or 10 g N m^?2 year^?1 improved the CO₂ sink of the grassland ecosystem, and increased grassland C sequestration.     

Changes in organic matter,nitrogen, phosphorus and cations in soil as a result of fire and water erosion in a Mediterranean landscape

Fire affects large parts of the dry Mediterranean shrubland, resulting in erosion and losses of plant nutrients. We have attempted to measure these effects experimentally on a calcareous hillside representative of such shrubland. Experimental fires were made on plots (4 m × 20 m) in which the fuel was controlled to obtain two different fire intensities giving means of soil surface temperature of 439°C and 232°C with temperatures exceeding 100°C lasting for 36 min and 17 min. The immediate and subsequent changes induced by fire on the soil's organic matter content and other soil chemical properties were evaluated, together with the impact of water erosion. Seven erosive rain events, which occurred after the experimental fires (from August 1995 to December 1996), were selected, and on them runoff and sediment produced from each plot were measured. The sediments collected were weighed and analysed. Taking into account the variations induced by fire on the soil properties and their losses by water erosion, estimates of the net inputs and outputs of the soil system were made. Results show that the greatest losses of both soil and nutrients took place in the 4 months immediately after the fire. Plots affected by the most intense fire showed greater losses of soil (4077 kg ha^?1) than those with moderate fire intensity (3280 kg ha^?1). The unburned plots produced the least sediment (72.8 kg ha^?1). Organic matter and nutrient losses by water erosion were related to the degree of fire intensity. However, the largest losses of N‐NH₄⁺ and N‐NO₃^– by water erosion corresponded to the moderate fire (8.1 and 7.5 mg N m^?2, respectively).     

Nitrous oxide emissions and nitrogen cycling in managed grassland in Southern Hokkaido,Japan

Abstract

Nitrous oxide (N₂O) emissions were measured and nitrogen (N) budgets were estimated for 2?years in the fertilizer, manure, control and bare plots established in a reed canary grass (Phalaris arundinacea L.) grassland in Southern Hokkaido, Japan. In the manure plot, beef cattle manure with bark was applied at a rate of 43–44?Mg fresh matter (236–310?kg?N)?ha^?1?year^?1, and a supplement of chemical fertilizer was also added to equalize the application rate of mineral N to that in the fertilizer plots (164–184?kg?N?ha^?1?year^?1). Grass was harvested twice per year. The total mineral N supply was estimated as the sum of the N deposition, chemical fertilizer application and gross mineralization of manure (GMm), soil (GMs), and root-litter (GMl). GMm, GMs and GMl were estimated by dividing the carbon dioxide production derived from the decomposition of soil organic matter, root-litter and manure by each C?:?N ratio (11.1 for soil, 15.5 for root-litter and 23.5 for manure). The N uptake in aboveground biomass for each growing season was equivalent to or greater than the external mineral N supply, which is composed of N deposition, chemical fertilizer application and GMm. However, there was a positive correlation between the N uptake in aboveground biomass and the total mineral N supply. It was assumed that 58% of the total mineral N supply was taken up by the grass. The N supply rates from soil and root-litter were estimated to be 331–384?kg?N?ha^?1?year^?1 and 94–165?kg?N?ha^?1?year^?1, respectively. These results indicated that the GMs and GMl also were significant inputs in the grassland N budget. The cumulative N₂O flux for each season showed a significant positive correlation with mineral N surplus, which was calculated as the difference between the total mineral N supply and N uptake in aboveground biomass. The emission factor of N₂O to mineral N surplus was estimated to be 1.2%. Furthermore, multiple regression analysis suggested that the N₂O emission factor increased with an increase in precipitation. Consequently, soil and root-litter as well as chemical fertilizer and manure were found to be major sources of mineral N supply in the grassland, and an optimum balance between mineral N supply and N uptake is required for reducing N₂O emission.     

Tillage Effects on Particulate and Mineral‐Associated Organic Matter in Two Tropical Brazilian Soils

Abstract

Two Ferralsols (350 and 600 g kg^?1 clay) from the Brazilian Cerrado Region were evaluated for long‐term effects (5 and 8 years) of no tillage on carbon (C) stocks in particulate (>53 µm) and mineral‐associated (<53 µm) soil organic matter (SOM) fractions. Carbon stocks in particulate SOM increased under no tillage compared with conventional tillage, and the rate was higher in the clayey soil (0.62 Mg C ha^?1 yr^?1) than in the sandy clay loam soil (0.31 Mg C ha^?1 yr^?1). In contrast, the mineral‐associated SOM in the top soil layer (0–20 cm) was not affected by tillage system. Sequestration of atmospheric C in tropical no‐tillage soils seems to be due to accumulation of C in labile SOM fractions, with highest rates in clayey soils probably due to physical protection.     

Application of wood ash accelerates soil respiration and tree growth on drained peatland

Forested peatlands contain large pools of terrestrial carbon. As well as drainage, forest management such as fertilizer application can affect these pools. We studied the effect of wood ash (application rates 0, 5 and 15 t ha^?1) on the heterotrophic soil respiration (CO₂ efflux), cellulose decomposition, soil nutrients, biomass production and amount of C accumulated in a tree stand on a pine‐dominated drained mire in central Finland. The ash was spread 13 years before the respiration measurements. The annual CO₂ efflux was statistically modelled using soil temperature as the driving variable. Wood ash application increased the amounts of mineral nutrients of peat substantially and increased soil pH in the uppermost 10 cm layer by 1.5–2 pH units. In the surface peat, the decomposition rate of cellulose in the ash plots was roughly double that in control plots. Annual CO₂ efflux was least on the unfertilized site, 238 g CO₂‐C m^?2 year^?1. The use of wood ash nearly doubled CO₂ efflux to 420–475 g CO₂‐Cm^?2 year^?1 on plots fertilized with 5–15 t ha^?1 of ash, respectively. Furthermore, ash treatments resulted also in increased stand growth, and during the measurement year, the growing stand on ash plots accumulated carbon 11–12 times faster than the control plot. The difference between peat C emission and amount of C sequestered by trees on the ash plots was 43–58 g C m^?2, while on the control plot it was 204 g C m^?2. Our conclusion is that adding wood ash as a fertilizer increases more C sequestration in the tree stand than C efflux from the peat.     

Mechanisms of short-term soil carbon storage in experimental grasslands

We investigated the fate of root and litter derived carbon in soil organic matter and dissolved organic matter in soil profiles, in order to explain mechanisms of short-term soil carbon storage. A time series of soil and soil solution samples was investigated at the field site of The Jena Experiment between 2002 and 2004. In addition to the main experiment with C3 plants, a C4 species (Amaranthus retroflexus L.) naturally labeled with ¹³C was grown on an extra plot. Changes in organic carbon concentration in soil and soil solution were combined with stable isotope measurements to follow the fate of plant carbon into the soil and soil solution. A split plot design with plant litter removal versus double litter input simulated differences in biomass input. After 2 years, the no litter and double litter treatment, respectively, showed an increase of 381 g C m^?2 and 263 g C m^?2 to 20 cm depth, while 71 g C m^?2 and 393 g C m^?2 were lost between 20 and 30 cm depth. The isotopic label in the top 5 cm indicated that 115 g C m^?2 and 156 g C m^?2 of soil organic carbon were derived from C4 plant material on the no litter and the double litter treatment, respectively. Without litter, this equals the total amount of 97 g C m^?2 that was newly stored in the same soil depth, whereas with double litter this clearly exceeded the stored amount of 75 g C m^?2. Our results indicate that litter input resulted in lower carbon storage and larger carbon losses and consequently accelerated turnover of soil organic carbon. Isotopic evidence showed that inherited soil organic carbon was replaced by fresh plant carbon near the soil surface. Our results suggest that primarily carbon released from soil organic matter, not newly introduced plant organic matter, was transported in the soil solution. However, the total flow of dissolved organic carbon was not sufficient to explain the observed carbon storage in deeper soil layers, and the existence of additional carbon uptake mechanisms is discussed.     

Black carbon in wildfire‐affected shrubland Mediterranean soils

Because Mediterranean ecosystems are prone to fire, their soils are expected to contain relevant amounts of black carbon (BC); nevertheless, quantitative information is scarce. Herein, we provide data on the abundance of BC in the surface soil (uppermost 5 cm) of shrubland plots on old agricultural fields diversely affected by fires (0, 1, or 2 wildfires in the last 25 y) and with contrasted land‐use histories (either never cropped, early abandoned, or recently abandoned). Black C and black nitrogen (BN) were quantified in the surface horizon (0–5 cm) as the residue of low‐temperature dichromate oxidation, after previous destruction of mineral matter with HF. The obtained amounts of BC ranged from 0.73 to 10.32 g (kg dw)^–1 (mean: 3.07, which corresponds to an average of 8.62% of the total organic C), while the amounts of BN ranged from 21.5 to 373.0 mg (kg dw)^–1 (mean: 97.1, or an average of 4.30% of the total N of the samples). Repeated fires did not consistently increase either the BC or the BN amounts. Black‐C and (especially) BN accumulation seems related to fine silt, whereas the effect of clay is unclear. Even though the amounts of BC obtained in this study are slightly higher than those from other ecosystems, including Mediterranean broad‐leaved forests, overall they are far from the very high values reported in the literature for chernozems from Germany or Canada. Thus, on the whole, in Mediterranean shrublands affected by wildfires, BC does not seem to be a dominant fraction in the soil organic C.     

Below‐ground and above‐ground production of vegetational organic matter along a climosequence in alpine grasslands

The distribution of vegetational organic matter above‐ and below‐ground and its productivity was analyzed in an alpine area along a climosequence ranging from subalpine to alpine climates. Emphasis is placed on the quantification of carbon (C) and nitrogen (N) fixed in the above‐ground and below‐ground vegetation and its annual input. Annual C‐input ranged from 17.9 to 60.2 g m^—2 year^—1 and the N‐input from 0.74 to 2.48 g m^—2 year^—1. Above‐ground phytomass and the annual production rate of organic matter showed a distinct correlation with the altitude and, thus, the climate. However, the measurement of the above‐ground phytomass is bound to methodological problems: the commonly used harvesting method seems to underestimate the real situation. The harvesting method yielded in its average 100 to 300 g m^—2 phytomass which was 35—83% of the values obtained by the soil core method. Thus, the calculation of turnover times of above‐ground vegetation greatly depends on the method used. Calculated turnover times based on the harvesting method did not correlate with the climate while a clear tendency of lower turnover times with increasing altitude could be observed using the soil core method. The amount of below‐ground phytomass was in the range of 1880 to 2469 g m^—2 and the corresponding annual C‐input (fixation in the roots) between 91.1 and 162 g m^—2 year^—1 and the N‐input between 2.68 and 4.99 g m^—2 year^—1. The below‐ground phytomass and its production rate in high alpine zones are of greater importance and exceed the above‐ground ones. With increasing altitude, furthermore, the importance of the below‐ground phytomass increases with respect to the biomass and to the C‐ and N‐input. For high alpine areas, the phytomass is concentrated in the uppermost soil horizons. About 88.7 to 94.5% of the below‐ground phytomass was found in the soil compartment 0‐20 cm. The below‐ground production rate of phytomass in alpine grassland is fundamental in order to calculate any C or N budgets and potential inputs to SOM: its neglection would introduce most significant errors in modeling any C or N cycles.     

Impact of tillage intensity on carbon and nitrogen pools in surface and sub‐surface soils of three long‐term field experiments

Management options such as the intensity of tillage are known to influence the turnover dynamics of soil organic matter. However, less information is available about the influence of the tillage intensity on individual soil organic matter pools with different turnover dynamics in surface as compared with sub‐surface soils. This study aimed to analyse the impact of no tillage (NT), reduced tillage (RT) and conventional tillage (CT) on labile, intermediate and stable carbon (C) and nitrogen (N) pools in surface and sub‐surface soils. We took surface and sub‐surface soil samples from the three tillage systems in three long‐term field experiments in Germany. The labile, intermediate and stable C and N pool sizes were determined by using the combined application of a decomposition experiment and a physical‐chemical separation procedure. For the surface soils, we found larger stocks of the labile C and N pool under NT and RT (C, 1.7 and 1.3 t ha^?1; N, 180 and 160 kg ha^?1) than with CT (C, 0.5 t ha^?1; N, 60 kg ha^?1). In contrast, we found significantly larger stocks of the labile C pool under CT (2.7 t ha^?1) than with NT and RT (2 t ha^?1) for the sub‐surface soils. The intermediate pool accounted for 75–84% of the soil organic C and total N stocks. However, the stocks of the intermediate N and C pools were only distinctly larger for NT than for CT in the surface soils. The stocks of the stable C and N pools were not affected by the tillage intensity but were positively correlated with the stocks of the clay‐size fraction and oxalate soluble aluminum, indicating a strong influence of site‐specific mineral characteristics on the size of these pools. Our results indicate soil depth‐specific variations in the response of organic matter pools to tillage of different intensity. This means that the potential benefits of decreasing tillage intensity with respect to soil functions that are closely related to organic matter dynamics have to be evaluated separately for surface and sub‐surface soils.     

Carbon sequestration in forest soils: effects of soil type, atmospheric CO2 enrichment, and N deposition

Soil contains the major part of carbon in terrestrial ecosystems, but the response of this carbon to enriching the atmosphere in CO₂ and to increased N deposition is not completely understood. We studied the effects of CO₂ concentrations at 370 and 570 μmol CO₂ mol^?1 air and increased N deposition (7 against 0.7 g N m^?2 year^?1) on the dynamics of soil organic C in two types of forest soil in model ecosystems with spruce and beech established in large open‐top chambers containing an acidic loam and a calcareous sand. The added CO₂ was depleted in ¹³C and thus the net input of new C into soil organic carbon and the mineralization of native C could be quantified. Soil type was the greatest determining factor in carbon dynamics. After 4 years, the net input of new C in the acidic loam (670 ± 30 g C m^?2) exceeded that in the calcareous sand (340 ± 40 g C m^?2) although the soil produced less biomass. The mineralization of native organic C accounted for 700 ± 90 g C m^?2 in the acidic loam and for 2800 ± 170 g C m^?2 in the calcareous sand. Unfavourable conditions for mineralization and a greater physico‐chemical protection of C by clay and oxides in the acidic loam are probably the main reasons for these differences. The organic C content of the acidic loam was 230 g C m^?2 more under the large than under the small N treatment. As suggested by a negligible impact of N inputs on the fraction of new C in the acidic loam, this increase resulted mainly from a suppressed mineralization of native C. In the calcareous sand, N deposition did not influence C concentrations. The impacts of CO₂ enrichment on C concentrations were small. In the uppermost 10 cm of the acidic loam, larger CO₂ concentrations increased C contents by 50–170 g C m^?2. Below 10 cm depth in the acidic loam and at all soil depths in the calcareous sand, CO₂ concentrations had no significant impact on soil C concentrations. Up to 40% of the ‘new’ carbon of the acidic loam was found in the coarse sand fraction, which accounted for only 7% of the total soil volume. This suggests that a large part of the CO₂‐derived ‘new’ C was incorporated into the labile and easily mineralizable pool in the soil.