长期不同施肥对新疆荒漠农田土壤碳含量及其剖面分布的影响

以建于1990年的新疆阜康荒漠生态系统观测研究站的长期施肥定位试验为平台，分析了不施肥（CK）、单施化肥（化肥常规NPK和化肥高量N2P2K）、化肥配施秸秆（化肥配施秸秆常规NPKS和化肥配施秸秆高量NPKS2）和化肥配施猪粪（NPKM）对农田土壤有机碳和无机碳含量与剖面分布及有机碳热稳定性的影响。结果表明：（1）长期施肥处理提高了土壤有机碳含量，其中耕层（0—20 cm）土壤有机碳响应最敏感，化肥配施秸秆还田处理下有机碳积累高于其他处理。（2）长期施肥处理均降低土壤有机碳稳定性，表明增加的土壤有机碳并不利于碳稳定。（3）长期施肥处理降低土壤无机碳含量，其降低程度随剖面呈下降趋势，可能由于灌溉淋洗导致深层土壤无机碳的净增加。（4）尽管长期施肥促进土壤有机碳累积，但由于降低无机碳含量及其储量，最终导致土壤全碳降低，表明土壤无机碳变化对干旱区土壤碳库变化具有决定性作用。结果表明，深入理解施肥措施对干旱区盐碱土的碳影响，需要同时考虑不同深度土壤有机碳和无机碳含量的变化。     

Estimates of carbon stock changes in Belgian cropland

Abstract. Article 3.4 of the Kyoto Protocol allows carbon emissions to be offset by demonstrable removal of carbon from the atmosphere by improved management of agricultural soils. To make use of this possibility, a good estimate of soil organic carbon (SOC) stocks and baseline emissions (in 1990) are crucial factors. Over 210 000 topsoil (0–24 cm) measurements have been made in Belgian cropland in the period 1989–1999, which are available for seven different agro-pedological regions and for three periods (1989–91, 1992–95, 1996–99). We used this extensive SOC data set to estimate SOC stocks and fluxes in Belgian cropland. The measurements of SOC were extrapolated to 1 m depth using an exponential SOC depth distribution model, based on another large data set of over 5184 fully described soil profiles on Belgian cropland made during the National Soil Survey. The SOC data were combined with cropland area figures to calculate SOC stocks to 1 m depth. The 1990 baseline SOC flux of Belgian cropland was then obtained using two different calculation methods, which each yielded similar results and showed that SOC stocks were decreasing in the 1990s at a mean rate of 608 kton OC yr⁻¹. Consequently, a large part of the Belgian cropland acted as a net source of CO₂ emissions during the period 1989–1999.     

干旱半干旱区农田土壤碳垂直剖面分布特征研究

以中国干旱半干旱区农田土壤为研究对象,通过收集自然农田和长期定位站点(178个剖面,0~100 cm土层)农田土壤碳的数据并对其进行整合,分析了农田土壤有机碳和无机碳含量的垂直剖面分布特征及其影响因素。结果表明,随土层深度增加,农田土壤有机碳呈下降趋势,表层含量高于底层;不同地区农田土壤无机碳含量变化趋势不一,随土壤深度增加整体呈现升高的趋势,但是也有一些地区呈现下降趋势。土壤剖面深度为100 cm的农田土壤有机碳和无机碳密度平均值分别为8.33和15.83 kg m-2,农田土壤无机碳储量大约是土壤有机碳的2倍。土壤深度为0~30 cm的有机碳占100 cm总有机碳含量的45%,无机碳仅占100 cm总无机碳含量的29%;土壤无机碳主要集中在30~100 cm土层,占100 cm总无机碳含量的71%,远高于有机碳在此土层占100 cm总有机碳含量的百分比(55%)。综合自然农田和长期定位站点农田土壤碳的数据,土壤容重与土壤p H是影响农田土壤有机碳和无机碳分布特征的重要因素:自然农田土壤有机碳与土壤p H(R2=0.61,p0.01)和土壤容重(R2=0.64,p0.01)呈显著负相关;长期定位站点土壤无机碳与土壤p H(R2=0.56,p0.01)和土壤容重(R2=0.63,p0.01)呈显著正相关。中国干旱半干旱区农田土壤有机碳和无机碳的分布特征与影响因素,将为陆地生态系统碳储量估算提供数据基础与理论支撑。     

Impact of land use changes on soil carbon, nitrogen and phosphorus and water pollution in an arid region of northwest China

Abstract. Physical, chemical and environmental consequences of land use change from cultivated land to desert grassland and vice-versa were monitored in the middle reaches of the Heihe River basin, which is one of the largest inland basins of arid northwest China. Levels of N and P in soils and surface waters and soil organic carbon were measured. After the first 3–5 years of cultivation the N and P contents of various former grassland soils, including mountain-meadow and plains-meadow grasslands, decreased significantly. After some 13 years of cultivation, soil nutrient content in former mountain meadow grasslands gradually stabilized, whereas those of desertified grassland, where cultivation had simply been abandoned, showed a notable decrease. Under these latter conditions, soil N and P were lost at a rate of 276 kg ha⁻¹ and 360 kg ha⁻¹, respectively, over the 13-year period. The transformation of grassland into cultivated land and that of cultivated land into desert grassland resulted in organic carbon emissions of 1.68 Tg C and 0.55 Tg C, respectively, over 13 years. Land use changes in the arid inland region clearly have a significant influence on the soil organic carbon pool and carbon cycle. Falls in soil N and P led to 63% and 34% mean enrichment of N and P, respectively, in downstream waters, thus posing a future environmental problem for the arid region of northwest China.     

A soil carbon and land use database for the United Kingdom

Abstract. The compilation of a database of soil carbon and land use is described, from which models of soil carbon dioxide emissions across the United Kingdom (UK) can be run. The database gives soil organic carbon, sand, silt and clay contents and bulk densities weighted to reference layers from 0 to 30 cm and from 30 to 100 cm depths. The data are interpolated from information on soil types and land use on a 1 km grid across the UK and are used to estimate soil carbon stocks. For 1990, the baseline year for the Kyoto Protocol on carbon emissions, the estimate is 4562 Tg soil organic carbon in the top 1 m of soil across the UK, with an average density of 18 kg m⁻². The data can be reported by layer (e.g. 54% in topsoils) and country (e.g. 48% in Scotland) as well as by soil and land type.     

Stock, turnover time and accumulation of organic matter in bulk and density fractions of a Podzol soil

Temperate forest soils store large amounts of organic matter and are considered as net sinks for atmospheric carbon dioxide. Information about the sink strength and the turnover time of soil organic carbon (SOC) is required to assess the potential response of soils to climate change. Here we report on stocks, turnover times (TT) and accumulation of SOC in bulk soil and density fractions from genetic horizons of a Podzol in the Fichtelgebirge, Germany. Stocks of SOC, total nitrogen and exchangeable cations determined in nine quantitative soil pits strongly varied with stone content and thickness of horizons in both the organic layer and the mineral soil. On the basis of radiocarbon signatures, mean turnover times of 4, 9 and 133 years, respectively, were calculated for Oi, Oe and Oa horizons from three soil pits, using a non-steady-state model. The Oa horizons accumulated 4–8 g C m⁻² year⁻¹ whereas the Oi and Oe horizons were close to steady-state during the past decade. Free particulate organic matter (FPOM) was the most abundant fraction in the Oa and EA horizons with TT of 70–480 years. In the B horizons, mineral associated organic matter (MAOM) dominated with over 40% of total SOC and had TT of 390–2170 years. In contrast to other horizons, MAOM in the Bsh and Bs horizon had generally faster TT than occluded particulate organic matter (OPOM), possibly because of sorption of dissolved organic carbon by iron and aluminium oxides/hydroxides. Our results suggest that organic horizons with relatively short turnover times could be particularly vulnerable to changes in climate or other disturbances.     

Carbon and nitrogen stocks in the soils of Central and Eastern Europe

Abstract. Soil organic carbon and total nitrogen stocks are presented for Central and Eastern Europe. The study uses the soil geographic and attribute data held in a 1:2 500 000 scale Soil and Terrain (SOTER) database, covering Belarus, Bulgaria, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Moldova, Poland, Romania, the Russian Federation (west of the Urals), Slovakia, and Ukraine. Means and coefficients of variation for soil organic carbon and total nitrogen are presented for each major FAO soil grouping. The mean content of organic carbon, to a depth of 1 m, ranges from 3.9 kg C m^–2 for coarse textured Arenosols to 72.9 kg C m^–2 for poorly drained Histosols. Mean carbon content for the mineral soils, excluding Arenosols, is 15.8 kg C m^–2. The top 1 m of soil holds 110 Pg C (Pg=10¹⁵ g), which corresponds to about 7% of the global stock of soil organic carbon. About 44% of this carbon pool is held in the top 0.3 m of the soil, the layer that is most prone to be changed by changes in soil use and management. About 166 million ha in Central and Eastern Europe have been degraded by compaction, erosion of topsoil, fertility decline and crusting. The achievable level of carbon sequestration for these soils, upon adoption of 'best' management practices or restorative measures, is estimated.     

Comparison of approaches for estimating carbon sequestration at the regional scale

Abstract. Many former estimates of regional scale C sequestration potential have made use of linear regressions based on long-term experimental data, whilst some have used dynamic soil organic matter (SOM) models linked to spatial databases. Few studies have compared the two methods. We present a case study in which the potential of different land management practices to sequester carbon in soil in arable land is estimated by different methods. Two dynamic SOM models were chosen for this study, RothC (a soil process model) and CENTURY (a whole ecosystem model with a SOM module). RothC and CENTURY are the two most widely used and validated SOM models worldwide. A Geographic Information System (GIS) containing soil, land use and climate layers, was assembled for a case study in central Hungary. GIS interfaces were developed for the RothC and CENTURY models, thus linking them to the spatial datasets at the regional level. This allowed a comparison of estimates of the C sequestration potential of different land management practices obtained using the two models and using regression based approaches. Although estimates obtained by the different approaches were of the same order of magnitude, differences were observed. Some of the land management scenarios studied here showed sufficient C mitigation potential to meet Hungarian CO₂ reduction commitments. For example, afforestation of 12% current arable land could sequester 0.042–0.092 Tg yr^–1 in the soil alone, or 0.285–0.588 Tg C yr^–1 in both soil and biomass; 1990 level CO₂ emissions for the study area were 4.7 Tg C with a corresponding reduction commitment of 0.282 Tg C. It is not, however, suggested that this is the only, or the most favourable way, in which to meet the commitments.     

Impacts of land use and salinization on soil inorganic and organic carbon in the middle-lower Yellow River Delta

Soil inorganic carbon (SIC) is an important reservoir of carbon (C) in arid, semi-arid, and semi-humid regions. However, knowledge is incomplete on the dynamics of SIC and its relationship with soil organic C (SOC) under different land use types in the semi-humid region, particularly in coastal zones impacted by soil salinization. We collected 170 soil samples from 34 profiles across various land use types (maize-wheat, cotton, paddy, and reed) in the middle-lower Yellow River Delta (YRD), China. We measured soil pH, electrical conductivity (EC), water-soluble salts, and SOC and SIC contents. Our results showed significant differences in both SOC and SIC among land use types. The dry cropland (maize-wheat and cotton) soils had significantly higher SOC and SIC densities (4.71 and 15.46 kg C m^-2, respectively) than the paddy soils (3.28 and 14.09 kg C m^-2, respectively) in the 0–100 cm layer. Compared with paddy soils, reed soils contained significantly higher SOC (4.68 kg C m^-2) and similar SIC (15.02 kg C m^-2) densities. There was a significant positive correlation between SOC and SIC densities over a 0–100 cm soil depth in dry cropland soils, but a negative relationship in the paddy soils. On average, SOC and SIC densities under maize-wheat cropping were 15% and 4% lower, respectively, in the salt-affected soils in the middle-lower YRD than the upper YRD. This study indicated that land use types had great influences on both SOC and SIC and their relationship, and salinization had adverse effect on soil C storage in the YRD.     

Impact of water percolation on nutrient leaching from an irrigated paddy field in Japan

Abstract. We examined the effect on soil nutrient status and sustainability of water percolation through an irrigated paddy field in Japan, to the depth of drainage (40 cm). The difference between amounts of nutrients leached by percolation and those supplied by irrigation indicated that 25–130 kg ha⁻¹ Ca, 8–24 kg ha⁻¹ Mg, from −1 to 9 kg ha⁻¹ K, and 8–17 kg ha⁻¹ Fe, respectively, were lost each year from the 0–40 cm soil layer during rice cultivation, when the supply from fertilization and rainfall and the loss in grain harvest were not accounted for. When the supply of K from rainfall and the loss in grain harvest were taken into account, a total K loss of about 10 kg ha⁻¹ was estimated. The electrical neutrality of inorganic ions in the percolating water was always maintained. From these results we estimate that the amounts of exchangeable Ca and Mg in the soil to a depth of 40 cm would decrease by 50% within 50–260 and 30–100 years, respectively, if similar management were continued without fertilization. The total amount of carbon dioxide (ΣCO₂) leached in percolating water during the period of rice cultivation was 120–325 kg C ha⁻¹, which corresponded to 0.47–0.94% of the soil organic carbon to 40 cm depth.     

Long-term changes in soil organic matter under conventional tillage and no-tillage systems in semiarid Morocco

Abstract. A no-tillage (NT) system was developed in semiarid Morocco to improve the soil fertility and stabilize yield through conservation of water. Results in two long-term trials (4 and 11 years) were able to show the effects of a no-tillage system in increasing total soil organic matter and total nitrogen. Over time, the quality of the NT soil surface was improved compared with that under conventional tillage (CT) with disc harrows. This effect was the result of an increase in soil organic carbon (SOC) and a slight decline in pH. However, over time, nitrogen decreased in both tillage practices, especially in the 0–25 mm layer (from 0.59 to 0.57 t ha⁻¹ and from 0.44 to 0.42 t ha⁻¹ under NT and CT, respectively). After 4 years of NT an extra 5.62 t ha⁻¹ of SOC was sequestered in the 0–25 mm layer, and after 11 years the SOC increased further to 7.21 t ha⁻¹.     

Methane consumption in a frequently nitrogen-fertilized and limed spruce forest soil after clear-cutting

Abstract. The effects of especially frequent nitrogen (N) additions (from 1959 to 1986, totalling 860 kg N ha⁻¹) and liming (in 1958 and 1980, totalling 6000 kg CaCO₃ ha⁻¹) on CH₄ uptake by a boreal forest soil were studied in a stand of Norway spruce. Except for a forested reference plot, the stand was clear-cut in January 1993 and the following year one-half of each clear-cut plot was prepared by mounding. Fluxes of CH₄ were measured with static chambers in the autumn before clear-cutting and during the following four summers. The average CH₄ uptake during 1993–96 in the forested reference plot was 82 μg CH₄ m⁻² h⁻¹(ranging from 10 to 147 units). In the first summer after clear-cutting, the cleared plot showed 42% lower CH₄ uptake rate than the forested reference plot, but thereafter the difference became less pronounced. The short-term decrease in CH₄ consumption after clear-cutting was associated with increases in soil NH₄⁺ and NO₃⁻concentrations. Mounding tended at first to stimulate CH₄ uptake but later to inhibit it. Neither liming nor N-fertilization had significant effects on CH₄ consumption. Our results suggest that over the long term, in N-limited upland boreal forest soils, N addition does not decrease CH₄ uptake by the soil.     

黄土高原丘陵区典型退耕恢复植被土壤碳分布特征及其影响因素

为深入了解植被恢复对土壤碳库的影响，本研究选取黄土高原丘陵区草地、沙棘、油松、山杏和山杏油松混交林0~100 cm土壤为研究对象，运用随机森林模型等方法，探究黄土高原丘陵区典型退耕恢复植被有机碳（SOC）、无机碳（SIC）、全碳（TC）含量分布特征及其影响因素。结果表明：（1）研究区各恢复植被平均土壤全碳含量为1.685~1.898 g/kg，平均土壤有机碳含量山杏(0.368 g/kg)>草地(0.299 g/kg)>沙棘(0.250 g/kg)>油松(0.233 g/kg)>油松山杏混交(0.209 g/kg) ，平均土壤无机碳含量为平均土壤有机碳含量的5.6倍。（2）所有恢复植被土壤深层（60~100 cm）无机碳含量均无显著差异（P>0.05）。除油松外，各恢复植被表层（0~20 cm）土壤有机碳含量显著高于其他土层（P<0.05）。（3）坡向、坡度、海拔、土地利用类型、土壤含水量、土壤黏粒、速效磷和速效氮共解释了78%、24%和77%的SOC、SIC和TC含量变化，其中海拔、坡向和土壤含水量为研究区土壤碳含量变化的主要影响因素（P<0.05）。在黄土高原植被恢复过程中应充分考虑地形因子和土壤理化性质的影响。本研究结果可为正确评估人工林土壤碳储量及其生态效益提供基础数据和科学参考。     

Soil Organic Carbon and Inorganic Carbon Accumulation Along a 30‐year Grassland Restoration Chronosequence in Semi‐arid Regions (China)

Carbon accumulation is an important research topic for grassland restoration. It is requisite to determine the dynamics of the soil carbon pools [soil organic carbon (SOC) and soil inorganic carbon (SIC)] for understanding regional carbon budgets. In this study, we chose a grassland restoration chronosequence (cropland, 0 years; grasslands restored for 5, 15 and 30 years, i.e. RG5, RG15 and RG30, respectively) to compare the SOC and SIC pools in different soil profiles. Our results showed that SOC stock in the 0‐ to 100‐cm soil layer showed an initial decrease in RG5 and then an increase to net C gains in RG15 and RG30. Because of a decrease in the SIC stock, the percentage of SOC stock in the total soil C pool increased across the chronosequence. The SIC stock decreased at a rate of 0·75 Mg hm⁻² y⁻¹. The change of SOC was higher in the surface (0–10 cm, 0·40 Mg hm⁻² y⁻¹) than in the deeper soil (10–100 cm, 0·33 Mg hm⁻² y⁻¹) in RG5. The accumulation of C commenced >5 years after cropland conversion. Although the SIC content decreased, the SIC stock still represented a larger percentage of the soil C pool. Moreover, the soil total carbon showed an increasing trend during grassland restoration. Our results indicated that the soil C sequestration featured an increase in SOC, offsetting the decrease in SIC at the depth of 0–100 cm in the restored grasslands. Therefore, we suggest that both SOC and SIC should be considered during grassland restoration in semi‐arid regions. Copyright © 2016 John Wiley & Sons, Ltd.     

评估烧失量测定法测定中国西北干旱土壤有机和无机碳的研究

There is a need for determinations of soil organic carbon (SOC) and inorganic carbon (SIC) due to increasing interest in soil carbon sequestration. Two sets of soil samples were collected separately from the Yanqi Basin of northwest China to evaluate loss-on-ignition (LOI) method for estimating SOC and SIC in arid soils through determining SOC using an element analyzer, a modified Walkley-Black method and a LOI method with combustion at 375℃ for 17 h and determining SIC using a pressure calcimeter method and a LOI procedure estimated by a weight loss between 375 to 800℃. Our results indicated that the Walkley-Black method provided 99%recovery of SOC for the arid soils tested. There were strong linear relationships(r > 0.93, P < 0.001) for both SOC and SIC between the traditional method and the LOI technique. One set of soil samples was used to develop relationships between LOI and SOC(by the Walkley-Black method), and between LOI and SIC(by the pressure calcimeter method), and the other set of soil samples was used to evaluate the derived equations by comparing predicted SOC and SIC with measured values. The mean absolute errors were small for both SOC (1.7 g C kg-1) and SIC(1.22 g C kg-1), demonstrating that the LOI method was reliable and could provide accurate estimates of SOC and SIC for arid soils.     

Carbon pools and soil biochemical properties in manure‐based organic farming systems of semi‐arid New Mexico

The economic benefits of organic agriculture and its wide adoption are well documented, but the impact of that practice on soil C dynamics in irrigated croplands of semi‐arid regions is less well understood. In manure‐based organic production systems, land applications of animal wastes not only provide nutrients but could also contribute to soil carbon sequestration. A study was conducted in irrigated cotton (Gossypium arboreum L) agro‐ecosystems of New Mexico (USA) under conventional (CONV; 100 kg N/ha as urea and NH₄NO₃) and organic farming practices (OF for 3–9 yr; 50 Mg dry manure/ha) to assess the effect of OF on soil C stocks (organic, inorganic) and biochemical indices [microbial biomass C (MBC); respiration; metabolic quotient (qCO₂)]. In the plough layer (0–30 cm), soil organic carbon (SOC) stocks tended to be higher (although not statistically) under OF (35.9 Mg C/ha) than CONV (33.5 Mg C/ha). However, when the entire 100‐cm soil profile was considered, the total SOC under CONV exceeded that under OF by 39.8 Mg C/ha, but this may be influenced by other factors. Accounting for 52% of the total C stock, inorganic C was significantly higher under CONV than OF and was positively correlated with soil respiration and the H/C ratio of soil organic matter. While OF duration had no consistent effect on soil biochemical properties, MBC was significantly higher (1.5 times) and the qCO₂ (3–6 times) was lower in the organically fertilized soils than under CONV. These results suggest the development, under OF, of a soil microbial community that is larger and processes added C substrates more efficiently compared with the community present in CONV practices.     

Methane and nitrous oxide fluxes from a farmed Swedish Histosol

Fluxes of the greenhouse gases methane (CH₄) and nitrous oxide (N₂O) from histosolic soils (which account for approximately 10% of Swedish agricultural soils) supporting grassley and barley production in Sweden were measured over 3 years using static chambers. Emissions varied both over area and time. Methane was both produced and oxidized in the soil: fluxes were small, with an average emission of 0.12 g CH₄ m⁻² year⁻¹ at the grassley site and net uptake of −0.01 g CH₄ m⁻² year⁻¹ at the barley field. Methane emission was related to soil water, with more emission when wet. Nitrous oxide emissions varied, with peaks of emission after soil cultivation, ploughing and harrowing. On average, the grassley and barley field had emissions of 0.20 and 1.51 g N₂O m⁻² year⁻¹, respectively. We found no correlation between N₂O and soil factors, but the greatest N₂O emission was associated with the driest areas, with < 60% average water-filled pore space. We suggest that the best management option to mitigate emissions is to keep the soil moderately wet with permanent grass production, which restricts N₂O emissions whilst minimizing those of CH₄.     

Leaching and balances of phosphorus and other nutrients in lysimeters after application of organic manures or fertilizers

Abstract. A long-term lysimeter experiment with undisturbed monoliths studied leaching behaviour and balances of phosphorus (P), potassium (K) and nitrogen (N) during a seven year crop rotation on four types of soil receiving inorganic fertilizers, manure and grass compost respectively. It was shown that application of manure did not lead to any direct change in nutrient leaching, unlike the application of fertilizers to soils of normal fertility. However, soil type considerably affected the nutrient concentrations in the drainage water.
Manure applied in amounts equal to the maximum animal density allowed by Swedish legislation slightly oversupplied P and N (0.5–3.5 and 18–38 kg ha⁻¹ y⁻¹ respectively) compared to the crop requirement and leaching losses for most of the soils. The relationship between lactate-soluble P in the topsoil and the concentrations of dissolved P in the drainage water was very strong. However the strength of this relationship was dependent on just one or two soils. P losses from a fertile sandy soil were large (1–11 kg ha⁻¹ y⁻¹) throughout the crop rotation and average crop removal (13 kg ha⁻¹ y⁻¹) plus the leaching losses were not balanced (average deficit 3–6 kg ha⁻¹ y⁻¹) by the addition of fertilizer, manure or grass compost. No decreasing trend was found in the P losses during seven years. However, the K deficit (average 26 kg ha⁻¹ y⁻¹) led to a significant reduction in the leaching trend from this soil. The other soils that had a smaller K deficit showed no significant reduction in the leaching of K.     

Environmental Factors Controlling Soil Organic Carbon Stocks in Two Contrasting Mediterranean Climatic Areas of Southern Spain

Managing soil carbon requires accurate estimates of soil organic carbon (SOC) stocks and its dynamics, at scales able to capture the influence of local factors on the carbon pool. This paper develops a spatially explicit methodology to quantify SOC stocks in two contrasting regions of Southern Spain: Sierra Norte de Sevilla (SN) and Cabo de Gata (CG). Also, it examines the relationship between SOC stocks and local environmental factors. Results showed that mean SOC stocks were 4·3 kg m⁻² in SN and 3·0 kg m⁻² in CG. Differences in SOC in both sites were not significant, suggesting that factors other than climate have a greater influence on SOC stocks. A correlation matrix revealed that SOC has the highest positive correlation with clay content and soil depth. Based on the land use, the largest SOC stocks were found in grassland soils (4·4 kg m⁻² in CG and 5·0 kg m⁻² in SN) and extensive crops (3·0 kg m⁻² in CG and 5·0 kg m⁻² in SN), and the smallest under shrubs (2·8 kg m⁻² in CG and 3·2 kg m⁻² in SN) and forests soils (4·2 kg m⁻² in SN). This SOC distribution is explained by the greatest soil depth under agricultural land uses, a common situation across the Mediterranean, where the deepest soils have been cultivated and natural vegetation mostly remains along the marginal sites. Accordingly, strategies to manage SOC stocks in southern Spain will have to acknowledge its high pedodiversity and long history of land use, refusing the adoption of standard global strategies. Copyright © 2015 John Wiley & Sons, Ltd.     

Effects of ridging on crop performance and symbiotic N2 fixation of fababean (Vicia faba L.)

Abstract. The success of organic cropping systems depends on symbiotic N₂ fixation by leguminous crops, and it is important to explore new management systems to improve the nitrogen input through N₂ fixation. During two growing seasons the possible advantage of growing fababean ( Vicia faba L.) in ridges was studied in comparison to the traditional method on flat soil. Differences in soil physical parameters resulted in a significantly greater microbial activity and a deeper root system at the flowering stage when grown in the ridge than on the flat. Consequently, the amount of fixed N at flowering was significantly greater in ridges than in flat soil. However, during the period from flowering until harvest, when the major part of the N uptake and N₂ fixation took place, the differences between the treatments disappeared. Average values for the growing season of fluorescein diacetate hydrolysis, arylamidase activity and arylsulphatase activity were significantly greater in the ridge than on the flat, and the microbial biomass-C, derived from substrate induced respiration (SIR), was on average 232 and 223 μg C g⁻¹ soil in the ridge and on the flat, respectively. Measured total-N uptake, including root N (0–30 cm depth), ranged from 206 to 247 kg N ha⁻¹, of which 182–201 kg N ha⁻¹ was fixed N. From 154 to 173 kg N ha⁻¹ was removed in grain resulting in a soil-N balance of +28 kg N ha⁻¹ in both years. However, by including estimates of total root N and rhizodeposition-N the soil-N balance ranged from +52 to +62 kg N ha⁻¹.