Soil physical quality of a Brazilian Oxisol under two tillage systems using the least limiting water range approach

Plant growth is directly affected by soil water, soil aeration, and soil resistance to root penetration. The least limiting water range (LLWR) is defined as the range in soil water content within which limitations to plant growth associated with water potential, aeration and soil resistance to root penetration are minimal. The LLWR has not been evaluated in tropical soils. Thus, the objective of the present study was to evaluate the LLWR in a Brazilian clay Oxisol (Typic Hapludox) cropped with maize (Zea mays L. cv. Cargil 701) under no-tillage and conventional tillage. Ninety-six undisturbed soil samples were obtained from maize rows and between rows and used to determine the water retention curve, the soil resistance curve and bulk density. The results demonstrated that LLWR was higher in conventional tillage than in no-tillage and was negatively correlated with bulk density for values above 1.02 g cm⁻³. The range of LLWR variation was 0–0.1184 cm³ cm⁻³ in both systems, with mean values of 0.0785 cm³ cm⁻³ for no-tillage and 0.0964 cm³ cm⁻³ for conventional tillage. Soil resistance to root penetration determined the lower limit of LLWR in 89% of the samples in no-tillage and in 46% of the samples in conventional tillage. Additional evaluations of LLWR are needed under different texture and management conditions in tropical soils.     

Surface runoff and soil erosion in a natural regeneration area of the Brazilian Cerrado

The Brazilian Cerrado has been converted to farmland, and there is little evidence that this expansion will decrease, mainly because agriculture is the country’s main economic sector. However, the impacts of intense modification of land use and land cover on surface runoff and soil erosion are still poorly understood in this region. Here, we assessed surface runoff and soil loss in a woodland Cerrado area under a former pasture area, which was abandoned and has undergone a natural regeneration process for 7 years (RC). Its results were compared with that found in an undisturbed area of woodland Cerrado (CE), 40-month-old eucalyptus (3.0 × 1.8 m) (EU), and pasture under rotational grazing (PA). The study was conducted on Red Acrisol located in the Brazilian Cerrado. We performed rainfall simulations on a plot of 0.7 m² and using three constant rainfall intensities of 60, 90, and 120 mm h⁻¹ for 1 h. For each rainfall intensity, we carried out four repetitions using different plots in each treatment, i.e. 12 plots per treatment studied and 48 plots in total. We noted that the soil physical properties were improved in RC and, consequently, water infiltration and soil erosion control; RC presented surface runoff and soil loss different from EU and PA (α = 0.05). The macroporosity and soil bulk density affected surface runoff in RC and PA because the RC was used as pasture and is currently regenerating back to the cerrado vegetation. As the rainfall intensity increased, EU became more similar to PA, which showed the highest surface runoff and soil loss. Our findings indicate that natural regeneration processes (pasture to the cerrado vegetation) tend to improve the soil ecosystem services, improving infiltration and reducing surface runoff and soil erosion.     

Tillage systems for a cotton–peanut rotation with winter-annual grazing: Impacts on soil carbon, nitrogen and physical properties

Integrating livestock with cotton (Gossypium hirsutum L.) and peanut (Arachis hypogaea L.) production systems by grazing winter-annuals can offer additional income for producers provided it does not result in yield-limiting soil compaction. We conducted a 3-year field study on a Dothan loamy sand (fine-loamy, kaolinitic, thermic plinthic kandiudults) in southern Alabama, USA to determine the influence of tillage system prior to cotton–peanut planting on soil properties following winter-annual grazing. Two winter-annual forages [oat (Avena sativa L.) and annual ryegrass (Lolium mutiflorum L.)] and four tillage practices [chisel + disk, non-inversion deep tillage (paratill) with and without disking and no-till] were evaluated in a strip-plot design of four replications. We evaluated cone index, bulk density, infiltration, soil organic carbon (SOC), and total nitrogen (N). Paratilling prior to cotton or peanut planting, especially without surface soil tillage, reduced compaction initially to 40 cm and residually to 30 cm through the grazing period in winter. There were no significant differences in cone index, bulk density, or infiltration between forage species. No-tillage resulted in the greatest bulk density (1.65 Mg m⁻³) and lowest infiltration (36% of water applied), while paratilling increased infiltration in no-tillage to 83%. After 3 years, paratilling increased SOC 38% and N 56% near the soil surface (0–5 cm), as compared to concentrations at the beginning of the experiment, suggesting an improvement in soil quality. For coastal plain soils, integrating winter-annual grazing in a cotton–peanut rotation using a conservation tillage system of non-inversion deep tillage (paratill) with no surface tillage can improve soil quality by reducing cone index, increasing infiltration, and increasing SOC in the soil surface.     

放牧对小嵩草草甸土壤酶活性及土壤环境因素的影响

观测了放牧对高寒小嵩草草甸土壤酶活性及土壤环境因素的影响,结果表明:①随着放牧压力的增大,植被盖度、地上生物量、土壤有机质、全氮、硝态氮、全磷、有效磷、土壤水分明显下降,而地下生物量、土壤容重、pH及根土比呈现增大趋势,不同放牧处理间植物群落特征及土壤理化特性显示出明显的差异(P0.05);②纤维素分解酶、多酚氧化酶、脲酶、蛋白酶、碱性磷酸酶和蔗糖酶活性均随放牧压力增大而下降,只有过氧化氢酶以轻牧最高,各处理间上述土壤酶活性差异程度不同;③随着土壤深度的增加,土壤水分含量、地下生物量、根土比、土壤有机质及氮、磷养分等明显下降,而土壤容重和pH逐渐增大,且不同层次之间存在显著的差异(P0.05);④除过氧化氢酶和多酚氧化酶外,其它土壤酶活性随土壤深度的加深显著减小。     

Impact of cattle grazing on temperate coastal salt marsh soils

Over the last two decades, grazing intensity has increased in the temperate salt marshes of Samborombón Bay (Argentina) due to agricultural expansion and the displacement of domestic livestock to these areas. We investigated the effect of cattle grazing on soil chemical and physical properties in the higher (HE), medium (ME) and lower (LE) elevation levels of this temperate salt marsh. Soil data were collected from both a National Park, where cattle grazing has been excluded for more than 35 yrs, and an adjacent commercial livestock farm continuously grazed by cattle. We found that soil salinity was greater on the grazed than on the ungrazed sites, especially those in the ME and LE. This could be related to the upward flow of salts from the saline groundwater, driven by the increase in the proportion of bare soil on grazed sites. The increase in soil salinity changed the plant community structure through the increase of salt‐tolerant and non‐palatable species and the decrease of palatable species. Soil physical variables (soil bulk density and soil bearing capacity) were also higher on the grazed than on the ungrazed sites, which can be related to the decrease in soil organic matter (SOM), and suggest an incipient compaction process; however, the values were still lower than those considered critical for plant growth in clay soils. These results suggest that continuous grazing management in this temperate salt marsh might have negative consequences for animal production and ecosystem conservation, mainly related to the increased soil salinity. Further research will be necessary to evaluate the suitability of switching to intermittent grazing management.     

SoilFlex‐LLWR: linking a soil compaction model with the least limiting water range concept

Soil compaction impacts growing conditions for plants: it increases the mechanical resistance to root growth and modifies the soil pore system and consequently the supply of water and oxygen to the roots. The least limiting water range (LLWR) defines a range of soil water contents within which root growth is minimally limited with regard to water supply, aeration and penetration resistance. The LLWR is a function of soil bulk density (BD), and hence directly affected by soil compaction. In this paper, we present a new model, ‘SoilFlex‐LLWR’, which combines a soil compaction model with the LLWR concept. We simulated the changes in LLWR due to wheeling with a self‐propelled forage harvester on a Swiss clay loam soil (Gleyic Cambisol) using the new SoilFlex‐LLWR model, and compared measurements of the LLWR components as a function of BD with model estimations. SoilFlex‐LLWR allows for predictions of changes in LLWR due to compaction caused by agricultural field traffic and therefore provides a quantitative link between impact of soil loading and soil physical conditions for root growth.     

封育与放牧对黄土高原天然草地土壤化学计量特征的影响

为揭示封育与放牧2种草地利用方式下土壤化学计量特征及其驱动因素,以宁夏固原云雾山封育草地和放牧草地为研究对象,定量分析了0—30 cm土壤化学计量特征及其影响因子。结果表明:(1)封育降低了土壤紧实度、容重和含水率,土壤孔隙度略有上升。(2)封育草地0—10 cm土层以0.25 mm水稳性团聚体为主,其中5 mm团聚体达到了46%,平均重量直径(MWD)和几何平均直径(GWD)分别为3.78,1.70,显著高于放牧草地(P0.05),但两者分形维数并无差异,封育草地土壤团聚体稳定性有所提高。(3)封育草地0—10 cm土层土壤有机碳(SOC)、全氮(TN)、全磷(TP)分别为17.714,2.018,0.659 g/kg,均高于放牧草地,其中土壤TN含量达到显著水平(P0.05)。(4)封育草地0—20 cm土层碳氮比(C/N)、碳磷比(C/P)、氮磷比(N/P)均高于放牧草地。(5)封育草地土壤SOC、TN、C/P和N/P与土壤容重和分形维数呈显著性负相关(P0.05),并且SOC和TN与MWD和GWD呈显著性正相关(P0.05),放牧草地土壤化学计量特征与物理特性相关性低于封育草地。封育与放牧相比,改善了土壤物理特性和团聚体结构,土壤养分有所提升,土壤化学计量特征与土壤物理特性密切相关,土壤物理结构可能是影响土壤化学计量特征的主控因子。     

Least limiting water range for different soil management practices in dryland farming in Iran

Integrated evaluation of soil physical properties using the least limiting water range (LLWR) approach may allow a better knowledge of soil water availability. We determined the LLWR for four tillage practices consisted of conventional tillage (CT), reduced tillage (RT), no-tillage (NT) and fallow no-tillage (NT_f). In addition, LLWR was determined for abandoned soils (i.e. control), compacted soils, ploughed compacted soils and abandoned soils with super absorbent polymers (SAPs) application. Soil water retention, penetration resistance (PR), air-filled porosity and bulk density were determined for the 0–5 and 0–25-cm depths. Mean LLWR (0.07–0.08 cm³ cm^?3) was lower in compacted soils than the soils under CT, NT, NT_f, RT, tilled, abandoned and SAP practices but it was not different among tillage practices. The values of LLWR were 0.12 cm³ cm^?3 for NT and CT. LLWR for tilled plots (0.12 cm³ cm^?3) became greater than compacted soils by 1.3 times. Analysis of the lower and upper limits of the LLWR further indicated that PR was the only limiting factor for soil water content, but aeration was not a limiting factor. The LLWR was more dependent on soil water content at permanent wilting point and at PR.     

Assessing the relationship between fire and grazing on soil characteristics and mite communities in a semi-arid savanna of northern Australia

Northern Australian tropical savannas are subjected to pressures from both grazing and planned and unplanned burning. We know little about the effects of these processes on the below-ground environment. The aim of this study was to investigate the effects of fire, grazing and season on environmental and biological properties of the soil at the base of grass tussocks in a semi-arid savanna rangeland of north Australia. A long-term fire and grazing exclusion experiment was used to test the effects of season, fire and grazing on soil physicochemical factors (soil organic carbon, total nitrogen, ammonium, nitrate levels and bulk density) and soil mite abundance and diversity. Grazed plots were associated with small but significant reductions in total soil nitrogen and organic carbon when compared to 30 year old plots where grazing and fire had been excluded. This suggests slow, long-term losses of nitrogen and soil carbon from an ecosystem with limited available nutrients. Fire had a limited impact on soil properties, but this may reflect the modest experimental fire intensity resulting from fuel reduction due to grazing. Treatment effects on soil bulk density were also negligible. Season had a significant impact on total soil mite abundance and diversity, whereas burning and grazing treatments had no impact on soil mites. Only two morpho-species, one each from the families Cunaxidae and Stigmaeidae, decreased in abundance as a result of grazing. Increased moisture levels in the wet season were associated with increased total nitrogen and the highly mobile nitrate. Changes in mite abundance and diversity reflected these changes in levels of nitrogen and it is possible that increasing total nitrogen availability and soil moisture, is a determinant of mite abundance.     

Carbon sequestration in two Brazilian Cerrado soils under no-till

A considerable proportion of the 200 million hectares of the Brazilian Cerrado is suitable for annual crops but little is known about the effects of tillage on the C dynamics of Cerrado soils. We evaluated the role of two representative Cerrado Oxisols (350 and 650 g clay kg⁻¹) as sources or sinks of atmospheric C when managed under three tillage systems (conventional tillage (CT), reduced tillage (RT), and no-till (NT)) in 8- and 5-year long-term experiments. A literature review was also carried out and the mean C sequestration rates in no-till soils of tropical and subtropical regions of Brazil were calculated and compared with values for soils from temperate regions of the world. The original C stocks in 0–20 cm layer of soils under native Cerrado were higher in the clayey (54.0 Mg ha⁻¹) than in the sandy clay loam soil (35.4 Mg ha⁻¹), suggesting a higher physical stability of organic matter associated with variable clay minerals in the clayey Oxisol. The original C stocks of the native Cerrado soils appear not to have decreased after 23 years of conventional tillage in the sandy clay loam Oxisol, except when the soil had been subjected to erosion (15% loss of C), or after 25 years in the clayey Oxisol. Compared to conventionally tilled soil, the C stocks in no-till sandy clay loam Oxisol increased by 2.4 Mg ha⁻¹ (C sequestration rate = 0.30 Mg ha⁻¹ year⁻¹) and in the clayey Oxisol by 3.0 Mg ha⁻¹ (C sequestration rate = 0.60 Mg ha⁻¹ year⁻¹). The mean rate of C sequestration in the no-till Brazilian tropical soils was estimated to be 0.35 Mg ha⁻¹ year⁻¹, similar to the 0.34 Mg ha⁻¹ year⁻¹ reported for soils from temperate regions but lower than the 0.48 Mg ha⁻¹ year⁻¹ estimated for southern Brazilian subtropical soils. Considering the large area (about 70 million hectares) of the Cerrado which is currently used and potentially available for cropland, the adoption of no-till systems could turn the Cerrado soils into a significant sink for atmospheric C and contribute to the mitigation of global climate change.     

放牧干扰对人工林土壤物理性状的影响

为揭示人工林群落在人为干扰下的退化状况,应用空间对比法研究了放牧干扰对黄土高原人工油松林下土壤物理性状的影响。结果表明,随放牧干扰强度的增加,土壤中砂粒含量增加,黏粒含量下降,土壤出现砂化趋势。放牧干扰下土壤容重增加了0．09～0．39g／cm^3。随放牧干扰强度的增加,土壤总孔隙、毛管孔隙和非毛管孔隙呈大幅下降的趋势,土壤持水量、含水量、入渗速率和导水率降低。重度干扰和中度干扰下土壤持水量、含水量与无放牧干扰时相比显著降低。随放牧干扰强度的增加,初渗率和稳渗速率下降,饱和导水率也随之下降。     

Effects of seasonal grazing on soil respiration in alpine meadow on the Tibetan plateau

Little research has been conducted on how to balance plant production and soil respiration (Rs) under seasonal grazing patterns in alpine meadows. Our results from 2009 to 2012 showed that warm season grazing (WG) from June to September significantly increased aboveground net primary production compared with no‐grazing (NG), except in 2010, and compared with cold season grazing (CG) except in 2012, while there were no significant differences between NG and CG except in 2009. In both WG and CG treatments, grazing increased root biomass at 0–40 cm depth compared with NG, except in 2011. WG and CG only significantly increased seasonal Rs in 2009. Daily Rs was mainly affected by soil temperature, which explained 40–49% of the variation in daily Rs for all grazing treatments. Seasonal Rs from July to September was significantly influenced by soil temperature and root biomass, which explained 55% of the variation in seasonal Rs for all grazing treatments. Therefore, relative to NG, regardless of WG and CG, moderate grazing significantly increased plant production and had little influence on soil respiration in this alpine region.     

Influence of topographic and edaphic factors on vulnerability to soil degradation due to cattle grazing in humid tropical mountains in northern Honduras

Low altitude humid tropical mountains in Central America have experienced a process of livestock expansion during recent decades. However, the use of sloping areas for cattle grazing may lead to significant soil degradation and therefore we examined the influence of the slope gradient on soil degradation in pastures in a humid tropical mountainous area in northern Honduras. Understanding this relationship permits estimates of the physical carrying capacity of the soil, which in turn may help to improve livestock use within the study area. Variables examined included soil bulk density, texture, organic matter content and consistency as well as visual indicators of soil and vegetation degradation. There is a significant positive correlation between the bulk density as a proxy for soil degradation and slope gradient. Furthermore, it was found that when soils are water-saturated grazing leads to severe degradation. Together with visual indicators, these data show that paddocks with slopes less than 30% have a carrying capacity between 900 and 1900 Animal Units (AU) ha^− 1 year^− 1 and many are currently underutilized. Paddocks with slopes between 30 and 50% have a carrying capacity between 400 and 600 AU ha^− 1 year^− 1. Paddocks with slopes over 50% have the lowest carrying capacity: less than 200 AU ha^− 1 year^− 1. The latter are frequently over-used; most of them show clear signs of soil and vegetation degradation. Land use in these areas needs to change or their grazing management needs to be reorganized to adjust actual stocking rate to physical carrying capacity of the soils to prevent further degradation.     

Land management effects on the near-surface physical quality of a clay loam soil

Although agricultural land management is known to affect near-surface soil physical quality (SPQ), the characteristics of these affects are poorly understood, and diagnostic SPQ indicators are not well-developed. The objective of this study was to measure a suite of potential SPQ indicators using intact soil cores and grab samples collected from the 0–10 cm depth of a clay loam soil with the treatments: (i) virgin soil (VS); (ii) long-term continuous bluegrass sod (BG); (iii) long-term maize (Zea mays L.)—soybean (Glycine max (L.) Merr.) rotation under no-tillage (NT); (iv) long-term maize–soybean rotation under mouldboard plough tillage (MP); (v) short-term (1–4 years) NT after long-term MP; (vi) short-term MP after long-term BG; (vii) short-term MP after long-term NT. Organic carbon content, dry bulk density, air capacity, relative water capacity and saturated hydraulic conductivity appeared to be useful SPQ indicators because they were sensitive to land management, and proposed optimum or critical values are available in the literature. Soil macroporosity was also sensitive to land management, but optimum or critical values for this parameter are not yet established. Soil matrix porosity and plant-available water capacity did not respond substantially or consistently to changes in land management, and were thus not useful as SPQ indicators in this study. Converting long-term BG to MP caused overall SPQ to decline to levels similar to long-term MP within 3–4 years. Converting long-term NT to MP or vice versa caused only minor changes in overall SPQ. With respect to the measured SPQ indicators and their optimum or critical values, both VS and BG produced “good” overall SPQ in the near-surface soil, while long-term maize–soybean rotation under NT and MP produced equally “poor” SPQ.     

放牧对希拉穆仁草原土壤入渗过程影响的定量评估

为定量评估放牧对草原土壤入渗的影响,了解和防治放牧造成的水土流失,科学评估草原水文调节功能,采用双环入渗法研究了放牧对典型草原土壤入渗性能的影响,并收集影响土壤入渗的环境特征变量.结果表明:(1)土壤入渗能力均受到放牧活动的干扰,随着放牧强度的增加,土壤初渗速率、稳渗速率减小,达到稳渗时的历时缩短,而轻度放牧对土壤入渗...     

放牧强度对中国内蒙古草原土壤水分状况与通量的影响

In the past few decades,the increase in grazing intensity has led to soil degradation and desertification in Inner Mongolia grassland,China,due to population growth and shift in the socio-economic system.Two sites with different grazing intensities,continuous grazing site(CG) with 1.2 sheep ha 1 year 1 and heavy grazing site(HG) with 2.0 sheep ha 1 year 1,were investigated at the Inner Mongolia Grassland Ecosystem Research Station(43 37 50 N,116 42 18 E) situated in the northern China to i) characterize the temporal distribution of soil water content along soil profile;and ii) quantify the water fluxes as affected by grazing intensity.Soil water content was monitored by time domain reflectometry(TDR) probes.Soil water retention curves were determined by pressure membrane extractor,furthermore processed by RETC(RETention Curve) software.Soil matric potential,plant available water and water flux were calculated using these data.Both sites showed an identical seasonal soil water dynamics within four defined hydraulic periods:1) wetting transition coincided with a dramatic water increase due to snow and frozen soil thawing from March to April;2) wet summer,rainfall in accordance with plant growth from May to September;3) drying transition,a decrease of soil water from October to November due to rainfall limit;and 4) dry winter,freezing from December to next February.Heavy grazing largely reduced soil water content by 43%-48% and plant available water by 46%-61% as compared to the CG site.During growing season net water flux was nearly similar between HG(242 mm) and CG(223 mm) sites between 5 and 20 cm depths.However,between 20 and 40 cm depths,the upward flux was more pronounced at HG site than at CG site,indicating that water was depleted by root uptake at HG site but stored at CG site.In semi-arid grassland ecosystem,grazing intensity can affect soil water regime and flux,particularly in the growing season.     

C and N stocks are not impacted by land use change from Brazilian Savanna (Cerrado) to agriculture despite changes in soil fertility and microbial abundances

Central Brazil is the region with the most dynamic agriculture expansion worldwide, where tropical forests and Cerrado (Brazilian savanna) are converted to pastures and crop fields. Following deforestation, agricultural practices, such as fertilization, tillage and crop rotations, alter soil parameters and affect microbial abundances and the C and N cycles. The objective of this study was to compare changes in soil fertility, stocks of soil C and N, microbial biomass, and abundance of bacteria, fungi and archaea in Cerrado soils following land use change to crops (soybean/corn/cotton) and pasture (the perennial forage grass Brachiaria brizantha A. Rich.). Agriculture increased soil fertility and conserved soil C and N since their absolute concentration values were highest in agriculture soils and the C and N stocks adjusted by soil density were similar to the native vegetation soils. At the same time, agriculture changed the microbial abundances (decrease of microbial biomass C and N, increase of archaea, and reduction of bacteria and fungi at the crop sites), and N dynamics (increase of soil ammonium and nitrate concentrations). Even if these changes can be beneficial for food and agricultural commodities production, all these soil alterations should be further investigated due to their possible unknown effects on biosphere–hydrosphere–atmosphere exchange processes such as greenhouse gases emissions and nitrate leaching.     

Disc harrowing intensity and its impact on soil properties and plant growth of agropastoral systems in the Llanos of Colombia

Oxisols such as those of the Colombian Eastern Plains (Llanos) are susceptible to physical, chemical and biological degradation once brought into cultivation, especially under intensive use of machinery. The main objective of this study was to determine the impact of intensive disc harrowing (2, 4 or 8 disc harrow passes per year over 3 years) on soil physical and chemical properties, soil phosphorus dynamics, plant growth and nutrient acquisition of contrasting agropastoral systems on an Oxisol. The three main systems tested after 2 years of upland rice cultivation were grass-alone pasture (Brachiaria dictyoneura), green manure (Crotalaria juncea), and maize (Zea mays). Native savanna treatment was used as a control. Intensive disc harrowing improved macroporosity values of 0–5 cm soil layer up to 59% for grass-alone pasture system compared to native savanna. Disc harrowing significantly reduced bulk densities for pasture and green manure systems compared to the native savanna in the 0–5 cm soil layer. Intensive disc harrowing significantly improved volumetric moisture content of green manure and maize systems at 5–10 cm soil depth. The distribution of biologically, moderately and sparingly available P, organic P and total P varied under green manure, maize and grass-alone pasture systems. Two passes of disc harrow per year were sufficient for grass-alone pasture, while maize showed greater aboveground production and nutrient acquisition at 8 passes of disc harrow per year. The maize and green manure cropping systems were better than the grass-alone pasture system at separating the effect of increased number of disc harrow passes on soil physical and chemical characteristics.     

放牧对荒漠草原土壤养分及微生物量的影响

[目的]探讨不同放牧强度对荒漠草原植被多样性、土壤理化性状、土壤养分及土壤微生物量的影响。[方法]以围封禁牧草地为对照,采用野外调查和室内分析的方法,对不同放牧强度下的草地土壤及植被展开调查。[结果]随放牧强度的增加,荒漠草原植被盖度、物种多样性、地上生物量、土壤养分和微生物量显著降低,土壤容重和pH值呈增加趋势,土壤电导率呈先增加后降低趋势,地下生物量则没有明显变化趋势;在植被作用下土壤养分和微生物量垂直方向表现递减规律并且在表层富集,"表聚性"较为明显;在放牧干扰下土壤全磷变异系数最高;放牧并没有改变荒漠草原土壤养分和微生物量的垂直分布特征;相关分析表明,放牧干扰下土壤微生物量与土壤养分之间具有较强的相关性,二者与土壤含水量也有较强的相关性。[结论]放牧强度对土壤全磷的空间变异影响较大,并且土壤微生物量对于放牧干扰的敏感性高于土壤养分全量;土壤养分和微生物量等地下生态系统各指标之间具有统一性。     

Tillage effects on soil organic matter in density fractions of a Cerrado Oxisol

Reclamation of Brazilian cerrados (savannas) has been intensified in the last decades, with implications for soil quality and soil organic matter (SOM) dynamics. Studying the impact of different tillage systems is essential to define better strategies for land use in Cerrado, which may favor C sequestration and improve soil quality. We used density fractionation and ¹³C natural abundance to assess changes in SOM in an Oxisol previously under a cerrado sensu-stricto following 30 years of cultivation. The objectives of the study were to: (i) evaluate the long-term impact of tillage systems on SOM stocks in a Dark Red Latosol (Oxisol) from the Cerrado Biome, and (ii) better understand the dynamics of SOM in different density fractions of this soil. Cultivation led to compaction, which significantly increased soil bulk density. This resulted in the systematic overestimation of C and N stocks in cultivated areas when compared to the natural cerrado. Conversion of the cerrado into cropland using plow tillage (PT) or no-tillage (NT) system did not alter the total C (100 Mg ha⁻¹) and N (7 Mg ha⁻¹) stocks in the first 45 cm depth at the end of 30 years of cultivation. However, about 22% of the total C was replaced by C from maize. The relative replacement of C decreased following the order: free light fraction (F-LF)>heavy fraction (HF)>occluded light fraction (O-LF). The low substitution in the O-LF was attributed to a possible presence of charcoal. Converting cerrado into cropland significantly decreased F-LF quantity. The proportions of C replacement in this fraction were higher in PT than NT, suggesting a faster turnover in PT. Nevertheless, because most C (95%) was held in the HF, C dynamics in the whole soil were controlled by the behavior of this fraction. The maintenance of C levels even at the end of 30 years of cultivation and the lack of differentiation between NT and PT were attributed to the high clay contents and Fe+Al oxi-hydroxides concentrations of the studied soil as well as to a sufficient C supply by the maize crop.