Effects of desertification on soil and crop growth properties in Horqin sandy cropland of Inner Mongolia, north China

Wind erosion and accumulated sand are recognized as the primary forms of cropland desertification in the arid and semiarid region, Inner Mongolia. However, there is a lack of knowledge on the effects of wind erosion and accumulated sand on soil properties and crop growth properties in this region. A field experiment was conducted from 2001 to 2002 on desertified cropland with gradients of wind erosion and accumulated sand to investigate changes in soil and crop growth properties resulting from desertification in the Horqin sandy land. Results indicated that the soil environment degraded significantly and crop growth and biomass were seriously restrained by wind erosion. In the severely eroded cropland, soil clay, organic matter, total N and P, available N and P, average soil moisture decreased by 59.6%, 71.2%, 67.4%, 31.4%, 64.5%, 38.8% and 51.8%, respectively. Erosion increased soil pH from 8.66 to 8.92; delayed plant life cycle by 6 days, decreased plant height and diameter by 15.5% and 29.1%, and decreased above and below ground biomass and seed yield by 87.3%, 47.9% and 96.5%, respectively. Effects of a little accumulated sand were fewer on soil properties and crop growth and seed yield compared to wind erosion, severe accumulated sand effects on the cropland remain to be studied further. The results of correlation analysis showed that crop biomass and seed production were significantly correlated to organic matter, total N, pH and soil moisture under circumstance of worsened soil environment. In the windy and sandy region in Inner Mongolia, some measures to control cropland erosion should be introduced including construction of windbreak, stubble cultivation, increased organic fertilizer and spring irrigation.     

我国北方半干旱地区土壤的沙漠化演变过程与机制

土地沙漠化是我国北方干旱半干旱地区土地退化最严重的类型之一。但迄今为止,人们对这一地区土地沙漠化过程中土壤沙化过程和机制的了解还不够透彻。2002-2003年,我们在科尔沁沙地选择具有明显沙漠化梯度的一个区域,调查和研究了土壤沙漠化演变过程和机制。结果表明,在科尔沁沙地,不同粒径土壤颗粒的理化特性具有较大差异,其中土壤粘粉粒和粗沙相比,具有较低的土壤容重和较高的土壤结持力、起沙风速、毛管持水力和养分含量。从土壤粘粉粒到粗沙,容重增加了10.32%,结持力、毛管持水量、有机碳、全氮分别下降了99.15%,51.23%,83.73%和80.24%。沙漠化过程中,土壤的理化性质随着土地沙漠化程度的增强而明显恶化。和非沙漠化土地相比,严重沙漠化土地的土壤粗沙含量、非毛管孔隙度和容重分别增加了35.04%,117.50%和21.7%,细沙含量、粘粉粒含量、总孔隙度、毛管孔隙度、田间持水量、毛管持水量、土壤有机碳、全氮、全磷、有效氮和有效磷分别下降了77.78%,70.00%,15.38%,27.49%,54.34%,37.54%,64.15%,70.77%,65.90%,66.32%和50.59%。相关分析结果表明,沙漠化过程中,土壤有机碳、全氮、全磷、土壤含水量和土壤结持力的变化与土壤粘粉粒的减少呈明显正相关,与土壤粗沙含量的增加呈明显负相关。这说明,沙漠化过程中,由于风蚀而导致的富含养分和具有较高持水能力的土壤细颗粒的损失,是沙漠化过程中土壤退化的主要机制。     

科尔沁沙地旱作农田土壤退化的过程和特征

在一个较小的时空尺度上对科尔沁沙地典型旱作农田土壤物理和化学特性在风蚀沙化过程中的演变特征及其退化过程进行了的探讨。结果表明 :旱作农田由于周边沙质草地的开垦而发生风蚀沙化 ,3～ 5年的时间就会使 5 %～ 15 %的细粒物质吹蚀 ,表土粗化 ,向风沙沉积特征发展 ,土体结构破坏 ,容重增加 0 .0 5～ 0 .15 g/ cm3,总孔隙度降低 3%～ 5 % ,持水性能变劣 ,养分也随之丧失 ;<0 .0 5 mm的粘粉粒吹蚀 1% ,土壤有机质和全氮含量下降 0 .2 5 9g/ kg和 0 .0 16 4 g/ kg;土壤退化的演变 ,既有系统本身的自然属性决定的内在原因 ,更重要的是人为的外部干扰体系的驱动 ;退化的过程既有渐变型 ,又有跃变型     

Effects of desertification on soil organic C and N content in sandy farmland and grassland of Inner Mongolia

Desertification is one of the most serious types of land degradation. A field experiment was conducted during 2002 and 2003 in Horqin Sand Land, China to investigate changes in soil C and N contents in relation to land desertification. Four primary results were derived from this work. First, land desertification characterized by wind erosion resulted in a significant decrease in soil fine particles (clay + silt) with a corresponding increase in sand content. In comparison to non-desertified land, soil fine particle content decreased by up to 89.2%, and sand content increased by up to 47.2%, in the severely desertified land. Second, the organic C and total N in soil were mainly associated with the soil fine particles, and decreased significantly with desertification development. Organic C decreased by 29.2% and total N by 31.5% in the severely desertified land compared to the non-desertified land. Third, the decrease in organic C and N content was greater in desertified grassland than in desertified farmland. Fourth, the changes in organic C and total N content had a significant positive correlation with the soil fine particle content (P < 0.01) and a significant negative correlation with coarse sand content (P < 0.01), indicating that land desertification by wind erosion is mediated through a loss of soil fine particles, with a resultant decrease in soil organic C and total N.     

Effects of animal manure and mineral fertilizer on the total carbon and nitrogen contents of soil size fractions

Summary Soil was sampled in autumn 1984 in the 132 field (sandy loam soil) of the Askov long-term experiments (started in 1894) and fractionated according to particle size using ultrasonic dispersion and sedimentation in water. The unmanured plot and plots given equivalent amounts of N (1923–1984 annual average, 121 kg N/ha) in either animal manure or mineral fertilizer were sampled to a depth of 15 cm, fractionated and analysed for C and N. Mineral fertilizer and animal manure increased the C and N content of whole soil, clay (<2 m) and silt (2–20 m) size fractions relative to unmanured samples, while the C content of the sand size fractions (fine sand 1, 20–63 m; fine sand 2, 63–200 m; coarse sand, 200–2000 m) was less affected. Clay contained 58% and 65°70 of the soil C and N, respectively. Corresponding values for silt were 30% and 26%, while sand accounted for 10% of the soil C. Fertilization did not influence this distribution pattern. The C : N ratio of the silt organic matter (14.3) was higher and that of clay (10.6) lower than whole-soil C:N ratios (12.0). Fertilization did not influence clay and silt C : N ratios. Animal manure caused similar relative increases in the organic matter content of clay and silt size fractions (36%). In contrast, mineral fertilizer only increased the organic matter content of silt by 21% and that of clay by 14%.     

风沙区雨养耕地水分与养分动态及其对作物产量的影响

风沙区由于林带的缺少,雨养耕地风蚀现象严重,就地起沙,形成了风蚀地以及风积地在短距离内交错分布的特点。通过对风沙区雨养耕地土壤水养状况的定位研究得出:没有防护林带保护的雨养耕地有3种类型,即风蚀地、积沙地、过渡地段。沿主害风方向,上风地段为风蚀沙地,下风地段为风积沙地。风蚀沙地水分状况最好,风积沙地最差;表层土壤(0~20cm)养分状况比下层(20~60cm)差,风积沙地肥力最低,过渡地段最好。沙区雨养耕地特点明显,一年中土壤含水率变幅比降水量小得多,水养同步作物生产力水平高;农作物产量与土壤中全N、全P、全K含量关系不密切,与土壤表层有机质、有效P、K含量及下层有效N的含量呈明显的正相关。     

Decomposability of organic matter in particle size fractions from field soils with straw incorporation

Soils from two field experiments on straw disposal were fractionated according to particle size using ultrasonic dispersion and gravity-sedimentation in water. Samples of whole soils, clay. silt and sand-size fractions were held for 49 days at 20°C and the CO₂ evolution measured on 14 dates by gas chromatography.Recovery of soil solids. C and N was 99, 98 and 93%, respectively. Most of the soil C and N was in the clay (<2μm). (loamy sand, 50% C and 56% N; sandy loam. 65% C and 68% N), the silt (2–20 μm) having smaller proportions (loamy sand, 41% C and 38% N; sandy loam. 29% C and 27% N). The sand fraction (20–6000 μm) accounted for 4–7% of the organic matter, and 1–2% of the C was water soluble. Straw incorporation generally increased the C and N content of whole soils and size fractions.The decomposition rate constants were higher for the sandy loam than for the loamy sand soil. For both soils, the decomposability of the organic matter decreased in the order: sand > clay ⩾ whole soil > silt. Straw incorporation increased the decomposition rate of whole soil and sand organic matter. whereas the effect of straw on clay and silt respiration was small.Between 58 and 73% of the respiration was from clay, 21–25% from silt and 6–19% from the sand size fraction.     

Soil properties, crop productivity and irrigation effects on five croplands of Inner Mongolia

In the Horqin Sand Land, more than half of the original pasture area has been converted to farmland over the last century. A field experiment was conducted from 2000 to 2001 on five croplands in the Horqin Sand Land of Inner Mongolia to examine differences in soil properties, crop productivity and irrigation effects across different soils in the region to assess their relative suitability for cultivation, in the face of continued pressure for conversion of these generally fragile, sandy soils to agriculture.Two irrigated croplands studied were originally sandy meadow (ISM) and sandy grassland (ISG), and three dry croplands were from sandy meadow (DSM), sandy grassland (DSG) and fixed sand dunes (DFD). Results showed that most measured properties of soils, and crop productivity, differed among the five croplands. The silt + clay fraction, bulk density, organic matter content, total N and P, available N and P, average soil moisture and temperature, plant height and aboveground biomass were as follows in the DSM|DSG|DFD soils: 51.1%|47.5%|24.3%; 1.44 g/cm³|1.49 g/cm³|1.58 g/cm³; 6.3 g/kg|4.6 g/kg|3.4 g/kg; 0.55 g/kg|0.33 g/kg|0.21 g/kg; 0.21 g/kg|0.17 g/kg|0.13 g/kg; 27.0 mg/kg|13.7 mg/kg|7.7 mg/kg; 2.9 mg/kg|2.9 mg/kg|3.0 mg/kg; 9.4%|7.0%|6.2%; 21.4 °C|21.7 °C|22.0 °C; 225 cm|220 cm|181 cm; and 2116 g/m²|1864 g/m²|1338 g/m². Corresponding values for ISM|ISG soils were: 54.3%|47.9%; 1.42 g/cm³|1.49 g/cm³; 8.5 g/kg|6.4 g/kg; 0.58 g/kg|0.42 g/kg; 0.20 g/kg|0.19 g/kg; 29.0 mg/kg|23.3 mg/kg; 4.7 mg/kg|7.9 mg/kg; 13.0%|10.1%; 21.0 °C|21.1 °C; 266 cm|245 cm; and 2958 g/m²|2702 g/m².In general, the ecological origin of a cropland was a stronger determinant of its current characteristics than was irrigation history, although irrigation was correlated with significantly increased organic matter content, some soil nutrient levels, and aboveground biomass productivity. Results indicate that fixed sand dunes should not be converted to cropland because of their very sandy and poorer soil, lower biomass productivity and greater wind-erosion risk. Although both the sandy meadow and sandy grassland may be reclaimed for farming, the cropland derived from the sandy meadow had higher resistance to wind erosion and higher crop productivity, so is somewhat more suitable than sandy grassland.     

退耕还湖后安庆沿江湿地土壤理化性质变化

以安庆沿江湿地不同历史利用方式(水耕和旱耕)和不同退耕还湖方式(自然水域且水产养殖-白荡湖和自然水域无水产养殖-菜子湖)的湿地土壤为研究对象,分析退耕还湖后土壤理化性质的变化。结果表明,退耕后白荡湖湿地土壤容重、粘粒含量和大多数养分含量均下降,而全磷含量有所增加;退耕后菜子湖湿地土壤粘粒含量和大多数养分含量均增加,而土壤容重和有效磷含量下降;历史水耕的菜子湖湿地土壤除有效磷含量外,土壤粘粒含量和大多数养分含量均显著高于历史旱耕湿地。退耕还湖为自然湿地相对于水产养殖更有利于湿地土壤生态恢复;相对于历史水耕土壤,历史旱耕湿地土壤生态恢复较为缓慢;土壤粘粒、有机质和全氮是评价湿地土壤生态恢复状况的良好指标。     

乌兰布和沙漠绿洲农田不同土地利用方式地表风蚀特征研究

土壤风蚀是干旱地区绿洲农田开发的一个突出生态问题,如何针对不同的立地单元采取保护性措施是人们长久以来关注的重点。本文选择乌兰布和沙漠绿洲农田流沙地、沙质耕地、黏质耕地、撂荒耕地及留茬地5种典型土地利用方式,对其风速特征、地表蚀积量进行野外原位测定,并分析了不同下垫面的地表粗糙度和沉积土壤粒径特征。结果表明:研究区土壤颗粒组成以细砂含量占主导地位,粉粒、中砂与粗砂含量相对较低。与对照相比,各土地利用方式均能有效增加地表粗糙度,表现为:留茬地(0.32 cm)撂荒耕地(0.29 cm)沙质耕地(0.25 cm)黏质耕地(0.19cm)流沙地(0.02 cm)。土壤风蚀深度整体表现为流沙地沙质耕地撂荒耕地黏质耕地留茬地,5种利用方式下,留茬地风蚀程度最轻,风蚀深度仅为0.04 cm/d,分别较流沙地、沙质耕地、黏质耕地及撂荒耕地降低了99.03%、96.83%、94.29%与90.24%。因而,留茬地具有较好的防风蚀效益,是一种值得推广的土壤风蚀防治措施。     

Effects of soil erosion on long-term soil productivity in the black soil region of northeastern China

China's northeastern Black Soil Region, one of the country's most important crop production areas, has been seriously affected by soil erosion. This study evaluated the effects of soil erosion on the long-term productivity of this region. We used a modified productivity index (MPI) model (MPI is a number between 0 and 1, with 1 indicating highest productivity) to assess the current effects of soil erosion on soil productivity, as well as to predict long-term change in productivity. Samples from 21 black soil profiles yielded varying MPI values, although most MPI values were indicative of moderate productivity. Organic matter content and available water capacity impact MPI values in the region, whereas soil clay content and pH were less important. Overall, organic matter content and available water capacity of soil profiles decreased consistently as depth of erosion increased. Modeling indicated that MPI in the region will decrease by 0.0052 for each centimeter of topsoil eroded; this rate represents 1% of the current average MPI for the study area. The model predicts a 9.6% productivity reduction over 100 years and a 48.3% reduction over 500 years.     

C and N turnover in structurally intact soils of different texture

The turnover of native and applied C and N in undisturbed soil samples of different texture but similar mineralogical composition, origin and cropping history was evaluated at −10 kPa water potential. Cores of structurally intact soil with 108, 224 and 337 g clay kg⁻¹ were horizontially sliced and ¹⁵N-labelled sheep faeces was placed between the two halves of the intact core. The cores together with unamended treatments were incubated in the dark at 20 °C and the evolution of CO₂-C determined continuously for 177 d. Inorganic and microbial biomass N and ¹⁵N were determined periodically. Net nitrification was less in soil amended with faeces compared with unamended soil. When adjusted for the NO₃-N present in soil before faeces was applied, net nitrification became negative indicating that NO₃-N had been immobilized or denitrified. The soil most rich in clay nitrified least N and ¹⁵N. The amounts of N retained in the microbial biomass in unamended soils increased with clay content. A maximum of 13% of the faeces ¹⁵N was recovered in the microbial biomass in the amended soils. CO₂-C evolution increased with clay content in amended and unamended soils. CO₂-C evolution from the most sandy soil was reduced due to a low content of potentially mineralizable native soil C whereas the rate constant of C mineralization rate peaked in this soil. When the pool of potentially mineralizable native soil C was assumed proportional to volumetric water content, the three soils contained similar proportions of potentially mineralizable native soil C but the rate constant of C mineralization remained highest in the soil with least clay. Thus although a similar availability of water in the three soils was ensured by their identical matric potential, the actual volume of water seemed to determine the proportion of total C that was potentially mineralizable. The proportion of mineralizable C in the faeces was similar in the three soils (70% of total C), again with a higher rate constant of C mineralization in the soil with least clay. It is hypothesized that the pool of potentially mineralizable C and C rate constants fluctuate with the soil water content.     

Nitrogen mineralization in relation to site history and soil properties for a range of Australian forest soils

Rates of N mineralization were measured in 27 forest soils encompassing a wide range of forest types and management treatments in south-east Australia. Undisturbed soil columns were incubated at 20°C for 68 days at near field-capacity water content, and N mineralization was measured in 5-cm depth increments to 30 cm. The soils represented three primary profile forms: gradational, uniform and duplex. They were sampled beneath mature native Eucalyptus sp. forest and from plantations of Pinus radiata of varying age (<1 to 37 years). Several sites had been fertilized, irrigated, or intercropped with lupins. The soils ranged greatly in total soil N concentrations, C:N ratios, total P, and sand, silt, and clay contents. Net N mineralization for individual soil profiles (0–30 cm depth) varied from 2.0 to 66.6 kg ha^-1 over 68 days, with soils from individual depths mineralizing from <0 (immobilization) to 19.3 kg ha^-1 per 5 cm soil depth. Only 0.1–3.1% of the total N present at 0–30 cm in depth was mineralized during the incubation, and both the amount and the percentage of total N mineralized decreased with increasing soil depth. N fertilization, addition of slash residues, or intercropping with lupins in the years prior to sampling increased N mineralization. Several years of irrigation of a sandy soil reduced levels of total N and C, and lowered rates of N mineralization. Considuring all soil depths, the simple linear correlations between soil parameters (C, N, P, C:N, C:P, N:P, coarse sand, fine sand, silt, clay) and N mineralization rates were generally low (r<0.53), but these improved for total N (r=0.82) and organic C (r=0.79) when the soils were grouped into primary profile forms. Prediction of field N-mineralization rates was complicated by the poor correlations between soil properties and N mineralization, and temporal changes in the pools of labile organic-N substrates in the field.     

科尔沁沙地不同风沙土的风蚀特征

通过风洞实验，研究了科尔沁沙地两种典型风沙土的风蚀特征。流动沙丘土样和农田土样的风蚀率随风速的增加均呈幂函数关系增长。风干的农田土样在风速3．7m／s时开始出现风蚀现象，但流沙土样的临界起沙风速是4．3m／s。在低风速段．农田土样的风蚀率大于流沙土样，但当风速增加到大约5．7m／s以上时，流沙土样的风蚀率开始大于农田土样，并且差值随风速的增加而加大。两种土样的风蚀率随土样含水率的增加呈负幂函数关系迅速减小。但流沙土样风蚀率的减小要比农田土样更迅速。流沙土样的临界起沙风速随含水量的增加呈线性关系增长，而农田土样的临界起沙风速随含水量的增加呈二次幂函数关系增长。两种风沙土在风蚀特征方面的差异，主要是由质地不同引起的。增加土壤含水量，是本地区风季减小风蚀的有效方法。     

Dust storm erosion and its impact on soil carbon and nitrogen losses in northern China

There is increased awareness of the environmental impacts of soil carbon (C) and nitrogen (N) losses through wind erosion, especially in areas heavily affected by dust storm erosion. This paper reviews the recent literature concerning dust storm-related soil erosion and its impact on soil C and N losses in northern China. The purpose of our study is to provide an overview of the area of erosion-affected soils and to estimate the magnitude of soil C and N losses from farmland affected by dust storm erosion.According to the second national soil erosion remote-sensing survey in 2000, the area affected by wind erosion was 1.91 million km², accounting for 20% of the total land area in China. This area is expanding quickly as the incidence of heavy dust storms has greatly increased over the last five decades, mainly as a result of the intensification of soil cultivation. The economic and ecological damage caused by wind erosion is considerable. Heavily affected areas show a loss of nutrients and organic carbon in soils and the heavily degraded soils are much less productive. Compared with the non-degraded soil, the C and N contents in degraded soils have declined by 66% and 73%, respectively. The estimated annual losses per cm toplayer of soil C and N by dust storm erosion in northern China range from 53 to 1044 kg ha^− 1 and 5 to 90 kg ha^− 1, respectively. Field studies suggest that soil losses by wind erosion can be reduced by up to 79% when farmers shift from conventional soil tillage methods to no-till. Thus shifting to no-till or reduced tillage systems is an effective practice for protecting soil and soil nutrients. Our study indicates that soil conservation measures along with improved soil fertility management measures should be promoted in dry-land farming areas of northern China. As erosion is a major mechanism of nutrient withdrawal in these areas, we plead for the development of accurate methods for its assessment and for the incorporation of erosion, as a nutrient output term, in nutrient budget studies.     

Effects of charcoal production on maize yield,chemical properties and texture of soil

The effects of charcoal production on soil textural and chemical properties were investigated in Ejura, Ghana. The aim was to study the effects of heating and charcoal residue on maize yield, soil texture and soil chemical properties. Composite samples were taken from the 0–10 cm layer of soil at charcoal-making sites and from adjacent fields (control). Twelve sites were randomly selected for the study across the range of the Kotokosu watershed. Maize was planted in four selected locations on charcoal site soils (CSS) and adjacent field soils (AFS) to assess the impact of charcoal production on crop yield. There was a significant increase in soil pH, base saturation, electrical conductivity, exchangeable Ca, Mg, K, Na and available P in the soil at the kiln sites as compared to the adjacent soils. A relative change of up to 329% was observed in K while organic C and total N decreased by 9.8% and 12.8%, respectively. Organic C and total N were highly correlated ( P <0.01) and both parameters significantly ( P <0.05) depended on clay minerals in the soils. Soil texture was also modified with a significantly higher sand content and lower clay fraction in the CSS. The grain and biomass yield of maize increased by 91% and 44%, respectively, on CSS as compared to AFS. Further research to ascertain the long-term effects of charcoal production on the soil environment and the fertility of tropical soils is needed.     

海伦黑土有机碳和速效养分空间异质性分析

应用地统计学的方法结合GIS空间分析技术,对海伦市黑土农田区域采集的65个耕层(0~20cm)土壤样本进行了数据处理,并对其全碳、碱解氮、速效磷和速效钾含量的空间分异特征进行了研究和探讨。结果表明,海伦黑土全碳、碱解氮和速效钾半变量函数曲线的理论模型符合高斯模型,速效磷的理论最佳模型为球状模型。全碳、碱解氮和速效磷表现为中度空间相关,其空间变异受结构性因子和随机因子的影响各占一半。速效钾属于高度空间相关,其变化主要受结构性因子的影响。土壤全碳与碱解氮具有显著的正相关,二者在空间分布上吻合度较高。全碳和碱解氮都与速效钾呈显著负相关。地统计学分析土壤养分的空间变异特征和1978年海伦县自然资源综合考察对黑土农田土壤肥力调查的结果具有相同的异质性。利用地统计学分析的方法,对区域农业生产合理布局、综合治理水土流失都具有重要的意义。     

Soil properties influencing the denitrification potential of Flemish agricultural soils

The denitrification potential of the soil horizons between 0- and 90-cm depth of 20 agricultural fields, representative of the most frequent combinations of agricultural crops and soil textures in Flanders (Belgium), and the factors affecting the denitrification potential were studied in the laboratory under controlled conditions. The denitrification potential in the presence of an added soluble C and N source was measured at 15°C after saturation of air-dried soil samples with water. The denitrification potential of the lower horizons was generally negligible compared to the upper horizons. The lower denitrification potential of the deeper horizons could partially be explained by their limited C availability. The denitrification potential of the upper horizons strongly depended on texture. Based on this parameter the soils could be divided into three groups: soils with a high clay content (>30% clay) were characterised by a high denitrification potential (>8.33 µg N g^-1 dry soil day^-1); soils with medium texture had a medium denitrification potential, between 0.41 and 7.25 µg N g^-1 dry soil day^-1; and soils with a high sand content (>80% sand) had a low denitrification potential (<2.58 µg N g^-1 dry soil day^-1). In most cases, extending the saturation period during pre-incubation increased the denitrification potential. Comparison of the denitrification potential of the upper horizons with and without addition of a soluble C source showed that the denitrification potential of the upper horizons of these soils was limited by their percentage of endogenous C. The measured denitrification potentials indicate that denitrification losses in soils high in clay content can be important when NO₃ ^- concentrations are high.     

Soil C and N changes on conservation reserve program lands in the Central Great Plains

The Conservation Reserve Program (CRP) was initiated to reduce water and wind erosion on marginal, highly erodible croplands by removing them from production and planting permanent, soil-conserving vegetation such as grass. We conducted a field study at two sites in Wyoming, USA, in order to quantify changes in soil C and N of marginal croplands seeded to grass, and of native rangeland plowed and cropped to wheat–fallow. Field plots were established on a sandy loam site and a clay loam site on wheat–fallow cropland that had been in production for 60+ years and on adjacent native rangeland. In 1993, 6 years after the study was initiated, the surface soil was sampled in 2.5 cm depth increments, while the subsurface soil was composited as one depth increment. All soil samples were analyzed for total organic C and N, and potential net mineralized C and N. After 60+ years of cultivation, surface soils at both study sites were 18–26% lower (by mass) in total organic C and N than in the A horizons of adjacent native range. Six years after plowing and converting native rangeland to cropland (three wheat–fallow cycles), both total and potential net mineralized C and N in the surface soil had decreased and NO₃–N at all depths had increased to levels found after 60+ years of cultivation. We estimate that mixing of the surface and subsurface soil with tillage accounted for 40–60% of the decrease in surface soil C and N in long-term cultivated fields; in the short-term cultivated fields, mixing with tillage may have accounted for 60–75% of the decrease in C, and 30–60% of the decrease in N. These results emphasize the need to evaluate C and N in the entire soil solum, rather than in just the surface soil, if actual losses of C and N due to cultivation are to be distinguished from vertical redistribution. Five years after reestablishing grass on the sandy loam soil, both total and potential net mineralized C and N in the surface soil had increased to levels equal to or greater than those observed in the A horizon of the native range. On the clay loam soil, however, significant increases in total organic C were observed only in the surface 2.5 cm of N-fertilized grass plots, while total organic N had not significantly increased from levels observed in the long-term cultivated fields.