稻麦轮作条件下长期不同土壤管理对磷、钾和pH的影响

免耕土壤速效磷含量在整个面均低于耕翻土壤；0～5cm土壤生物量磷含量免耕高于耕翻，5～15cm免耕低于耕翻；施肥和休闲土壤均维持了较高的有效磷和生物量磷水平。种植作物的免耕土壤缓效钾含量与耕翻土壤没有显差异，0～5cm速效钾含量呈免耕高于耕翻的趋势，休闲的免耕土壤整个剖面缓效钾和速效钾含量都高于耕翻；免耕土壤缓效钾的分布无明显的层次性，速效钾则相反；施肥明显提高土壤速效钾含量，但对缓效钾的影响不大。长期免耕没有导致土壤酸化；免耕土壤pH与耕翻土壤没有差异。     

长期施用氮磷肥对塿土钾素的影响

以长期定位施肥与小麦—玉米轮作田间试验为平台,研究了施肥对塿土土壤钾素的影响以及冬小麦生育期内土土壤速效钾、缓效钾及微生物量钾的变化。结果表明:氮磷肥不同施用量对土壤全钾基本没影响,而对速效钾、缓效钾及微生物量钾的影响较大。土壤速效钾和缓效钾均随施肥量的增加而减少,而微生物量钾随施肥量的增加而增加,且0—20cm土层土壤速效钾、缓效钾及微生物量钾含量均高于20—40cm土层。小麦生育期内,土壤速效钾和微生物量钾含量的变化一致,均是先上升再下降,再上升,又下降的变化。土壤中微生物量钾与有机碳、全氮均呈显著正相关关系,而与土壤速效钾和缓效钾无显著相关关系。氮磷肥在一定施用量范围内,促进了土壤无机态钾素的消耗,而提高了土壤微生物量钾的含量。土壤钾素的这种变化是作物吸收、施肥等因素共同作用的结果。     

长期施用氮磷肥对(土娄)土钾素的影响

以长期定位施肥与小麦一玉米轮作田间试验为平台,研究了施肥对(土娄)土土壤钾素的影响以及冬小麦生育期内(土娄)土土壤速效钾、缓效钾及微生物量钾的变化.结果表明:氮磷肥不同施用量对土壤全钾基本没影响,而对速效钾、缓效钾及微生物量钾的影响较大.土壤速效钾和缓效钾均随施肥量的增加而减少,而微生物量钾随施肥量的增加而增加,且0-20 cm土层土壤速效钾、缓效钾及微生物量钾含量均高于20-40 cm土层.小麦生育期内,土壤速效钾和微生物量钾含量的变化一致,均是先上升再下降,再上升,又下降的变化.土壤中微生物量钾与有机碳、全氮均呈显著正相关关系,而与土壤速效钾和缓效钾无显著相关关系.氮磷肥在一定施用量范围内,促进了土壤无机态钾素的消耗,而提高了土壤微生物量钾的含量.土壤钾素的这种变化是作物吸收、施肥等因素共同作用的结果.     

在不同水土保持耕作中磷和钾的分层研究

通过对25cm以上土层精确的定量样品分析以及在实验室对有效磷和有效钾进行必要的测定,研究了在长期水土保持耕作措施下土壤中磷和钾的分布状况。结果表明在垄作、沟种及平耕地的磷和钾均具有明显的垂直分层性。一般地,最上层5cm土层中的有效磷、钾含量是5~15cm土层中的3.5倍多。     

等高固氮植物篱模式对坡耕地土壤养分的影响

对坡耕地土壤剖面养分的研究结果表明，等高固N植物篱模式对养分的影响表现在对养分的归还和在土壤剖面的再分配。培植植物篱5年后，该系统中与植物篱不同距离位点0－40cm土层土壤有机质和全N含量分别比对照高1.2-2.3倍和0．5－0．7倍，有效磷和速效钾显著增加，表明植物篱可有效改善土壤养分状况。研究结果还表明在植物篱模式下0－60cm土层土壤有机质和全N含量均比种植带相应土层高，0－20cm土层有效磷和有效钾含量明显比种植带各位点相应土层高，表明植物篱与农作物之间无养分竞争；种植带下部0－40cm土层养分状况比种植带中部和上部相应土层优越，种植带上部各土层养分状况最佳；植物篱内40cm以下土层中有效磷和速效钾含量比种植带下相同土层低，说明植物篱根系可将土壤深层的P、K吸收并通过刈割枝叶返还种植带，发挥养分泵的功能。因此，植物篱通过生物固N和养分泵的功能提高种植带上层土壤养分含量和促进矿物养分循环。     

冀东滨海平原稻区土壤钾库状况与变化

通过对大面积土壤各形态钾的定位监测,研究了1982～2016年30余年间冀东滨海平原稻区土壤钾库的现状、变化及其影响因素,分析了成土母质、耕种施肥等对该区域土壤钾库的影响。主要结果:(1)冀东滨海平原稻区土壤全钾、缓效钾、速效钾平均含量分别为23.04 g/kg、1 615.94 mg/kg、247.37 mg/kg。种稻40～70余年间,各年限土壤全钾、速效钾分别维持在23.96 g/kg、257.41 mg/kg左右。与种稻40年相比,缓效钾年均减少20.19 mg/kg。(2)不同母质土壤全钾、缓效钾、速效钾含量大小顺序为:海相沉积物、泻湖沉积物河流冲积物;土壤缓效钾/全钾和速效钾/全钾均为海相沉积物、泻湖沉积物河流冲积物,3种母质发育的土壤速效钾/缓效钾差异不显著;海相沉积物、泻湖沉积物二者之间的各形态钾含量及比值均无显著差异。(3)土壤速效钾与土壤盐分、有机质均呈显著正相关关系;土壤速效钾与有效磷无显著相关关系。主要结论:随种植年限增加,土壤全钾、速效钾无显著变化,缓效钾显著下降。海相沉积物、泻湖沉积物发育的土壤全钾、缓效钾、速效钾的含量均显著高于河流冲积物,土壤速效钾与盐分呈显著正相关关系。稻区土壤钾输入量小于水稻钾的输出量,随着种稻年限的增加,逐渐消耗土壤缓效钾。     

植茶年限降低土壤团聚体稳定性并促进大团聚体中钾素释放

  【目的】   研究植茶年限对土壤团聚体稳定性以及供钾和释钾能力的影响，以期为指导茶园土壤施肥、促进茶园生态系统可持续发展提供科学依据。   【方法】   采集四川省雅安市草坝镇茶园农业生态区植茶年限分别为5 a、10 a、15 a和30 a的原状土 (0—15和15—30 cm)，利用湿筛法分离出粒径> 2 mm、0.25～2 mm、0.053～0.25 mm和 < 0.053 mm的土壤样品，分析其稳定性、各粒级土壤速效钾和缓效钾含量，并采用四苯硼钠 (NaTPB) 浸提法探讨了其有效钾释放特征。   【结果】   各植茶年限土壤均以大团聚体 (粒径> 0.25 mm) 为主，质量占比为75.87%～95.75%，但随植茶年限增加，土壤大团聚体比例显著减少 (P < 0.05)，且各土层土壤团聚体平均重量直径 (MWD) 均明显减小。15 a和30 a的茶园土壤中同一粒级团聚体的土壤速效钾和缓效钾含量均高于5 a和10 a茶园。5 a和10 a茶园土壤中各粒级团聚体中速效钾含量分布较为均匀，15 a和30 a的茶园土壤速效钾含量随大粒级团聚体的增加而增高。土壤缓效钾在各年限茶园均表现为微团聚体 (粒径< 0.25 mm) 高于其他团聚体组分。各茶龄土壤不同粒级团聚体有效钾累积释放量在102.3～236.5 mg/kg，且呈现前期快、后期较稳定的趋势，释放过程均以扩散模型拟合程度最好。进一步比较表明，植茶15 a和30 a的土壤团聚体有效钾累积释放量明显高于植茶5 a和10 a时的土壤。   【结论】   茶园土壤中以大团聚体 (粒径> 0.25 mm) 的比例最高，但随植茶年限的增加，微团聚体 (粒径< 0.25 mm) 比例增加，土壤结构稳定性降低，特别是15—30 cm土层土壤。大团聚体的减少促进了土壤速效钾、缓效钾的释放，因而，植茶15 a和30 a的土壤速效钾含量较5 a和10 a的茶园高，但是会耗竭土壤钾库，不利于茶园的可持续利用。     

水旱轮作条件下长期自然免耕对土壤理化性质的影响

通过实地调查，采集不同免耕年限土壤进行对比分析，研究了水早轮作条件下长期自然免耕对土壤理化性质的影响。结果表明：水早轮作条件下，不进行秸秆覆盖长期双免耕的农耕措施对土壤理化性质有较大的影响。0～15cm土层容重随免耕年限的增加而减小，免耕7～8年后趋于稳定，而15～30cm土层的容重几乎不受影响；土壤团聚度随免耕年限的增加而增加，表层增加更明显；土壤有机质、全氮、速效氮和磷含量均随免耕年限增加而逐年上升，并且有明显表聚现象，但对速效钾的影响则不明显。     

水旱轮作制下连续秸秆覆盖对土壤理化性质和作物产量的影响

采用田间定位试验研究方法,于2008～2010连续3年研究了水旱轮作制下连续秸秆覆盖对土壤理化性质和作物产量的影响。结果表明,连续秸秆覆盖显著降低了土壤表层（05cm）容重,提高了05cm和515cm土层土壤含水量。同时连续秸秆覆盖还田还可以显著提高025cm土层土壤有机质、碱解氮、速效磷和速效钾含量。秸秆覆盖对表层（05cm）土壤养分状况的效应更明显。秸秆连续覆盖5季后,05cm土层土壤速效钾含量的增幅（7.64%～15.33%）速效磷（7.52%～10.03%）碱解氮（7.30%～8.74%）有机质（6.08%～7.53%）。秸秆覆盖还田后,可以提高作物产量。其中旱季作物（小麦、油菜）的增产效应要高于水季作物（水稻）,并且随着秸秆还田年限和用量的增加,作物的增产幅度也随之提高。起主要作用的产量构成因素是小麦、水稻的有效穗数以及油菜单株角果数和每角粒数。     

控释肥养分释放对坡耕地土壤磷钾损失及花生产量的影响

以沂蒙山区坡耕地花生为研究对象，探究控释肥养分释放对土壤磷钾流失特征、花生产量及磷钾利用率的影响。试验设五个处理，分别为全量和减量30%控释肥（CRF1，CRF2），全量和减量30%普通复合肥（CF1，CF2）及不施肥处理。结果表明：控释肥在田间土壤中的释放规律与花生植株磷钾吸收特征曲线相吻合，配合土壤磷钾供应量，满足了不同时期花生植株养分需求。等量施肥下，控释肥处理荚果产量较普通复合肥处理显著增加5.75-12.01%，且减量30%控释肥处理较CF1增产3.75%；控释肥处理中植株磷钾吸收量分别较普通复合肥提高11.46-11.78%和10.44-12.38%，其中CRF2磷钾表观利用率最高，分别为45.48%和51.84%。土壤有效磷和速效钾含量均随深度的增加而下降，CF1和CF2苗期时0-40 cm的表层土中有效磷含量高于控释肥处理，花针期以后趋势相反，但40 cm以下土层中各处理差异不显著；控释肥处理显著提高了花针期以后0-60 cm土层速效钾含量，各处理不同时期60-100 cm土层速效钾差异不显著。前三次产流事件中，控释肥处理显著减少了地表径流水中有效磷和速效钾含量，各处理后期径流水中水溶性磷和钾含量降低且趋于稳定，处理间差异不显著。因此，控释肥能够减少坡耕地土壤有效磷和速效钾的径流和淋溶损失，提高花生养分利用率和产量，促进生态农业的可持续发展。     

稻田系统管理措施对微生物生物量C、N、P的长期影响

A 12-year field experiment was conducted to investigate the effect of different tillage methods and fertil-ization systems on microbial biomass C,N and P of a gray fluvo-aguic soil in rice-based cropping system .Five fertilization treatments were designed under conventional tillae(CT) or on tillage(NT) system:no fertilizer(CK) ; chemical fertilizer only(CF) ; combining chemical fertilizer with pig manure(PM); combining chemical fertilizer with crop straw (CS) and fallow (F). The results showed that biomass C,N and P were enriched in the surface layer of no-tilled soil,whereas they distributed relatively evenly in the tilled soil,which might result from enrichment of crop resdue,organic manure and mineral fertilzer,and surficial developent of root systems under NT.Under the cultivation system NT had slightly greater biomass C,N and P at 0-5 cm depth ,significantly less biomass C,N and P at 5-15 cm depth ,less microbial biomass C,N and equivalent biomass P at 15-30 cm depth as compared to CT,indicating hat tillage was beneficial for the multiplication of organims in the plowed layer of soil.Under the fallow system,biomass C,N and P in the surface layer were significantly greater for NT than CT while their differences between the two tillage methods were neligible in the deeper layers.In the surface layer,biomass C,N and P in the soils amended with oranic manure combined with mineral fertilizers were significantly greater than those of the treatments only with mineral fertilizers and the control.Soils without fertilzer had the least biomass nutrient contents among the five fertilization treatments.Obviously,the long-term application of organic manure could maintain the higher activity of microorganisms in soils.The amounts of biomass C,N and P in the fallowed soils varied with the tillage methods;they were much greater under NT than under CT,especially in the surface layer,suggesting that the frequent plowing could decrease the content of organic matter in the surface layer of the fallowed soil.     

Improvement of the physical fertility of a degraded Alfisol with planted and natural fallows under humid tropical conditions

Abstract. Topsoil (0–15 cm) bulk density, aggregate stability, soil dispersibility, water retention and infiltration were measured between 1989 and 1996 on an Alfisol under rehabilitation in southwestern Nigeria. The planted leguminous species were Pueraria phaseoloides, Senna siamea, Leucaena leucocephala, Acacia leptocarpa and A. auriculiformis. Also, plots with natural fallow and maize/cassava intercropping were included. Level (minimum) and mound tillage with hoes was adopted for the cultivated areas under study after 4 and 6 year fallow periods. Under fallow, the soil bulk density decreased from1.56 to 1.11 t m73.The continuously cropped treatment (level tillage) had significantly higher bulk density than the fallowed subplots after 6 years. Mean soil penetrometer resistance ranged from 75 to 157 kPa for fallowed plots and from 192 to 295 kPa for the continuously cropped (level tillage) subplot. Surface soilwater contentswere similar for all the treatments during the soil strength measurements. Although soil aggregates were generally of low stability and not well formed, they were improved by fallowing.
Soil structural improvement by planted fallows was similar to that by natural fallow, but the trees were more promising for long-term fallow (>6 years) than the herbaceous P. phaseoloides. However, the improvement in soil structure after 4 or 6 year fallow could not be maintained in subsequent cropping. Furthermore, the significant improvement in soil bulk density caused by A. auriculiformis and natural fallow was more rapidly lost on the cultivated subplots compared with other fallow treatments. Thus, soil structure recovery under a fallow does not imply a sustained improvement when stress is applied to this soil. Post-fallow soil management options such as residue incorporation and tillage to ameliorate compaction or soil strength will be necessary to enhance the improvements by fallow species.     

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.     

Cultivation affected the level of inorganic phosphorus more than the organic pool of the Pampas soils in Argentina

The effects of land use on soil organic and inorganic phosphorus (P) stocks were assessed in the Pampas, Argentina. Three hundred and eighty-six paired sites widely distributed over an area of ca. 50 Mha were sampled. Land use types included soils under trees, uncropped soils, cropped soils at the pasture phase of a mixed rotation, cropped soils at the crop phase of a mixed rotation, and flooded soils. Slight differences in organic P stocks were found among land uses. Organic P was 21–35% lower in flooded soil than in the other treatments in the 0–100 cm depth. Inorganic P was significantly lower (ca. 27%) in pasture and cropped soils than in the uncropped controls at 0–25 cm depth. The ratios of organic P/inorganic P and organic C/organic P decreased with depth and did not significantly differ among the sites. The influence of cultivation on inorganic P to a depth of 100 cm depended on the initial phosphorus content of the soil. Soils rich in phosphorus lost substantial amounts of their phosphorus stocks, in some cases losses were as high as 70%, whereas phosphorus-poor soils presented only small changes in their inorganic P levels.     

温室蔬菜栽培对土壤DTPA浸提态铁锰铜锌含量的影响

对沈阳市大民屯镇蔬菜生产基地温室番茄栽培条件下0~30 cm土层土壤DTPA浸提态微量元素含量的调查表明,温室土壤DTPA浸提态Mn、Cu、Zn含量高于相邻的露地菜田,0~20 cm土层DTPA-Fe含量低于露地土壤。与露地菜田相比,高量施肥条件下温室土壤DTPA-Zn富集是微量元素变化的一个主要特征,11年和5年温室土壤0~20 cm土层DTPA-Zn平均含量分别是露地菜田的4.0和4.8倍。温室栽培条件下高量施用有机肥、土壤pH降低及土壤Eh变化是引起土壤DTPA浸提态微量元素变化的主要因素。     

东北黑土有机碳的分布及其损失量研究

为了分析东北黑土土壤有机碳(SOC)的分布特征及其开垦以来黑土SOC的损失程度,我们于2004～2005年在黑龙江和吉林两省采集了32个自然黑土剖面样品,在每个自然黑土样品附近对应采集32个景观条件相似的耕作黑土样品。结果表明,自然黑土样品0～30cm土层SOC含量平均为32.20 g kg-1,最高可达63.46 g kg-1,黑龙江省自然黑土SOC含量(34.55 g kg-1)高于吉林省(23.80 g kg-1)。耕作土壤SOC平均含量为22.71 g kg-1,远低于自然土壤。受温度的影响,随着纬度的增加,自然黑土与耕作黑土SOC含量逐渐递增。由于土壤侵蚀以及耕垦和去除作物残留物等农业管理措施的综合作用,使得耕作黑土表层SOC含量小于自然黑土。与自然黑土相比,耕作黑土0～10cm土层SOC损失量在26.84%～46.57%之间,亚表层损失相对较少。黑土SOC含量下降也是土壤水土流失致使黑土层变薄的一个直接表现。耕作黑土表层流失厚度可以通过自然与耕作黑土剖面SOC含量的分异差值来估算。通过对土壤剖面上SOC的分布进行校正剔除土壤侵蚀的影响后得到的同等深度SOC含量的差值才可视为由耕作以及有机质输入量差异等因素造成的SOC损失量。未经校正而进行的自然黑土和耕作黑土同一深度SOC含量的比较可能过高估计了农业管理措施对土壤SOC损失量的影响。     

Ecological study on the dynamics of soil organic matter and its related properties in shifting cultivation systems of Northern Thailand

Abstract

There is a large number of hill people in northern Thailand, who practices shifting cultivation. In order to analyze the soil ecological problems involved in the transition from traditional shifting cultivation to more intensive upland farming, the authors carried out comparative studies on the dynamics of organic matter and its related properties in soils both in the traditional shifting cultivation systems adopted by Karen people and more intensive upland farming practiced by Thai and Hmong people in the area. The contents of organic matter and available N in the surface 10 cm layers of soil from the fields continuously cultivated were lower than those in soils under prolonged fallow (more than 10 y) or natural forest. Based on the rate of soil respiration, the amount of organic matter decomposed within 1 y was estimated to reach nearly 10% of that stored in the upper 50 cm layers of the soil profile in the upland crop fields. These results indicate that the organic matter-related resources markedly decreased under continuous cropping. The contents of C, N, and P in the microbial biomass of the surface 10 cm layers of soil ranged from 0.37 to 2.09 mg C g^?l soil, from 22.7 to 188 µg N g^?l soil, and from 6.1 to 65.7 µg P g^?l soil, respectively. Since the contents of microbial C, N, and P in the surface soils were generally higher under prolonged fallow and natural forests than in the fields continuously cultivated, the microbial activity and/or the amounts of C, N, and P available for biological activity seemed to have declined under continuous upland farming. The incubation experiment to assess the N mineralization pattern showed two remarkable characteristics: 1) there was an initial time lag until active mineralization of N occurred in the soils from young fallow forest and 2) the soil burning effect was observed after burning in the fields under prolonged fallow. The active process of nitrification after N mineralization was always associated with a sharp fall in soil pH, suggesting that soil acidification was promoted and basic cations were lost from the soils. In conclusion, rapid deterioration of the soil organic matter-related properties in cropping fields can be considered to be one of the ecological reasons why upland fields must be returned to fallow again a few years after forest reclamation in traditional shifting cultivation systems. Therefore, in alternative farming systems with more intensive land use, it is essential to apply organic materials into soils to decrease the rate of soil degradation, or to improve the soil fertility, in avoiding soil acidification along with nitrification.     

西安市郊区日光温室大棚番茄施肥现状及土壤养分累积特性

调查了西安市郊区100余个日光温室栽培番茄的施肥现状,并采集了60个日光温室土壤剖面样品,测定了土壤养分及盐分含量。结果表明,当地番茄的氮磷钾化肥的平均施用量分别为N 600 kg hm-2,623 kg P2O5hm-2和497 kgK2O hm-2。过量施肥特别是过量施用磷肥比较普遍。日光温室土壤0~20 cm和20~40 cm土层有机质、有效磷、速效钾含量均明显高于露地。不同地点日光温室土壤0~100 cm土层硝态氮含量及土壤电导率存在差别,但其含量均高于露地土壤,其中日光温室土壤0~100 cm土层硝态氮含量较露地的增加幅度在127%~433%之间。由于研究地区土壤多发育在河流冲积物上,地下水位埋藏浅,土壤质地相对较粗,因此,不合理施肥引起的土壤养分累积及损失,以及由此带来的环境问题值得关注。     

Long-term cultivation effects on hydraulic properties of a Walla Walla silt loam

Hydraulic properties of a Walla Walla silt loam were significantly changed by 50 years or more of cultivation under either a wheat-peas rotation (tillage depth 30 cm) or a wheat-summerfallow rotation (tillage depth 15 cm) as compared with no cultivation. Soil pH was reduced to depths as great as 60 cm in the cultivated sites; dry bulk density was increased to depths as great as 40 cm. Expressions of these changes were greater in the wheat-peas rotation because tillage was deeper than in the wheat-summerfallow rotation. Small reductions in soil organic matter were also noted in the cultivated sites. In the 60- to 90-cm depth, all three sites had similar bulk density, pH, cation exchange capacity, soil texture, desorption water characteristic, and hydraulic conductivity. In the upper 40-cm layer the desorption water characteristic showed that cultivation produced more smaller pores at the expense of large pores; in the upper 30-cm layer of the cultivated soils hydraulic conductivity was reduced at least 10-fold for water potentials > −100 cm of H₂O. Steady-state drainage profiles and associated assumptions suggest that long-term cultivation increased the hydraulic gradient in the upper 35 cm, and that the low saturated conductivity of the 0- to 15-cm layer had an overall drying effect on the 15- to 35-cm layer. In the cultivated soils increased runoff and denitrification in the plow layer should both be expected and water relations in the 15- to 35-cm layer should favor microorganisms sensitive to high water potentials. Simulations suggested that long-term cultivation decreased evaporation rates an estimated 40% and in wet soil, increased the drying time needed to attain optimum moisture for tillage.     

Wheat and barley straw decomposition under field conditions: Effect of soil type and plant cover on weight loss,nitrogen and potassium content

The decomposition of wheat and barley straw buried in a coarse sand and a sandy loam soil was followed under field conditions using chopped straw enclosed in mesh bags. The straw was buried at 10 cm depth in early autumn. Plots were either kept fallow or covered in plants during the summer and autumn following straw burial. Samples were recovered on 10 dates over 15 months and were analyzed for weight loss, C, N and K content. Results obtained were corrected for organic matter and nutrients introduced to the straw by soil entering the mesh bags. Soil temperatures and soil moisture contents were determined.The plant cover reduced soil temperatures, soil moisture contents and straw weight loss rates during the summer. The overall weight loss pattern was similar for the two straw types. In both soils, the straw showed an initial weight loss of 30% during the first month after burial. After 6 months (spring), the straw had lost about 50% of its weight. The weight loss then proceeded faster in fallow than in the planted plots. Seasonal changes in weight loss rates were relatively small. The absolute weight loss rates, determined by a simple linear regression model, ranged from 0.10% day^?1in planted plots of the coarse sand to 0.17% day^?1 in uncovered plots of the sandy loam. For planted plots, weight losses after 15 months of exposure were 67% in the coarse sand and 78% in the sandy loam. Corresponding values for fallowed plots were 75 and 92%, respectively.Initial straw N contents differed, but overall N dynamics was less influenced by straw type than by soil type. In the sandy loam, net immobilization began just after straw burial and reached a maximum in the spring, where 3 mg N g^?1 initial straw was immobilized. The N release was more rapid in fallowed than in planted plots. In the coarse sand, N was initially leached from the straw, and the immobilization of N was less extensive than in the sandy loam. Further, the release of N from the straw was grossly similar in fallowed and planted plots. For all treatments, the net release of N from the straw began when the C-to-N ratio was between 28 and 35.Most of the straw K content was leached during the first month after straw burial. Subsequent straw K content was not affected by initial K content or plant cover.